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Intercropping and whitefly (Homoptera: Aleyrodidae) management

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Intercropping and whitefly (Homoptera: Aleyrodidae) management
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Smith, Hugh Adam, 1963-
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viii, 163 leaves : ; 29 cm.

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Beans ( jstor )
Corn ( jstor )
Crops ( jstor )
Eggs ( jstor )
Intercropping ( jstor )
Intercrops ( jstor )
Mulches ( jstor )
Nymphs ( jstor )
Tomatoes ( jstor )
Trap crops ( jstor )
Agricultural pests ( lcsh )
Aleyrodidae -- Control ( lcsh )
Dissertations, Academic -- Entomology and Nematology -- UF ( lcsh )
Entomology and Nematology thesis, Ph. D ( lcsh )
Intercropping ( lcsh )
City of Gainesville ( local )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1999.
Bibliography:
Includes bibliographical references (leaves 143-162).
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Printout.
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Vita.
Statement of Responsibility:
by Hugh Adam Smith.

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INTERCROPPING AND WHITEFLY (HOMOPTERA: ALEYRODIDAE)
MANAGEMENT















By

HUGH ADAM SMITH


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1999














For George














ACKNOWLEDGMENTS

I am extremely grateful to Dr. Robert McSorley for serving as the chairman of my

committee. I feel very fortunate to have benefited from the depth of his knowledge and

his guidance. I am also very grateful to Dr. Heather McAuslane, who has helped with this

research project from its initial stages to the bitter end. I want to thank Debbie Boyd,

who gave me my first orientation in working with whiteflies, and my friend Dr. Rose

Koenig, without whom chapter 2 would not have happened. I would also like to thank

Dr. Jon Allen and Dr. Raymond Gallaher for serving on my committee.

I am grateful to Dr. Don Dickson for allowing his crew to help me with the

research at Green Acres. Without the help I received from Reggie Wilcox, the studies

described in chapters 3 and 5 would have been far more difficult, if not impossible, to

carry out. I am grateful to Dr. Jerry Stimac for taking the time to help me understand

sampling theory, one of my objectives when I started the PhD program. I am heavily

indebted to Jay Harrison, formerly of IFAS statistics, for many hours of assistance. I am

grateful to Dr. Greg Evans of the Division of Plant Industry for help with identification of

whitefly parasitoids and to Dr. Avas Hamon (also of DPI) and Dr. Andrew Jensen

(formerly of the USDA, Beltsville) for identification of whiteflies from Guatemala. I

want to thank John Frederick for all sorts of help. I thank Clay Scherer for his friendship.

As always, I am grateful to Dr. John Capinera for support and good advice.








I want to thank Ing. Baltasar Moscoso, formerly head of ICTA, for facilitating my

research with that organization in 1998. I would not have been able to overcome the

various logistical hurdles of carrying out field research in Guatemala without the constant

support of Ing. Arnoldo Sierra, the head of the ICTA station in San Jer6nimo. It was a

pleasure getting to know Dr. Robert MacVean of the Bucks County Organization for

Intercultural Advancement, who cleared all kinds of diplomatic hurdles for me and my

vehicle and without whom my research in Guatemala would have been very difficult.

Lic. Margarita Palmieri, Lic. Carolina Muhioz, Estela de Flores, Dr. Chuck MacVean,

Lic. Catherine Cardona, and Dr. Jack Schuster of the Universidad del Valle all

contributed to my research with their resources, expertise, and kindness. I am very

grateful to Rodolfo Guzman and Rend Santos of Altertec and to Juan, Leo, Felix, and

Don Tancho of the ICTA station in San Jer6nimo for their friendship during my stay. My

friend Antonio Garcia Torres managed the field plots for the research in Guatemala and

contributed greatly to the success of that research. Special thanks go to Chuck and

Rodolfo who have been there since the beginning.

The Southeastern Sustainable Agriculture Research and Education program of the

USDA provided the funds for the research reported in chapter 2. The research described

in chapters 4 and 5 was funded by a fellowship provided by the National Security

Education Program. I am extremely grateful to the reviewers of the original research

proposals who recommended them for funding.

Finally, I thank my mother, Nancy Smith, and my grandparents, Ruth Freeman

and Anselm Fisher, without whom I could not have done any of this.














TABLE OF CONTENTS

page

ACKNOW LEDGM ENTS ................................................ iii

A B STR A CT .......................................................... vii

CHAPTERS

I LITERATURE REVIEW AND RESEARCH GOALS ...................... 1

W hiteflies ......................................................... 1
Intercropping ...................................................... 11
Research O bjectives ................................................ 21

2 THE EFFECT OF SILVER REFLECTIVE MULCH AND A SUMMER
SQUASH (CUCURBITA PEPO L.) TRAP CROP ON DENSITIES OF
IMMATURE BEMISIA ARGENTIFOLII (HOMOPTERA:
ALEYRODIDAE) ON ORGANIC BEAN (PHASEOLUS VULGARIS L.) ... 23

Introduction ....................................................... 23
M aterial and M ethods ............................................... 24
R esults ........................................................... 27
D iscussion ........................................................ 30
C onclusion ....................................................... 33

3 POTENTIAL OF FIELD CORN (ZEA MA YS L.) AS A BARRIER CROP
AND EGGPLANT (SOLANUM MELONGENA L.) AS A TRAP CROP
FOR MANAGEMENT OF THE SILVERLEAF WHITEFLY, BEMISIA
ARGENTIFOLIL (HOMOPTERA: ALEYRODIDAE) ON BEAN
(PHASEOLUS VULGARIS L.) IN NORTH FLORIDA .................. 49

Introduction ....................................................... 49
M aterials and M ethods .............................................. 51
Results and D iscussion .............................................. 57
C onclusion ....................................................... 6 1








4 THE ROLE OF CROP DIVERSITY IN THE MANAGEMENT OF A
WHITEFLY (HOMOPTERA: ALEYRODIDAE) SPECIES COMPLEX
ON BEAN (PHASEOLUS VULGARIS L.) AND TOMATO
(LYCOPERSICON ESCULENTUM MILL.) IN THE SALAMA VALLEY,
BAJA VERAPAZ, GUATEMALA ................................. 68

Introduction ....................................................... 68
M aterials and M ethods .............................................. 73
Results and D iscussion .............................................. 86
C onclusions ....................................................... 96
Sum m ary ......................................................... 99

5 A COMPARISON OF SOME ARTHROPOD GROUPS ON
MONOCROPPED AND INTERCROPPED TOMATO (LYCOPERSICON
ESCULENTUM MILL.) IN BAJA VERAPAZ, GUATEMALA .......... 112

Introduction ...................................................... 112
M aterials and M ethods ............................................. 113
R esults .. .. .. ....... .. ..... ..... ....... ....... ..... .... ... .. .. .. 116
D iscussion ....................................................... 118

6 METHODS FOR SAMPLING IMMATURE STAGES OF
BEMISIA ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE)
ON BEAN (PHASEOLUS VULGARIS L.) ........................... 122

Introduction ...................................................... 122
M aterials and M ethods ............................................. 124
Results and D iscussion ............................................. 127
Tim e Costs and Conclusions ......................................... 131
Sum m ary ..... .................................................. 132

7 SUMMARY AND CONCLUSIONS .................................. 139

APPENDIX SOME WHITEFLY HOSTS AT DIFFERENT ELEVATIONS
IN EASTERN GUATEMALA ............................ 142

REFEREN CES ....................................................... 143

BIOGRAPHICAL SKETCH ............................................. 163














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

INTERCROPPING AND WHITEFLY (HOMOPTERA: ALEYRODIDAE)
MANAGEMENT

By

Hugh Adam Smith

December 1999

Chairman: Robert McSorley
Major Department: Entomology and Nematology

Field studies were carried out in north central Florida and central Guatemala to

examine the effect of intercropping on numbers of whiteflies (Homoptera: Aleyrodidae).

Squash (Cucurbitapepo) and eggplant (Solanum melongena) were tested as trap crops,

and field corn (Zea mays) was tested as a barrier crop, for management of the silverleaf

whitefly (Bemisia argenti blii) on common bean (Phaseolus vulgaris) in Florida between

1995 and 1997. Three distinct mixed and row-intercropping arrangements with poor and

non-host crops were tested in 1998 in Guatemala to reduce densities of immature

greenhouse whitefly (Trialeurodes vaporariorum) and sweetpotato whitefly (Bemisia

tabaci) on common bean and tomato (Lycopersicon esculentum). In addition, a plastic

mulch painted with a reflective aluminum strip was tested to reduce immature stages of








B. argentifolii alone and in combination with the squash trap crop in Florida, and two

pesticides were tested as subplot treatments in two intercropping studies in Guatemala.

Counts from yellow sticky traps in the barrier test in Florida indicated that wind

direction was the primary factor determining movement of adult B. argentifolii, and that

the presence of a corn barrier only marginally affected the penetration of adults into test

plots. None of the intercropping treatments consistently reduced densities of immature

whiteflies compared to densities on crops grown in monoculture. Some intercropping

treatments in the Guatemala studies reduced plant quality, making it difficult to interpret

results. The reflective aluminum mulch treatment significantly reduced egg counts during

the first week of sampling in two out of three years in Florida. Imidacloprid protected

bean from damage by whiteflies and other sucking insects during the dry season in

Guatemala, and reduced densities of immature whiteflies on tomato during the rainy

season. A detergent and oil spray rotation did not protect bean from whitefly or other

sucking insects during the dry season. Combining aluminum mulch or imidacloprid with

intercropping treatments did not provide any additional advantage over using them alone.

The lack of effect of intercropping on whitefly counts is discussed in relation to whitefly

host-finding mechanisms and mobility. Methods for sampling immature stages of

whiteflies on common bean are compared to determine the preferred sample unit and

location within the plant canopy for sampling.














CHAPTER 1
LITERATURE REVIEW AND RESEARCH GOALS

Whiteflies

According to the system of classification commonly used in the United States,

whiteflies (family Aleyrodidae) belong to the order Homoptera (Borror et al. 1989). As

members of the suborder Sternorrhyncha, whiteflies are closely related to the psyllids,

aphids, and scale insects (Campbell et al. 1996). They are considered by some to be the

tropical equivalent of the aphids (Byrne and Bellows 1991). They occur throughout

warm regions of the world, and under certain conditions, in temperate regions (Bink-

Moenen and Mound 1990. Mound and Halsey 1978). The center of origin for aleyrodids

is unknown, although Pakistan is considered likely because of the diversity of whitefly

parasitoids in that region (Brown et al. 1995, Mound and Halsey 1978).

All known whiteflies are phloem-feeders (Byrne and Bellows 1991). Of the more

than 1,200 species described (Bink-Moenen and Mound 1990), the majority are

monophagous or oligophagous (Brown et al. 1995). However, polyphagy is common

among economically important species, of which there are probably fewer than 20 (Byrne

et al. 1990). Whiteflies cause crop losses by extracting water, amino acids and

carbohydrates from the phloem, and by the production of honeydew, a sugar-rich excreta

which accumulates on foliage and serves as a substrate for sooty molds (Hendrix et al.

1996). Sooty molds impede photosynthesis and reduce the quality of cotton (Gossypium

hirsutum L.) lint and fruit (Byrne et al. 1990). In addition to causing mechanical damage,









at least three whitefly species cause widespread crop losses by vectoring plant viruses.

Bemisia tabaci (Gennadius), the sweetpotato whitefly, vectors dozens of debilitating

geminiviruses to a broad range of agronomic and horticultural crops (Brown 1994).

Bemisia tabaci, Trialeurodes vaporariorum (Westwood), the greenhouse whitefly, and

Trialeurodes abutilonea (Haldeman), the banded-wing whitefly, vector closteroviruses

(Duffus 1996). Geminiviruses are transmitted in a persistent, circulative manner (Polston

and Anderson 1997), and closteroviruses in a semi-persistent manner (Duffus 1996).

Bemisia tabaci and T vaporariorum are the most economically damaging species

of whitefly. Both species attack members of most major crop groups (Mound and Halsey

1978, Naresh and Nene 1980, Russell 1963, 1977). Trialeurodes vaporariorum has

traditionally been a pest of greenhouse crops in Europe and the United States (Lloyd

1922, Vet et al. 1980), although in recent decades it has expanded its range, affecting

glasshouse agriculture in Japan since 1974 (Yano 1983) and in Crete since 1979

(Roditakis 1990). It is a major pest of tomato (Lycopersicon esculentum Mill.) and

cucumber (Cucumis sativus L.) grown in greenhouses, although successful biological

control programs using parasitoids have been developed (Vet et al. 1980). In Central

America, T vaporariorum tends to be more common above 500 meters, and B. tabaci

below 500 meters (Caballero 1994). Trialeurodes vaporariorum is a serious pest of

tomato and other horticultural crops grown at higher elevations in Central America, while

Bemisia and Bemisia-vectored geminiviruses are limiting factors at lower elevations

(Hilje 1993).

Bemisia tabaci was first described in 1889 as a tobacco (Nicotiana tobacurn L.)

pest in Greece (Gennadius 1889). It was responsible for virus-induced crop losses during









the first decades of the century in Africa, Asia, India, and Latin America, primarily in

cotton, tobacco, cassava (Manihot esculenta Krantz), and various legumes (Costa 1975).

Large-scale monocultures of cotton in Central America and cotton and soybean (Glycine

max L.) in Brazil favored massive increases in B. tabaci populations in those regions in

the 1960s (Costa 1975, Dard6n 1992). Until the early 1980s, B. tabaci outbreaks were

largely sporadic (Bedford et al. 1994). By the end of the 1980s, a strain of B. labaci, later

described as a new species, had become one of the most important agricultural pests

around the globe.

In Puerto Rico in the 1950s, researchers established that morphologically

indistinct populations of B. tabaci existed with different host ranges. Strains or biotypes

of B. tabaci based on host range were later recognized in Brazil and West Africa (Brown

et al. 1995). In the mid-1980s, a strain of B. tabaci was introduced from the

Mediterranean into the western hemisphere via the Caribbean, probably on ornamental

plants (Brown et al. 1995, Polston and Anderson 1997). This strain was designated the

B-biotype, or B strain, to distinguish it from the A-biotype, the prevalent North American

strain (Costa and Brown 1990, 1991). The B-biotype appeared in Arizona, California,

Texas, and Florida between 1988 and 1989, and within a few years had largely displaced

the A-biotype throughout much of this region (Brown et al. 1995). By 1993, the B-

biotype had been recorded throughout Central America and in Brazil (Brown et al. 1995).

The B-biotype has a broader host range than indigenous strains, causing serious

infestations of poinsettia (Euphorbia pulcherrima (Willd.)), tomato, bell pepper

(Capsicum annuum L.), broccoli (Brassica oleracea L.), cauliflower (Brassica oleracea

L.), and alfalfa (Medicago sativa L.), none of which had been seriously affected by the A

strain (Perring 1996). The new strain demonstrated greater rates of oviposition and








feeding on some crops (Bethke et al. 1991, Cohen et al. 1992). Byrne and Miller (1990)

found that the B strain produced more honeydew than the A strain, and suggested that it

might have better access to the phloem. Feeding by the B strain has been associated with

the silvering of squash (Cucurbita pepo L.) and irregular ripening of tomato (Maynard

and Cantliffe 1989), as well as other previously unknown plant disorders (Shapiro 1996).

The B strain introduced dozens of new geminiviruses to the New World, primarily on the

Solanaceae and Cruciferae. Many of these are still uncharacterized (Brown et al. 1995,

Polston and Anderson 1997). Epidemics of bean golden mosaic geminivirus increased in

Central America after the arrival of the B-biotype (Rodriguez 1994). In 1993, the first

epidemic of bean golden mosaic was reported in south Florida (Blair et al. 1995). The B-

biotype also exhibited high levels of resistance to carbamate, organophosphate,

pyrethroid, and other pesticide groups compared to the A-biotype (Denholm et al. 1996,

Dittrich et al. 1990).

Based on DNA differentiation tests, allozymic frequency analysis, crossing

experiments, and mating behavior, Perring et al. (1993) reported that the B-biotype was a

new species. Presenting differences in pupal case morphology and allozymic characters,

Bellows et al. (1994) described the new species as Bemisia argentijblii Bellows &

Perring, the silverleaf whitefly. The name was derived from the ability of the whitefly to

induce silvering of leaves in certain cucurbits (Yokomi et al. 1990).

The elevation of the B-biotype to species has been disputed. Liu et al. (1993)

reported that, based on esterase isozyme analysis, populations of the A- and B-biotypes

mixed over time under laboratory conditions. Bartlett and Gawel (1993) argued that the

molecular analysis carried out by Perring et al. (1993) was insufficient to demonstrate the

existence of a new species. Brown et al. (1995) suggested that allozyme markers are






5

useful for tracking the spread of B. tabaci strains, but that they are not appropriate for the

designation of species. They added that other distinct B. tabaci populations show

significant variability in pupal case morphology, esterase banding profiles, and mating

behavior. They reasoned therefore that if the B-biotype were a new species, other B.

tabaci strains must be described as new species as well.

There seems to be consensus among many whitefly workers that the designation

of B. argentifolii as a new species is "premature" (Bedford et al. 1994). The data suggest,

however, that B. tabaci may be a species complex undergoing evolutionary change

(Brown et al. 1995, Drost et al. 1998). Brown et al. (1995) believe that the A-biotype

belongs to the New World group of B. tabaci, and that the B-biotype belongs to the Old

World group. Brown et al. (1995) and Byrne et al. (1990) suggest that the B-biotype may

have risen to predominance under the selective pressure of large-scale, heavily-sprayed

monocultures, particularly cotton monocultures.

Crucial aspects of whitefly movement, host finding, and host acceptance have

been described. Whiteflies are weak fliers and have been described as aerial "plankton,"

which move with the wind currents, probing plants as they are encountered (Byrne and

Bellows 1991 ). Mound (1962) first reported that B. tabaci oriented toward either

yellowish or blue/ultraviolet light, and suggested that this phenomenon might be related

to colonizing and migratory behavior. Byrne et al. (1996) determined that B. tabaci has

two distinct adult morphs, which engage in either trivial or long-distance movement.

Trivial fliers orient toward the yellowish-green range of light spectra emitted by most

vegetation, and seem to be predisposed to colonize. Long-distance fliers are attracted to

ultraviolet light associated with the sky, and are apparently predisposed to migrate (Byrne








et al. 1996). Trialeurodes vaporariorum demonstrates similar orientation behavior to

these two wavelength ranges (Coombe 1981, 1982, Vaishampayan et al. 1975a).

Neither B. tabaci nor T. vaporariorum respond to host-specific visual or olfactory

cues (Mound 1962, van Lenteren and Noldus 1990, Vaishampayan et al. 1975a, 1975b).

Feeding behavior studies and examinations of precibarial and cibarial chemosensilla of B.

tabaci and T vaporariorum indicate that the two species must probe a plant in order to

determine if it is an acceptable host (Hunter et al. 1996, Lei et al. 1998, van Lenteren and

Noldus 1990). Oviposition and longevity for each species vary on different crops. This

has led to rankings of host suitability for T vaporariorum (van Boxtel et al. 1978, van

Lenteren and Noldus 1990, van de Meredonk and van Lenteren 1978), B. tabaci (Aslam

and Gebara 1995, Costa et al. 1991, Coudriet et al. 1985, Naresh and Nene 1980,

Simmons 1994), B. argentiJolii (Chu et al. 1995, Tsai and Wang 1996, Wang and Tsai

1996), and to comparisons of host plant suitability for both species or biotypes of Bemisia

(Blua et al. 1995, Drost et al. 1998). Survival and host plant selection by a whitefly

female may be influenced by the plant species on which she was reared (Costa et al. 1991,

van Boxtel et al. 1978). Both B. tabaci and T. vaporariorum emigrate from some host

species more quickly than from others (Costa et al. 1991, van Lenteren and Noldus 1990,

Verschoor-van der Poel and van Lenteren 1978). This may influence host-specific rates

of oviposition.

Bemisia tabaci and T. vaporariorum females usually oviposit on the abaxial side

of young leaves (Noldus et al. 1986a, Simmons 1994). Bemisia tabaci females seem to

prefer a moderate degree of pubescence to either glabrous or extremely hairy leaf surfaces

for oviposition (Butler et al. 1986, McAuslane 1996). First-instar nymphs tend to move a








short distance from the egg to find a feeding site, (Byrne and Bellows 1991, Price and

Taborsky 1992), although they are capable of moving within and between plants to find

healthy feeding sites (Summers et al. 1996). Subsequent instars are sessile. For this

reason, nymph age tends to correlate with leaf age (Ekbom and Rumei 1990).

Researchers have taken advantage of this behavior to develop stratified sampling

plans for "pupal" and parasitized stages of T vaporariorum in greenhouses (Martin and

Dale 1989, Martin et al. 1991, Noldus et al. 1986b) and egg and nymph stages of Bemisia

on cantaloupe (Cucumis melo L.) (Gould and Naranjo 1999, Tonhasca et al. 1994a,

1994b), cotton (Naranjo and Flint 1994, Ohnesorge and Rapp 1986a, von Arx et al.

1984), peanut (Arachis hypogea L.) (Lynch and Simmons 1993, McAuslane et al. 1993),

and tomato (Schuster 1998). Bemisia eggs and nymphs exhibit a highly aggregated

distribution on leaves and across plants (Naranjo 1996). Sampling plans have been

developed for whiteflies, primarily B. tabaci, to determine economic injury levels and to

compare the efficacy of control measures (Butler et al. 1986, Ekbom and Rumei 1990,

Naranjo 1996, Ohnesorge and Rapp 1986b).

Bemisia tabaci has demonstrated some degree of resistance to most classes of

broad-spectrum pesticides (Denholm et al. 1996, Dittrich et al. 1990), although novel

compounds (Horowitz and Ishaaya 1996) and "biorational" insecticides such as

detergents and oils (Stansly et al. 1996, Veierov 1996) continue to provide some measure

of control. One of the most effective and widely used compounds for whitefly control at

the time of writing is imidacloprid, a systemic pesticide which inhibits nicotinergic

acetylcholine receptors, produced by Bayer (Polston et al. 1994).








Host plant resistance to whiteflies is primarily derived from leaf characteristics

such as pubescence or the presence of glandular trichomes (Berlinger 1986). Some

degree of host plant resistance to Bemisia has been found in cotton (Flint and Parks 1990,

Wilson et al. 1993), soybean (McAuslane 1996) and tomato (Heinz and Zalom 1995).

Resistance to T. vaporariorum has been found in sweet pepper (Capsicum annuum L.)

(Laska et al. 1986) and melon (Cucumis melo L. var. agrestis) (Soria et al. 1996).

Progress has been achieved recently in developing resistance to Bemisia-transmitted

geminiviruses in tomato (Scott et al. 1996, Nateshan et al. 1996).

The sessile habit of immature whiteflies renders them susceptible to many

pathogens (Fransen 1990), predators, and parasitoids (Gerling 1990). Successful

biological control programs have been developed to manage T vaporariorum in

greenhouses, primarily with the parasitoid Encarsiajbrmosa (Gahan) (Hymenoptera:

Aphelinidae) (Vet et al. 1980). In Florida, high rates of parasitism have been found on

weeds, organically grown vegetables (Stansly et al. 1997) and unsprayed peanuts

(McAuslane et al. 1994). However, the intensive use of broad-spectrum pesticides and the

rapid rate of increase of Bemisia prevents its suppression by natural enemies in most

agricultural systems (Hoelmer 1996). Exotic parasitoids have been introduced into

Arizona, California, Florida, and Texas to control Bemisia with little success (Hoelmer

1996, Nguyen 1996). Releases of predators in California (Brazzle et al. 1994, Legaspi et

al. 1994, Roltsch and Pickett 1994) and attempts to establish refugia for natural enemies

of Bemisia in the desert southwest have been similarly unfruitful (Roltsch and Pickett

1995, 1996). The arid conditions, heavy pesticide regimes, and continuous cropping

cycles that characterize agriculture in the southwestern United States may place biological









control agents at a disadvantage (Hoelmer 1996). Crops tested as refugia include kenaf

(Hibiscus cannibinus L.) and rosa de jamaica (Hibiscus sabdaritft L.) (Malvaceae)

(Roltsch and Pickett 1995). Rosa de jamaica, also called roselle and sorrel in English,

possesses extra-floral nectaries at the base of the leaf (Standley and Steyermark 1949).

Cultural methods used to reduce whitefly damage include manipulation of

planting dates, use of short-season varieties, reflective mulches (Czizinsky et al. 1997,

Powell and Stofella 1993), and floating row covers (Norman et al. 1993). Trap crops and

intercropping have also been suggested as methods for management of Bemisia (Faust

1992).

Attempts to reduce whitefly damage with trap crops have produced unclear

results. Squash (Cucurbita pepo L.) (Schuster et al. 1996), cantaloupe (Cucumis melo L.)

(Ellsworth et al. 1994, Perring et al. 1995), soybean (McAuslane et al. 1995) and

Wright's groundcherry (Physalis wrightii Gray) (Ellsworth et al. 1994) have been tested

as trap crops for Bemisia. Whitefly densities on the main crop were either unaffected by

the presence of the trap crop candidate, or were reduced on only a few sampling dates.

Puri et al. (1996) intercropped cotton with wild brinjal (Solanum khasianum Clarke),

which traps arthropods with a sticky exudate, without significantly reducing Bemisia

densities in cotton. However, Al-Musa (1982) and Schuster et al. (1996) delayed the

onset of virus in tomato by trap cropping with cucumber and squash, respectively. Al-

Musa reported reductions of virus incidence of greater than 30% in tomato interplanted

with cucumber.

Several tall-growing non-host plants, primarily in the family Gramineae. have

been tested as barrier crops or intercrops to reduce whitefly colonization and virus








transmission among main crops. Results have been mixed. Morales et al. (1993)

reported that a sorghum [Sorghum bicolor (L.) Moench] barrier slightly reduced Bemisia

densities and transmission of virus on tomato in the Motagua Valley, Guatemala. A pearl

millet [Penniselum typhoides (Burm. f.) Stapf & Hubbard] barrier prevented whitefly

virus transmission on cowpea [ Vigna unguiculata (L.) Walp.] (Sharma and Varma 1984)

and reduced it on soybean (Rataul et al. 1989) in India. In Colombia, Gold et al. (1990)

found reduced densities of Aleurotrachelis socialis Bondar and Trialeurodes variabilis

(Quaintance) on cassava intercropped with maize (Zea mays L.) and cowpea, but

attributed this in part to reduced host quality due to intercrop competition. Ahohuendo

and Sarkar (1995) reduced density of B. tabaci by more than 50% and incidence of

cassava virus by 40% on cassava by intercropping with maize and cowpea in Benin.

Fargette and Farquet (1988), whose study in the Ivory Coast included the effect of wind

direction, found densities of B. tabaci and virus incidence were sometimes higher on

cassava intercropped with maize than on monocropped cassava.

Successful management of Bemisia may require coordinated efforts throughout

agricultural regions, such as the government-imposed host-free periods attempted in the

Dominican Republic (Polston and Anderson 1997). Integrated pest management plans

for tomato growers have been developed in Central America (Hilje 1993), and attempts to

develop a collaborative model for whitefly management throughout the region are on-

going (Hilje 1998). Ellsworth et al. (1996) describe efforts to develop a community-

based Bemisia management program in Arizona. Kogan (1996) discusses the difficulties

of adapting the integrated pest management for Bemisia to a region-wide management

program.









Intercropping

Intercropping is the agronomic practice of growing two or more crops

simultaneously in the same field (Andrews and Kassam 1976). Crops may be planted

without regard to rows (mixed intercropping), in alternating rows, or with different crops

alternating within the same row. Relay intercropping refers to planting of one intercrop

species before another so that their life cycles partially overlap (Kass 1978). The broader

term "polyculture" includes intercropping, but also encompasses intentionally combining

crops and weeds, and combining crops with beneficial non-crop plants, such as cover

crops or nursery crops (Andow 1991 a). Perrin and Phillips (1978) include mixtures of

crop cultivars and multilines in their definition of intercropping, because such

combinations may possess some of the advantages associated with conventional

intercropping.

Traditional food-production systems in tropical Africa, Asia, and Latin America

are usually characterized by some degree of intercropping (Perrin and Phillips 1978). In

agricultural areas where labor is the primary resource and reduction of risk the primary

concern, polyculture systems have been developed which give higher and more secure

yields than monoculture (Perrin 1977). Successful intercropping systems are

characterized by greater efficiency in the use of solar radiation, nutrients, and soil

moisture, as well as higher yields, compared to monocropped production under the same

conditions (Andow 1991 b, Kass 1978, Perrin 1977, Vandermeer 1989). In the first

decades of this century, intercropping was common in temperate regions (Andow 1983).

While generally considered inappropriate for the mechanized, chemical-intensive

agriculture of industrialized nations, intercropping methods might improve the production








of high-value, labor-intensive fruits and vegetables in places like the United States

(Capinera et al. 1985, Risch et al. 1983).

Among the agronomic benefits attributed to some systems of polyculture is the

reduction of damage from arthropod pests (Altieri 1994, Altieri and Letourneau 1982,

Andow 1983, 1991a, Kass 1978, Litsinger and Moody 1976. Perrin 1977, Perrin and

Phillips 1978, Risch et al. 1983, Vandermeer 1989). This phenomenon was first

discussed extensively in Western scientific literature in the earlier part of the twentieth

century, based on observations of pest behavior in temperate and tropical silvicultural

systems (reviewed by Andow 1983. and Pimentel 1961). Additional information came

from traditional systems of polyculture in the tropics. Working in India, Aiyer (1949)

proposed three ways intercropping might reduce pest damage: 1) individual plants might

be more difficult to find because they are usually more dispersed in intercropped systems;

2) certain species might serve as trap crops, diverting pests from other crops; and 3) some

crops might have a repellent effect on herbivores.

Elton (1927, 1958) suggested that the ability of natural enemies to suppress

herbivores in naturally diverse agroecosystems was lost in simple systems, and promoted

the idea that diverse systems should be more stable than simple ones. Diverse

environments would offer a greater variety of habitats and victims to natural enemies

(Huffaker 1958), as well as alternate food sources such as pollen and nectar (van Emden

1963, 1965), enabling natural enemies to suppress herbivore populations more efficiently

than in simple environments. Drawing on Elton, Nicholson (1933), and his own work

with pests of Brassica oleracea L. in simple and diverse systems, Pimentel (1961) refined

this idea. He proposed that the varied but limited resources of diversified cropping









systems would support diverse but limited populations of both herbivores and natural

enemies. Competition over resources would dampen oscillations among all trophic

levels, creating a stable system, free from the pest outbreaks that characterized

monocultures.

Root (1973) found that herbivores were less dense in B. olereacea grown in

diverse than in simple stands, but determined that this could not be explained by

increased activity of natural enemies. Summarizing the literature, Root explicitly defined

the generally accepted "enemies" hypothesis, and added to it the "resource concentration"

hypothesis to explain the reduction in herbivore load he had observed. According to the

resource concentration hypothesis, herbivores with a narrow host range are more likely to

find and remain on hosts grown in pure stands, and will attain higher relative densities in

simple environments (Root 1973). Trenbath (1976, 1977) outlined the "fly-paper effect,"

a variation on the resource concentration hypothesis, which states that the time spent

searching and probing diversionary intercrops will reduce the time and energy invested in

damaging main crops, and may increase mortality among potential pests before they

affect the main crop.

Vandermeer (1989) proposed three hypotheses to encompass all of the

mechanisms suggested by Aiyer (1949), Root (1973), and Trenbath (1976, 1977). The

"disruptive crop" hypothesis states that certain intercrop species will disrupt the ability of

a pest to attack the main crop. The "'trap crop" hypothesis refers to the ability of a more

attractive intercrop to draw the pest away from the main crop. Intercrop systems which

reduce herbivore densities by attracting more natural enemies than monocrops are

examples of the "enemies" hypothesis.









The idea that diversity in itself reduces pest damage has been abandoned as

inconsistent with empirical data (Andow 1991 a). As Risch et al. (1983) point out,

stability in pest populations is desirable only below economically damaging levels.

However, reviews of the intercropping literature indicate that, relative to monoculture,

herbivores were less dense in more than 50% of studies, more dense in 15 to 18 % of the

cases, and variable in about 20 % (Andow 199 1a, Risch et al. 1983). About 9% showed

no difference in density between cropping systems. Recent analysis has focused on

rigorous examination of the two hypotheses defined by Root (1973) in an attempt to

determine under which conditions polyculture might be useful for pest management

(Andow 1991a, Corbett and Plant 1993, Kareiva 1983, Power 1990, Risch et al. 1983,

Russell 1989, Sheehan 1986, Stanton 1983). The trap cropping mechanism has been

ignored by all reviewers except Vandermeer (1989).

Neither the resource concentration hypothesis nor the enemies hypothesis has

proven to be consistently useful for predicting how crop density and diversity will affect

arthropod density or diversity (Kareiva 1983, Russell 1989). Andow (1991 a) and Risch

et al. (1983) state that, based on reviews of the literature, the resource concentration

hypothesis tends to account for herbivore response to polyculture more often than the

enemies hypothesis. However, given the high degree of variability in response by some

herbivores, Andow (1991 a) suggests that this generalization is of limited predictive value.

Russell (1989) writes that studies which have compared insect abundance in simple and

diverse systems have uncovered little evidence to support the enemies hypothesis.

The inability to explain arthropod response to vegetative diversity with a few

broad mechanisms has been attributed in part to the many adaptive variations that








characterize arthropod behavior. Kareiva (1983) states the need for research that

identifies "species specific traits.. .that govern the responses of herbivores to vegetation

texture." The ability of an herbivore to colonize a given cropping system, diverse or

simple, will depend largely on the range of its diet, the nature and sophistication of its

host-finding mechanisms, and its relative mobility (Kareiva 1983, Stanton 1983). The

same holds true for plant disease vectors (Power 1990) and natural enemies (Russell

1989, Sheehan 1986). The specific nature of vegetative diversity will also determine

arthropod response. Vegetative texture can vary in terms of density, patch size, spatial

array, and temporal overlap (Andow 1991 a, Kareiva 1983), as well as the ratio of host to

non-host plants, which will have a greater influence on herbivore abundance than the

actual number of plant species (Power 1990, Stanton 1983).

Vegetative diversity can affect arthropod damage and densities by influencing the

rate at which an arthropod immigrates into a cropped area, its population dynamics once

it has entered, and the rate at which it emigrates from the area (Stanton 1983). The extent

to which immigration can be influenced depends on the host-finding mechanisms and

mobility of the arthropod. Intercropping with certain crops may interfere with the

olfactory cues certain insects rely on for host-finding (Perrin 1977, Stanton 1983). For

instance, Tahvanainen and Root (1972) demonstrated that tomato volatiles interfered with

host-finding by Phyllotreta cruciferae Goeze, a flea beetle, and led to reduced oviposition

on collards. Interference with visual host-finding cues has been suggested (Perrin 1977,

Stanton 1983). However, most examples of manipulation of visual perception concern

increased attraction of insects such as aphids to sparsely planted crops which stand out

against bare ground (Kennedy et al. 1961, Smith 1976).









The extent to which vegetative diversity will interfere with immigration also

depends on the range at which an insect detects the host, and whether this detection

mechanism is specific or general (Kareiva 1983, Stanton 1983). Host-specific orientation

cues tend to be characteristic of monophagous insects (Prokopy and Owen 1978), which

may in addition evolve sophisticated searching ability in order to find rare hosts.

Polyphagous insects such as certain whiteflies and aphids do not rely on host-specific

visual or olfactory cues, and respond generally to the spectra of yellowish light emitted by

most vegetation (van Lenteren and Noldus 1990, Power 1990). Whiteflies, aphids, and

thrips have limited ability to control their flight, and have been described as "aerial

plankton" (Byrne and Bellows 1991, Price 1976). The "flypaper effect" (Trenbath 1976)

suggests a mechanism by which weak fliers with unsophisticated host-finding

mechanisms such as whiteflies and aphids might be reduced in polyculture. Simply by

alighting on and probing diversionary intercrops, such insects may invest less time in

damaging main crops. However, this mechanism has not been demonstrated

scientifically.

Trap cropping is a method of pest suppression that relies on manipulating host-

finding mechanisms. The herbivore's decision-making must be influenced before it finds

and damages the main crop. Vandermeer (1989) writes that trap cropping should affect

generalist herbivores. However, the sensitive host-finding cues of monophagous

herbivores are presumably more vulnerable to manipulation than the general attraction to

most vegetation demonstrated by some polyphagous insects. Hunter and Whitfield

(1996) almost doubled yields and reduced densities of the Colorado potato beetle

(Leptinotarsa decemlineata (Say)) by more than half by using potato as a trap crop with









tomato. Trap cropping has been used to manage the cotton boll weevil (Anthomonus

grandis Boheman) in Nicaragua (Swezey and Daxl 1988) and Arizona (Moore and

Watson 1990). The ability to support higher densities than a main crop does not make a

"preferred" crop a trap crop; the trap crop must actually reduce densities on the main crop

when the two are interplanted (Ali and Karim 1989). Trap crops are often treated with

pesticides to prevent damaging herbivores from building up and spilling over onto main

crops (Srinivasan and Moorthy 1991). Effective control often depends on the timing of

pesticide applications to the trap crop (Todd and Schumann 1988) or the timing of

planting for the trap crop in relation to the main crop (Kloen and Altieri 1990).

There are several ways herbivore density, damage, and growth may be affected by

vegetative diversity once an insect has entered a polyculture. Polycultures which support

high densities of natural enemies may increase predation and parasitism of herbivores

(Altieri and Letourneau 1982). For example, Letourneau (1987) found parasitism of

Diaphania hyalinata L. higher in squash intercropped with corn and bean (Phaseolus

vulgaris L.) than in monocropped squash. Intercropping may affect herbivore health by

affecting the suitability of individual plants, or by repelling certain insects because of

increased shading (Kareiva 1983). Hawkes and Coaker (1976) reported that Delia

brassicae (Wied.), the cabbage root fly, oviposited less on Brassica sp. intercropped with

clover (Trifolium sp.) than in pure stands. This was apparently due to higher rates of

departure from hosts within the patch rather than to increased difficulty finding them

(Coaker 1980).

The effect of polyculture on the transmission of arthropod-vectored pathogens

may vary according to the epidemiology of the pathogen. Incidence of non-persistent






18

viruses has been reduced on main crops by diverting aphid vectors to protection crops"

(Jenkinson 1955, Broadbent 1969). Crop combinations which cause vectors to probe

more frequently but for shorter periods of time may increase the incidence of non-

persistent viruses, and reduce the incidence of persistent viruses (Power 1990).

Rates of arthropod emigration from a vegetatively diverse patch may be

influenced by searching behavior. Insects which restrict their search area upon finding a

host ("patch restricted searching") may be more likely to remain within a diverse area

than insects whose movement is unaffected by encountering a host ("uniform searching")

(Stanton 1983). Highly mobile insects may leave a diverse area after encountering a

number of non-hosts in succession. Bach (1980a, 1980b) and Risch (1980, 1981) fund

that leaf beetles emigrated more quickly from patches of hosts mixed with non-hosts than

from pure stands, and were able to show that increased emigration was responsible for

lower beetle densities in polyculture compared to monoculture. Being weak fliers,

aphids, whiteflies, and thrips (the "aerial plankton" group) may simply move short

distances from plant to plant until they find acceptable hosts. This passive method of

searching may cause such insects to accumulate in higher densities on hosts in

polyculture, if these hosts are planted at a lower density than in monoculture.

Root's (1973) hypothesis that crop diversity would tend to reduce densities of

monophagous herbivores rather than polyphagous ones is supported by the preceding

summary, and by reviews of the intercropping literature (Andow 1991 a, Risch et al.

1983). Andow (1991a) found that 28% of polyphagous herbivores studied had lower

densities in polyculture, while 40% had higher densities. Only 8% of monophagous

herbivores had higher densities in polyculture, while 59% had lower densities.








The success and efficiency of natural enemies in polyculture relative to

monoculture is largely determined by the specifics of behavior, much as it is for

herbivores. The enemies hypothesis implicitly refers to generalist natural enemies, in that

it suggests polyculture will offer alternate prey or hosts, and alternate food sources, such

as pollen and nectar (Root 1973). Like specialist herbivores, specialist natural enemies

such as host-specific parasitoids may rely on sensitive visual, olfactory, and tactile cues to

find hosts. These cues are more likely to be obscured in polyculture than in monoculture

(Sheehan 1986). The disruption of plant patches may cause a specialist enemy to leave a

diverse area more quickly than a simple one. Host-feeding is essential for some

parasitoids, and alternate protein and carbohydrate sources such as nectar or pollen may

not serve as a substitute (Sheehan 1986).

There are many examples of predators achieving higher densities in monoculture

than polyculture (Corbett and Plant 1993). For instance, Schultz (1988) found

significantly fewer lacewing (Chrysopidae) eggs on cotton intercropped with corn or

weeds than on monocropped cotton. The assumption that predators will move from a

resource-rich intercrop to the main crop that the agriculturalist intends to protect may not

always be valid. Bugg et al. (1987) found that predators on knotweed (Polygnum

aviculare) did not tend to move from it to adjacent crops.

Few robust generalizations can be made to predict how polyculture will affect

arthropod density (Andow 1991 a, Kareiva 1983). However, the literature suggests that

polyculture will reduce densities of monophagous herbivores more often than densities of

polyphagous herbivores (Andow 1991 a). In addition, polyculture may favor some








generalist predators, but is more likely to impede the efficiency of specialist parasitoids

(Pimentel 1961, Sheehan 1986).

Inadequate research methods have contributed to the ambiguity surrounding the

effect of polyculture on arthropods. Intercropping often reduces plant quality relative to

monoculture (Andow 1991 a, Kareiva 1983). Some authors include the effect of reduced

plant quality in their analysis (for instance Gold et al. 1990, Schultz 1988), but many do

not (Kareiva 1983). Stanton (1983) remarks that there may be significant differences in

how researchers and insects perceive "diversity." In addition, Andow (1991 a) writes that

results of polyculture studies have varied depending on whether polyculture treatments

were substitutive or additive, i.e. whether host plant density was different in monocrop

and intercrop treatments.

The greatest difficulty in designing field tests of intercropping effects on

arthropods is determining the appropriate scale of plots and distance between plots

(Russell 1989, Stanton 1983). Some arthropods may perceive a patchwork of

monocropped and intercropped plots as one large polyculture. Small clustered plots will

increase the influence of patch borders on searching, and the likelihood that arthropod

density in one treatment plot is influenced by the arthropod's attraction to or rejection of a

distinct adjacent treatment plot (Andow 1991a, Stanton 1983). Plot size will affect the

ability of herbivores and natural enemies to find hosts, as well as their foraging behavior

within the plot, and the rate at which they leave it (Corbett and Plant 1993, Kareiva 1983,

Stanton 1983, Russell 1989).








Research Objectives

The objective of the following research was to determine if intercropping could be

used to reduce densities of immature whiteflies compared to densities on crops grown in

monoculture. Intercropping studies were designed to test the reduction of whitefly

densities on bean and tomato. It was hoped that results from these crops would apply to

other economically-important crops. Population densities, and in some cases yield, were

measured to estimate whitefly incidence and damage under simple and mixed systems,

although damage was not measured directly.

Summarizing the literature, Vandermeer (1989) proposed three all-encompassing

hypotheses to explain how intercropping might reduce pest damage (trap crops, disruptive

crops, and increased natural enemies). The following field experiments focused on

testing two of these hypotheses, the trap crop hypothesis and the disruptive crop

hypothesis. The first set of experiments, carried out on an organic farm near Gainesville,

examined squash as a trap crop. The second set of experiments, carried out on a

University of Florida agricultural research farm near Gainesville, tested eggplant as a trap

crop and field corn as a barrier crop. The final set of experiments tested the potentially

disruptive effect of intercropping bean or tomato with poor or non-hosts of whitefly.

These last studies took place at a government agricultural research station in central

Guatemala. Data were gathered on parasitism in most of these studies, and on predators

in a few studies, but only the final experiment in Guatemala attempted to test the third

intercropping hypothesis, the enemies hypothesis, by intercropping with cilantro to

augment densities of generalist predators.






22

An additional objective of the research was to determine if whitefly suppression

through intercropping could be enhanced by integration with other control strategies. In

the first set of studies, plastic mulch with a strip of reflective aluminum paint was tested

alone and in combination with the trap crop. Imidacloprid and a detergent/oil rotation

were tested as subplot pesticide treatments in some intercropping studies in Guatemala.

The final study in Guatemala included an initial evaluation of methods for protecting

tomato seedlings from whitefly damage in the nursery stage.














CHAPTER 2
THE EFFECT OF SILVER REFLECTIVE MULCH AND A SUMMER SQUASH
(CUCURBITA PEPO L.) TRAP CROP ON DENSITIES OF IMMATURE BEMISIA
ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE) ON ORGANIC BEAN
(PHASEOL US VULGARIS L.)

Introduction

Bemisia argentifolii Bellows & Perring, the silverleaf whitefly (also known as

Bemisia tabaci (Gennadius) strain B), has become a serious pest of horticultural and

agronomic crops throughout warm regions of the world (Brown et al. 1995). Since the

mid 1980s, the Florida vegetable industry has lost millions of dollars due to a variety of

diseases and disorders associated with B. argentifolii (Norman et al. 1993). These

include the tomato mottle and bean golden mosaic geminiviruses (Hiebert et al. 1996), as

well as irregular ripening of tomato and squash silverleaf (Maynard and Cantliffe 1989).

Bemisia has developed resistance to most classes of pesticides (Denholm et al. 1996,

Dittrich et al. 1990), forcing conventional growers to seek non-chemical alternatives to

Bemisia management. Synthetic pesticides are not an option for organic growers, who

face special challenges in the management of virus vectors.

The present study was undertaken to assess the efficacy of reflective plastic mulch

and yellow summer squash (Cucurbitapepo L.) as a trap crop for management of B.

argentifolii on snap bean (Phaseolus vulgaris L.) on an organic farm in north central

Florida. Florida is the foremost producer of snap bean in the United States (National

Agricultural Statistics Service 1998). In 1995, revenue from fresh market snap bean in

Florida exceeded $50 million (Florida Statistical Abstract 1996). Plastic mulches which







24

reflect ultra-violet rays are disorienting to certain insects (Prokopy and Owens 1983) and

have been used to repel virus vectors such as aphids (Smith and Webb 1969, Jones 1991)

and thrips (Smith et al. 1972, Scott et al. 1990). Schuster and Kring (1988) reported

some success using reflective mulch to manage whiteflies.

Trap crops are preferred host plants which are used to draw herbivores away from

a less-preferred main crop (Vandermeer 1989). Trap crops are sometimes sprayed with

pesticides to prevent the damaging herbivores from building up and spreading to the main

crop (Ellsworth et al. 1994). Several crops have been tested as trap crops for

management of Bemisia (Al-Musa 1982, Ellsworth et al. 1994, McAuslane et al. 1995,

Schuster et al. 1996). By the early 1990s, squash had been singled out as a promising trap

crop candidate for management of Bemisia (Faust 1992).

Material and Methods

The study was carried out on a 4-ha certified organic farm, 6 km northwest of

Gainesville, Florida (290 40'N, 82' 30'W). Four treatments were compared: 1) bean

grown on bare soil ("bean"), 2) bean grown with reflective polyethylene mulch

("mulch"), 3) bean mixed with squash grown on bare soil ("squash") and 4) bean mixed

with squash grown with reflective mulch ("squash/mulch").

'Espada' garden bean seed and 'Multipik' yellow summer squash seed from

Harris Seed (60 Saginaw Drive, Rochester, New York) were used. Seed had been

previously treated with captan (N-(trichloromethyl)thio-4-cyclohexene-1,2-

dicarboxamide), metalaxyl (N-(2,6-dimethylphenyl)-N-(methoxyacetyl)alanine methyl

esther), streptomycin and chloroneb. It is acceptable for organic growers to use treated

seeds if untreated seeds are unavailable (Organic Materials Review Institute 1998). To

ensure uniformity among covered and exposed beds, all beds were formed using a Rainflo







25

plastic mulch layer (model no. 560, Rainflo Irrigation, East Earl, PA). Plastic mulch and

drip irrigation tubing were laid over all beds, which were 1.22 m wide. After planting,

plastic mulch was removed from the bare soil treatments.

Beans were planted 15 cm apart within the row. Squash replaced every fifth bean

plant in the squash treatments. Beds were 3.5 m long, and the space between beds was 2

m. Each treatment plot contained two beds with two rows of plants per bed. The

reflective mulch was a white polyethylene mulch with a central stripe of silver pigment,

61 cm wide (product 60-64S/W125PR, North American Film Corporation, 19 Depot

Road, Bridgeport PA).

There was concern that whiteflies might colonize certain borders of the

experimental area before others because of wind direction or migration from adjacent

host plants. To control for two potential extraneous sources of variation, treatments were

arranged in a 4 x 4 latin square design.

Plots were irrigated as needed using drip irrigation. Plants were fertilized 3 weeks

after emergence and at flowering with approximately 250 g per row of 3-2-3 (N-P205-

K20) North Florida Brand composted chicken manure. Plots were hand-weeded as

needed. No pest control products were applied to the experimental area.

The study was repeated in 1995, 1996 and 1997. In 1995, crops were planted on

October 15. The following years, crops were planted on September 2.

Sampling

Sampling for whiteflies began one week after crop emergence. Sampling was

stopped after 4 weeks in 1995 because of a freeze. Bean and squash were sampled for 6

weeks in 1996 and 1997. Four or 5 plants were sampled per plot each week. The sample

unit was a 3.34 cm2 leaf disc cut from upper and lower leaves using a number 13 nickel








cork borer (McAuslane et al 1995). Discs were taken from the underside of the leaf, in

the lower half of the leaf to the right of the mid-vein. Samples were examined using a

dissecting stereoscope set at 20x and numbers of whitefly eggs, nymphs, parasitized

nymphs, and red-eyed nymphs were recorded.

Yield

Pods were harvested week and fresh weight was recorded for weeks 7, 8, and 9

after planting.

Virus Screening

After harvest, leaf tissue from 6 plants from each plot was collected and tested

with a dot blot hybridization technique for the presence of geminivirus (Rojas et al.

1993). Analysis was conducted by the laboratory of Dr. E. Hiebert at the Department of

Plant Pathology at the University of Florida. Bean tissue (50 mg) was extracted in 200

mM NaOH with 1% SDS. Geminivirus DNA-A component was amplified by PCR with

Maxwell degenerate primers (PALlv1978 and PARlc496). The amplified DNA was

used for a 32P random-primed labeling reaction (Life Technologies RTS RadPrime DNA

Labeling Systems). The membrane was hybridized with 32P labeled probe in 6x SSC, 5x

Denhardts solution and 0.5% SDS at 65' C for 16 hrs. The membrane was then washed

under high stringency conditions with 0.2x SSC and 0.lx SDS at 650 C. Finally the

membrane was exposed to X-ray film for 16 hrs.

Statistical Analysis

Whitefly counts were transformed by log,0(x+l) because of low counts during the

first year and unequal variance over time. Treatment comparisons were made of upper

leaf counts, lower leaf counts, and of the average of the two strata. Treatments were

compared with time as a variable, and then by individual week, using analysis of variance









(PROC MIXED, SAS version 6.11, SAS Institute 1996). When appropriate, treatment

means were compared using Tukey's Studentized Range test with an adjusted

experiment-wise error rate of ot =0.05. Yield data were analyzed using the same analysis

of variance and mean separation procedures. Counts in upper and lower strata within the

same treatment were compared using a pairwise t-test. Bean samples which tested

positive for the presence of bean golden mosaic virus were assigned a value of 1, and

negative responses were assigned a value of 0. Responses were then analyzed using

logistic regression.

Results

Research Design

A latin square design was used because of the concern that some blocks might

become colonized by whiteflies before others due to their proximity to infested hosts or

their orientation to prevailing winds. It was observed during this and concurrent studies

that populations of whitefly adults require minutes rather than days or weeks to move

from one end of an experimental area to the other. It was decided therefore that the latin

square design was unnecessarily complicated for studying whiteflies, and that a

randomized complete block design would be adequate for future studies. However, the

data was analyzed using analysis of variance for latin square. A November freeze killed

all crops in 1995 after only 4 weeks of sampling. During the next two years the study was

initiated during the first week of September to reduce the risk of freezes.

Treatment Comparisons

Egg densities on the squash trap crop were significantly higher than on bean

throughout the three years of the study (Tables 1-3). Otherwise there were no consistent

trends among treatments from year to year. When treatment differences occurred, egg






28

and nymph densities tended to be highest on bean alone. However no treatment showed a

clear advantage over bean alone in reducing densities of eggs or nymphs.

Eggs. While egg densities tended to be lowest in the two treatments containing

squash in 1995 (Table 2-1), these densities were significantly lower than those in bean

alone only during the second week of sampling. Egg densities tended to be highest in the

reflective mulch treatment in 1995, though mean egg counts in the reflective mulch

treatment were never significantly different from those on bean alone.

Egg counts in the reflective mulch treatment were 25% lower than bean alone

during week 1 in 1996 (Table 2-2), and 32% lower than bean alone during the first week

of sampling in 1997 (Table 2-3). There were no significant differences in egg counts

among treatments during the subsequent five weeks of sampling in 1996 or 1997.

Nymphs. In 1995, there were differences in nymph densities among treatments

only during the second week of sampling, when nymph densities in the mulch-and-squash

treatment were significantly lower than in bean alone (Table 2-4). Nymph densities

tended to be lowest in the mulch treatment when nymphs first appeared in 1996 and 1997

(Tables 5-6). However on the fifth week of sampling in 1996, nymph counts were

fourteen times higher in the mulch treatment than in the squash treatment, and twelve

times higher than in bean alone (Table 2-5). On the fifth week of sampling in 1997,

nymph counts were significantly higher in the mulch treatment than in the three other

treatments (Table 2-6).

Parasitized nymphs. No parasitism was recorded in 1995. Little parasitism was

observed in 1996, and was observed only in the lower stratum. During the final week of

sampling in 1996, parasitism was significantly higher in the squash treatment (0.12 +

0.23) than in the squash/mulch treatment (0; p < 0.05). Parasitism was intermediate in






29

the mulch (0.07 0.18) and bean (0.05 0.16) treatments. Parasitism was much higher

in 1997. Parasitized nymphs were observed in all treatments beginning with the third

week in 1997 (Table 2-7). During the sixth week of sampling, mean parasitism in the

bean alone treatment was 262% greater than in the mulch treatment.

Red-eyed nymphs. Red-eyed nymphs were not observed in 1995. Red-eyed

nymphs were observed sporadically in 1996. During the fourth week of sampling that

year, densities of red-eyed nymphs were significantly higher in the mulch treatment (0.29

0.38) than in the squash treatment ( 0.05 0.16; p < 0.05). Densities were intermediate

in the bean (0.24 0.50) and squash/mulch (0.14 0.29) treatments. Red-eyed nymphs

were present in all treatments from the third week of sampling in 1997 until the final

week of sampling (Table 2-8). There were no significant differences in densities of red-

eyed nymphs among treatments.

Stratum Comparisons

On bean, there were significant differences in density between strata only during

1996, when eggs tended to be higher in the upper stratum (Table 2-2). Nymphs in the

same year tended to be higher in the lower stratum (Table 2-5). No parasitized nymphs or

red-eyed nymphs were recorded in 1995, probably because of the early freeze. In the

following two years low densities of parasitized or red-eyed nymphs were observed

primarily in the lower stratum.

On squash, egg densities tended to be highest on younger leaves early in the

season and to shift to predominance in older leaves in the last few weeks of sampling

(Table 2-9). Nymphs were found primarily on the older leaves each year (Table 2-10).

Yield






30

Crops froze in 1995 before yield could be harvested. In 1996 yields were highest

in the mulched treatments. Yields were extremely low in 1997, presumably due to high

whitefly pressure (Table 2-11).

Virus

In 1996, only I plant (in the squash treatment) tested positive for the presence of

bean golden mosaic geminivirus. Virus presence was much higher in 1997. There were

no significant differences in virus presence (percent of plants testing positive for virus)

among treatments (bean: 56 51%; mulch: 55 + 51%; squash: 27 46%; squash/mulch:

38 49%).

Discussion

Reflective Mulch

The loss of effectiveness of reflective mulch after the first week of 1996 and 1997

may be attributed to accumulation of dust on the mulch and shading by growing plants.

Bemisia tabaci engages in most flight activity in the middle of the day (Bellows et al.

1988, Byrne and von Bretzel 1987), when mulch should be reflecting repellent UV rays.

However, it is not unusual to see adults moving with early morning breezes in agricultural

fields. Adults may colonize crops planted with reflective mulch before the mulch

receives strong sunlight.

Most studies compare reflective plastic mulch with mulches of other colors rather

than with bare soil (Csizinsky et al. 1997, Powell and Stofella 1993). Researchers

generally conclude that reflective mulch is insufficient as a sole method of control

(Natwick and Mayberry 1994, Schuster et al. 1989). While reflective mulch does not

appear to provide season-long reduction of whitefly densities, the use of reflective mulch








has resulted in delays in the onset of virus in tomatoes (Csizinsky et al. 1997) and

reduction in viral disease in tomatoes and squash (Fehmy et al. 1994).

Crops grown with plastic mulches experience reduced weed competition and

increased water and nutrient availability compared to crops grown on bare soil. In our

studies, crops grown with mulch were visibly more robust than crops grown on bare

ground. This clearly had a direct effect on yield (Table 2-11). The improved plant

quality of crops grown with mulch may have enhanced their ability to support higher

populations of nymphs as was observed during week 5 of 1996 and 1997.

Trap Crop

Egg densities were consistently far higher on squash with or without mulch than

on bean in the same treatments (Tables 2-1 to 2-3). However egg densities on bean

planted with squash were not lower than on bean alone. This indicates that squash did

not function as a trap crop.

High densities of Bemisia on a given crop have been interpreted as a preference'

for that crop, in some cases leading it to be tested as a trap crop. Squash (Schuster et al.

1996), cantaloupe (Cucumis melo L.) (Ellsworth et al. 1994, Perring et al. 1995), soybean

(Glycine max L.) (McAuslane et al. 1995) and Wright's groundcherry (Physalis wrightii

Gray) (Ellsworth et al. 1994) have been tested as trap crops for Bemisia with unclear

results. Whitefly densities on the main crop were either unaffected by the presence of the

trap crop candidate, or reduced on a few isolated sampling dates, as occurred with our

study. Puri et al. (1996) intercropped cotton (Gossypium hirsutum L.) with wild brinjal

(Solanum khasianum Clarke), which traps arthropods with a sticky exudate, without

significantly reducing Bemisia densities in cotton.






32

A successful trap crop will draw a herbivore away from the main crop before the

herbivore has damaged the main crop by oviposition, feeding, or inoculation with a

pathogen. The limited success achieved managing Bemisia with trap crops may be due to

the mechanisms by which whiteflies find and accept hosts.

Whiteflies seeking hosts respond to the yellowish range of light spectra emitted by

most vegetation (Mound 1962, van Lenteren and Noldus 1990, Byrne and Bellows 1991).

Trialeurodes vaporariorum, B. tabaci and Aleurocanthus woglumi apparently do not

respond to crop-specific olfactory or visual cues (van Lenteren and Noldus 1990).

Trialeurodes vaporariorum must probe before accepting or rejecting a plant (van Sas et

al. 1978, Noldus et al. 1986a). Bemisia also seems to require gustatory information to

judge host suitability (Byrne and Bellows 1991). Examination of the precibarial and

cibarial chemosensillae by Hunter et al. (1996) indicates that B. tabaci can test plant sap

without ingesting it. This supports the notion that host discrimination by Bemisia occurs

after the host has been tasted.

Host 'preference' by whiteflies among crops may not be apparent until after adults

have invested time in colonizing the less suitable crop. Trialeurodes vaporariorum will

leave certain acceptable hosts after a few hours, while spending days on other hosts (van

Sas et al. 1978, Verschoor-van der Poel and van Lenteren 1978). Similarly, T

vaporariorum tends to accumulate in greater density on some hosts than others over a

given time period (Verschoor-van der Poel 1978 cited in van Lenteren and Noldus 1990).

If host preference for a given crop, such as a trap crop candidate, does not affect whitefly

behavior until after whitefly adults have oviposited and fed on the main crop, trap

cropping may have limited benefit for whitefly management.








However, Al-Musa (1982) and Schuster et al. (1996) reduced the incidence of

virus in tomato (Lycopersicon esculentum Mill.) by trap cropping with cucumber

(Cucumis sativus L.) and squash, respectively. Meena et al. (1984) reported a reduction

in Bemisia-vectored yellow mosaic of moth bean (Vigna aconitijolia (Jacqu.) Marechal)

by trap cropping with guar (Cyanopsis tetragonoloba (Linn.) Taub), sesame (Sesamum

indicum L), millet (Pennisetum typhoides (Burm, F.) Stapf. and Hubb.) or sorghum

(Sorghum vulgare L). The latter two crops are not hosts of Bemisia, however, so it is

possible that a different mechanism was involved. These studies indicate that trap

cropping can be used to reduce transmission of virus by whiteflies.

Conclusion

In our study squash did not function as a trap crop either by reducing density of

whitefly or presence of virus on adjacent bean. Oviposition was consistently higher on

squash than on bean. Oviposition was significantly less on bean in plots with reflective

silver mulch during the first week of sampling in 2 of the 3 years of this study. Mulch

improved plant quality and increased yield compared to unmulched plants. Neither

squash, reflective mulch nor the combination of the 2 provided significantly greater

protection from B. argentifolii than bean planted alone on bare soil.









Table 2-1. Egg density of B. argentifolii (mean SD /cm2) on beans and squash, 1995

Bean Squash

Week Treatment Lower stratum Upper stratum Mean Mean

I Bean 0.37 0.53 0.95 0.88ab' 0.66 0.78ab
Mulch 1.06 0.79 0.96 0.92a 1.01 0.85a
Squash 0.53 0.55 0.44 0.41 be 0.49 0.48ab
Squash/mulch 0.38 0.60 0.33 0.47c 0.36 0.54b

2 Bean 0.40 0.48 0.63 0.6lab 0.52 0.56a
Mulch 0.80 1.30 1.26 1.28a 1.03 1.30a
Squash 0.13 0.24 0.13 0.22b 0.13 0.23b 2.80 3.95a**
Squash/mulch 0.07 0.15 0.19 0.26b 0.13 0.22b 1.60 2.80b**

3 Bean 0.39 0.56b 0.40 0.85 0.40 0.7 lab
Mulch 0.81 0.75a 0.57 0.73 0.70 0.74a
Squash 0.04 0.1Oc 0.11 0.20 0.07 0.16b 1.24 1.83**
Squash/mulch 0.19 0.34bc 0.15 0.38 0.17 0.36b 0.50 0.70**

4 Bean 0.12 0.20 0.18 0.35 0.15 0.28
Mulch 0.32 0.39 0.24 0.35 0.28 0.37
Squash 0.10 0.16 0.13 0.29 0.11 0.23 0.51 0.78**
Squash/mulch 0.06 0.15 0.08 0.16 0.07 0.15 0.32 0.59**

'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, **indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively.










Table 2-2. Egg density of B. argentilblii (mean SD/cm2) on beans and squash, 1996


Bean


Treatment


Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch


Lower Stratum

4.16 3.28a'
0.97 1.14c
4.03 1.80ab
1.86 1.72bc


0.69 0.78
1.71 2.02
0.36 0.50
1.31 1.59

0.45 0.43
0.33 0.40
0.12 0.26
0.45 0.44

0.02 0.08
0.17 0.41
0.02 0.08
0.05 0.11

0.31 0.43
0.86 0.75
0.41 0.85
0.29 0.39


Upper stratum


Mean


0.96 0.72**
0.07 0.21 **


1.59 0.60
1.40 0.95
1.38 0.77
1.19 1.01

1.33 0.91
1.02 0.92
0.86 0.68
1.19 1.07

0.43 0.66
0.88 0.98
0.48 1.05
0.43 0.52

0.24 0.58
0.31 0.39
0.38 0.75
0.14 0.26


1.14 1.06
1.56 1.55
0.87 0.82
1.24 1.31

0.89 0.83
0.68 0.78
0.49 0.63
0.82 0.89

0.23 0.51
0.52 0.82
0.25 0.77
0.24 0.42

0.27 0.50
0.58 0.65
0.39 0.79
0.22 0.33


5.19 8.89*
5.02 8.28*



3.67 6.24*
6.34 7.74**


5.00 6.87**
3.76 5.62**



0.51 0.81
0.63 0.93*


Week


Squash

Mean






6 Bean 0.07 0.13 0.26 0.49 0.17 0.37
Mulch 0.17 0.19 0.26 0.33 0.22 0.27
Squash 0.09 0.25 0.33 0.29 0.21 0.30 0.66 0.77*
Squash/mulch 0.14 0.26 0.22 0.28 0.18 0.27 1.30 1.89**

'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively.









Table 2-3. Egg density of B. argentifblii (mean SD/cm2) on beans and squash, 1997


Bean


Lower stratum


UDDer stratum


Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch


15.32 10.73a'
4.89 4.17b
9.18 4.14ab
5.93 3.48b

16.77 10.44
11.25 8.18
14.48 10.27
13.29 16.62

1.71 2.27#
0.30 0.54#
2.39 3.49
0.36 0.62#

0.25 0.27#
0.55 0.78#
0.39 0.85#
0.36 0.45#

0.57 1.29
0.75 1.55#
0.09 0.20#
0.11 0.17#


27.37 33.31 **
23.07 27.53 **


27.11 13.47
29.86 15.15
34.95 22.13
26.46 18.91

8.50 5.09#
15.55 9.45#
7.36 6.80
12.87 11.16#

7.73 9.04#
11.45 11.36#
6.29 4.01#
7.71 2.46#

3.45 3.59
10.39 13.21#
8.43 4.50#
7.52 7.57#


21.94
20.55
24.71
19.88


12.81
15.19
19.74
18.50


5.11 5.17
7.93 10.19
4.88 5.82
6.61 10.00

3.99 7.28
6.00 9.60
3.34 4.14
4.04 4.17

2.01 3.00
5.57 10.37
4.26 5.29
3.81 6.43


123.71 137.66**
134.92 163.37**


47.52 58.68**
45.70 44.85**



36.22 33.11 *
24.09 21.08**


37.67 36.67**
46.50 37.98**


Week


Treatment


Mean


Squash


Mean


Week






6 Bean 0.04 + 0.07 0.88 0.74 0.46 0.67
Mulch 0.02 + 0.05 1.80 3.14 0.91 2.34
Squash 0 0.82 1.25 0.41 + 0.96 17.05 16.74**
Squash/mulch 0.07 0.20 0.91 0.60 0.49 + 0.61 20.24 18.00**

'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type 1 experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.









Table 2-4. Nymph density of B. argentffolii (mean SD/cm2) on beans and squash, 1995

Bean Squash

Week Treatment Lower stratum Upper stratum Mean Mean

I Bean 0.20 0.59 0.11 0.29 0.15 0.46
Mulch 0.15 + 0.29 0.25 0.62 0.20 0.48
Squash 0.18 0.36 0.33 0.50 0.26 0.44
Squash/mulch 0.13 0.34 0.06 0.17 0.10 0.27

2 Bean 0.96 0.84a' 0 0.48 0.76a
Mulch 0.42 0.36ab 0.20 0.35 0.30 0.37ab
Squash 0.21 0.27b 0.07 0.17 0.14 0.24ab 0.16 0.77
Squash/mulch 0.21 0.40b 0.04 0.10 0.13 0.30b 0*

3 Bean 0.27 0.472 02 0.14 0.35
Mulch 0.25 0.40 0 0.13 0.31
Squash 0.06 0.15 0 0.03 0.11 0.20 0.75
Squash/mulch 0.05 0.14 0.02 0.08 0.04 0.11 0.07 0.27

4 Bean 0.13 0.24 0 0.07 0.18
Mulch 0.20 0.57 0 0.10 0.41
Squash 0.01 0.06 0 0.01 0.04 0.13 0.56
Squash/mulch 0.01 0.06 0 0.01 0.04 0.32 1.04*

'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. Upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.10. *, *
indicate that nymph densities were significantly different between bean and squash at p < 0.05 and p < 0.01 according to the pairwise
t-test.









Table 2-5. Nymph density of B. argentifolii (mean SD/cm2) on bean and squash, 1996

Bean Squash
Week Treatment Lower stratum Upper stratum Mean Mean
2 Bean 0.52 0.57 0.57 0.60ab' 0.55 0.57a
Mulch 0.05 0.16 0.19 0.33a 0.12 0.27b
Squash 0# 1.28 0.87b# 0.64 0.89a 0*
Squash/mulch 0 0.29 0.30a 0.14 0.25b 0.01 0.06*

3 Bean 2.29 1.36# 0.36 0.49# 1.32 1.40
Mulch 2.31 1.52# 0.62 1.33# 1.46 1.64
Squash 1.97 1.13# 0.21 0.28# 1.10 1.21 1.01 2.40
Squash/mulch 1.86 1.02# 0.31 0.39# 1.08 1.09 0.63 1.33

4 Bean 1.41 0.86# 0.33 0.56# 0.87 0.90a
Mulch 1.76 1.51# 0.19 0.43# 0.98 1.35a
Squash 0.83 0.70 0.79 0.88 0.81 0.78ab 0.12 0.58**
Squash/mulch 0.86 0.89 0.02 0.08 0.44 0.75b 0.25 0.63

5 Bean 0.17 0.19 0 0.08 0.16a
Mulch 0.55 0.51 0.02 0.08 0.29 0.45b
Squash 0.05 0.11 0 0.02 0.08a 0.37 1.22
Squash/mulch 0.17 0.33 0.10 0.19 0.13 0.27ab 0.42 1.21

6 Bean 0.41 0.51 0 0.20 0.41
Mulch 1.05 1.32# 0# 0.52 1.06
Squash 0.26 0.28 0 0.13 0.24 0.27 1.05
Squash/mulch 0.50 0.61 0.07 0.25 0.29 0.51 0.16 0.45
'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.









Table 2-6. Nymph density of B. argentifolii (mean SD/cm2) on bean and squash, 1997


Bean


Week
2


Treatment
Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch

Bean
Mulch
Squash
Squash/mulch


Mean
4.48 5.19ab'
2.12 2.03b
6.87 8.07a
3.19 3.83ab

11.70 14.91
5.38 4.71
11.51 10.88
6.51 5.91

3.96 3.78
4.30 6.19
5.02 8.93
1.24 1.65

2.12 2.69b
9.59 7.47a
1.21 1.70b
2.20 2.49b


Mean


Lower stratum
8.29 4.81#
3.03 1.38
11.09 6.82#
5.98 3.62

13.16 12.04
7.54 3.26
12.84 7.54
6.97 3.92

3.59 A 2.77
6.12 7.10
3.34 2.77
2.46 1.55

0.95 0.91
7.89 5.52
1.93 1.99
2.55 2.78

0.89 1.03
0.86 A 0.99
0.48 0.44
2.21 2.61


Upper stratum
0.68 1.26#
1.20 2.23
2.66 7.24#
0.39 0.69

10.50 18.06
3.21 5.12
10.18 A 13.88
6.05 7.68

4.32 A 4.75
2.50 A 4.91
6.70 12.52
0.02 0.05

3.29 3.4lab
11.29 9.09a
0.50 A 1.03b
1.84 2.3l ab

2.86 A 2.98
4.05 A 5.22
4.92 A 5.26
3.27 6.27


'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.


1.88 2.38
2.46 3.99
3.33 4.07
2.74 4.68


Squash


12.47 24.15
6.25 10.40



61.69 176.54
13.04 32.00



14.98 29.31@
5.51 13.60



3.35 6.14a
0.79 1.93b


24.89 + 40.84**
16.49 29.18


1.4 ......18.









Table 2-7. Parasitized nymph density (mean SD/cm2) of B. argentifolii on bean, 1997

Week Treatment Lower stratum Upper stratum Mean

3 Bean 0.32 0.36 0 0.16 0.29
Mulch 0.25 0.28 0 0.13 0.23
Squash 0.13 0.19 0 0.06 0.15
Squash/mulch 0.34 0.37 0 0.17 0.31

4 Bean 0.61 0.55 0.02 0.05 0.31 0.48
Mulch 0.61 0.70 0.02 + 0.05 0.31 0.57
Squash 0.89 +0.86# 0.11 0.30# 0.50 0.74
Squash/mulch 0.34 0.42 0 0.17 0.33

5 Bean 0.41 0.67 0.02 0.05 0.21 0.50
Mulch 1.00 0.92# 0.09 0.25# 0.54 0.80
Squash 0.79 0.67# 0# 0.39 0.61
Squash/mulch 0.70 0.97 0 0.34 0.75

6 Bean 0.48 0.59 0.20 0.56 0.34 0.57a'
Mulch 0.25 0.50 0 0.13 0.37b
Squash 0.48 0.44 0 0.24 0.39ab
Squash/mulch 0.45 0.76 0.04 0.10 0.24 0.57ab
'Means in the same column with the same letter are not significantly different according to Tukey's Studentized Range test with
controlled type I experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. # indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p <
0.05.









Table 2-8. Red-eyed nymph density (mean SD/cm2) of B. argentifolii on bean, 1997

Week Treatment Lower stratum Upper stratum Mean

3 Bean 0.43 0.60 0 0.21 0.47
Mulch 0.32 0.50 0 0.16 0.38
Squash 0.14 0.30 0 0.07 0.22
Squash/mulch 0.46 0.48 0 0.23 0.41

4 Bean 0.71 1.08 0 0.36 0.82
Mulch 0.20 0.44 0 0.10 0.32
Squash 0.68 0.77 0.02 0.05 0.35 0.63
Squash/mulch 0.38 0.40 0 0.19 0.34

5 Bean 0.55 1.45 0 0.28 1.03
Mulch 1.13 1.38# 0.18 0.51# 0.65 1.11
Squash 0.22 0.30 0 0.11 0.23
Squash/mulch 0.46 0.87 0.04 0.10 0.25 0.63

6 Bean 0.41 0.48 0.04 0.10 0.22 0.39
Mulch 0.09 0.20 0 0.04 0.14
Squash 0.34 0.37 0 0.10 0.22
Squash/mulch 0.20 0 0.17 0.31

# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05.











Table 2-9. Egg density (mean SD/cm2) of B. argentifolii by stratum on squash.

Year Week Treatment Lower stratum Upper stratum

1995 2 Squash 1.00 + 1.67@ 4.61 4.72@
Squash/mulch 0.14 0.28 3.05 + 3.40

3 Squash 2.15 2.20@ 0.33 0.48@
Squash/mulch 0.83 0.81# 0.16 0.34#

4 Squash 0.92 0.92# 0.10 0.20#
Squash/mulch 0.60 0.74# 0.05 + 0.14#

1996 1 Squash 0.96 + 0.72
Squash/mulch 0.07 + 0.21

2 Squash 0.02 0.08# 10.36 10.35#
Squash/mulch 0.14 0.33# 9.91 9.56#

3 Squash 1.98 2.11 5.35 .8.41
Squash/mulch 2.36 2.21 10.33 9.52

4 Squash 2.07 4.23# 7.93 7.88#
Squash/mulch 1.17 1.43@ 6.36 7.02@

5 Squash 0.31 0.67 0.72 0.92
Squash/mulch 0.36 0.70 0.91 1.07

6 Squash 0.64 0.72 0.67 0.84
Squash/mulch 1.50 2.45 1.10 1.15






1997


Squash
Squash/mulch
Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch


0.73 0.39#
0.11 0.17#
0. 18 0.26#
0.07 0.07#

78.16 67.16#
70.32 37.31#

46.95 42.90
11.02 18.10

66.31 30.69#
70.21 34.20

24.86 19.28
25.50 17.85


54.00 27.50#
46.03 20.47#
247.25 75.65#
269.77 125.02#

16.88 26.85#
21.09 39.17#

25.50 15.70
14.00 14.04

9.04 8.14#
22.79 25.22

7.95 6.50
14.98 18.10


# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05. @ indicates that
upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0. 10.











Table 2-10. Nymph density (mean SD/cm2) on B. argentifblii by stratum on squash.


Treatment


Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch


Lower stratum


0.13 0.58
0

0.39 1.04
0.13 0.37

0.26 0.78
0.64 1.41


2.02 3.13#
1.26 1.69#

0.24 0.83
0.45 0.84

0.74 1.68
0.83 1.64

0.45 1.48
0.31 0.04


Upper stratum


0.19 0.93
0

0.01 0.06
0


0
0.02 0.08


0
0.05 0.16


0.10 0.19
0


Year


Week


1995


1996






1997


0.07 0.20
0.02 0.05


Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch

Squash
Squash/mulch


24.91 29.94
12.46 11.98

17.57 32.49
5.64 15.96


0.04 0.10
0.04 0.10

105.80 247.56
20.45 42.59

29.96 36.44
11.02 18.09

6.70 7.44
1.59 2.56

38.96 49.21
29.48 37.37


# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05.


5.20 11.15
3.50 8.43









Table 2-11. Total bean yield (kg).


Year


Treatment


Total bean yield (kg/plot)


1996 Bean 6.22b
Mulch 15.68a
Squash 4.52bc
Squash/mulch 11.42ab

1997 Bean 0.57
Mulch 0.68
Squash 0
Squash/mulch 1.24

'Means in the same column with the same letter are not significantly different according
to Tukey's Studentized Range test with controlled type I experimentwise error rate
(a=0.05). The absence of letters in a column indicates the lack of significant differences
among any means.














CHAPTER 3
POTENTIAL OF FIELD CORN (ZEA MAYS L.) AS A BARRIER CROP AND
EGGPLANT (SOLANUMMELONGENA L.) AS A TRAP CROP FOR MANAGEMENT
OF THE SILVERLEAF WHITEFLY, BEMISIA ARGENTIFOLII (HOMOPTERA:
ALEYRODIDAE) ON BEAN (PHASEOLUS VULGARIS L.) IN NORTH FLORIDA

Introduction

Bemisia argentifolii Bellows & Perring, the silverleaf whitefly (also known as

Bemisia tabaci strain B (Gennadius)), causes significant economic damage to agronomic

and horticultural crops throughout warm regions of the world (Brown et al. 1995).

Bemisia argentifolii is a phloem-feeder which vectors numerous geminiviruses and

inflicts a variety of plant disorders as well as mechanical damage (Byrne et al. 1990,

Hiebert et al. 1996, Shapiro 1996). Bemisia has demonstrated resistance to most classes

of pesticides (Denholm et al. 1996), forcing growers and reseachers to evaluate

alternative methods of control. Attempts to manage whiteflies by cultural means have

included the use of trap crops (Al-Musa 1982, Ellsworth et al. 1994, McAuslane et al.

1995, Schuster et al. 1996) and barrier crops (Fargette and Fauquet 1988, Morales et al.

1993, Rataul et al. 1989, Sharma and Varma 1984).

Trap crops are preferred host plants which are used to draw an herbivore away

from a less-preferred main crop (Vandermeer 1989). Bemisia argentiJolii has been

observed to oviposit heavily on eggplant (Solanum melongena L.), leading researchers to

suggest eggplant as a promising trap crop candidate (Faust 1992).

Whiteflies are weak fliers, relying on air currents for both short and long distance

migration (Byrne and Bellows 1991, Byrne et al. 1996). Several tall-growing non-host







50

plants, primarily in the family Gramineae, have been tested as barrier crops or intercrops

to reduce whitefly colonization and virus transmission among main crops. Results have

been mixed. Morales et al. (1993) reported that a sorghum (Sorghum bicolor (L.)

Moench) barrier reduced Bemisia densities, but not transmission of virus, on tomatos

(Lycopersicon esculentum Mill.). A pearl millet (Pennisetum typhoides (Burm. f.) Stapf

& Hubbard) barrier reduced whitefly virus transmission on cowpea (Vigna unguiculata

(L.) Walp.) (Sharma and Varma 1984) and soybean (Glycine max (L) Merrill) (Rataul et

al. 1989). Gold et al. (1990) found reduced densities of Aleurotrachelis socialis Bondar

and Trialeurodes variabilis (Quaintance) on cassava (Manihot esculenta Crantz)

intercropped with maize (Zea mays L.) and cowpea, but attributed this in part to reduced

host quality due to intercrop competition. Fargette and Farquet (1988), whose study

included the effect of wind direction, found densities of B. tabaci and virus incidence

were sometimes higher on cassava intercropped with maize than on monocropped

cassava.

These studies have been carried out primarily in the tropics, where safe,

inexpensive cultural control measures are a priority for low resource farmers. Extension

material from Central America promotes the use of crop barriers as a component of

whitefly management programs (Salguero 1993; Pan-American School of Agriculture

(Zamorano) poster: 'Reconozca y controle la mosca blanca'). The present study was

undertaken in 1996 to test the usefulness of eggplant as a trap crop and field corn as a

barrier crop for management of B. argenti'bflii on snap bean (Phaseolus vulgaris L.). It

was continued in 1997 focusing only on the barrier crop treatment and including the

effects of wind direction and barrier row orientation.








Materials and Methods

1996

Research design and plot management. The experiment was carried out at the

University of Florida Green Acres Agronomy Research Farm northwest of Gainesville,

FL (2940'N, 8230'W). Four treatments were compared: 1) bean planted in

monoculture, 2) bean intercropped with eggplant, 3) bean intercropped with field corn,

and 4) bean monoculture treated with imidacloprid (Provado 1.6F, Bayer, Kansas City,

MO), a systemic insecticide. The imidacloprid treatment was included for yield

comparison only. It was not sampled for whiteflies.

Crop varieties used were 'Espada' garden bean (Harris Seed, Rochester, NY),

'Black Beauty' eggplant (Ferry-Morse Seed, Fulton, KY), and the subtropical field corn

hybrid Howard IIIST (Gallaher et al. 1998). Plant spacing within the row was 10 cm for

bean, 15 cm for corn and 46 cm for eggplant. Each plot contained 14 rows, 6.1 m in

length with 0.9 m between rows. Monoculture bean plots contained only beans.

Intercropped plots were planted in a 2:4:2:4:2 pattern, with corn or eggplant in the

outermost and central 2 rows, surrounding 2 four-row patches of bean. Each treatment

was replicated 5 times and arranged in a randomized complete block design.

Corn was planted 26 July and fertilized with 0.68 kg 15-0-14 (N-P2O5-KO) per

row. Corn received 0.3 kg 15-0-14 per row on 9 August. Heavy Spodopterafrugiperda

(JE Smith) damage threatened the barrier crop treatment in August. Corn was treated

with 1.74 liter/ha methomyl (Lannate, DuPont Corp., Newark, DE) on 9 August and 29

August. Eggplant was transplanted 22 August when 3 wks old. Eggplant received 0.23

kg per row 15-0-14 fertilizer 27 August, and 0.8 kg on 27 September and 10 October.









Beans were planted 15 September and fertilized with 0.37 kg 15-0-14 per row on 23

September and 12 October.

The experimental area was treated with 0.19 liter/ha paraquot (Gramoxone,

Zeneca) on 26 July. Subsequent weed control was mechanical or by hand. The

imidacloprid-treated beans received 52.6 g/ha ai imidacloprid on 4 October and 12

October. This is the recommended rate for most vegetables. Imidacloprid is not

registered for use on beans but was included so that yield from intercropping treatments

could be compared with yield from chemically-protected beans.

Sampling. Whole plant examinations were made of 1 or 2 bean plants per plot

each week from 22 September through 11 November except for 29 September. Only the

underside of the leaf was examined. The area of each leaf was recorded using a LI-COR

portable leaf area meter (model LI-3000A, LI-COR Inc., Lincoln, NE). Bean treatment

comparisons were made on the basis of whole plant counts. Leaf counts from upper,

middle, and lower plant strata were used for comparison with eggplant on 21 October and

4 November. On 29 September bean and eggplant comparisons were based on the

average of counts taken from one 3.35 cm2 disc from a leaf in the upper and lower stratum

of two plants per plot (McAuslane et al. 1995).

Whole plant examinations were made of I to 3 eggplants per block each week

from 25 August through 8 October. After that time, plants became too large for whole

plant examinations. Whole leaf counts from upper, middle and lower strata were made of

eggplant on 21 October and 4 November.

Leaves were examined using a stereoscope and fiber-optic light. Total number of

B. argentifolii eggs, nymphs, parasitized nymphs, and red-eyed nymphs (pharate adults)

was recorded for each leaf. Leaves with nymphs showing symptoms of parasitism were






53

placed in unwaxed cylindrical 0.95 liter cardboard cartons (Fonda Group Inc., Union, NJ)

to allow parasitoids to emerge.

Corn height. The height of five corn plants per row was measured on 4 October to

assess the barrier effect.

Yield. Bean was harvested from two 2.0-m sections from each plot on 22

November. Fresh weight was recorded.

Statistical analysis. Densities of B. argentijblii eggs, nymphs, parasitized nymphs

and red-eyed nymphs were compared among bean treatments using analysis of variance

(PROC GLM, SAS version 6.11, SAS Institute 1996). Densities of whitefly immatures on

bean and eggplant in the trap crop test were compared using the same test, as was bean

yield. When appropriate, mean separation was carried out using Tukey's Studentized

Range test.

1997

Research design and plot management. In 1997 the corn barrier treatment was

repeated on a larger scale. Three treatments were compared to evaluate the influence of

the barrier crop and the effect of barrier row orientation to wind direction on adult

whitefly movement. Prevailing winds in August in the area tend to be from the east. The

treatments were 1) bean planted in monoculture ('bean alone'), 2) alternating rows of

bean and corn planted north to south ('barrier') and 3) alternating rows of bean and corn

planted east to west (*open*) (Figure 3-1).

Treatments were arranged in a randomized complete block strip split plot design.

Each treatment was replicated four times. The four blocks were arranged in pairs on

either side of a 12 m-wide path running north to south. Treatment plots were 15.25 m x






















V Release
point


Barrier


Open

Bean



I Wind


Figure 3-1. Plot Plan, Green Acres 1997









30.5 m, with the shorter side parallel to the central path. This design was used to allow

for a release of whitefly adults from points spaced evenly along the central path.

Corn was planted 25 March. It was fertilized with 67 kg/ha 15-0-14 (N- PO5-

K20) on 1 April, 26 April and 14 May. Bean was planted 1 July and fertilized with 33

kg/ha 15-0-14 (N-PO5-K20) at planting, on 10 July and 20 July. Overhead irrigation was

used to supplement rainfall. Plots were weeded mechanically and by hand.

Mass-rearing of B. argentifolii. About 30 senescing broccoli (Brassica olerecea

L.) plants infested with B. argentifolii were removed from an organic farm near

Gainesville on 1-6 June. They were potted and placed with 36 flowering hibiscus

(Hibiscus rosa-sinensis L.) plants in a greenhouse at the Department of Entomology and

Nematology at the University of Florida. Hibiscus plants were watered regularly and

fertilized with Purcell's Sta-Green plant food (18-6-12 N-P2O5-K20) (Purcell Industries,

Inc., Sylacauga, AL). By early August, the hibiscus plants were heavily infested with

whiteflies.

Trap preparation. Yellow sticky traps have been used in many instances to

monitor and sample whitefly adults (Ekbom and Rumei 1990). In the evening of 7

August, 180 plastic yellow 710-ml Solo Party cups (Solo Cup Company, Urbana, IL)

were coated with Tangle-Trap Insect Trap Coating (product 95010, Tanglefoot Company,

Grand Rapids, MI), an aerosol adhesive, for use as whitefly traps. The traps were

arranged in 5 rows within each plot at 1.5, 7.6, 14, 20, and 26 m from the edge of the plot

bordering the central path. Three traps were placed in each row. One trap was placed 3.8

m in from either side of the plot, and one was placed 7.6 m within the plot, at the center

of the row.








Dust-and-release procedure. Byrne et al. (1996) developed a method of dusting

whitefly adults with a fluorescent pigment in the field and trapping them at a distance as a

means to monitor movement. We modified this method to distinguish the released

whitefly adults which were caught on the traps from trapped members of the naturally-

occurring field population.

Before dawn on 8 August the infested hibiscus plants were enclosed in 113.5 liter

plastic leaf litter bags. The nozzle of a Lesco technical duster (product 1964, Lesco Inc.,

Cleveland, OH) was forced through the plastic and approximately 8.5-14 g Day-Glo Fire

Orange fluorescent AX-14-N pigment (Day-Glo Color Corp., Cleveland, OH) was puffed

from the duster into the bag onto the infested plants. The hibiscus plants were transported

to the experimental area enclosed in plastic bags and arranged in 6 clusters of 6 plants

along the central path and between pairs of treatment plots. The plastic bags were

removed between 7:30 and 7:50 AM to allow a unified release of dyed whitefly adults.

The traps were removed and replaced at dusk. The second set of traps was removed at

dusk on 9 August. After removal, traps were kept refrigerated until examined.

On 10 August, the hibiscus plants were returned to the greenhouse. Traps were

placed in the plots from 8:00 AM to 5:00 PM on 14 August to determine that whitefly

adults from the first release were no longer measurably present in the area. On 24 August

the dust-and-release procedure was repeated. Traps were set out from 8:00 AM to 8:00

PM on 24 August, and replaced with traps that were recovered at dusk on 25 August.

Hibiscus plants were removed after the second set of traps had been retrieved. Traps

were examined using a Spectroline 365 nm black light (model B-14N, Spectronics Corp.,

Westbury, NY). The number of fluorescing whitefly adults on each trap was recorded.






57

Corn height. The height of 15 com plants per plot was measured on 27 August to

evaluate the barrier effect.

Statistical analysis. The effect of treatment, block, and trap position on trap count

was analyzed using analysis of variance (PROC GLM, SAS version 6.11, SAS Institute

1996). Orthogonal contrasts were then used to compare trap counts in the same treatment

east and west (upwind and downwind) of the release point, and to compare trap counts

among treatments in blocks west of the release point. Wind direction data collected at the

site were provided by Dr. E. B. Whitty, Agronomy Department, University of Florida,

Gainesville, FL.

Results and Discussion

1996

Whitefly densities. Densities of eggs were highest on bean when sampling began

and declined over subsequent weeks (Table 3-1). Nymph densities were highest during

weeks 3 and 4. Observations of parasitized nymphs and red-eyed nymphs were low

throughout, although parasitism increased slightly over time.

There were no differences (p < 0.10) in egg density among treatments during the

first six weeks of sampling. Egg densities on bean alone were higher (p < 0.05) than on

bean intercropped with corn or eggplant during weeks 7 and 8. No differences (p < 0.10)

in nymph densities occurred among treatments. Densities of red-eyed nymphs were

higher (p < 0.05) on bean alone than on the corn and eggplant treatments during week 4.

During week 6, parasitism was higher (p < 0.1) in the corn treatment than in the bean

alone treatment. During week 7, parasitism was more than twice as high in the eggplant

treatment as in the other two treatments.






58

Whitefly adults were observed on eggplant the day following transplanting on 22

August, and eggs were observed in the 25 August sample (Table 3-2). When bean plants

were emerging, eggplants were quite large: they had an average of 7.0 1.3 branches, a

mean height of 17.33 0.28 cm and mean leaf area of 485 156 cm2 (n=5).

Bean vs. eggplant. Densities of eggs and nymphs peaked on eggplant 4 weeks

after transplanting and declined during the following weeks (Table 3-2). Egg densities

were one and a half times higher on eggplant than on bean during the first week that bean

was sampled (22 September). On all subsequent sampling dates, however, egg densities

were significantly higher on bean than on eggplant.

During the week that nymphs were first observed on bean, densities were

significantly lower on bean than on eggplant. During subsequent sampling dates, nymph

densities were either higher on bean or not statistically different. Observations of

parasitized and red-eyed nymphs were either higher on eggplant than on bean or not

significantly different on the two hosts.

Parasitoid species. All parasitoids reared from bean and eggplant were

hymenopterans from the family Aphelinidae.

Thirty-nine parasitoid individuals were recovered from bean leaves. Thirty-two of

these were Encarsia nigricephala Dozier (82%), 4 were Eretmocerus sp. (10.3%), and 3

were Encarsia pergandiella Howard (7.7%).

One hundred twenty-one parasitoid individuals were reared from eggplant leaves.

Fifty-one of these were Encarsia pergandiella (42.1%), 48 were Encarsia nigricephala

(3 9.7%), 13 were Eretmocerus sp. (10.7%), 6 were Encarsia transvena (Timberlake)

(5%), and 3 were Encarsia sp. (2.5%).






59

The greater parasitism and variety of parasitoid species on eggplant may be due to

the greater number of weeks that eggplant was in the field.

Bean yield. Bean yield per 2 m of row was not different among the three

treatments and the imidacloprid-treated bean plants (imidacloprid: 0.95 kg + 0.71; bean:

0.87 kg 0.58; corn: 0.47 kg 0.28; eggplant: 1.14 kg 0.77).

Eggplant as a trap crop. Eggplant did not reduce oviposition on adjacent bean

early in the season, and so did not function as a trap crop. Oviposition was not

consistently higher on eggplant than on bean as reported elsewhere (Tsai and Wang

1996). Eggplant leaves may have been less suitable for oviposition because they were

several weeks older than the bean leaves. Treatment differences were not statistically

significant, but egg densities tended to be higher on bean planted with eggplant than on

the other bean treatments during the first weeks of sampling. Proximity to colonized

eggplant may tend to increase rather than decrease oviposition on bean.

A concurrent test of squash (Cucurbitapepo L.) as a trap crop for whiteflies also

produced negative results (Smith et al., unpublished). It is possible that host-finding

mechanisms used by whitefly adults prevent them from being drawn away from one host

plant by the presence of another. Bemisia does not respond to host-specific visual or

olfactory cues (Mound 1962). It apparently requires gustatory information in order to

accept or reject a host (van Lenteren and Noldus 1990). Whitefly adults tend to leave

some host plant species more quickly than others (Verschoor-van der Poel 1978). The

observed differences in host-specific oviposition density by Bemisia may be due to length

of tenure on the plant rather than to some preference expressed in the host-finding stage.

Many trap crop studies have not resulted in consistent reductions of whitefly

densities on the main crop (Ellsworth et al. 1994, McAuslane et al. 1995, Perring et al.







60

1995, Puri et al. 1995, Schuster et al 1996). However, Al-Musa (1982) and Schuster et al.

(1996) reported a reduction in virus incidence on tomato (Lycopersicon esculentum Mill.)

using cucumber (Cucumis sativus L.) and squash, respectively, as trap crops.

Corn as a barrier crop. The corn did not grow well in 1996 due to insufficient

fertilizer. It attained a mean height of 1.18 m 0.34 (n = 150) and a density of 27 7

plants per 6.1 m row (n = 30). We decided to re-evaluate the barrier effect in 1997 with

larger, properly fertilized plots. Eggplant did not appear to be a promising trap crop, and

so was not included in the field experiment the following year.

1997

Release of adult whiteflies. Average corn height was 2.45 1.97 m when

whitefly releases were made. The effect of treatment on trap count was not significant

(p < 0.10) on any of the four collection dates (Table 3-3). The block effect was highly

significant, and the interaction between treatment and block was significant or highly

significant on three of the collection dates. Wind direction was from the east or northeast

during the 4 days that collections were made (Table 3-4). Trap counts in plots to the west

of the release point were significantly higher than trap counts in plots to the east of the

release point for each treatment on each collection date (Table 3-4). When treatments

were compared on the basis of downwind plots only, counts were significantly lower in

the barrier treatment than in the other two treatments on two of the four collection dates.

Wind direction appeared to be the primary factor determining where whitefly adults were

trapped. This is consistent with observations that whitefly adults move passively with

wind currents as 'aerial plankton' (Byrne and Bellows 1991). Among downwind plots,

the barrier treatment tended to have the lowest counts, indicating that the arrangement of

corn rows perpendicular to the prevailing wind direction did have some effect on the








movement of adults within the plot. However the overall trap counts in this study were

low. The contribution made by corn barriers to reducing whiteflies may depend on the

density of the whitefly population. Crop barriers such as corn may be more effective

when used with other control measures. Short of employing manufactured barriers such

as floating row covers or fine mesh screens, whitefly adults probably cannot be excluded

from a cropped area (Norman et al. 1993).

Trap position had a significant effect on trap count (Table 3-3). The number of

whiteflies caught decreased as trap distance from the release point increased. The

interaction of treatment and trap position interaction was not significant, suggesting that

this decline was not different among treatments.

Data derived from attractive traps may be ambiguous. A gravid or hungry

whitefly adult which is surrounded by non-hosts, such as corn, may be more sensitive to a

distant patch of bright yellow than an adult in similar condition surrounded by acceptable

hosts, such as bean. It is conceivable that the whitefly adults in the corn treatments spent

more time searching and so were drawn from a greater area than the whitefly adults

trapped in the monocropped bean treatments. It is possible that fewer whitefly adults

entered the corn treatments than the monocropped bean, but that a higher proportion of

those entering the corn treatments were trapped. However, these considerations do not

alter the overall impression that where air currents can enter, whitefly adults can follow.

Conclusion

Eggplant, transplanted a few weeks before bean was planted, did not serve as a

trap crop for B. argentifolii. Wind direction was the overwhelming factor determining

movement of whitefly adults into experimental plots with or without barrier crops. In

downwind plots, corn rows planted perpendicular to the predominant wind direction






62

marginally reduced penetration of whitefly adults into plots on some dates when

compared to bean monoculture and corn rows planted parallel to the wind. Corn barriers

planted perpendicular to the wind may be useful at certain whitefly population densities

when used with other control tactics.










Table 3-1. Mean (SD) number of immature B. argentifolii/cm2 on bean, 1996.


Wk Treatment Egg


Nymph


Para. Nymph2


I Bean
Corn
Eggplant


3 Bean
Corn
Eggplant


4 Bean
Corn
Eggplant
X

5 Bean
Corn
Eggplant


6 Bean
Corn
Eggplant


7 Bean
Corn
Eggplant
5x


8 Bean
Corn
Eggplant


0.790.58
1.040.73
1.270.68
1.030.67

0.620.40
0.930.26
1.000.58
0.850.44

0.400.30
0.670.49
0.600.27
0.560.36

0.360.20
0.390.10
0.440.15
0.400.14

0.410.34
0.430.16
0.460.14
0.430.22

0.540.58a'
0.220.18b
0.340.32b
0.370.39


0.260.16a
0.060.04b
0.1 10.16b
0.140.15


0.640.29
0.860.33
1.310.87
0.940.59

0.790.30
1.100.65
0.800.25
0.900.43

0.480.30
0.800.51
0.610.33
0.630.39

0.460.23
0.580.22
0.670.26
0.570.24

0.510.26
0.440.15
0.410.22
0.460.20

0.450.35
0.310.20
0.330.24
0.360.26


0
0.0020.004
0.0060.01
0.0030.008

0.0100.008
0.0100.004
0.0040.005
0.0100.008

0.0060.005
0.0160.015
0.0060.008
0.0090.010

0.0040.005a4
0.0200.015b
0.01 00.007ab
0.0110.012

0.0100.01 a
0.0160.015a
0.0360.027b
0.0210.021

0.0460.049
0.0520.043
0.0240.018
0.0410.038


0
0
0.0040.008
0.0010.005

0.0 100.02a
0.0020.004b
Ob
0.0040.013

0.0060.005
0.0080.013
0.0040.005
0.0060.008

0.0120.011
0.0180.016
0.0160.011
0.0150.012

0.0100.010
0.0160.013
0.0140.008
0.0130.010


0.0060.008
0.0080.008
0.0020.004
0.0050.007


'Means assigned different letters in the same column and week of sampling are
significantly different according to Tukey's Studentized Range test with an adjusted
experiment-wise error rate of a=0.05. @ indicates a=0. 1. 2Parasitized nymphs. 3Red-
eyed nymphs (pharate adults).


REN3










Table 3-2. Immature B. argentilblii (mean SD/cm2) on bean and eggplant, 1996.


Egg Nymph Parasitized nymph Red-eyed nymph
Date Bean Eggplant Bean Eggplant Bean Eggplant Bean Eggplant
Aug. 25 0.660.46 0 0 0

Sept. 1 0.891.02 1.3 11.60 0 0

Sept. 8 1.030.65 0.520.33 0 0.0030.006

Sept. 16 3.530.72 2.390.33 0 0.0070.006

Sept.22 1.661.67@ 2.741.72@ 0* 1.841.72* 0 0 0 0

Sept.29 5.523.44* 1.681.72* 0.880.62* 2.131.78* 0 0 0@ 0.0310.104@

Oct. 8 0.650.31* 0.240.41* 1.590.83* 0.290.19* 0.0050.016* 0.0350.037* 0.0050.016 0.0120.015

Oct. 21 0.640.54* 0.230.22* 0.450.35 0.490.65 0.0090.014 0.0250.038 0.0030.009 0.0460.078

Nov. 4 0.260.26* 0.020.03* 0.280.19* 0.110.10* 0.0240.043@ 0.0690.067@ 0.0060.012* 0.0480.043*

*indicates that numbers on bean and eggplant are significantly different on a given date according to analysis of variance at w=0.05.
(@ indicates a=0.1.









Table 3-3. Analysis of variance for whitefly release data, 1997
August 8


Source


Block

Treatment

Trap position

Block*treatment

Block*trap position

Treatment*trap position

**p < 0.01; *p < 0.05; @p<0.1.


10.74**

0.02

4.67*

4.10**

1.48

0.59


August 9

F


FF F


32.76**

0.86

4.65*

2.54*

1.23

0.94


August 24


56.24**

1.78

2.99@

5.28**

7.84**

0.13


0.13 1.77


August 25


12.34**

2.01

2.86@

1.81

0.80

1.77










Table 3-4. Whitefly adults (mean SD) per trap under 3 cropping systems, August 1997.


Bean Alone


Corn: Barrier to Wind


Corn: Open to Wind


Row Downwind


Release 11
Aug. 8


Aug. 9 I
2
3
4
5
R


Release 2'
Aug. 24


1.672.25
1.331.97
0.670.52
0.500.55
0.330.52
0.901.40*

2.001.79
1.670.82
1.501.22
0.500.55
0.500.84
1.231.22*a2


1 3.002.00
2 1.671.21
3 0.830.75
4 0.500.55
5 0.330.52
R 1.271.46*b


0.330.52
0
0
0.330.52
0
0.130.35*

0.170.41
0.330.52
0
0
0.170.41
0.130.35*


0.330.52
0.170.41
0.170.41
0.170.41
0.170.41
0.200.41 *


2.331.03
1.000.63
1.331.51
0.330.52
0.170.41
1.031.16*

1.170.75
1.000.89
0.830.98
0.500.84
0.670.82
0.830.83*b


2.833.25
1.501.05
0.500.84
0.170.41
0
1.001.82*b


0.330.52
0
0
0
0.160.41
0.100.31*

0
0.170.41
0
0
0
0.030.18*


0
0.170.41
0
0
0
0.030.18*


2.331.21
0.330.52
0.170.41
0.670.82
0.671.03
0.831.12*

1.831.47
1.171.17
0.671.21
1.000.89
0.500.84
1.031.16*ab


3.502.17
2.501.05
1.831.33
1.170.41
1.331.21
2.101.52*a


0.500.84
0.170.41
0.170.41
0
0
0.170.46*

0.330.52
0
0
0
0
0.070.25*


0.170.41
0.170.41
0
0
0
0.070.25*


Date


Upwind


Downwind


Upwind


Downwind


Upwind









Aug. 25 1 3.33+1.97
2 1.00+1.10
3 1.170.98
4 0.670.82
5 0.330.52
,x 1.30+1.53*a


0.170.41
0
0
0
0
0.030.18*


0.330.52
1.001.10
0.170.41
0.330.82
0.170.41
0.400.72b@


1.171.17
1.000.89
0.500.55
0.831.17
1.00+0.89
0.90+0.92"a@


'Wind direction on release dates: Aug.8: 750; Aug.9: 970; Aug. 24: 610; Aug. 25: 55'.
*indicates mean trap counts in the same treatment upwind and downwind of the release point are significantly different at p < 0.05
according to F-test for contrasts.
2Different letters indicate that mean trap counts in blocks downwind of release point are significantly different at p < 0.05 according to
F-test for contrasts.
@ indicates means are significantly different at p < 0.1 according to F-test for contrasts.
3Row refers to trap location (1 =nearest, 5- farthest from release point; see text). R = mean across all 5 row locations.














CHAPTER 4
THE ROLE OF CROP DIVERSITY IN THE MANAGEMENT OF A WHITEFLY
(HOMOPTERA: ALEYRODIDAE) SPECIES COMPLEX ON BEAN (PHASEOLUS
VULGARIS L.) AND TOMATO (LYCOPERSICON ESCULENTUM MILL.) IN THE
SALAMI VALLEY, BAJA VERAPAZ, GUATEMALA

Introduction

Intercropping is the agronomic practice of growing two or more crops in a field at

the same time (Andrews and Kassam 1976). Intercrop arrangements include growing

crops in alternating rows (row intercropping), mixing crops within a row or without

regard to rows (mixed intercropping), and relay intercropping, which allows partial

overlap of crop cycles (Andrews and Kassam 1976). Among the advantages attributed to

some intercropping systems is reduced pest damage (Kass 1978, Litsinger and Moody

1976, Perrin 1977). Reviews of the intercropping literature indicate that, relative to

monoculture, herbivore numbers were lower in more than 50 percent of the intercropping

systems studied, greater in 15 to 18 percent of the cases, and variable in about 20 percent

of studies (Andow 1991 a, Risch et al. 1983).

Several theories have been proposed to explain how intercropping may reduce

pest damage (Altieri 1994, Andow 1991a,Vandermeer 1989). Pimentel (1961) articulated

the idea that diverse cropping systems will support arthropod communities which are

more diverse and comprised of populations which are less dense and more stable than

arthropod communities in monocultures. It was hypothesized that natural enemies might

be more efficient in diverse agroecosystems than in simple ones, and that by damping









oscillations in arthropod populations, crop diversity would reduce pest outbreaks (Elton

1927, 1958, Pimentel 1961). This "enemies" hypothesis was summarized by Root

(1973), who added to it the "resource concentration" hypothesis to explain reduced

herbivore damage in some complex agroecosystems. The "resource concentration"

hypothesis suggests that exploitation of crops by specialist herbivores can be reduced by

breaking up monocultures. Damage by polyphagous herbivores may also be reduced by

the presence of poor or non-hosts in mixed systems by the "flypaper effect" (Trenbath

1976, 1977). Finally, trap crops can be used in intercropping to draw herbivores away

from a main crop (Vandermeer 1989).

The theory that diversity in itself will reduce pest damage has been largely

discarded as inconsistent with empirical data (Andow 1991a, Risch et al 1983). More

recent analysis suggests that the interaction between a cropping system and its arthropod

community is determined largely by the specific characteristics of each (Andow 1991 a,

Kareiva 1983, Sheehan 1986, Stanton 1983). The ratio of host to non-host species will

have a greater effect on herbivore abundance than the actual number of crop species

(Power 1990, Stanton 1983). The response of both herbivores and natural enemies to a

given cropping system will depend on their host range, their host-finding mechanisms,

and their mobility (Kareiva 1983, Power 1990, Russell 1989, Sheehan 1986, Stanton

1983).

Many small farmer cropping systems in the tropics rely on the principles of

intercropping to produce a range of goods for the home and market (Altieri and Hecht

1990, Kass 1978). Efforts by low resource farmers to improve income by concentrating






70

on higher-value market and export crops have resulted in an increase in pesticide use and

pesticide-related health problems in Central America (Murray 1991, Nicholls

and Altieri 1997). In Guatemala, the cultivation of non-traditional export crops has been

associated with reduced nutrition (Barrett 1995) and increased debt in some communities

(Glover and Kuterer 1990, Rosset 1991). The present series of studies was undertaken

with the intention of developing an intercropping system which helped meet the

economic and nutritional needs of low resource farmers by including both subsistence

crops (bean, Phaseolus vulgaris L.; and corn, Zea mays L.) and a market crop (tomato,

Lycopersicon esculentum Mill.) while reducing pesticide use.

Whiteflies cause economic damage to agronomic and horticultural crops

throughout the tropics (Brown et al. 1995, Byrne et al. 1990, Byrne and Bellows 1991).

Trialeurodes vaporariorum (Westwood), the greenhouse whitefly, Bemisia tabaci

(Gennadius), the sweetpotato whitefly, and Bemisia argentijblii Bellows and Perring (also

known as B. tabaci strain B), the silverleaf whitefly, are among the most damaging

species on annual crops. These three whitefly taxa reduce yields by vectoring viruses,

inflicting plant disorders, and causing mechanical damage to members of most crop

groups except the grasses (Byrne et al. 1990). Whiteflies have developed some degree of

resistance to most classes of pesticides (Denholm et al. 1996, Dittrich et al. 1990),

forcing growers and researchers to evaluate alternative methods of control. Imidacloprid

(Bayer) is a systemic insecticide which is currently effective against whiteflies and other

sucking insects (Polston et al. 1994). Detergents and oils have been used successfully to

manage whiteflies under certain conditions (Stansly 1995).






71

Attempts to manage whiteflies with intercropping have produced variable results.

Al Musa (1982) and Schuster et al. (1996) reduced Bemisia-vectored geminivirus on

tomato by trap cropping with cucumber (Cucmis sativus L.) and squash (Cucurbita pepo

L.), respectively. However, efforts to reduce whitefly densities with trap crops have

generally been unsuccessful (Ellsworth et al. 1994, McAuslane et al. 1995, Puri 1996).

Barrier crops have been used to reduce densities of Bemisia (Morales et al. 1993) and

incidence of whitefly-transmitted virus on cowpea (Vigna unguiculata (L.) Walp.)

(Sharma and Varma 1984) and soybean (Glycine max (L.) Merrill) (Rataul et al. 1989).

Gold et al. (1990) found that densities of immature cassava whiteflies Aleurotrachelus

socialis Bondar and Trialeurodes variabilis (Quaintaince) were lower on cassava

(Manihot esculenta Crantz) intercropped with cowpea than on monocropped cassava, but

attributed this in part to reduced host quality in intercropped treatments. Ahohuendo and

Sarkar (1995) reduced density of B. tabaci and incidence of cassava virus on cassava by

intercropping with maize (Zea mays L.) and cowpea. Fargette and Farquet (1988) found

that densities of B. tabaci and virus incidence were sometimes higher on cassava

intercropped with maize than on cassava grown alone.

The origin of the whitefly problem in Central America is associated with the

dense populations that developed on large-scale cotton (Gossypium hirsutum L.)

plantations along the region's Pacific coastal plain in the 1960s (Dard6n 1992). Bean

golden mosaic geminivirus, vectored by B. tabaci (Costa 1975), was first described in

Guatemala in 1963 (Scheiber 1983). After peaking in the late 1970s, bean golden mosaic

declined in importance until 1989, when it decimated bean crops throughout Central

America (Rodriquez 1994). Devastating whitefly-transmitted geminiviruses spread






72

throughout tomato-producing areas of Central America and the Caribbean during the late

1980s (Brown 1994), severely impacting Guatemala in 1987 (Dard6n 1992). This

explosion of tomato geminiviruses is attributed to the arrival of the 'B' strain of B. tabaci,

also known as B. argentijolii, throughout the region (Polston and Anderson 1997).

Whitefly problems in Central America tend to be attributed to Bemisia, but there

are at least 15 genera of whiteflies in the region with varying degrees of economic

importance (Caballero 1994). According to Caballero (1994), Trialeurodes

vaporariorum tends to be found in areas more than 1000 m above sea level, whereas B.

tabaci is rarely found above 1000 m. Trialeurodes vaporariorum does not vector

geminiviruses (Brown and Bird 1992), but its importance relative to Bemisia at higher

elevations may be underestimated.

The following experiments were a component of a broader effort to evaluate the

potential of intercropping for management of whiteflies. Prior field studies at the

University of Florida in Gainesville, Florida, indicated that trap cropping with squash or

eggplant (Solanum melongena L.) was ineffective in reducing densities of B. argentifolii,

and that using corn as a barrier crop was only marginally effective (see Chapters 2 and 3).

After consulting with pest management specialists from the Guatemala, San Jer6nimo

was chosen as a suitable site in which to test intercropping and whitefly management in

the context of small farmer cropping systems. San Jer6nimo is at the eastern end of the

Salamd valley, a major tomato-producing area in central Guatemala. A system of gravity-

fed irrigation canals was built in this portion of the valley in the mid- 1 970s, permitting

year-round cultivation of tomato and other crops. This has improved the local economy,







73

but may have contributed to the unmitigated build-up of whitefly populations in the area

since the 1980s.

The current study was undertaken to determine if whitefly numbers on bean and

tomato could be reduced by intercropping with crops that were either poor hosts or non-

hosts for whitefly. Pesticide treatments were included in some studies to determine if

intercropping combined with pesticide application offered any advantage over either

control measure alone. The last study included a comparison of mechanical and

chemical methods of whitefly protection for tomato in the nursery stage, prior to

transplanting into monocropped and intercropped environments.

Materials and Methods

Location

This series of experiments was carried out at the Instituto de Ciencia y Tecnologia

Agricolas (ICTA) field station in San Jer6nimo (150 03' 40" N, 900 15'00" W), Baja

Verapaz, Guatemala. ICTA is the government agricultural research institute of

Guatemala. The station is 1000 m above sea level. The area is classified as subtropical

dry forest under the Holdridge system (Holdridge 1967, de la Cruz 1982). The dry season

is from November to April. The soils on the station belong to the Salamdi series and are

characterized as loose and friable, with a low cation exchange capacity and a substratum

of volcanic ash (Krug 1993, Sharer and Sedat 1987).

Overview

Three sets of experiments were carried out between March and December 1998 to

evaluate the effect of three distinct intercropping arrangements on the densities of

immature whiteflies on bean and tomato. Numbers of whitefly eggs and nymphs on







74

intercropped plants were compared with numbers on monocropped plants for each study.

These studies are referred to as the diversity, mosaic, and corn/cilantro studies. The

corn/cilantro study included a comparison of two methods of tomato production in the

nursery.

Diversity Study

This study was initiated in March toward the end of the dry season, when whitefly

populations are at their highest. Bean or tomato was intercropped in alternating rows

with corn, cabbage (Brassica oleracea L.), cilantro (Coriandrum sativum L.), rosa de

jamaica (Hibiscus sabdariffa L.), and velvetbean (Mucuna deeringiana (Bort.) Small)

(Figure 4-1). These crops are either poor or non-hosts for whiteflies, and were chosen

from crops grown regionally to represent a diverse range of plant architecture and plant

chemistry. All have dietary and market value, except for velvetbean, which is primarily

used as a forage and green manure. The purpose of the study was to determine if the

presence of varied poor and non-hosts affected whitefly densities on bean and tomato

when compared to densities on bean and tomato grown in monoculture. This study

included subplots with pesticide treatments.

After the first bean crop had been harvested, a second bean crop was planted on

smaller scale. Whitefly numbers on monocropped and intercropped bean were compared

without pesticide subplot treatments.

The bean variety used was 'ICTA-Santa Gertudis,' a cultivar developed and

promoted by ICTA as resistant to bean golden mosaic. 'Elios' tomato seedlings

(Petoseed, Saticoy, CA) were purchased from Safil Vasquez, Estancia La Virgen, El

Progreso. The field corn hybrid used was 'ICTA HB-83' (ICTA 1993). 'Costanza'
















































Figure 4-1. Intercrop Pattern: Diversity Experiment









cabbage (Petoseed, Saticoy, CA) was used. Cultivar information was not available for

cilantro, velvet bean, and rosa de jamaica, which were grown from locally-acquired seed.

A tractor was used to cultivate the experimental area and form rows at the

beginning of the dry season (March 19) and rainy season (August 13) experiments.

Application of fertilizer, weeding and all other aspects of plot management were carried

out manually. Crops were fertilized according to local recommendations (ICTA 1993,

Superb 1997). Fungicides and pesticides were applied with a 16-liter Matabi "Super 16"

backpack sprayer (Goizper S. Coop., Guipuzcoa, Spain). Fungicides were applied on a

weekly basis to tomato to control for foliar and root pathogens once the rains began in

May. Water from a furrow irrigation system was made available to the station every 6

days for 3 days during the dry season and upon request during the rainy season.

A split plot design was used with 2 whole plot treatments (monocrop, intercrop)

and 3 subplot pesticide treatments (imidacloprid, detergent/oil, control). Each treatment

was replicated 4 times.

Whole plots contained 17 rows, 17 m in length. Monocrop plots consisted of 8

rows of bean and 8 rows of tomato separated by one bare row. Intercrop plots consisted

of 8 rows of a bean/intercrop mix next to 8 rows of tomato/intercrop mix. A row of

velvetbean separated the bean and tomato sections in the intercrop plots. The other 4

intercrop species were planted in alternating rows with bean or tomato to either side of

the velvetbean in the following order: rosa de jamaica, cilantro, cabbage, corn.

Spacing between plants was 20 cm for bean, corn, cilantro, and velvetbean and 40

cm for tomato and cabbage. Space between rows was 1.0 m. Rows were planted north to

south. Corn, cabbage, cilantro and rosa dejamaica were planted 25 March. Velvetbean







77

was planted 26 March. Beans were planted 5 and 6 April. Tomatoes in the untreated and

detergent/oil plots were transplanted 6 May.

Each whole plot was divided into 3 sections of 5.67 m in length. These sections

were demarcated with nylon cord supported by stakes. Each section was randomly

assigned to the imidacloprid treatment, the detergent and oil treatment, or the control.

Imidacloprid (Confidor 70 WG) was prepared at a rate of 0.73 g/liter of water.

Approximately 10 cc of this mixture (73 mg imidacloprid) was applied to the base of

each plant at each application. Imidacloprid was applied to bean at emergence, 1 week

after emergence and 3 weeks after emergence. Imidacloprid is not registered for bean,

and was included for comparison only. Commercially-produced tomato seedlings

received 1 imidacloprid application in the nursery, and were treated 1 and 3 weeks after

transplanting.

Olmecag vegetable oil (Olmeca S.A., Guatemala) and Unox laundry detergent

(Quimicas Lasser S.A., El Salvador) were applied at a rate of 1% or 16 cc/16 liter spray

tank (Calder6n et al. 1993). An elbowed nozzle attachment was used to apply the

mixture to the lower surface of leaves. Detergent or oil was applied in rotation every 5

days.

Whitefly Identification

Plants were examined under a dissecting microscope and the numbers of whitefly

eggs, nymphs, parasitized nymphs, and fourth-instar nymphs were recorded. The eyes of

the pharate adult become apparent in the final stage of fourth-instar Bemisia nymphs.

This stage was used to estimate the proportion of Bemisia relative to T. vaporariorum in

the nymph population. Earlier instars of Bemisia and T. vaporariorum can be






78

distinguished, but this is prohibitively time-consuming when high numbers of nymphs are

being counted.

In each study and for all crops, only the underside of leaves was examined for

whitefly immatures (Ekbom and Rumei 1990).

Bean was sampled on 6 occasions: 17 April (1 week after emergence), 25 April, 3

May, 12 May, 19 May, and 17 June. The sample unit on weeks I through 3 and week 5

was a 3.35 cm2 disc removed with a cork borer from upper and lower leaves (McAuslane

et al. 1995). The disc was removed from the underside of the central leaflet to the right of

the mid-vein. Five plants per plot were sampled on these weeks. The average of the 2

discs was used in treatment analysis. On weeks 4 and 6, one whole plant per subplot

replicate was sampled. Five plant heights per plot were measured on weeks 3, 5, and 6.

Five plants per plot were weighed on weeks 4, 5, and 6.

During week 4, five whole bean plants per plot were enclosed quickly in plastic

bags and refrigerated. These plants were sampled to estimate the number of generalist

predators on the bean plants as well as whitefly immatures.

Tomato was sampled on 4 occasions: 19 May, 1 June, 28 June, and 17 July. Disc

samples were taken from upper and lower strata on the first 2 sample dates. Whole

branches were examined for whitefly immatures from upper, middle and lower plant

strata on the latter two dates. Whole plants and branches were weighed to estimate the

percentage of the whole plant represented by the 3 strata. Five plants per plot were

sampled on the first two sampling dates, and one plant per plot was sampled during the

second two dates. Height and weight data on five plants per plot were gathered on weeks

2 and 3.









Fourth-instar whitefly nymphs were mounted in the laboratory of Lic. Margarita

Palmieri at the Universidad del Valle in Guatemala City and sent to Dr. Avas Hamon of

the Division of Plant Industry for identification. Dr. Andrew Jensen of the United States

Department of Agriculture in Beltsville, MD, kindly identified nymphs on dried plant

material. Leaves or whole plants with nymphs showing symptoms of parasitism were

placed in unwaxed cylindrical 0.95-liter cardboard cartons (Fonda Group Inc., Union, NJ,

USA) for parasitoid emergence. Several weeks later, dead parasitoids were placed on

cotton in gel capsules and sent to Dr. Greg Evans of the Division of Plant Industry,

Gainesville, FL, for identification.

Tissue from bean and tomato plants exhibiting symptoms of bean golden mosaic

or tomato leaf curl was analyzed using ELISA (Agdia Inc., Elkhart, IN) in the laboratory

of Lic. Margarita Palmieri. The total number of plants per row and number of plants with

bean golden mosaic symptoms was counted for all even-numbered rows in each bean

study. The total number of plants per row was counted in even-numbered rows for the

tomato treatments. Attempts to estimate percentage tomato leaf curl visually were

abandoned because virus symptoms are easily confused with other tomato disorders

(Polston and Anderson 1997).

Five velvetbean plants were examined for whitefly immatures on 3 May and 9

May. The leaves were traced onto paper, and this area was measured using a LI-COR

portable leaf area meter (model LI-3000A, LI-COR Inc., Lincoln, NE) in the United

States. Whole plant examinations were made of 12 cabbages on 6 June and 10 rosa de

jamaica plants on 8 June.








Imidacloprid-treated bean was harvested 29 June. Detergent/oil bean and

untreated bean was harvested 6 July. Tomato was harvested each week from 15 July

through 12 August and classified as large, medium, small, and reject.

On 10 July a second bean crop was planted in the former imidacloprid subplots.

A randomized complete block design with 4 replications was used to compare whitefly

immatures on bean grown under 2 treatments: monocropped and intercropped with the

five mature and senescing poor and non-host crops.

Spacing between bean plants was 20 cm. Bean was sampled weekly for 6 weeks

from 19 July through 23 August. Eight whole bean plants per plot were sampled during

week 1, four plants per plot on week 2, and two plants per plot for the remaining weeks.

The number of trifoliate leaves per plant was recorded each week. Bean was harvested 20

September.

Statistical Analysis

Treatments were compared using analysis of variance for split plot or randomized

complete block, followed by mean separation when appropriate (SAS Institute 1996).

Mosaic Experiment

This study was carried out toward the end of the rainy season. A mixed

intercropping pattern was used to evaluate corn and rosa de jamaica as crops which might

offer a cryptic environment for bean and tomato when intercropped in a mosaic pattern.

The same crop cultivars were used as in the diversity experiment. Tomato seedlings were

bought from Piloncito Verde, Chimaltenango.

Densities of immature whiteflies were compared on bean and tomato grown under

two treatments: bean and tomato grown in monoculture, and bean and tomato








intercropped with corn and rosa de jamaica. Each treatment was replicated 4 times and

arranged in a randomized complete block design. Monocrop plots contained 4 rows of

tomato adjacent to 4 rows of bean. Intercrop plots consisted of 8 rows of mixed crops

(Figure 4-2). The order of crop species within the row for the intercrop treatment was

corn, rosa de jamaica, bean, corn, rosa de jamaica, tomato. The first crop in consecutive

rows was staggered so that each bean or tomato plant was surrounded by corn, rosa de

jamaica and the other main crop, but was not immediately adjacent to a conspecific.

Rows were 8 m in length and between row spacing was 1.0 m. Between plant

spacing was 40 cm for all intercrop plants and the monocrop tomato, and 20 cm for

monocrop bean. Corn and rosa de jamaica were planted 18 August. Bean was planted 8

October. Tomato seedlings were transplanted 20 October.

Whole plant counts were taken for bean each week from 18 October through 17

November. Six plants per plot were sampled during the first week, 4 plants per plot

during weeks 2-4, and 2 plants per plot for the last 2 weeks. Plant height was measured

each week. Number of branches was recorded during weeks 3-6, and plants were

weighed in weeks 4-6. Number of plants per row and number of plants with bean golden

mosaic symptoms was counted 2 December.

Whole plant counts were taken for tomato for 4 weeks from 21 October through

12 November. On 22 November and 4 December, only the lower third of the plant was

sampled because the plants were too large for whole plant counts. During the first 2

weeks, 4 plants per plot were sampled. During week 2, two plants per plot were sampled.

During the remaining 3 weeks, 3 plants per plot were sampled. Plant heights were

measured during the first 5 weeks of sampling. Number of branches per plant was













o Field corn
Rosa de
jamaica

Bean

*Tomato



Figure 4-2. Intercrop Arrangement, Mosaic Experiment









recorded for weeks 2-5, and fresh plant weights were taken during weeks 3-5. On 2

December the number of tomato plants per row was recorded.

On 7 October one whole rosa de jamaica plant per block was examined for

whitefly immatures.

Statistical Analysis

Numbers of whitefly immatures and plant size characteristics were compared

between treatments using analysis of variance with SAS software (SAS 1996).

Nursery and Corn/Cilantro Study

In the final study, carried out toward the end of the rainy season, an attempt was

made to develop an overall management program for whitefly on tomato. Two methods

of tomato seedling production were compared in a nursery study. Seedlings were either

treated with imidacloprid or grown under protective mesh in covered nurseries. The

seedlings produced in this nursery study were then used in the corn/cilantro study.

Tomatoes in the corn/cilantro study were grown under four treatments: monocropped

with and without imidacloprid, and intercropped with and without imidacloprid. The

seedlings used in the imidacloprid treatments were those which had been treated with

imidacloprid in the nursery. The untreated seedlings were those which had been grown

under protective mesh.

The intercrop treatment consisted of tomato intercropped with corn and cilantro.

High numbers of generalist predators had been observed on flowering cilantro in the

diversity study, and an attempt was made to increase densities of predators on tomato by

intercropping with cilantro. In the intercrop treatment, corn was used to anchor the nylon

cord which supports growing tomato, replacing the wooden stakes which are normally







84
employed for this purpose. Intercropping tomato with mature field corn is not uncommon

among small farmers in Guatemala (Eduardo Landeverri, ICTA agronomist, personal

communication). The corn was widely spaced, and specifically managed to reduce

shading: lower leaves were removed from the corn early in November, and corn was

harvested in the fresh (elote") stage on 19 November, after which the top of the each

corn plant was removed.

Nursery Study

Tomato plants used in this study were grown individually in containers made from

newspaper ("cartuchos") on the research site (Rufino 1998). Seeds were planted in

cartuchos on 21 September. About 300 seedlings were dusted with imidacloprid (Gaucho

70 WC; Bayer, Germany) before planting and grown in an exposed nursery. Another 300

seedlings were grown in a nursery protected from whiteflies by fine nylon mesh (Rivas et

al. 1994) and received no pesticide treatment. The treated seedlings received

approximately 73 mg imidacloprid (Confidor 70 WG) on 8 October. The height of eight

tomatoes from each nursery treatment was measured on 18 October, when the nursery

covering was removed. Eight plants from the two nursery treatments were examined for

whitefly immatures on 18 October. Tomato seedlings were transplanted into the

corn/cilantro study 19 October.

Corn/Cilantro Study

A randomized complete block split plot design was used with 2 wholeplot

treatments (monocrop and intercrop) and 2 subplot treatments (imidacloprid treatment

and control). The imidacloprid treatment consisted of tomato plants which received

imidacloprid in the nursery study and in two post-transplant applications. The control








treatment was comprised of tomato seedlings produced under protective mesh in the

nursery study which received no pesticide applications before or after transplanting.

Each treatment was replicated 4 times. Main treatment plots were 6 m2. Each

main treatment plot was divided in half with nylon cord to produce two subplots, each 6

x 3 m. Monocrop and intercrop whole plots were separated by a 6 m2 patch of corn.

Approximately 73 mg imidacloprid (Confidor 70 WG) was applied to tomatoes in the

imidacloprid treatment on 22 October and 5 November.

Corn was planted 18 August and spaced every 2 m on the east side of the bed.

Cilantro was planted in a nursery 20 August and transplanted into the intercrop plots 2

October. Cilantro was planted every 12 cm on the west side of the bed. Tomato was

planted every 40 cm.

Tomato plants in the corn/cilantro study were sampled for 6 weeks, from 21

October through 2 December. Whole plant counts were made during weeks 1-4. During

weeks 5 and 6, only the lower third of the plant was sampled because of plant size. Four

plants per plot were sampled during weeks 1-3. Two plants per plot were sampled during

week 4, and 3 plants per plot were sampled during weeks 5 and 6.

Number of branches per plant was recorded for weeks 1-5. Plant heights were recorded

weeks 2-5, and weights were measured weeks 3-5.

On 5 November, two beat cloth samples per subplot were taken from tomato to

estimate generalist predators. A 1.0 m x 0.75 cm plastic sheet was spread out on a

wooden board at the base of two adjacent tomato plants. The plants were struck swiftly 4

times toward the sheet, which was then folded into a ball and sealed with masking tape.









The samples were first refrigerated, then transported to the Universidad del Valle in

Guatemala City for identification.

Weather data was provided by the Instituto Nacional de Sismologia,

Vulcanologia, Meteorologia e Hidrologia, San Jer6nimo station.

Statistical Analysis

Treatments were compared using analysis of variance for split plot or randomized

complete block, followed by mean separation when appropriate (SAS Institute 1996).

Results and Discussion

The predominant whitefly species in the Salamd valley was determined to be T.

vaporariorum. Whitefly populations were highest at the end of the dry season (March-

May), dropped with the first cool, wet months of the rainy season (June-August), but rose

again to high levels by the end of the rainy season (October-November). Relatively few

fourth-instar B. wbaci nymphs were observed throughout the 10-month study.

Observations of fourth-instar B. tabaci and geminivirus symptoms on bean and tomato

were highest at the end of the dry season and rare at the end of the rainy season. In the

middle of the rainy season, when overall whitefly populations were at their lowest, almost

50% of observed fourth-instar nymphs were Bemisia. The strain or strains of B. tabaci

present in the Salamd valley were not determined.

Diversity Study

Differences in levels of whitefly immatures, predators, plant density, percent bean

golden mosaic geminivirus and yield were not significant between monocropped and

intercropped treatments on any sampling date (p < 0.1). Statistical differences in the

diversity study occurred among subplot pesticide treatments only.









During week 1, egg counts were lower (p < 0.05) in the imidacloprid-treated

intercrop than in the control intercrop (Table 4-1). During week 2, egg counts were lower

(p < 0.05) in the imidacloprid and detergent/oil treatments than the control. Nymph

counts during week 2 were different (p < 0.05) among all treatments, with the lowest

counts in the imidacloprid treatment and the highest in the detergent/oil treatment.

Three weeks after germination, bean plants treated with imidacloprid were clearly

larger and more robust than those in the detergent/oil treatment and control. Plants in the

detergent/oil treatment showed symptoms of phytotoxicity. In addition, plants in the

detergent/oil and control treatments were stunted, with shortened stems and petioles. A

chlorotic burn appeared along the leaf border and tip, typical of leafhopper damage.

Whole plant examinations during week 4 revealed high densities of thrips

(Thysanoptera) and leafhoppers (Homoptera: Cicadellidae) on plants in the detergent/oil

treatment and the control. Size differences between the imidacloprid-treated bean and the

other two treatments increased during subsequent weeks.

The imidacloprid-treated plants tended to have more eggs and nymphs than the

stunted plants in other treatments during weeks 3 and 4 (Table 4-2). There were no

subplot treatment differences during weeks 5 and 6 as plants senesced and whitefly

populations declined.

Fourth-instar B. tabaci nymphs were observed for the first time during whole

plant examinations on week 4. Densities of fourth-instar B. tabaci were lower (p < 0.10)

in the imidacloprid treatment (0.13 0.35/plant) than in the control (7.62 12.22).

Densities in the detergent/oil treatment were intermediate (0.38 0.52). The ratio of

fourth-instar Bemisia to Trialeurodes from all treatments during week 4 was 65: 573.









Incidence of Bemisia during the following two weeks was not high enough for

meaningful comparison.

Generalist predators collected from whole plant bean samples during week 4

included Geocoris sp. (Hemiptera: Lygaeidae), Coccinellidae (Coleoptera), Thysanoptera,

Neuroptera, syrphid larvae (Diptera: Syrphidae), and spiders. Only Geocoris sp. was

present in sufficient quantities for statistical comparison. Levels of Geocoris sp. were

higher (p < 0.001) on imidacloprid-treated bean (0.60 0.87/plant) than on bean in the

detergent/oil treatment (0.05 0.22) and the control (0.25 0.16).

The parasitoids reared from bean and tomato in the diversity experiment were

almost entirely Encarsia pergandiella Howard (Hymenoptera: Aphelinidae), although a

few individuals from the Encarsia meritoria species complex were reared from the

second bean crop early in August. Sex ratios for E. pergandiella ranged from a low of

about 15% males in mid-May, when host and parasitoid populations were high, to 33%

males in July and August, when overall populations were low, to about 26% males in

November and December, when both populations were high again.

There were no statistical differences (p < 0.1) among treatments in levels of

parasitized nymphs during week 4 (12.33 16.79/plant). Parasitism was higher (p <

0.05) in the imidacloprid treatment than the other two treatments during week 5

(imidacloprid: 2.98 4.19/cm2, detergent/oil: 0.13 0.34, control: 0.78 1.26) and week

6 (imidacloprid: 29.50 21.23/plant, detergent/oil: 6.00 6.30, control: 4.75 6.86).

Percent parasitism, calculated as the percentage of parasitized nymphs to parasitized and

fourth-instar nymphs combined, ranged from about 33% during week four to 80% during

week 6. Parasitism and numbers of Geocoris were presumably highest on imidacloprid-









treated plants because these plants were larger and supported more hosts/prey than

untreated plants.

There were more (p < 0.05) plants per row in the imidacloprid treatment (26.50 +

4.88) than in the control (22.28 + 6.75). Plant density in the detergent/oil treatment was

intermediate (24.00 7.76).

The percentage of plants with bean golden mosaic symptoms was different (p <

0.05) among all subplot treatments (imidacloprid: 7.27 7.03 %; detergent/oil: 14.86

10.53 %; control: 21.99 15.86 %). Eight bean plants out often showing symptoms of

bean golden mosaic tested positive for the presence of geminivirus.

The bean yield per row was higher (p < 0.05) in the imidacloprid treatment (0.29

+ 0.09 kg) than in the detergent/oil treatment (0.05 0.03 kg) and the control (0.02 0.03

kg), neither of which produced marketable yield.

Because of delays in planting, the imidacloprid treatment could not be included in

the analysis. We learned when the tomato seedlings were delivered that all

commercially-produced tomato seedlings are treated with imidacloprid in the nursery.

Both detergent/oil and control seedlings received a pre-transplant imidacloprid treatment.

Whitefly populations on tomato remained low throughout the diversity study.

This may be partially explained by the effect of imidacloprid and other chemicals applied

in the nursery. In the third sample (June 28), there were more (p < 0.05) fourth-instar T.

vaporariorum on the control (3.38 3.07) than on the detergent/oil plants (0.71 1.11).

Observations of Bemisia were too few for analysis. There were no statistical differences

(p < 0.1) among subplot treatments in density of whitefly immatures (Table 4-3),






90

parasitized nymphs (week 3: 6.53 7.93/branch; week 4: 0.60 + 0.91 /branch), plants per

row (11.16 + 1.87), or total yield per row (5.69 + 4.29 kg).

Seven tomato plants out of 10 showing geminivirus symptoms tested positive for

the presence of geminivirus.

Very few whitefly eggs or nymphs were found on cabbage, rosa de jamaica and

velvetbean. Cabbage plants were large (254.25 180.88 g) with well-formed heads when

sampled. Mean egg count was 0.17 0.58/plant and mean nymph count was 3.25 5.43.

Two fourth-instar T. vaporariorum nymphs were found. Rosa de jamaica plants weighed

164.67 150.92 g and were 49.33 12.14 cm tall. No whitefly eggs were found on the

rosa de jamaica. Average per plant count for nymphs and fourth-instar B. tabaci was 7.67

6.89 and 0.89 1.36 respectively. Velvetbean sampled on 3 May averaged 0.08 + 0.06

eggs and 0.03 0.04 nymphs/cm2. Velvetbean sampled on 9 May averaged 0.01 0.01

eggs and 0.05 0.06 nymphs/cm2.

Diversity Study: Second Bean Crop

Number of eggs was higher in the monocrop than the intercrop treatment during

weeks 3 (p < 0.05) and 4 (p < 0.01) (Table 4-4). Egg numbers did not differ by treatment

on other dates. However, intercrop plants had fewer trifoliates during week 5 (p < 0.05)

and week 6 (p < 0.01). Overall egg and nymph densities were therefore higher on the

intercrop plants during these weeks, since intercrop plants were smaller than monocrop

plants. The smaller size of intercrop beans was probably due to shading from intercrops,

particularly the rosa de jamaica, which was about 1.5 m tall in August.

There were no treatment differences (p < 0.1) on any sampling date for the second

bean crop between numbers of nymphs (Table 4-4), parasitized nymphs (week 4: 0.25








0.77/plant, week 5:1.75 2.89, week 6: 6.12 + 8.61), or fourth-instar T. vaporariorum

(week 4: 0.44 0.81, week 5: 0.94 1.12, week 6: 4.38 7.82). There were no

statistical differences (p < 0.1) between treatments in the numbers of fourth-instar B.

tabaci during week 4 (0.37 0.81) or week 6 (0.37 0.81). The number of fourth-instar

B. tabaci was lower (p < 0.05) in the monocrop treatment (0.50 0.53) than in the

intercrop treatment (1.13 0.99) during week 5.

During weeks 4 and 5, B. tabaci made up 46% of the observed fourth-instar

whitefly immatures (ratio of B. tabaci to T. vaporariorum was 6: 7 on week 4 and 13: 15

on week 5). On week 6, B. tabaci comprised 7% of the observed fourth-instar whitefly

immatures (1: 15).

There were fewer (p < 0.001) plants per row in the intercrop treatment (60.13

22.53) than in the monocrop treatment (82.19 9.16). Yield per row was higher (p <

0.05) in the monocrop treatment (2.47 0.53 kg) than in the intercrop treatment (1.25 +

0.39 kg). The reason for the lower number of plants per row in the intercrop treatments is

not clear. Possibly the weeding and fertilizing of the bean plants was impeded by the

presence of intercrop plants, leading to reduced survival.

Leafhoppers and thrips were barely discernable on this second bean crop, although

high populations of these insects decimated unprotected bean in the dry season.

However, dense populations of chrysomelids (Coleoptera), primarily Cerotoma and

Diabrotica spp., attacked the second bean crop early. Cerotoma and Diabrotica spp. are

among the vectors of severe mosaic of bean, a comovirus (Morales and Cardona 1998).

Dr. Francisco Morales of International Center for Tropical Agriculture, Cali, Colombia,

identified symptoms of severe mosaic of bean among experimental plants in the field.









Leaf beetles typically build up on field corn, then move on to young beans as the corn

senesces in the first months of the rainy season. Incidence of the virus was high among

experimental plants. Leaf necrosis and deformation from severe mosaic of bean masked

symptoms of bean golden mosaic, preventing an estimate of presence of bean golden

mosaic at the end of the season.

Mosaic Study

Egg counts were lower (p < 0.05) on intercropped than monocropped bean during

the first four weeks of sampling (Table 4-5). Nymph counts were lower (p < 0.05) on

intercropped than monocropped bean on weeks 2, 4, and 5.

Lower numbers of eggs and nymphs among intercrop bean early in the study may

be attributed to the emergence of intercrop plants into a cryptic environment. However,

bean size and health were affected by shading from corn and rosa de jamaica soon after

emergence, and the overall plant area available for colonization was presumably less than

in the monocrop treatment by week 3. From weeks 3-6, intercrop bean was stunted

compared to monocrop bean, and whitefly densities were correspondingly lower.

A few Encarsia pergandiella individuals and one member of the Encarsia

meritoria species complex were reared from bean in the mosaic experiment. There were

no treatment differences among numbers of parasitized nymphs (week 4: 0.88

2.03/plant, week 5: 0.88 2.03, week 6:1.44 2.66), fourth-instar T vaporariorum

(week 4: 0.13 0.71, week 5: 0.88 1.71, week 6: 4.19 5.76), or fourth-instar B. tabaci

(week 5: 0.06 0.25, week 6: 0.06 0.25). During week 5, B. tabaci fourth-instars

comprised 7% of fourth-instar nymphs on bean. During week six, 1.5% of fourth-instar

nymphs on bean were B. tabaci. Number of plants per row averaged 5.15 + 2.49 in the




Full Text
86
The samples were first refrigerated, then transported to the Universidad del Valle in
Guatemala City for identification.
Weather data was provided by the Instituto Nacional de Sismologa,
Vulcanologa, Meteorologa e Hidrologa, San Jernimo station.
Statistical Analysis
Treatments were compared using analysis of variance for split plot or randomized
complete block, followed by mean separation when appropriate (SAS Institute 1996).
Results and Discussion
The predominant whitefly species in the Salam valley was determined to be T.
vaporariorum. Whitefly populations were highest at the end of the dry season (March-
May), dropped with the first cool, wet months of the rainy season (June-August), but rose
again to high levels by the end of the rainy season (October-November). Relatively few
fourth-instar B. tabaci nymphs were observed throughout the 10-month study.
Observations of fourth-instar B. tabaci and geminivirus symptoms on bean and tomato
were highest at the end of the dry season and rare at the end of the rainy season. In the
middle of the rainy season, when overall whitefly populations were at their lowest, almost
50% of observed fourth-instar nymphs were Bemisia. The strain or strains of B. tabaci
present in the Salam valley were not determined.
Diversity Study
Differences in levels of whitefly immatures, predators, plant density, percent bean
golden mosaic geminivirus and yield were not significant between monocropped and
intercropped treatments on any sampling date (p < 0.1). Statistical differences in the
diversity study occurred among subplot pesticide treatments only.


95
Seedlings produced under mesh received heavy whitefly pressure as soon as they
were removed from the covered nursery. This led to stunting of untreated seedlings.
Imidacloprid-treated tomato was taller and heavier (p < 0.05) than untreated seedlings 3
weeks after transplanting (Table 4-8).
Egg numbers were lower (p < 0.01) in the imidacloprid treatment during weeks 1
and 6, and in the intercrop imidacloprid treatment during week 2 (Table 4-9). Egg
numbers were lower (p < 0.1) in the monocrop than in the intercrop treatments during
week 2. Nymph numbers were lower (p < 0.05) on the imidacloprid-treated intercrop
plants during each week of sampling. Nymph numbers were lower (p <0.01) on
imidacloprid-treated plants in both cropping systems during week 3. During week 4,
nymph densities were higher (p < 0.05) on the imidacloprid-treated tomato than on
untreated tomato in the monocrop treatment. This was because imidacloprid-treated
plants were considerably larger.
Parasitoids reared from tomato consisted of Encarsia pergandiella, members of
the Encarsia meritoria species complex, and Amitus fuscipennis (Table 4-10). The overall
parasitoid diversity in this experiment was low. Diversity indices (H1) for parasitoid
complexes reared from the four treatments were: monocrop plus imidacloprid: 0.113;
monocrop untreated: 0.026; intercrop plus imidacloprid: 0.313; intercrop untreated:
0.176.
Densities of parasitized nymphs were higher (p <0.01) in the untreated subplots
(15.65 14.79/plant) than the imidacloprid-treated subplots (1.38 2.37) during week 5.
During week 6, densities of parasitized nymphs were higher (p < 0.05) in the intercrop
treatments (35.87 40.73) than in the monocrop treatments (24.24 28.55).


57
Corn height. The height of 15 com plants per plot was measured on 27 August to
evaluate the barrier effect.
Statistical analysis. The effect of treatment, block, and trap position on trap count
was analyzed using analysis of variance (PROC GLM, SAS version 6.11, SAS Institute
1996). Orthogonal contrasts were then used to compare trap counts in the same treatment
east and west (upwind and downwind) of the release point, and to compare trap counts
among treatments in blocks west of the release point. Wind direction data collected at the
site were provided by Dr. E. B. Whitty, Agronomy Department, University of Florida,
Gainesville, FL.
Results and Discussion
1996
Whiteflv densities. Densities of eggs were highest on bean when sampling began
and declined over subsequent weeks (Table 3-1). Nymph densities were highest during
weeks 3 and 4. Observations of parasitized nymphs and red-eyed nymphs were low
throughout, although parasitism increased slightly over time.
There were no differences (p <0.10) in egg density among treatments during the
first six weeks of sampling. Egg densities on bean alone were higher (p < 0.05) than on
bean intercropped with com or eggplant during weeks 7 and 8. No differences (p < 0.10)
in nymph densities occurred among treatments. Densities of red-eyed nymphs were
higher (p < 0.05) on bean alone than on the corn and eggplant treatments during week 4.
During week 6, parasitism was higher (p < 0.1) in the com treatment than in the bean
alone treatment. During week 7, parasitism was more than twice as high in the eggplant
treatment as in the other two treatments.


o
Field corn
ABbk
Rosa de
jamaica
O
Bean

Tomato
Figure 4-2. Intercrop Arrangement, Mosaic Experiment
OO
to


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
INTERCROPPING AND WHITEFLY (HOMOPTERA: ALEYRODIDAE)
MANAGEMENT
By
Hugh Adam Smith
December 1999
Chairman: Robert McSorley
Major Department: Entomology and Nematology
Field studies were carried out in north central Florida and central Guatemala to
examine the effect of intercropping on numbers of whiteflies (Homoptera: Aleyrodidae).
Squash (Cucrbita pepo) and eggplant (Solanum melongend) were tested as trap crops,
and field corn (Zea mays) was tested as a barrier crop, for management of the silverleaf
whitefly (Bemisia argentifolii) on common bean (Phaseolus vulgaris) in Florida between
1995 and 1997. Three distinct mixed and row-intercropping arrangements with poor and
non-host crops were tested in 1998 in Guatemala to reduce densities of immature
greenhouse whitefly (Trialeurodes vaporariorum) and sweetpotato whitefly (Bemisia
tabaci) on common bean and tomato (Lycopersicon esculentum). In addition, a plastic
mulch painted with a reflective aluminum strip was tested to reduce immature stages of
Vll


Table 6-2. Immature B. argentifolii on bean sampled with disc punches from three strata -mean numbers per cm2, coefficients of
variation, and r2 values'
Eggs Nymphs2 Parasitized nymphs Red-eye nymphs Total
Wk
Stratum
Mean
CV
r
Mean
CV
9
r
Mean
CV
7
r
Mean
CV
r
Mean
CV
.2
r
1
first true
leaves
1.38
92
ns4
0
-
-
0
-
"
0
-
-
1.38
92
ns
3
upper
1.35a3
97
ns
0.19a
331
ns
0a
-
-
0a
-
-
1.54a
109
ns
middle
1.07a
78
0.27*
3.37b
61
0.26*
0.02a
401
0.68**
0.03a
774
ns
4.47b
52
0.42**
4
upper
1.35a
128
ns
0.21a
209
ns
0a
-
ns
0a
-
-
1.56a
125
0.43**
middle
0.72b
119
0.31*
1.46b
117
0.44**
0a
-
ns
0a
-
-
2.16a
96
0.61**
lower
0.49b
149
ns
2.97c
68
ns
0.10b
274
0.21*
0.13a
323
0.78**
3.58b
68
ns
5
upper
1.55a
109
ns
0.21a
216
ns
0a
-
ns
0a
-
-
1.76a
97
ns
middle
0.56b
172
0.23+
1.04b
123
0.53**
0a
-
ns
0.03a
543
ns
1.62a
96
0.36*
lower
0.07c
334
ns
1.50b
107
ns
0.05b
292
0.25*
0.10a
354
ns
1.65a
100
ns
6
upper
2.21a
1 17
0.68**
0.25a
218
0.41*
0a
-
ns
0a
-
-
2.47a
104
0.47**
middle
0.63b
229
ns
1.51b
131
ns
0.01a
543
ns
0.03a
543
ns
2.16a
108
ns
lower
0.13c
523
ns
1.02b
89
0.54**
0.04a
377
0.62**
0.33b
219
ns
1.28b
97
ns
7
upper
1.09a
161
0.31*
0.23a
229
ns
0a
-
ns
0a
-
-
1.32a
138
ns
middle
0.43b
182
0.32*
0.86b
132
ns
0.01a
573
ns
0.03ab
543
0.31*
1.31a
110
ns
lower
0.11c
280
ns
0.63b
93
ns
0.03a
319
0.31*
0.25b
300
ns
0.85a
92
ns
8
upper
0.35a
221
0.67**
0.35a
215
ns
0a
-
ns
0a
-
-
0.70a
156
0.50**
middle
0.14a
236
0.52**
0.76b
116
0.20+
0.04ab
323
0.32*
0a
-
-
0.94a
107
0.30*
lower
0.02b
401
ns
0.41a
119
ns
0.14b
243
0.71**
0.25a
342
ns
0.65a
126
0.26+
'r2 values for regression equation between whole plant data and disc punch data.
2Includes all nymphs except parasitized and red-eyed nymphs.
hneans followed by letters are significantly different (p < 0.05) according to pair-wise t-test.
4** + ¡n(iicate ^ significant at p < 0.01, p < 0.05, and p < 0.10, respectively; ns = not significant at p < 0.10.


147
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patterns and virus transmission among populations of Bemisia tabaci in Arizona.
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esterase patterns among populations of Bemisia tabaci and the association of one
population with silverleaf symptom development. Entomol. Exp. Appl. 61:211-
219.
Costa, H. S., J. K. Brown, and D. N. Byrne. 1991. Host plant selection by the whitefly,
Bemisia tabaci, under greenhouse conditions. J. Appl. Ent. 112: 146-152.
Coudriet, D. L., N. Prabhaker, A. N. Kishaba, and D. E. Meyerdirk. 1985. Variation in
developmental rate on different hosts and overwintering of the sweetpotato
whitefly, Bemisia tabaci. Environ. Entomol. 14:516-519.
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geminivirus transmitidos por la mosca blanca {Bemisia tabaci). Manejo Integrado
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Csizinsky, A. A., D. J. Schuster, and J. B. Kring. 1997. Evaluation of color mulches and
oil sprays for yield and for control of silverleaf whitefly, Bemisia argentifolii
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5
useful for tracking the spread of B. tabaci strains, but that they are not appropriate for the
designation of species. They added that other distinct B. tabaci populations show
significant variability in pupal case morphology, esterase banding profiles, and mating
behavior. They reasoned therefore that if the B-biotype were a new species, other B.
tabaci strains must be described as new species as well.
There seems to be consensus among many whitefly workers that the designation
of B. argentifolii as a new species is premature (Bedford et al. 1994). The data suggest,
however, that B. tabaci may be a species complex undergoing evolutionary change
(Brown et al. 1995, Drost et al. 1998). Brown et al. (1995) believe that the A-biotype
belongs to the New World group of B. tabaci, and that the B-biotype belongs to the Old
World group. Brown et al. (1995) and Byrne et al. (1990) suggest that the B-biotype may
have risen to predominance under the selective pressure of large-scale, heavily-sprayed
monocultures, particularly cotton monocultures.
Crucial aspects of whitefly movement, host finding, and host acceptance have
been described. Whiteflies are weak fliers and have been described as aerial plankton,
which move with the wind currents, probing plants as they are encountered (Byrne and
Bellows 1991). Mound (1962) first reported that B. tabaci oriented toward either
yellowish or blue/ultraviolet light, and suggested that this phenomenon might be related
to colonizing and migratory behavior. Byrne et al. (1996) determined that B. tabaci has
two distinct adult morphs, which engage in either trivial or long-distance movement.
Trivial fliers orient toward the yellowish-green range of light spectra emitted by most
vegetation, and seem to be predisposed to colonize. Long-distance fliers are attracted to
ultraviolet light associated with the sky, and are apparently predisposed to migrate (Byrne


150
Hoelmer, K. A. 1996. Whitefly parasitoids: can they control field populations of
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taxonomy, biology, damage, control, and management. Intercept, Andover,
Hants, UK.
Holdridge, L. 1967. Life zone ecology. Tropical Science Center, San Jos, Costa Rica.
Horowitz, A. R., and I. Ishaaya. 1996. Chemical control of Bemisia management and
application. Pages 537-556 in D. Gerling and R. Mayer, eds. Bemisia 1995:
taxonomy, biology, damage, control, and management. Intercept Ltd., Andover,
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Huffaker, C. B. 1958. Experimental studies on predation: dispersion factors and
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Hunter, D. W. A., and G. Whitfield. 1996. Potato trap crops control of Colorado potato
beetle in tomatoes. Canadian Entomol. 128: 407-412.
Hunter, W. B., E. Hiebert, S. E. Webb, J. E. Polston, and J.H.Tsai. 1996. Precibarial and
cibarial chemosensilla in the whitefly, Bemisia tabaci (Gennadius)(Homoptera:
Aleyrodidae). Intemat. J. Insect Morphol. Embryol. 25: 295-304.
ICTA, 1993. Recomendaciones tcnicas agropecuarias para los departamentos de Alta y
Baja Verapaz. Insituto de Ciencia y Tecnologa Agrcolas, Brcenas, Villa Nueva,
Guatemala.
Jenkinson, J. G. 1955. The incidence and control of cauliflower mosaic in broccoli in
south-west England. Ann. Appl. Biol. 43: 409-422.
Jones, R. A. C. 1991. Reflective mulch decreases the spread of of two non-persistently
aphid transmitted viruses to narrow-leafed lupin (Lupinus angustifolius). Ann.
Appl. Biol. 118: 79-86.
Kareiva, P. 1983. Influence of vegetation texture on herbivore populations: resource
concentration and herbivore movement. Pages 259-289 in R. F. Denno and M. S.
McClure, eds. Variable plants and herbivores in natural and managed systems.
Academic Press, New York.
Kass, D. C. L. 1978. Polyculture cropping systems: review and analysis. Cornell
International Agricultural Bulletin 32, Cornell University, Ithaca, NY.
Kennedy, J. S., C. O. Booth, and W. J. S. Kershaw. 1961. Host finding by aphids in the
field. III. Visual attraction. Ann. Appl. Biol. 49: 1-21.
Kloen, H., and M. A. Altieri. 1990. Effect of mustard as a non-crop plant on competition
and insect pests in broccoli. Crop Protection 9: 90-96.


Table 2-8. Red-eyed nymph density (mean SD/cnr) of B. argentifolii on bean, 1997
Week
Treatment
Lower stratum
Upper stratum
Mean
3
Bean
0.43 0.60
0
0.21 0.47
Mulch
0.32 0.50
0
0.16 0.38
Squash
0.14 0.30
0
0.07 0.22
Squash/mulch
0.46 0.48
0
0.23 0.41
4
Bean
0.71 1.08
0
0.36 0.82
Mulch
0.20 0.44
0
0.10 0.32
Squash
0.68 0.77
0.02 0.05
0.35 0.63
Squash/mulch
0.38 0.40
0
0.19 0.34
5
Bean
0.55 1.45
0
0.28 1.03
Mulch
1.13 1.38#
0.18 0.51#
0.65 1.11
Squash
0.22 0.30
0
0.11 0.23
Squash/mulch
0.46 0.87
0.04 0.10
0.25 0.63
6
Bean
0.41 0.48
0.04 0.10
0.22 0.39
Mulch
0.09 0.20
0
0.04 0.14
Squash
0.34 0.37
0
0.10 0.22
Squash/mulch
0.20
0
0.17 0.31
# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05.


Table 4-4. Whitefly immatures and number of trifoliate leaves per plant on bean
grown among senescing intercrops vs. monocrop. Diversity study, second bean
crop.
105
Week
Egg
Nymph
Trifoliate
1
Monocrop
1.25.76
_
Intercrop
2.002.38
-
-
Mean
1.632.11
-
-
2
Monocrop
3.443.29
0.942.11
-
Intercrop
2.752.11
1.25.34
-
Mean
3.092.74
1.09.75
-
3
Monocrop
16.25i8.14a1
8.506.50
-
Intercrop
8.256.32 b
6.133.64
-
Mean
12.258.16
7.315.24
-
4
Monocrop
17.386.99a
22.253.02
7.88i2.53
Intercrop
6.004.38 b
21.13ilO.56
7.00i2.51
Mean
11.698.12
21.69ill.47
7.44i2.48
5
Monocrop
6.636.02
37.50i26.27
11.25i3.94a
Intercrop
6.256.11
34.75i29.48
8.50i2.20 b
Mean
6.445.86
36.13i27.01
9.88i3.16
6
Monocrop
4.505.83
78.50i66.33
23.50i7.01a
Intercrop
7.754.86
41.00i24.87
13.50i3.02b
mean
6.135.45
59.75i52.12
18.50i7.34
1 Means in the same column for a given week followed by a different letter are significantly
different (p < 0.05) according to analysis of variance. The absence of letters indicates no
treatment differences (p > 0.1).


73
but may have contributed to the unmitigated build-up of whitefly populations in the area
since the 1980s.
The current study was undertaken to determine if whitefly numbers on bean and
tomato could be reduced by intercropping with crops that were either poor hosts or non
hosts for whitefly. Pesticide treatments were included in some studies to determine if
intercropping combined with pesticide application offered any advantage over either
control measure alone. The last study included a comparison of mechanical and
chemical methods of whitefly protection for tomato in the nursery stage, prior to
transplanting into monocropped and intercropped environments.
Materials and Methods
Location
This series of experiments was carried out at the Instituto de Ciencia y Tecnologa
Agrcolas (ICTA) field station in San Jernimo (15 03' 40" N, 90 1500" W), Baja
Verapaz, Guatemala. ICTA is the government agricultural research institute of
Guatemala. The station is 1000 m above sea level. The area is classified as subtropical
dry forest under the Holdridge system (Holdridge 1967, de la Cruz 1982). The dry season
is from November to April. The soils on the station belong to the Salam series and are
characterized as loose and friable, with a low cation exchange capacity and a substratum
of volcanic ash (Krug 1993, Sharer and Sedat 1987).
Overview
Three sets of experiments were carried out between March and December 1998 to
evaluate the effect of three distinct intercropping arrangements on the densities of
immature whiteflies on bean and tomato. Numbers of whitefly eggs and nymphs on


48
Table 2-11. Total bean yield (kg).
Year
Treatment
Total bean yield (kg/plot)
1996
Bean
6.22b1
Mulch
15.68a
Squash
4.52bc
Squash/mulch
11.42ab
1997
Bean
0.57
Mulch
0.68
Squash
0
Squash/mulch
1.24
1 Means in the same column with the same letter are not significantly different according
to Tukeys Studentized Range test with controlled type 1 experimentwise error rate
(a=0.05). The absence of letters in a column indicates the lack of significant differences
among any means.


98
arthropods in polyculture were higher than in monoculture 40.3% of the time, and lower
in 28.4% of reported studies. However, some efforts to manage whiteflies through
intercropping have been partially successful, particularly in the reduction of virus
incidence (Al-Musa 1982, Schuster et al. 1996).
Intercropping with poor and non-hosts may not interfere with host-finding
mechanisms of T. vaporariorum and B. [abaci. Trialeurodes vaporariorum and B. tabaci
apparently do not respond to host-specific visual or olfactory cues (Mound 1962, Woets
and van Lenteren 1976, van Lenteren and Woets 1977). They are attracted to the
yellowish range of light spectra emitted by most vegetation (van Lenteren and Noldus
1990). Trialeurodes vaporariorum and B. tabaci require gustatory information to judge
the suitability of a host, and do not reject a host until they have probed it (van Lenteren
and Noldus 1990). It is unlikely that whiteflies can be confused or repelled from a
cropped area by the volatiles or appearance of non-host plants. If host-finding cannot be
manipulated at a distance, it also may be unlikely that whiteflies can be drawn away from
a crop by the presence of a trap crop.
It seems probable that whitefly movement cannot be manipulated by intercropping
with poor and non-hosts. Whiteflies are weak fliers and rely on wind for both short and
long distance migration (Byrne et al. 1996). They move passively with air currents,
probing plants as they come into contact with them (Byrne and Bellows 1991). Unlike
stronger fliers, which might leave a cropped area after encountering a few non-hosts in
succession (Bach 1980a, Risch 1981, Power 1990), whiteflies move from plant to plant
within the field until they find an acceptable host (Byrne and Bellows 1991). Whitefly


94
Unlike bean, which emerged into the shaded intercrop environment, tomato
seedlings were produced under the optimal conditions of a commercial nursery. Tomato
size was not affected by intercrop shading until 5 weeks after being transplanted (Table 4-
6).
The mean number of tomato plants per row was 1.72 0.85 for intercrop
treatments and 15.25 2.32 for monocrop treatments at the end of the study. The low
number among the intercrop treatment is due to whole plant sampling of an initially small
population.
As observed in the dry season, rosa de jamaica was a poor whitefly host. The
mean weight and height of the 4 rosa de jamaica plants examined on 4 October was
150.75 57.04 g and 70.75 4.35 cm, and average leaf number was 68.75 36.59. The
average number of eggs and nymphs per plant was 5.50 5.80 and 3.25 1.50
respectively. One fourth-instar B. tabaci was found.
Nursery' Study
Seedlings produced in covered nurseries were taller (p < 0.1) than imidacloprid-
treated seedlings (19.94 4.85 cm vs. 15.31 4.99 cm) due to increased shade under
mesh. The number of eggs per plant was not different (p < 0.1) between the covered
nursery (0) and the imidacloprid-treated seedlings (0.13 0.35).
Com/Cilantro Study
Unseasonably high precipitation resulted in a high incidence of disease among
both cilantro and tomato. Few cilantro plants per row reached the flowering stage. The
com/cilantro' study therefore became a study comparing monocropped tomato with
tomato intercropped with corn, with and without imidacloprid.


162
Woets, J., and van Lenteren, J. C. 1976. The parasite-host relationship between
Encarsia formosa (Hymenoptera: Aphelinidae) and Trialeurodes vaporariorum
(Homoptera: Aleyrodidae). VI. The influence of host plant on the greenhouse
whitefly and its parasite Encarsia formosa. IOBC/WPRS Bulletin 4: 151-54.
Yano, E. 1983. Spatial distribution of greenhouse whitefly and a suggested sampling
plan for estimating its density in greenhouses. Res. Pop. Ecol. (Kyoto) 25: 309-
320.
Yokomi, R. K., K. A. Hoelmer, and L. S. Osborne. 1990. Relationship between the
sweetpotato whitefly and the squash silverleaf disorder. Phytopathology 80: 895-
900.


For George


6
Bean 0.07 0.13 0.26 0.49 0.17 0.37
Mulch 0.17 0.19 0.26 0.33 0.22 0.27
Squash 0.09 0.25 0.33 0.29 0.21 0.30 0.66 0.77*
Squash/mulch 0.14 0.26 0.22 0.28 0.18 0.27 1.30 1.89**
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively.


20
generalist predators, but is more likely to impede the efficiency of specialist parasitoids
(Pimentel 1961, Sheehan 1986).
Inadequate research methods have contributed to the ambiguity surrounding the
effect of polyculture on arthropods. Intercropping often reduces plant quality relative to
monoculture (Andow 1991a, Kareiva 1983). Some authors include the effect of reduced
plant quality in their analysis (for instance Gold et al. 1990, Schultz 1988), but many do
not (Kareiva 1983). Stanton (1983) remarks that there may be significant differences in
how researchers and insects perceive diversity. In addition, Andow (1991a) writes that
results of polyculture studies have varied depending on whether polyculture treatments
were substitutive or additive, i.e. whether host plant density was different in monocrop
and intercrop treatments.
The greatest difficulty in designing field tests of intercropping effects on
arthropods is determining the appropriate scale of plots and distance between plots
(Russell 1989, Stanton 1983). Some arthropods may perceive a patchwork of
monocropped and intercropped plots as one large polyculture. Small clustered plots will
increase the influence of patch borders on searching, and the likelihood that arthropod
density in one treatment plot is influenced by the arthropods attraction to or rejection of a
distinct adjacent treatment plot (Andow 1991a, Stanton 1983). Plot size will affect the
ability of herbivores and natural enemies to find hosts, as well as their foraging behavior
within the plot, and the rate at which they leave it (Corbett and Plant 1993, Kareiva 1983,
Stanton 1983, Russell 1989).


CHAPTER 6
METHODS FOR SAMPLING IMMATURE STAGES OF
BEMIS1A ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE)
ON BEAN (PHASEOLUS VULGARIS L.)
Introduction
Bemisia argentifolii Bellows & Perring (also known as Bemisia tabaci
(Gennadius) strain B) is a serious economic pest of agronomic and horticultural crops
throughout warm regions of the world (Brown et al. 1995). Bemisia argentifolii vectors
numerous geminiviruses (Hiebert et al. 1996), inflicts mechanical damage (Schuster et al.
1996), and causes plant disorders (Shapiro 1996) among a wide range of plant groups.
Methods for sampling adult and immature whiteflies have been developed on
some crops to establish economic thresholds and to test hypotheses related to control
measures (Butler et al. 1989, Ekbom and Rurnei 1990, Naranjo 1996, Ohnesorge and
Rapp 1986a). Sampling plans for Bemisia egg and nymph stages have been developed
for cotton (Gossypium hirsntum L.) (von Arx et al. 1984, Naranjo and Flint 1994,
Ohnesorge and Rapp 1986b), cantaloupe (Cucumis mel L.) (Gould and Naranjo 1999,
Tonhasca et al. 1994a. 1994b), tomato (Lycopersicon esculentum Mill.) (Carnero and
Gonzlez-Andujar 1994, Ohnesorge et al. 1980, Pernezny et al. 1995, Schuster 1998) and
peanut (Arachis hypogea L.) (Lynch and Simmons 1993, McAuslane et al. 1993).
Sampling methods for Bemisia are determined by the behavior and biology of the
insect and by the phenology of the crop being sampled (Naranjo 1996). Female whiteflies
tend to oviposit on the underside of young leaves in the upper plant canopy (van Lenteren
122


121
Table 5-5. Numbers of arthropods per 0.75 nr beat cloth sample collected from tomato1
under 3 pesticide regimes, Baja Verapaz, Guatemala
Pesticide Spiders Hemipteran herbivores Beetles
Imidacloprid 0.810.98a2 9.678.76a 0.400.74a
Detergent/oil 1.071.27a 13.3614.32b 0.290.61a
Control 2.121.67b 7.505.87a 0.330.62a
' There were no differences (p < 0.1) between monocropped and intercropped tomato.
2 Data are means SD of eight replications (two main plot treatments and four
replications). Means in columns followed by the same letter do not differ (p < 0.1)
according to Tukeys studentized range test.
Table 5-6. Numbers of arthropods per 0.75 m2 sample collected on four crops Baja
Verapaz, Guatemala
Spiders
Insect
predators
Hemipteran
herbivores
Beetles
Tomato
1.751.39a2
0.750.71b
6.254.89a
0.330.82b
Cilantro
3.753.40a
6.505.26a
2.250.96b
2.002.16ab
Rosa de jamaica
0.740.50a
1.250.50b
2.253.20b
5.503.1 la
Velvetbean
1.250.50a
1.501.00b
1.250.96b
4.001.83a
'Comparisons based on samples taken from unsprayed intercropped tomato and
associated crops.
2 Data are means SD of four replications. Means in columns followed by the same
letter do not differ (p < 0.1) according to Tukeys studentized range test.


91
0.77/plant, week 5: 1.75 2.89, week 6: 6.12 8.61), or fourth-instar T. vaporariorum
(week 4: 0.44 0.81, week 5: 0.94 1.12, week 6: 4.38 7.82). There were no
statistical differences (p < 0.1) between treatments in the numbers of fourth-instar B.
tabaci during week 4 (0.37 0.81) or week 6 (0.37 0.81). The number of fourth-instar
B. tabaci was lower (p < 0.05) in the monocrop treatment (0.50 0.53) than in the
intercrop treatment (1.13 0.99) during week 5.
During weeks 4 and 5, B. tabaci made up 46% of the observed fourth-instar
whitefly immatures (ratio of B. tabaci to T. vaporariorum was 6: 7 on week 4 and 13:15
on week 5). On week 6, B. tabaci comprised 7% of the observed fourth-instar w'hitefly
immatures (1: 15).
There were fewer (p < 0.001) plants per row in the intercrop treatment (60.13
22.53) than in the monocrop treatment (82.19 9.16). Yield per row was higher (p <
0.05) in the monocrop treatment (2.47 0.53 kg) than in the intercrop treatment (1.25
0.39 kg). The reason for the lower number of plants per row in the intercrop treatments is
not clear. Possibly the weeding and fertilizing of the bean plants was impeded by the
presence of intercrop plants, leading to reduced survival.
Leafhoppers and thrips were barely discernable on this second bean crop, although
high populations of these insects decimated unprotected bean in the dry season.
However, dense populations of chrysomelids (Coleptera), primarily Cerotoma and
Diabrotica spp., attacked the second bean crop early. Cerotoma and Diabrotica spp. are
among the vectors of severe mosaic of bean, a comovirus (Morales and Cardona 1998).
Dr. Francisco Morales of International Center for Tropical Agriculture, Cali, Colombia,
identified symptoms of severe mosaic of bean among experimental plants in the field.


158
Scott, S. J., P. J. McLeod, F. W. Montgomery, and C. A. Hander. 1990. Influence
of reflective mulch on incidence of thrips (Thysanoptera: Thripidae:
Phlaeothripidae) in staked tomatoes. J. Entomol. Science. 24 : 422-427.
Scott, J. W., M. R. Stevens, J. H. M. Barten, C. R. Thome, J. E. Polston, D. J. Schuster,
and C. Serra. 1996. Introgression of resistance to whitefly-transmitted
geminiviruses from Lycopersicon chilense to tomato. Pages 357-367 in D.
Gerling and R. Mayer, eds. Bemisia 1995: taxonomy, biology, damage, control,
and management. Intercept Ltd., Andover, Hants, UK.
Shannon, C. E., and W. Weaver. 1949. The mathematical theory of communication.
University of Illinois Press, Urbana, IL.
Shapiro, J.P. 1996. Insect-plant interactions and expression of disorders induced by the
silverleaf whitefly, Bemisia argentifolii. Pages 167-177 in D. Gerling and R.
Mayer, eds. Bemisia 1995: taxonomy, biology, damage, control, and
management. Intercept Ltd., Andover, Hants, UK.
Sharer, R. J., and D. W. Sedat. 1987. Archaeological investigations in the northern
Maya highlands, Guatemala. Interaction and development of maya civilization.
University of Pennsylvania, Philadelphia.
Sharma, S. R., and A. Varma. 1984. Effect of cultural practises on virus infection in
cowpea. Z. Acker- Pflanzenbau 153: 23-31.
Sheehan, W. 1986. Response by specialist and generalist natural enemies to
agroecosystem diversification: a selective review. Environ. Entomol. 15: 456-61.
Simmons, A. 1994. Oviposition on vegetables by Bemisia tabaci: temporal and leaf
surface factors. Environ. Entomol. 23: 381-389.
Slater, J. A., and R. M. Baranowski. 1978. How to know the true bugs. W.C. Brown
and Co., Dubuque, IA.
Smith, F. F., A. L. Boswell, and R. E. Webb. 1972. Repellent mulches for control of
gladiolus thrips. Environ. Entomol. 1: 672-673.
Smith, F. F., and R. E. Webb. 1969. Repelling aphids by reflective surfaces: a new
approach to the control of insect-transmitted viruses. Pages 631-639 in K.
Maramorosch, ed. Viruses, vectors, and vegetation. Wiley Interscience, New
York.
Smith, J. G. 1976. Influence of crop background on aphids and other phytophagous
insects on Brussels sprouts. Ann. Appl. Biol. 83: 1-13.


I want to thank Ing. Baltasar Moscoso, formerly head of ICTA, for facilitating my
research with that organization in 1998. I would not have been able to overcome the
various logistical hurdles of carrying out field research in Guatemala without the constant
support of Ing. Amoldo Sierra, the head of the ICTA station in San Jernimo. It was a
pleasure getting to know Dr. Robert Mac Vean of the Bucks County Organization for
Intercultural Advancement, who cleared all kinds of diplomatic hurdles for me and my
vehicle and without whom my research in Guatemala would have been very difficult.
Lie. Margarita Palmieri, Lie. Carolina Muoz, Estela de Flores, Dr. Chuck MacVean,
Lie. Catherine Cardona, and Dr. Jack Schuster of the Universidad del Valle all
contributed to my research with their resources, expertise, and kindness. I am very
grateful to Rodolfo Guzman and Ren Santos of Altertec and to Juan, Leo, Felix, and
Don Tancho of the ICTA station in San Jernimo for their friendship during my stay. My
friend Antonio Garcia Torres managed the field plots for the research in Guatemala and
contributed greatly to the success of that research. Special thanks go to Chuck and
Rodolfo who have been there since the beginning.
The Southeastern Sustainable Agriculture Research and Education program of the
USDA provided the funds for the research reported in chapter 2. The research described
in chapters 4 and 5 was funded by a fellowship provided by the National Security
Education Program. I am extremely grateful to the reviewers of the original research
proposals who recommended them for funding.
Finally, I thank my mother, Nancy Smith, and my grandparents, Ruth Freeman
and Anselm Fisher, without whom I could not have done any of this.
IV


52
Beans were planted 15 September and fertilized with 0.37 kg 15-0-14 per row on 23
September and 12 October.
The experimental area was treated with 0.19 liter/ha paraquot (Gramoxone,
Zeneca) on 26 July. Subsequent weed control was mechanical or by hand. The
imidacloprid-treated beans received 52.6 g/ha ai imidacloprid on 4 October and 12
October. This is the recommended rate for most vegetables. Imidacloprid is not
registered for use on beans but was included so that yield from intercropping treatments
could be compared with yield from chemically-protected beans.
Sampling. Whole plant examinations were made of 1 or 2 bean plants per plot
each week from 22 September through 11 November except for 29 September. Only the
underside of the leaf was examined. The area of each leaf was recorded using a LI-COR
portable leaf area meter (model LI-3000A, LI-COR Inc., Lincoln. NE). Bean treatment
comparisons were made on the basis of whole plant counts. Leaf counts from upper,
middle, and lower plant strata were used for comparison with eggplant on 21 October and
4 November. On 29 September bean and eggplant comparisons were based on the
average of counts taken from one 3.35 cm2 disc from a leaf in the upper and lower stratum
of two plants per plot (McAuslane et al. 1995).
Whole plant examinations were made of 1 to 3 eggplants per block each week
from 25 August through 8 October. After that time, plants became too large for whole
plant examinations. Whole leaf counts from upper, middle and lower strata were made of
eggplant on 21 October and 4 November.
Leaves were examined using a stereoscope and fiber-optic light. Total number of
B. argentifolii eggs, nymphs, parasitized nymphs, and red-eyed nymphs (pharate adults)
was recorded for each leaf. Leaves with nymphs showing symptoms of parasitism were


Table 6-1. Immature B. argentifolii on whole bean plants, fall 1996 -- mean numbers/cm2 and coefficients of variation (CV)
Week
Plant
area1
Eggs
Nymphs2
Parasitized
nymphs
Red-eye nymphs
Total
mean
CV
mean
CV
mean
CV
mean
CV
mean
CV
1
24
1.03
65
0
-
0
-
0
-
1.03
65
3
315
0.85
51
0.94
63
0.003
299
0.001
387
1.79
55
4
386
0.56
63
0.89
47
0.01
75
0.004
324
1.47
49
5
525
0.39
36
0.63
61
0.009
117
0.006
138
1.04
42
6
464
0.43
49
0.57
41
0.01
104
0.02
81
1.03
34
7
552
0.37
106
0.46
44
0.02
102
0.01
78
0.86
63
8
486
0.14
106
0.36
71
0.04
94
0.005
139
0.55
76
'Represents mean leaf area in cm2 for 15 bean plants,
includes all nymphs except parasitized and red-eyed nymphs.


89
treated plants because these plants were larger and supported more hosts/prey than
untreated plants.
There were more (p < 0.05) plants per row in the imidacloprid treatment (26.50
4.88) than in the control (22.28 6.75). Plant density in the detergent/oil treatment was
intermediate (24.00 7.76).
The percentage of plants with bean golden mosaic symptoms was different (p <
0.05) among all subplot treatments (imidacloprid: 7.27 7.03 %; detergent/oil: 14.86
10.53 %; control: 21.99 15.86 %). Eight bean plants out of ten showing symptoms of
bean golden mosaic tested positive for the presence of geminivirus.
The bean yield per row was higher (p < 0.05) in the imidacloprid treatment (0.29
0.09 kg) than in the detergent/oil treatment (0.05 0.03 kg) and the control (0.02 0.03
kg), neither of which produced marketable yield.
Because of delays in planting, the imidacloprid treatment could not be included in
the analysis. We learned when the tomato seedlings were delivered that all
commercially-produced tomato seedlings are treated with imidacloprid in the nursery.
Both detergent/oil and control seedlings received a pre-transplant imidacloprid treatment.
Whitefly populations on tomato remained low throughout the diversity study.
This may be partially explained by the effect of imidacloprid and other chemicals applied
in the nursery. In the third sample (June 28), there were more (p < 0.05) fourth-instar T.
vaporariorum on the control (3.38 3.07) than on the detergent/oil plants (0.71 1.11).
Observations of Bemisia were too few for analysis. There were no statistical differences
(p < 0.1) among subplot treatments in density of whitefly immatures (Table 4-3),


126
Bean was sampled each week from 22 September through 11 November, except
29 September. On 22 September, when plants were in the cotyledonary stage, eight
plants were collected from each plot. Disc and whole leaf counts were taken from each
cotyledon leaf. From 8 October through 11 November, stratified disc punch samples
were taken from four plants. Whole leaf samples were gathered from two of these plants,
using the same leaves from which disc counts had been taken. One of these two plants
was used to gather whole plant counts.
The area of each leaf used for whole leaf or whole plant sampling was recorded
using a LI-COR portable leaf area meter (model LI-3000A, LI-COR Inc., Lincoln, NE).
Disc, whole leaf, and whole plant counts were analyzed on a per cm2 basis.
Leaves were examined using a stereoscope and fiber-optic light. Total numbers of
B. argentifolii eggs, nymphs, parasitized nymphs, red-eyed nymphs, and total immature
stages were recorded for each disc, leaf, and plant.
Statistical Analysis
Densities of B. argentifolii eggs, nymphs, parasitized nymphs, red-eyed nymphs
and total immature stages on disc and whole leaves were compared across strata using
analysis of variance. Densities of immature stages were then compared between strata
using a pair-wise t-test (PROC GLM. SAS version 6.11, SAS Institute 1996). Because of
the large number of comparisons made, the a-level was adjusted using the Bonferroni
procedure (SAS 1996). Means and coefficients of variation (cv) were compared in order
to determine the most appropriate stratum for sampling different immature stages. The
regressions of whole plant counts on whole leaf counts and on disc counts were
determined for each response on each stratum. Regression equations and size of r2 values


72
throughout tomato-producing areas of Central America and the Caribbean during the late
1980s (Brown 1994), severely impacting Guatemala in 1987 (Dardn 1992). This
explosion of tomato geminiviruses is attributed to the arrival of the B strain of B. tabaci,
also known as B. argentifolii, throughout the region (Polston and Anderson 1997).
Whitefly problems in Central America tend to be attributed to Bemisia, but there
are at least 15 genera of whiteflies in the region with varying degrees of economic
importance (Caballero 1994). According to Caballero (1994), Trialeurodes
vaporariorum tends to be found in areas more than 1000 m above sea level, whereas B.
tabaci is rarely found above 1000 m. Trialeurodes vaporariorum does not vector
geminiviruses (Brown and Bird 1992), but its importance relative to Bemisia at higher
elevations may be underestimated.
The following experiments were a component of a broader effort to evaluate the
potential of intercropping for management of whiteflies. Prior field studies at the
University of Florida in Gainesville, Florida, indicated that trap cropping with squash or
eggplant (Solarium melongena L.) was ineffective in reducing densities of B. argentifolii,
and that using corn as a barrier crop was only marginally effective (see Chapters 2 and 3).
After consulting with pest management specialists from the Guatemala, San Jernimo
was chosen as a suitable site in which to test intercropping and whitefly management in
the context of small farmer cropping systems. San Jernimo is at the eastern end of the
Salam valley, a major tomato-producing area in central Guatemala. A system of gravity-
fed irrigation canals was built in this portion of the valley in the mid-1970s, permitting
year-round cultivation of tomato and other crops. This has improved the local economy,


Table 4-1. Whitefly eggs and nymphs on bean under 2 cropping systems and 3 pesticide regimes. Diversity study, first bean crop.
Egg Nymph
Wk Pesticide Monocrop Intercrop mean Monocrop Intercrop mean
]1 Imidacloprid
Detergent/oil
Control
mean
2 Imidacloprid
Detergent/oil
Control
mean
3 Imidacloprid
Detergent/oil
Control
mean
4 Imidacloprid
Detergent/oil
Control
mean
46.3844.68
68.9043.74
56.3050.31
57.1946.48
17.70115.86
31.33124.62
62.43149.36
35.21136.38
11.80114.27
3.4115.20
6.5718.92
7.26110.59
279.501535.02
8.75113.00
6.2515.68
98.171309.93
43.18135.08a2
74.93i53.57ab
108.3190.84b
75.46168.53
17.10115.40
28.93118.62
62.90143.88
36.31134.52
16.70118.95
4.2714.76
10.55124.57
10.51118.54
2533.7512292.44a
15.00117.63 b
3.2512.87 b
850.6711725.84
44.78139.68
71.91148.37
82.30177.1 1
17.40il5.45a
29.96121.09a
62.70145.60b
14.25116.74a
3.8414.94 b
8.56118.36 ab
1406.6311956.23
11.88114.72
4.7514.46
5.5315.00
42.83142.26
27.60130.70
23.34132.36
8.08111.40
20.32121.42
28.15121.85
18.85120.32
621.001613.60
30.25131.49
205.751136.32
285.671418.28
13.23117.24
59.15161.52
29.35127.28
33.94143.82
7.0519.28
25.15118.92
37.58138.60
23.26127.98
1547.2511293.92
47.50162.00
64.25190.17
553.001999.56
9.43113.13a
52.16154.01c
28.60128.37b
7.57110.28 a
22.73120.10b
32.87131.33b
1084.1311060.19a
38.88146.44 b
135.001131.03b


31
has resulted in delays in the onset of virus in tomatoes (Csizinsky et al. 1997) and
reduction in viral disease in tomatoes and squash (Fehmy et al. 1994).
Crops grown with plastic mulches experience reduced weed competition and
increased water and nutrient availability compared to crops grown on bare soil. In our
studies, crops grown with mulch were visibly more robust than crops grown on bare
ground. This clearly had a direct effect on yield (Table 2-11). The improved plant
quality of crops grown with mulch may have enhanced their ability to support higher
populations of nymphs as was observed during week 5 of 1996 and 1997.
Trap Crop
Egg densities were consistently far higher on squash with or without mulch than
on bean in the same treatments (Tables 2-1 to 2-3). However egg densities on bean
planted with squash were not lower than on bean alone. This indicates that squash did
not function as a trap crop.
High densities of Bemisia on a given crop have been interpreted as a preference
for that crop, in some cases leading it to be tested as a trap crop. Squash (Schuster et al.
1996), cantaloupe (Cucumis mel L.) (Ellsworth et al. 1994, Perring et al. 1995), soybean
('Glycine max L.) (McAuslane et al. 1995) and Wrights groundcherry (Physalis wrightii
Gray) (Ellsworth et al. 1994) have been tested as trap crops for Bemisia with unclear
results. Whitefly densities on the main crop were either unaffected by the presence of the
trap crop candidate, or reduced on a few isolated sampling dates, as occurred with our
study. Puri et al. (1996) intercropped cotton (Gossypium hirsutum L.) with wild brinjal
(Solarium khasianum Clarke), which traps arthropods with a sticky exudate, without
significantly reducing Bemisia densities in cotton.


30
Crops froze in 1995 before yield could be harvested. In 1996 yields were highest
in the mulched treatments. Yields were extremely low in 1997, presumably due to high
whitefly pressure (Table 2-11).
Virus
In 1996, only 1 plant (in the squash treatment) tested positive for the presence of
bean golden mosaic geminivirus. Virus presence was much higher in 1997. There were
no significant differences in virus presence (percent of plants testing positive for virus)
among treatments (bean: 56 51%; mulch: 55 51%; squash: 27 46%; squash/mulch:
38 49%).
Discussion
Reflective Mulch
The loss of effectiveness of reflective mulch after the first week of 1996 and 1997
may be attributed to accumulation of dust on the mulch and shading by growing plants.
Bemisia tabaci engages in most flight activity in the middle of the day (Bellows et al.
1988, Byrne and von Bretzel 1987), when mulch should be reflecting repellent UV rays.
However, it is not unusual to see adults moving with early morning breezes in agricultural
fields. Adults may colonize crops planted with reflective mulch before the mulch
receives strong sunlight.
Most studies compare reflective plastic mulch with mulches of other colors rather
than with bare soil (Csizinsky et al. 1997, Powell and Stofella 1993). Researchers
generally conclude that reflective mulch is insufficient as a sole method of control
(Natwick and Mayberry 1994, Schuster et al. 1989). While reflective mulch does not
appear to provide season-long reduction of whitefly densities, the use of reflective mulch


CHAPTER 4
THE ROLE OF CROP DIVERSITY IN THE MANAGEMENT OF A WHITEFLY
(HOMOPTERA: ALEYRODIDAE) SPECIES COMPLEX ON BEAN (PHASEOLUS
VULGARIS L.) AND TOMATO (L YCOPERSICON ESCULENTUM MILL.) IN THE
SALAM VALLEY, BAJA VERAPAZ, GUATEMALA
Introduction
Intercropping is the agronomic practice of growing two or more crops in a field at
the same time (Andrews and Kassam 1976). Intercrop arrangements include growing
crops in alternating rows (row intercropping), mixing crops within a row or without
regard to rows (mixed intercropping), and relay intercropping, which allows partial
overlap of crop cycles (Andrews and Kassam 1976). Among the advantages attributed to
some intercropping systems is reduced pest damage (Kass 1978, Litsinger and Moody
1976, Perrin 1977). Reviews of the intercropping literature indicate that, relative to
monoculture, herbivore numbers were lower in more than 50 percent of the intercropping
systems studied, greater in 15 to 18 percent of the cases, and variable in about 20 percent
of studies (Andow 1991a, Risch et al. 1983).
Several theories have been proposed to explain how intercropping may reduce
pest damage (Altieri 1994, Andow 1991a,Vandermeer 1989). Pimentel (1961) articulated
the idea that diverse cropping systems will support arthropod communities which are
more diverse and comprised of populations which are less dense and more stable than
arthropod communities in monocultures. It was hypothesized that natural enemies might
be more efficient in diverse agroecosystems than in simple ones, and that by damping
68


26
cork borer (McAuslane et al 1995). Discs were taken from the underside of the leaf, in

the lower half of the leaf to the right of the mid-vein. Samples were examined using a
dissecting stereoscope set at 20x and numbers of whitefly eggs, nymphs, parasitized
nymphs, and red-eyed nymphs were recorded.
Yield
Pods were harvested week and fresh weight was recorded for weeks 7, 8, and 9
after planting.
Virus Screening
/
After harvest, leaf tissue from 6 plants from each plot was collected and tested
with a dot blot hybridization technique for the presence of geminivirus (Rojas et al.
1993). Analysis was conducted by the laboratory of Dr. E. Hiebert at the Department of
Plant Pathology at the University of Florida. Bean tissue (50 mg) was extracted in 200
mM NaOH with 1% SDS. Geminivirus DNA-A component was amplified by PCR with
Maxwell degenerate primers (PALlvl978 and PARlc496). The amplified DNA was
used for a 32P random-primed labeling reaction (Life Technologies RTS RadPrime DNA
Labeling Systems). The membrane was hybridized with 32P labeled probe in 6x SSC, 5x
Denhardts solution and 0.5% SDS at 65 C for 16 hrs. The membrane was then washed
under high stringency conditions with 0.2x SSC and 0. lx SDS at 65 C. Finally the
membrane was exposed to X-ray film for 16 hrs.
Statistical Analysis
Whitefly counts were transformed by logl0(x+l) because of low counts during the
first year and unequal variance over time. Treatment comparisons were made of upper
leaf counts, lower leaf counts, and of the average of the two strata. Treatments were
compared with time as a variable, and then by individual week, using analysis of variance


85
treatment was comprised of tomato seedlings produced under protective mesh in the
nursery study which received no pesticide applications before or after transplanting.
Each treatment was replicated 4 times. Main treatment plots were 6 m2. Each
main treatment plot was divided in half with nylon cord to produce two subplots, each 6
x 3 m. Monocrop and intercrop whole plots were separated by a 6 m2 patch of com.
Approximately 73 mg imidacloprid (Confidor 70 WG) was applied to tomatoes in the
imidacloprid treatment on 22 October and 5 November.
Corn was planted 18 August and spaced every 2 m on the east side of the bed.
Cilantro was planted in a nursery 20 August and transplanted into the intercrop plots 2
October. Cilantro was planted every 12 cm on the west side of the bed. Tomato was
planted every 40 cm.
Tomato plants in the corn/cilantro study were sampled for 6 weeks, from 21
October through 2 December. Whole plant counts were made during weeks 1 -4. During
weeks 5 and 6, only the lower third of the plant was sampled because of plant size. Four
plants per plot were sampled during weeks 1-3. Two plants per plot were sampled during
week 4, and 3 plants per plot were sampled during weeks 5 and 6.
Number of branches per plant was recorded for weeks 1-5. Plant heights were recorded
weeks 2-5, and weights were measured weeks 3-5.
On 5 November, two beat cloth samples per subplot were taken from tomato to
estimate generalist predators. A 1.0 m x 0.75 cm plastic sheet was spread out on a
wooden board at the base of two adjacent tomato plants. The plants were struck swiftly 4
times toward the sheet, which was then folded into a ball and sealed with masking tape.


63
Table 3-1. Mean (SD) number of immature B. argentifolii/cm2 on bean, 1996.
Wk Treatment Egg Nymph Para. Nymph2 RENJ
1
Bean
0.790.58
0
0
0
Corn
1.040.73
0
0
0
Eggplant
1.270.68
0
0
0
X
1.030.67
0
0
0
3
Bean
0.620.40
0.640.29
0
0
Corn
0.930.26
0.860.33
0.0020.004
0
Eggplant
1.000.58
1.31 0.87
0.0060.01
0.0040.008
X
0.850.44
0.940.59
0.0030.008
0.000.005
4
Bean
0.400.30
0.790.30
0.0100.008
0.0100.02a
Corn
0.670.49
1.10=1=0.65
0.0100.004
0.0020.004b
Eggplant
0.600.27
0.800.25
0.0040.005
0b
X
0.560.36
0.900.43
0.0100.008
0.0040.013
5
Bean
0.360.20
0.480.30
0.0060.005
0.0060.005
Corn
0.39=1=0.10
0.800.51
0.0160.015
0.0080.013
Eggplant
0.440.15
0.60.33
0.0060.008
0.0040.005
X
0.400.14
0.630.39
0.0090.010
0.0060.008
6
Bean
0.41 0.34
0.460.23
0.0040.005a
0.0120.011
Corn
0.430.16
0.580.22
0.0200.015b
0.018=t0.016
Eggplant
0.46=1=0.14
0.670.26
0.0100.007ab
0.0160.011
X
0.430.22
0.570.24
0.011 0.012
0.0150.012
7
Bean
0.540.58a'
0.510.26
0.0100.01 a
0.0100.010
Corn
0.220.18b
0.440.15
0.0160.015a
0.0160.013
Eggplant
0.340.32b
0.40.22
0.0360.027b
0.0140.008
X
0.370.39
0.460.20
0.0210.021
0.0130.010
8
Bean
0.260.16a
0.450.35
0.0460.049
0.0060.008
Corn
0.060.04b
0.310.20
0.0520.043
0.0080.008
Eggplant
0.1 l0.16b
0.330.24
0.0240.018
0.0020.004
X
0.14=1=0.15
0.360.26
0.0410.038
0.0050.007
'Means assigned different letters in the same column and week of sampling are
significantly different according to Tukeys Studentized Range test with an adjusted
experiment-wise error rate of oc=0.05. @ indicates a=0.1. Parasitized nymphs. Red-
eyed nymphs (pharate adults).


99
populations that might therefore accumulate at higher densities on intercropped than
monocropped hosts, if the host is planted at a lower density in the intercropped system.
It seems unlikely that intercropping alone can protect crops from whitefly damage.
Crops such as tomato must be kept virus-free for 40-60 days after emergence in order to
protect yield (Cubillo et al. 1994), and this can probably only be achieved over limited
areas with fabricated devices such screened nurseries and floating row covers (Norman et
al. 1993). Whitefly management in the tropics may require region-wide coordination of
host-free periods, as practiced in the Dominican Republic (Polston and Anderson 1997),
combined with the integrated pest management programs specifically designed for
medium-scale and low resource farmers, such as those currently being implemented for
tomato growers in Costa Rica (Hilje 1993, 1998).
Summary
The predominant whitefly species in the Salani valley is T. vaporariorum.
Whitefly populations were highest at the end of the dry season (May), lowest early in the
rainy season (June-September), and increasing at the end of the rainy season (October-
November). Observations of B. tabaci were low throughout the year. Bemisia tabaci on
bean was estimated at 11% of the total population at the end of the dry season, 46% in the
middle of the rainy season, when overall populations were at their lowest, and 0.5% on
tomato at the end of the rainy season.
Treatment comparisons were difficult to interpret due to high variability among
samples, and by intercrop competition, which reduced bean and tomato size and health in
some studies. The intercropping arrangements examined either did not reduce whitefly
densities consistently relative to monoculture, reduced whitefly numbers by stunting the


144
Bach, C. E. 1980a. Effects of plant density and diversity in the population dynamics of a
specialist herbivore, the striped cucumber beetle, Acalymma vittata. Ecology 61:
1515-1530.
Bach, C. E. 1980b. Effects of plant diversity and time of colonization on an herbivore-
plant interaction. Oecologia 44:319-326.
Barrett, B. 1995. Commentary: plants, pesticides and production in Guatemala;
nutrition, health and nontraditional agriculture. Ecol. Food Nutrit. 33: 293-309.
Bartlett, A. C., and N. J. Gawel. 1993. Determining whitefly species. Science 261:
1333-1334.
Bedford, I. D., R. W. Briddon, J. K. Brown, R. C. Rosell, and P. G. Markham. 1994.
Geminivirus transmission and biological characterisation of Bemisia tabaci
biotypes from different geographic regions. Ann. Appl. Biol. 125: 311-325.
Bellows, T. S., Jr., T. M. Perring, R. J. Gill, and H. D. Headrick. 1994. Description of a
species of Bemisia. Ann. Entomol. Soc. Am. 87:195-206.
Berlinger, M. J. 1986. Host plant resistance to Bemisia tabaci. Agr. Ecosys. Env. 17: 69-
82.
Bethke, J. A., T. D. Paine, and G. S. Nuessly. 1991. Comparative biology,
morphometries, and development of two populations of Bemisia tabaci on cotton
and poinsettia. Ann. Entomol. Soc. Am. 84: 407-411.
Bink-Moenen, R. M., and L. A. Mound. 1990. Whiteflies: diversity, biosystematics, and
evolutionary patterns. Pages 1-11 in D. Gerling, ed. Whiteflies: their bionomics,
pest status and management. Intercept, Ltd., Andover, Hants, UK.
Blair, M. W., M. J. Bassett, A. M. Abouzid, E. Hiebert, J. E. Polston, R. T. McMillan, Jr.,
W. Graves, and M. Lamberts. 1995. Occurrence of bean golden mosaic virus in
Florida. Plant Dis. 79: 529-533.
Blua, M. J., H. A. Yoshida. and N. C. Toscano. 1995. Oviposition preference of two
Bemisia species. Environ. Entomol. 24: 88-93.
\
Borror, D. J., C. A. Triplehorn, and N. F. Johnson. 1989. An introduction to the study of
insects. Holt, Rhinehart and Winston, Orlando, FL.
Brazzle, J. R., K. M. Heinz, M. P. Parrella, and C. H. Pickett. 1994. Field evaluations of
Delphastus pusillus for control of Bemisia tabaci infesting upland cotton. Page
123 in T. J. Henneberry, N. C. Toscano, R. M. Faust, and J. R. Coppedge, eds.
Silverleaf whitefly: 1994 supplement to the 5-year national research and action
plan. Agricultural Research Service No. 125, U. S. Department of Agriculture,
Washington, DC.


Table 4-5. Whitefly immatures (x SD/plant) and plant parameters of bean monocropped and mix intercropped with field com and
rosa de jamaica. Mosaic study.
Egg Nymph Height (cm) Trifoliates Weight (g)
Wk Monocrop Intercrop Monocrop Intercrop Monocrop Intercrop Monocrop Intercrop Monocrop Intercrop
1 31.83
51.10
10.71
13.32@'
11,56
2.97
2
46.56
71.62
10.81
11.84@
8.00
11.31
2.12
2.75*
14.34
2.71
3
75.25
9.88
17.69
5.81
17.19
120.44
18.22*'
30.24
7.79
3.17
4
111.13
21.88
43.19
8.31
21.13
175.62
18.02*
63.19
7.89*
3.54
5
33.88
22.75
65.88
10.13
22.19
54.44
51.55
55.22
5.46*
3.07
6
7.25
4.63
133.25
29.50
27.75
4.59
5.78
259.35
40.02
6.25
16.86
2.49* *1
17.09
2.31**
-
-
-
-
18.00
4.43
1.13
0.81
0.50
0.52**
-
-
22.03
4.17
2.88
1.23
1.63
0.62**
10.97
5.58
4.02
1.56 **
22.00
4.06
4.50
1.31
2.50
0.76**
13.94
6.51
5.84
2.66**
24.38
7.86
6.25
2.38
4.13
2.17(3),
27.38
17.85
11.14
11.55**
'**, *, @ indicate that intercrop mean is significantly different from corresponding monocrop mean at p < 0.01. p < 0.05, and p < 0.1
respectively.
o
o\


69
oscillations in arthropod populations, crop diversity would reduce pest outbreaks (Elton
1927, 1958, Pimentel 1961). This enemies hypothesis was summarized by Root
(1973), who added to it the resource concentration hypothesis to explain reduced
herbivore damage in some complex agroecosystems. The resource concentration
hypothesis suggests that exploitation of crops by specialist herbivores can be reduced by
breaking up monocultures. Damage by polyphagous herbivores may also be reduced by
the presence of poor or non-hosts in mixed systems by the flypaper effect (Trenbath
1976, 1977). Finally, trap crops can be used in intercropping to draw herbivores away
from a main crop (Vandermeer 1989).
The theory that diversity in itself will reduce pest damage has been largely
discarded as inconsistent with empirical data (Andow 1991a, Risch et al 1983). More
recent analysis suggests that the interaction between a cropping system and its arthropod
community is determined largely by the specific characteristics of each (Andow 1991a,
Kareiva 1983, Sheehan 1986, Stanton 1983). The ratio of host to non-host species will
have a greater effect on herbivore abundance than the actual number of crop species
(Power 1990, Stanton 1983). The response of both herbivores and natural enemies to a
given cropping system will depend on their host range, their host-finding mechanisms,
and their mobility (Kareiva 1983, Power 1990, Russell 1989, Sheehan 1986, Stanton
1983).
Many small farmer cropping systems in the tropics rely on the principles of
intercropping to produce a range of goods for the home and market (Altieri and Hecht
1990, Kass 1978). Efforts by low resource farmers to improve income by concentrating


117
primarily of Engytatus modesta (Distant) (Miridae), the tomato bug, found predominantly
on tomato (Table 5-3). About 68% of the Coleptera recovered belonged to the
Chrysomelidae and Elateridae (Table 5-4). Proportions of various groups were
influenced by the fact that four times as many samples were taken from tomato (48) as
from each of the other crops (12). Sweep netting and other sampling methods would
have provided estimates of different arthropod groups, such as the Hymenoptera observed
on rosa de jamaica.
There were no differences (p > 0.1) between monocropped and intercropped
tomato in numbers of spiders or Coleptera. There were too few insect predators on
intercropped and monocropped tomato for meaningful comparisons. Spiders were the
primary predatory group found on tomato. Spider levels were higher (p < 0.05) on
unsprayed tomato than on tomato that had been treated with imidacloprid or the detergent
and oil rotation (Table 5-5). Numbers of hemipteran herbivores were higher (p < 0.1) on
tomato treated with detergent and oil than the other two treatments. Hemipteran
herbivores consisted primarily of Engytatus modesta, which was higher (p < 0.05) on
tomato treated with the detergent and oil rotation (12. 50 14.13 per beat cloth) than on
untreated tomato (6.06 5.37). Densities of E. modesta on tomato treated with
imidacloprid were intermediate (8.56 7.68).
There was no difference (p >0.1) in the number of spiders among cilantro, rosa de
jamaica, velvetbean, and unsprayed, intercropped tomato (Table 5-6). Levels of insect
predators were higher (p < 0.05) on cilantro than on the other three crops. Hemipteran
herbivores were more numerous (p < 0.05) on intercropped, unsprayed tomato than on the
other three crops. This difference was due to high densities of Engytatus modesta on
tomato. Beetle densities were higher (p < 0.05) on rosa de jamaica and velvetbean than
on unsprayed, intercropped tomato.


14
The idea that diversity in itself reduces pest damage has been abandoned as
inconsistent with empirical data (Andow 1991a). As Risch et al. (1983) point out,
stability in pest populations is desirable only below economically damaging levels.
However, reviews of the intercropping literature indicate that, relative to monoculture,
herbivores were less dense in more than 50% of studies, more dense in 15 to 18 % of the
cases, and variable in about 20 % (Andow 1991a, Risch et al. 1983). About 9% showed
no difference in density between cropping systems. Recent analysis has focused on
rigorous examination of the two hypotheses defined by Root (1973) in an attempt to
determine under which conditions polyculture might be useful for pest management
(Andow 1991a, Corbett and Plant 1993, Kareiva 1983, Power 1990, Risch et al. 1983,
Russell 1989, Sheehan 1986, Stanton 1983). The trap cropping mechanism has been
ignored by all reviewers except Vandermeer (1989).
Neither the resource concentration hypothesis nor the enemies hypothesis has
proven to be consistently useful for predicting how crop density and diversity will affect
arthropod density or diversity (Kareiva 1983, Russell 1989). Andow (1991a) and Risch
et al. (1983) state that, based on reviews of the literature, the resource concentration
hypothesis tends to account for herbivore response to polyculture more often than the
enemies hypothesis. However, given the high degree of variability in response by some
herbivores, Andow (1991a) suggests that this generalization is of limited predictive value.
Russell (1989) writes that studies which have compared insect abundance in simple and
diverse systems have uncovered little evidence to support the enemies hypothesis.
The inability to explain arthropod response to vegetative diversity with a few
broad mechanisms has been attributed in part to the many adaptive variations that


3
the first decades of the century in Africa, Asia, India, and Latin America, primarily in
cotton, tobacco, cassava (Manihot esculenta Krantz), and various legumes (Costa 1975).
Large-scale monocultures of cotton in Central America and cotton and soybean (Glycine
max L.) in Brazil favored massive increases in B. tabaci populations in those regions in
the 1960s (Costa 1975, Dardn 1992). Until the early 1980s, B. tabaci outbreaks were
largely sporadic (Bedford et al. 1994). By the end of the 1980s, a strain of B. tabaci, later
described as a new species, had become one of the most important agricultural pests
around the globe.
In Puerto Rico in the 1950s, researchers established that morphologically
indistinct populations of B. tabaci existed with different host ranges. Strains or biotypes
of B. tabaci based on host range were later recognized in Brazil and West Africa (Brown
et al. 1995). In the mid-1980s, a strain of B. tabaci was introduced from the
Mediterranean into the western hemisphere via the Caribbean, probably on ornamental
plants (Brown et al. 1995, Polston and Anderson 1997). This strain was designated the
B-biotype, or B strain, to distinguish it from the A-biotype, the prevalent North American
strain (Costa and Brown 1990, 1991). The B-biotype appeared in Arizona, California,
Texas, and Florida between 1988 and 1989, and within a few years had largely displaced
the A-biotype throughout much of this region (Brown et al. 1995). By 1993, the B-
biotype had been recorded throughout Central America and in Brazil (Brown et al. 1995).
The B-biotype has a broader host range than indigenous strains, causing serious
infestations of poinsettia (.Euphorbia pulcherrima (Willd.)), tomato, bell pepper
(Capsicum annuum L.), broccoli (Brassica olercea L.), cauliflower (Brassica olercea
L.), and alfalfa (Medicago sativa L.), none of which had been seriously affected by the A
strain (Perring 1996). The new strain demonstrated greater rates of oviposition and


Table 2-5.
Nymph density of B. argentifolii (mean SD/cm2)
on bean and squash, 1996
Bean
Squash
Week
Treatment
Lower stratum
Upper stratum
Mean
Mean
2
Bean
0.52 0.57
0.57 0.60ab
0.55 0.57a
Mulch
0.05 0.16
0.19 0.33a
0.12 0.27b
Squash
0#
1.28 0.87b#
0.64 0.89a
0**
Squash/mulch
0
0.29 0.30a
0.14 0.25b
0.01 0.06*
3
Bean
2.29 1.36#
0.36 0.49#
1.32 1.40
Mulch
2.31 1.52#
0.62 1.33#
1.46 1.64
Squash
1.97 1.13#
0.21 0.28#
1.10 1.21
1.01 2.40
Squash/mulch
1.86 1.02#
0.31 0.39#
1.08 1.09
0.63 1.33
4
Bean
1.41 0.86#
0.33 0.56#
0.87 0.90a
Mulch
1.76 1.51#
0.19 0.43#
0.98 1.35a
Squash
0.83 0.70
0.79 0.88
0.81 0.78ab
0.12 0.58**
Squash/mulch
0.86 0.89
0.02 0.08
0.44 0.75b
0.25 0.63
5
Bean
0.17 0.19
0
0.08 0.16a
Mulch
0.55 0.51
0.02 0.08
0.29 0.45b
Squash
0.05 0.11
0
0.02 0.08a
0.37 1.22
Squash/mulch
0.17 0.33
0.10 0.19
0.13 0.27ab
0.42 1.21
6
Bean
0.41 0.51
0
0.20 0.41
Mulch
1.05 1.32#
0#
0.52 1.06
Squash
0.26 0.28
0
0.13 0.24
0.27 1.05
Squash/mulch
0.50 0.61
0.07 0.25
0.29 0.51
0.16 0.45
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.
4^
O


INTERCROPPING AND WHITEFLY (HOMOPTERA: ALEYRODIDAE)
MANAGEMENT
By
HUGH ADAM SMITH
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1999


115
Imidacloprid (Confdor 70 WG, Bayer, Germany) was prepared at a rate of 0.73
g/liter of water. Approximately 10 cc of this mixture (73 mg imidacloprid) was applied
to the base of each tomato plant at each application. Tomato seedlings received one
imidacloprid application in the nursery, and were treated one and three weeks after
transplanting.
Olmeca vegetable oil (Olmeca S.A., Guatemala) and Unox laundry detergent
(Qumicas Lasser S.A., El Salvador) were applied at a rate of 1%, or 16 cc/16 liter spray
tank (Caldern et al. 1993). An elbowed nozzle attachment was used to apply the
mixture to the lower surface of leaves. Detergent or oil was applied in rotation every five
days.
Crop Management
Crops were managed according to local practices (ICTA 1993, Superb 1997).
Fungicides were applied with a backpack sprayer on a weekly basis to tomato to control
foliar and root pathogens once the rains began in May. No additional pesticides were
applied to tomato other than those from assigned treatments. No pesticides were applied
to cilantro, rosa de jamaica, or velvetbean. A furrow irrigation system was used as
needed.
Beat Cloth Samples
On 3 July, one beat cloth sample per subplot was taken from each row of cilantro,
rosa de jamaica, and velvetbean. One beat cloth sample was taken from intercrop tomato
in rows two and four of each subplot. These rows were situated between velvetbean and
rosa de jamaica and rosa de jamaica and cilantro, respectively. One beat cloth sample
was taken from monocrop tomato in rows two and four of each subplot. Tomato was
producing flowers and green fruit, and cilantro was flowering when the samples were
collected. Rosa de jamaica and velvetbean had foliage only.


Imidacloprid
19.21 32.05
23.4145.87
21.31 38.98
3.002.65
11.7229.46
7.3621.04
Detergent/oil
3.91 10.76
11.1931.36
7.5523.36
1.31 1.25
3.357.72
2.425.56
Control
0.31 0.44
14.7544.22
7.5331.63
1.692.17
2.785.82
2.234.36
mean
7.81 20.82
16.4540.45
2.002.19
6.01 17.99
Imidacloprid
0
0.250.50
0.130.35
30.2538.02
17.25 15.13
23.7527.68
Detergent/oil
0
0.250.50
0.130.35
2.503.11
14.00 16.73
8.25 12.73
Control
0
0
0
13.2523.19
3.002.45
8.13 16.22
Mean
0
0.160.39
15.3326.19
11.42 13.45
'Means from weeks 1,2, 3, and 5 represent the average count from one 3.35cm2 upper stratum leaf disc punch and one 3.35 cm- lower
stratum leaf disc punch.
2Data are means of 5 replications. Means in columns for a given week followed by the same letter are not significantly different (p <
0.05) according to Tukeys Studentized Range test. No letters present indicate no differences for that week. No differences (p > 0.1)
between means in monocrop vs. intercrop treatments on any sampling date.
Means from weeks 4 and 6 represent counts from 1 entire plant per replicate.
o
K>


59
The greater parasitism and variety of parasitoid species on eggplant may be due to
the greater number of weeks that eggplant was in the field.
Bean yield. Bean yield per 2 m of row was not different among the three
treatments and the imidacloprid-treated bean plants (imidacloprid: 0.95 kg 0.71; bean:
0.87 kg 0.58; corn: 0.47 kg 0.28; eggplant: 1.14 kg 0.77).
Eggplant as a trap crop. Eggplant did not reduce oviposition on adjacent bean
early in the season, and so did not function as a trap crop. Oviposition was not
consistently higher on eggplant than on bean as reported elsewhere (Tsai and Wang
1996). Eggplant leaves may have been less suitable for oviposition because they were
several weeks older than the bean leaves. Treatment differences were not statistically
significant, but egg densities tended to be higher on bean planted with eggplant than on
the other bean treatments during the first weeks of sampling. Proximity to colonized
eggplant may tend to increase rather than decrease oviposition on bean.
A concurrent test of squash (Cucrbita pepo L.) as a trap crop for whiteflies also
produced negative results (Smith et al., unpublished). It is possible that host-finding
mechanisms used by whitefly adults prevent them from being drawn away from one host
plant by the presence of another. Bemisia does not respond to host-specific visual or
olfactory cues (Mound 1962). It apparently requires gustatory information in order to
accept or reject a host (van Lenteren and Noldus 1990). Whitefly adults tend to leave
some host plant species more quickly than others (Verschoor-van der Poel 1978). The
observed differences in host-specific oviposition density by Bemisia may be due to length
of tenure on the plant rather than to some preference expressed in the host-finding stage.
Many trap crop studies have not resulted in consistent reductions of whitefly
densities on the main crop (Ellsworth et al. 1994, McAuslane et al. 1995, Perring et al.


9
control agents at a disadvantage (Hoelmer 1996). Crops tested as refugia include kenaf
(Hibiscus cannibinus L.) and rosa de jamaica (Hibiscus sabdariffa L.) (Malvaceae)
(Roltsch and Pickett 1995). Rosa de jamaica, also called roselle and sorrel in English,
possesses extra-floral nectaries at the base of the leaf (Standley and Steyermark 1949).
Cultural methods used to reduce whitefly damage include manipulation of
planting dates, use of short-season varieties, reflective mulches (Czizinsky et al. 1997,
Powell and Stofella 1993), and floating row covers (Norman et al. 1993). Trap crops and
intercropping have also been suggested as methods for management of Bemisia (Faust
1992).
Attempts to reduce whitefly damage with trap crops have produced unclear
results. Squash (Cucrbita pepo L.) (Schuster et al. 1996), cantaloupe (Cucumis mel L.)
(Ellsworth et al. 1994, Perring et al. 1995), soybean (McAuslane et al. 1995) and
Wright's groundcherry (Physalis wrightii Gray) (Ellsworth et al. 1994) have been tested
as trap crops for Bemisia. Whitefly densities on the main crop were either unaffected by
the presence of the trap crop candidate, or were reduced on only a few sampling dates.
Puri et al. (1996) intercropped cotton with wild brinjal (Solatium khasianum Clarke),
which traps arthropods with a sticky exudate, without significantly reducing Bemisia
densities in cotton. However, Al-Musa (1982) and Schuster et al. (1996) delayed the
onset of virus in tomato by trap cropping with cucumber and squash, respectively. Al-
Musa reported reductions of virus incidence of greater than 30% in tomato interplanted
with cucumber.
Several tall-growing non-host plants, primarily in the family Gramineae, have
been tested as barrier crops or intercrops to reduce whitefly colonization and virus


24
reflect ultra-violet rays are disorienting to certain insects (Prokopy and Owens 1983) and
have been used to repel virus vectors such as aphids (Smith and Webb 1969, Jones 1991)
and thrips (Smith et al. 1972, Scott et al. 1990). Schuster and Kring (1988) reported
some success using reflective mulch to manage whiteflies.
Trap crops are preferred host plants which are used to draw herbivores away from
a less-preferred main crop (Vandermeer 1989). Trap crops are sometimes sprayed with
pesticides to prevent the damaging herbivores from building up and spreading to the main
crop (Ellsworth et al. 1994). Several crops have been tested as trap crops for
management of Bemisia (Al-Musa 1982, Ellsworth et al. 1994, McAuslane et al. 1995,
Schuster et al. 1996). By the early 1990s, squash had been singled out as a promising trap
crop candidate for management of Bemisia (Faust 1992).
Material and Methods
The study was carried out on a 4-ha certified organic farm, 6 km northwest of
Gainesville, Florida (29 40N, 82 30W). Four treatments were compared: 1) bean
grown on bare soil (bean), 2) bean grown with reflective polyethylene mulch
(mulch), 3) bean mixed with squash grown on bare soil (squash) and 4) bean mixed
with squash grown with reflective mulch (squash/mulch).
Espada garden bean seed and Multipik yellow summer squash seed from
Harris Seed (60 Saginaw Drive, Rochester, New York) were used. Seed had been
previously treated with captan (N-(trichloromethyl)thio-4-cyclohexene- 1,2-
dicarboxamide), metalaxyl (N-(2,6-dimethylphenyl)-N-(methoxyacetyl)alanine methyl
esther), streptomycin and chloroneb. It is acceptable for organic growers to use treated
seeds if untreated seeds are unavailable (Organic Materials Review Institute 1998). To
ensure uniformity among covered and exposed beds, all beds were formed using a Rainflo


2
at least three whitefly species cause widespread crop losses by vectoring plant viruses.
Bemisia tabaci (Gennadius), the sweetpotato whitefly, vectors dozens of debilitating
geminiviruses to a broad range of agronomic and horticultural crops (Brown 1994).
Bemisia tabaci, Trialeurodes vaporariorum (Westwood), the greenhouse whitefly, and
Trialeurodes abutilonea (Haldeman), the banded-wing whitefly, vector closteroviruses
(Duffus 1996). Geminiviruses are transmitted in a persistent, circulative manner (Polston
and Anderson 1997), and closteroviruses in a semi-persistent manner (Duffus 1996).
Bemisia tabaci and T. vaporariorum are the most economically damaging species
of whitefly. Both species attack members of most major crop groups (Mound and Halsey
1978, Naresh and Nene 1980, Russell 1963, 1977). Trialeurodes vaporariorum has
traditionally been a pest of greenhouse crops in Europe and the United States (Lloyd
1922, Vet et al. 1980), although in recent decades it has expanded its range, affecting
glasshouse agriculture in Japan since 1974 (Yano 1983) and in Crete since 1979
(Roditakis 1990). It is a major pest of tomato (Lycopersicon esculentum Mill.) and
cucumber (Cucumis sativus L.) grown in greenhouses, although successful biological
control programs using parasitoids have been developed (Vet et al. 1980). In Central
America, T. vaporariorum tends to be more common above 500 meters, and B. tabaci
below 500 meters (Caballero 1994). Trialeurodes vaporariorum is a serious pest of
tomato and other horticultural crops grown at higher elevations in Central America, while
Bemisia and Bemisia-vectored geminiviruses are limiting factors at lower elevations
(Hilje 1993).
Bemisia tabaci was first described in 1889 as a tobacco (Nicotiana tobacum L.)
pest in Greece (Gennadius 1889). It was responsible for virus-induced crop losses during


27
(PROC MIXED, SAS version 6.11, SAS Institute 1996). When appropriate, treatment
means were compared using Tukeys Studentized Range test with an adjusted
experiment-wise error rate of a =0.05. Yield data were analyzed using the same analysis
of variance and mean separation procedures. Counts in upper and lower strata within the
same treatment were compared using a pairwise t-test. Bean samples which tested
positive for the presence of bean golden mosaic virus were assigned a value of 1, and
negative responses were assigned a value of 0. Responses were then analyzed using
logistic regression.
Results
Research Pesian
A latin square design was used because of the concern that some blocks might
become colonized by whiteflies before others due to their proximity to infested hosts or
their orientation to prevailing winds. It was observed during this and concurrent studies
that populations of whitefly adults require minutes rather than days or weeks to move
from one end of an experimental area to the other. It was decided therefore that the latin
square design was unnecessarily complicated for studying whiteflies, and that a
randomized complete block design would be adequate for future studies. However, the
data was analyzed using analysis of variance for latin square. A November freeze killed
all crops in 1995 after only 4 weeks of sampling. During the next two years the study was
initiated during the first week of September to reduce the risk of freezes.
Treatment Comparisons
Egg densities on the squash trap crop were significantly higher than on bean
throughout the three years of the study (Tables 1-3). Otherwise there were no consistent
trends among treatments from year to year. When treatment differences occurred, egg


4
feeding on some crops (Bethke et al. 1991, Cohen et al. 1992). Byrne and Miller (1990)
found that the B strain produced more honeydew than the A strain, and suggested that it
might have better access to the phloem. Feeding by the B strain has been associated with
the silvering of squash (Cucrbita pepo L.) and irregular ripening of tomato (Maynard
and Cantliffe 1989), as well as other previously unknown plant disorders (Shapiro 1996).
The B strain introduced dozens of new geminiviruses to the New World, primarily on the
Solanaceae and Cruciferae. Many of these are still uncharacterized (Brown et al. 1995,
Polston and Anderson 1997). Epidemics of bean golden mosaic geminivirus increased in
Central America after the arrival of the B-biotype (Rodriguez 1994). In 1993, the first
epidemic of bean golden mosaic was reported in south Florida (Blair et al. 1995). The B-
biotype also exhibited high levels of resistance to carbamate, organophosphate,
pyrethroid, and other pesticide groups compared to the A-biotype (Denholm et al. 1996,
Dittrich et al. 1990).
Based on DNA differentiation tests, allozymic frequency analysis, crossing
experiments, and mating behavior, Perring et al. (1993) reported that the B-biotype was a
new species. Presenting differences in pupal case morphology and allozymic characters,
Bellows et al. (1994) described the new species as Bemisia argentifolii Bellows &
Perring, the silverleaf whitefly. The name was derived from the ability of the whitefly to
induce silvering of leaves in certain cucurbits (Yokomi et al. 1990).
The elevation of the B-biotype to species has been disputed. Liu et al. (1993)
reported that, based on esterase isozyme analysis, populations of the A- and B-biotypes
mixed over time under laboratory conditions. Bartlett and Gawel (1993) argued that the
molecular analysis carried out by Perring et al. (1993) was insufficient to demonstrate the
existence of a new species. Brown et al. (1995) suggested that allozyme markers are


25
plastic mulch layer (model no. 560, Rainflo Irrigation, East Earl, PA). Plastic mulch and
drip irrigation tubing were laid over all beds, which were 1.22 m wide. After planting,
plastic mulch was removed from the bare soil treatments.
Beans were planted 15 cm apart within the row. Squash replaced every fifth bean
plant in the squash treatments. Beds were 3.5 m long, and the space between beds was 2
m. Each treatment plot contained two beds with two rows of plants per bed. The
reflective mulch was a white polyethylene mulch with a central stripe of silver pigment,
61 cm wide (product 60-64S/W125PR, North American Film Corporation, 19 Depot
Road, Bridgeport PA).
There was concern that whiteflies might colonize certain borders of the
experimental area before others because of wind direction or migration from adjacent
host plants. To control for two potential extraneous sources of variation, treatments were
arranged in a 4 x 4 latin square design.
Plots were irrigated as needed using drip irrigation. Plants were fertilized 3 weeks
after emergence and at flowering with approximately 250 g per row of 3-2-3 (N-P205-
K20) North Florida Brand composted chicken manure. Plots were hand-weeded as
needed. No pest control products were applied to the experimental area.
The study was repeated in 1995, 1996 and 1997. In 1995, crops were planted on
October 15. The following years, crops were planted on September 2.
Sampling
Sampling for whiteflies began one week after crop emergence. Sampling was
stopped after 4 weeks in 1995 because of a freeze. Bean and squash were sampled for 6
weeks in 1996 and 1997. Four or 5 plants were sampled per plot each week. The sample
unit was a 3.34 cnr leaf disc cut from upper and lower leaves using a number 13 nickel


84
employed for this purpose. Intercropping tomato with mature field com is not uncommon
among small farmers in Guatemala (Eduardo Landeverri. ICTA agronomist, personal
communication). The corn was widely spaced, and specifically managed to reduce
shading: lower leaves were removed from the corn early in November, and corn was
harvested in the fresh (elote) stage on 19 November, after which the top of the each
corn plant was removed.
Nursery Study
Tomato plants used in this study were grown individually in containers made from
newspaper (cartuchos) on the research site (Rufino 1998). Seeds were planted in
cartuchos on 21 September. About 300 seedlings were dusted with imidacloprid (Gaucho
70 WC; Bayer, Germany) before planting and grown in an exposed nursery. Another 300
seedlings were grown in a nursery protected from whiteflies by fine nylon mesh (Rivas et
al. 1994) and received no pesticide treatment. The treated seedlings received
approximately 73 mg imidacloprid (Confidor 70 WG) on 8 October. The height of eight
tomatoes from each nursery treatment was measured on 18 October, when the nursery
covering was removed. Eight plants from the two nursery treatments were examined for
whitefly immatures on 18 October. Tomato seedlings were transplanted into the
com/cilantro study 19 October.
Corn/Cilantro Study
A randomized complete block split plot design was used with 2 wholeplot
treatments (monocrop and intercrop) and 2 subplot treatments (imidacloprid treatment
and control). The imidacloprid treatment consisted of tomato plants which received
imidacloprid in the nursery study and in two post-transplant applications. The control


159
Soria, C., A. I. L. Sese, and M. L. Gomez-Guillamon. 1996. Resistance mechanisms of
Cucumis mel var. agrestis against Trialeurodes vaporariorum and their use to
control a closterovirus that causes a yellowing disease of melon. Plant Pathol. 45:
761-766.
Srinivasan, K., and P. N. K. Moorthy. 1991. Indian mustard as a trap crop for
management of major lepidopterous pests on cabbage. Trop. Pest Management
37: 26-32.
Standley, P., and J. Steyermark. 1949. Flora of Guatemala. Fieldiana botany, Vol. 24,
Part VI. Natural History Museum, Chicago, IL.
Stansly, P. A. 1995. Non-toxic control of whiteflies on vegetables. Citrus and Veg.
Magazine (March): 12-13.
Stansly, P. A., T. X. Liu, D. J. Schuster, and D. E. Dean. 1996. Pages 605-615 in D.
Gerling and R. Mayer, eds. Bemisia 1995: taxonomy, biology, damage, control,
and management. Intercept Ltd., Andover, Hants, UK.
Stansly, P. A., D. J. Schuster, and T.X. Liu. 1997. Apparent parasitism of Bemisia
argentifolii by Aphelinidae on vegetable crops and associated weeds in south
Florida. Biological Control 9: 49-57.
Stanton, M. L. 1983. Spatial patterns in the plant community and their effects upon
insect search. Pages 125-157 in S. Ahmad, ed. Herbivorous insects: host-seeking
behavior and mechanisms. Academic Press, New York.
Summers, C. G., A. S. Newton, Jr., and D. Estrada. 1996. Intraplant and interplant
movement of Bemisia argentifolii crawlers. Environ. Entomol. 25: 1360-1364.
Superb. 1997. Manual agrcola. Superb Agrcola, S. A. Guatemala City, Guatemala.
Swezey, S. L., and R. G. Daxl. 1988. Area-wide suppression of boll weevil populations
in Nicaragua. Crop Protection 7: 168-176.
Tahvanainen, J. O., and R. B. Root. 1972. The influence of vegetational diversity on the
population ecology of a specialized herbivore, Phyllotreta cruciferae. Oecologia
10: 321-346.
Taylor, L. R. 1961. Aggregation, variance and the mean. Nature (London) 189: 732-
735.
Taylor, L. R. 1984. Assessing and interpreting the spatial distributions of insect
populations. Annu. Rev. Entomol. 29: 321-357.


70
on higher-value market and export crops have resulted in an increase in pesticide use and
pesticide-related health problems in Central America (Murray 1991, Nicholls
and Altieri 1997). In Guatemala, the cultivation of non-traditional export crops has been
associated with reduced nutrition (Barrett 1995) and increased debt in some communities
(Glover and Kuterer 1990, Rosset 1991). The present series of studies was undertaken
with the intention of developing an intercropping system which helped meet the
economic and nutritional needs of low resource farmers by including both subsistence
crops (bean, Phaseolus vulgaris L.; and corn, Zea mays L.) and a market crop (tomato,
Lycopersicon esculentum Mill.) while reducing pesticide use.
Whiteflies cause economic damage to agronomic and horticultural crops
throughout the tropics (Brown et al. 1995. Byrne et al. 1990, Byrne and Bellows 1991).
Trialeurodes vaporariorum (Westwood), the greenhouse whitefly, Bemisia tabaci
(Gennadius), the sweetpotato whitefly. and Bemisia argentifolii Bellows and Perring (also
known as B. tabaci strain B), the silverleaf whitefly, are among the most damaging
species on annual crops. These three whitefly taxa reduce yields by vectoring viruses,
inflicting plant disorders, and causing mechanical damage to members of most crop
groups except the grasses (Byrne et al. 1990). Whiteflies have developed some degree of
resistance to most classes of pesticides (Denholm et al. 1996, Dittrich et al. 1990),
forcing growers and researchers to evaluate alternative methods of control. Imidacloprid
(Bayer) is a systemic insecticide which is currently effective against whiteflies and other
sucking insects (Polston et al. 1994). Detergents and oils have been used successfully to
manage whiteflies under certain conditions (Stansly 1995).


133
Distribution patterns of immature stages were similar for disc punch and whole
leaf sample units. However, the additional time required to process whole leaf samples
was not justified by the quality of the estimates when compared to estimates from disc
punch samples. Some whole plant sampling may be necessary to determine the range of
branches from which to sample life stages of interest with disc punches.


127
were used to evaluate disc and leaf samples as predictors of whole plant counts. For
comparison with existing literature, Taylors Power Law parameters (Taylor 1961, 1984)
were calculated for egg and nymph data from upper stratum disc units, lower stratum
whole leaf units, and whole plant sample units.
Results and Discussion
Whole Plant Samples
Total leaf area per plant increased from weeks 1-7 (Table 6-1). Egg and nymph
densities were highest during the first and third weeks of sampling, respectively, and
declined over subsequent weeks. Observations of parasitized nymphs occurred first
during week 3 and increased slightly over time. Observations of red-eyed nymphs
increased from weeks 3-6, then declined. Density of total immature stages peaked during
week 3 and declined over subsequent weeks. Coefficients of variation for whole plant
data did not show a clear trend over time for any immature stage (Table 6-1), although
they tended to be lowest for most stages during weeks 5-7.
Disc Samples
Egg densities were highest in the upper stratum across sampling dates (Table 6-2).
Coefficients of variation were lowest in the middle stratum when the crop was young
(weeks 3-4), but lowest in the upper stratum from week 5 onward. Regression of whole
plant on disc punch counts produced significant regression equations in middle strata
from weeks 3-5, and in upper or mid strata from weeks 6-8.
Nymph densities were consistently higher (p < 0.05) in mid and lower strata than
in the upper stratum. Nymph densities were not usually significantly different between
middle and lower strata, but there was a tendency in later weeks for the highest nymph


145
Broadbent, L. 1969. Disease control through vector control. In K. Maramorosch, ed.
Viruses, vectors, and vegetation. Interscience, New York.
Brown, J. K. 1994. Current status of Bemisia tabaci as a plant pest and virus vector in
agroecosystems worldwide. FAO Plant Prot. Bull. 42: 3-32.
Brown, J. K., and J. Bird. 1992. Whitefly-transmitted geminiviruses and associated
disorders in the Americas and the Caribbean basin. Plant Dis. 76: 220-225.
Brown, J. K., D. R. Frohlich, and R. C. Rosell. 1995. The sweetpotato or silverleaf
whiteflies: biotypes of Bemisia tabaci or a species complex? Ann. Rev.
Entomol. 40: 511-534.
Bugg, R. L., L. E. Ehler, and L. T. Wilson. 1987. Effect of common knotweed on
abundance and efficiency of insect predators of crop pests. Hilgardia 55: 1-51.
Butler, G. D., Jr., T. J. Henneberry and W. D. Hutchison. 1989. Biology, sampling and
population dynamics of Bemisia tabaci. Pages 83-111 in G. E. Russell, ed.
Biology and population dynamics of invertebrate crop pests. Intercept Ltd.,
Andover, Hants, UK.
Butler, G. D., Jr., T. J. Henneberry, and F. D. Wilson. 1986. Bemisia tabaci on cotton:
adult activity and cultivar oviposition preference. J. Econ. Entomol. 79: 350-354.
Byrne, D. N., and T. S. Bellows. 1991. Whitefly biology. Ann. Rev. Entomol. 36:
431-57.
Byrne, D. N., T. S. Bellows, Jr., and M. P. Parrella. 1990. Whiteflies in agricultural
systems. Pages 227-261 in D. Gerling, ed. Whiteflies: their bionomics, pest
status and management. Intercept Ltd, Andover, Hants, UK.
Byrne, D. N., and W. B. Miller. 1990. Carbohydrate and amino acid composition of
phloem sap and honeydew produced by Bemisia tabaci. J. Insect. Physiol. 36:
433-439.
Byrne, D. N., R. J. Rathman, T. V. Orum, and J. C. Palumbo. 1996. Localized migration
and dispersal by the sweet potato whitefly, Bemisia tabaci. Oecologia 105: 320-328.
Byrne, D. N., and P. K. von Bretzel. 1987. Similarity in flight activity rhythms in
coexisting species of Aleyrodidae, Bemisia tabaci and Trialeurodes abutilonea.
Entomol. Exp. Appl. 43: 215-219.
Caballero, R. 1994. Clave de campo para inmaduros de moscas blancas de
Centroamrica. Escuela Agrcola Panamericana, Zamorano, Honduras.


81
intercropped with com and rosa de jamaica. Each treatment was replicated 4 times and
arranged in a randomized complete block design. Monocrop plots contained 4 rows of
tomato adjacent to 4 rows of bean. Intercrop plots consisted of 8 rows of mixed crops
(Figure 4-2). The order of crop species within the row for the intercrop treatment was
corn, rosa de jamaica, bean, com, rosa de jamaica, tomato. The first crop in consecutive
rows was staggered so that each bean or tomato plant was surrounded by com, rosa de
jamaica and the other main crop, but was not immediately adjacent to a conspecific.
Rows were 8 m in length and between row spacing was 1.0 m. Between plant
spacing was 40 cm for all intercrop plants and the monocrop tomato, and 20 cm for
monocrop bean. Corn and rosa de jamaica were planted 18 August. Bean was planted 8
October. Tomato seedlings were transplanted 20 October.
Whole plant counts were taken for bean each week from 18 October through 17
November. Six plants per plot were sampled during the first week, 4 plants per plot
during weeks 2-4, and 2 plants per plot for the last 2 weeks. Plant height was measured
each week. Number of branches was recorded during weeks 3-6, and plants were
weighed in weeks 4-6. Number of plants per row and number of plants with bean golden
mosaic symptoms was counted 2 December.
Whole plant counts were taken for tomato for 4 weeks from 21 October through
12 November. On 22 November and 4 December, only the lower third of the plant was
sampled because the plants were too large for whole plant counts. During the first 2
weeks, 4 plants per plot were sampled. During week 2, two plants per plot were sampled.
During the remaining 3 weeks, 3 plants per plot were sampled. Plant heights were
measured during the first 5 weeks of sampling. Number of branches per plant was


CHAPTER 3
POTENTIAL OF FIELD CORN {ZEA MAYS L.) AS A BARRIER CROP AND
EGGPLANT (SOLANUMMELONGENA L.) AS A TRAP CROP FOR MANAGEMENT
OF THE SILVERLEAF WHITEFLY, BEMIS1A ARGENTIFOL1I (HOMOPTERA:
ALEYRODIDAE) ON BEAN (PHASEOLUS VULGARIS L.) IN NORTH FLORIDA
Introduction
Bemisia argentifolii Bellows & Perring, the silverleaf whitefly (also known as
Bemisia tabaci strain B (Gennadius)), causes significant economic damage to agronomic
and horticultural crops throughout warm regions of the world (Brown et al. 1995).
Bemisia argentifolii is a phloem-feeder which vectors numerous geminiviruses and
inflicts a variety of plant disorders as well as mechanical damage (Byrne et al. 1990,
Hiebert et al. 1996, Shapiro 1996). Bemisia has demonstrated resistance to most classes
of pesticides (Denholm et al. 1996), forcing growers and reseachers to evaluate
alternative methods of control. Attempts to manage whiteflies by cultural means have
included the use of trap crops (Al-Musa 1982, Ellsworth et al. 1994, McAuslane et al.
1995, Schuster et al. 1996) and barrier crops (Fargette and Fauquet 1988, Morales et al.
1993, Rataul et al. 1989, Sharma and Varma 1984).
Trap crops are preferred host plants which are used to draw an herbivore away
from a less-preferred main crop (Vandermeer 1989). Bemisia argentifolii has been
observed to oviposit heavily on eggplant {Solanum melongena L.), leading researchers to
suggest eggplant as a promising trap crop candidate (Faust 1992).
Whiteflies are weak fliers, relying on air currents for both short and long distance
migration (Byrne and Bellows 1991, Byrne et al. 1996). Several tall-growing non-host
49


18
viruses has been reduced on main crops by diverting aphid vectors to protection crops
(Jenkinson 1955, Broadbent 1969). Crop combinations which cause vectors to probe
more frequently but for shorter periods of time may increase the incidence of non-
persistent viruses, and reduce the incidence of persistent viruses (Power 1990).
Rates of arthropod emigration from a vegetatively diverse patch may be
influenced by searching behavior. Insects which restrict their search area upon finding a
host (patch restricted searching) may be more likely to remain within a diverse area
than insects whose movement is unaffected by encountering a host ("uniform searching)
(Stanton 1983). Highly mobile insects may leave a diverse area after encountering a
number of non-hosts in succession. Bach (1980a, 1980b) and Risch (1980, 1981) found
that leaf beetles emigrated more quickly from patches of hosts mixed with non-hosts than
from pure stands, and were able to show that increased emigration was responsible for
lower beetle densities in polyculture compared to monoculture. Being weak fliers,
aphids, whiteflies, and thrips (the aerial plankton group) may simply move short
distances from plant to plant until they find acceptable hosts. This passive method of
searching may cause such insects to accumulate in higher densities on hosts in
polyculture, if these hosts are planted at a lower density than in monoculture.
Roots (1973) hypothesis that crop diversity would tend to reduce densities of
monophagous herbivores rather than polyphagous ones is supported by the preceding
summary, and by reviews of the intercropping literature (Andow 1991a, Risch et al.
1983). Andow (1991a) found that 28% of polyphagous herbivores studied had lower
densities in polyculture, while 40% had higher densities. Only 8% of monophagous
herbivores had higher densities in polyculture, while 59% had lower densities.


CHAPTER 5
A COMPARISON OF SOME ARTHROPOD GROUPS ON MONOCROPPED AND
INTERCROPPED TOMATO (LYCOPERSICONESCULENTUM MILL.) IN BAJA
VERAPAZ, GUATEMALA
Introduction
Intercropping is the agronomic practice of growing two or more crops
simultaneously in the same field (Andrews and Kassam 1976). Intercropping and other
forms of polyculture have been associated with reduced pest damage in some cropping
systems (Andow 1991a). Root (1973) proposed two theories to explain why herbivore
damage may be reduced by polyculture. The resource concentration hypothesis states
that the ability of specialist herbivores to exploit a crop is reduced when that crop is
mixed with non-hosts. Polyphagous herbivores may also be affected if intercropping with
less attractive or non-hosts dilutes the time and energy invested in searching for
acceptable main crops (Trenbath 1976). The enemies hypothesis suggests that the
varied habitats and resources associated with polyculture may provide a stable supply of
hosts and prey for natural enemies. Populations of beneficial insects might therefore be
more stable in diverse cropping systems than in monoculture, enabling parasitoids and
predators to reduce herbivore populations on main crops before they become
economically damaging (Root 1973). Crops which support populations of natural
enemies do so by offering alternative prey or hosts, or energy sources such as pollen or
extra-floral nectaries (Sheehan 1986).
112


29
the mulch (0.07 0.18) and bean (0.05 0.16) treatments. Parasitism was much higher
in 1997. Parasitized nymphs were observed in all treatments beginning with the third
week in 1997 (Table 2-7). During the sixth week of sampling, mean parasitism in the
bean alone treatment was 262% greater than in the mulch treatment.
Red-eved nymphs. Red-eyed nymphs were not observed in 1995. Red-eyed
nymphs were observed sporadically in 1996. During the fourth week of sampling that
year, densities of red-eyed nymphs were significantly higher in the mulch treatment (0.29
0.38) than in the squash treatment ( 0.05 0.16; p < 0.05). Densities were intermediate
in the bean (0.24 0.50) and squash/mulch (0.14 0.29) treatments. Red-eyed nymphs
were present in all treatments from the third week of sampling in 1997 until the final
week of sampling (Table 2-8). There were no significant differences in densities of red
eyed nymphs among treatments.
Stratum Comparisons
On bean, there were significant differences in density between strata only during
1996, when eggs tended to be higher in the upper stratum (Table 2-2). Nymphs in the
same year tended to be higher in the lower stratum (Table 2-5). No parasitized nymphs or
red-eyed nymphs were recorded in 1995, probably because of the early freeze. In the
following two years low densities of parasitized or red-eyed nymphs were observed
primarily in the lower stratum.
On squash, egg densities tended to be highest on younger leaves early in the
season and to shift to predominance in older leaves in the last few weeks of sampling
(Table 2-9). Nymphs were found primarily on the older leaves each year (Table 2-10).
Yield


BIOGRAPHICAL SKETCH
Hugh Smith was bom February 12, 1963, in Boston, Massachusetts. He was
educated at Michael Hall, a Rudolf Steiner school in southern England, and received a
bachelors degree in classics from Brown University in 1985. Hugh received a masters
degree in entomology in 1993 from the University of Florida working under Dr. John
Capinera on the biological control of Diaphania spp. He started the Ph.D. program in
entomology at the University of Florida in 1995.
163


92
Leaf beetles typically build up on field corn, then move on to young beans as the corn
senesces in the first months of the rainy season. Incidence of the virus was high among
experimental plants. Leaf necrosis and deformation from severe mosaic of bean masked
symptoms of bean golden mosaic, preventing an estimate of presence of bean golden
mosaic at the end of the season.
Mosaic Study
Egg counts were lower (p < 0.05) on intercropped than monocropped bean during
the first four weeks of sampling (Table 4-5). Nymph counts were lower (p < 0.05) on
intercropped than monocropped bean on weeks 2, 4, and 5.
Lower numbers of eggs and nymphs among intercrop bean early in the study may
be attributed to the emergence of intercrop plants into a cryptic environment. However,
bean size and health were affected by shading from corn and rosa de jamaica soon after
emergence, and the overall plant area available for colonization was presumably less than
in the monocrop treatment by week 3. From weeks 3-6, intercrop bean was stunted
compared to monocrop bean, and whitefly densities were correspondingly lower.
A few Encarsia pergandiella individuals and one member of the Encarsia
meritoria species complex were reared from bean in the mosaic experiment. There were
no treatment differences among numbers of parasitized nymphs (week 4: 0.88
2.03/plant, week 5: 0.88 2.03, week 6: 1.44 2.66), fourth-instar T. vaporariorum
(week 4: 0.13 0.71, week 5: 0.88 1.71, week 6: 4.19 5.76), or fourth-instar B. tabaci
(week 5: 0.06 0.25, week 6: 0.06 0.25). During week 5, B. tabaci fourth-instars
comprised 7% of fourth-instar nymphs on bean. During week six, 1.5% of fourth-instar
nymphs on bean were B. tabaci. Number of plants per row averaged 5.15 2.49 in the


1
Squash
0.07 0.20
0
Squash/mulch
0.02 0.05
0
2
Squash
0.04 0.10
24.91 29.94
Squash/mulch
0.04 0.10
12.46 11.98
3
Squash
105.80 247.56
17.57 32.49
Squash/mulch
20.45 42.59
5.64 15.96
4
Squash
29.96 36.44
0
Squash/mulch
11.02 18.09
0
5
Squash
6.70 7.44
0
Squash/mulch
1.59 2.56
0
6
Squash
38.96 49.21
5.20 11.15
Squash/mulch
29.48 37.37
3.50 8.43
# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05.


Table 3-4. Whitefly adults (mean SD) per trap under 3 cropping systems, August 1997.
ON
ON
Bean Alone Com: Barrier to Wind Corn: Open to Wind
Date
Row
Downwind
Upwind
Downwind
Upwind
Downwind
Upwind
Release 11
Aug. 8
1
1.672.25
0.330.52
2.33.03
0.330.52
2.331.21
0.500.84
2
1.331.97
0
1.000.63
0
0.330.52
0.170.41
3
0.670.52
0
1.33.51
0
0.170.41
0.170.41
4
0.500.55
0.330.52
0.330.52
0
0.670.82
0
5
0.330.52
0
0.170.41
0.160.41
0.67.03
0
x3
0.901.40*
0.130.35*
1.03.16*
0.100.31*
0.83.12*
0.170.46*
Aug. 9
1
2.00.79
0.170.41
1.170.75
0
1.83.47
0.330.52
2
1.670.82
0.330.52
1.000.89
0.170.41
1.17.17
0
3
1.501.22
0
0.830.98
0
0.671.21
0
4
0.500.55
0
0.500.84
0
1.000.89
0
5
0.500.84
0.170.41
0.670.82
0
0.500.84
0
X
1.231.22*a2
0.130.35*
0.830.83*b
0.030.18*
1.031.16*ab
0.070.25*
Release 2'
Aug. 24
1
3.002.00
0.330.52
2.833.25
0
3.502.17
0.170.41
2
1.671.21
0.170.41
1.501.05
0.170.41
2.50.05
0.170.41
3
0.830.75
0.170.41
0.500.84
0
1.83.33
0
4
0.500.55
0.170.41
0.170.41
0
1.170.41
0
5
0.330.52
0.170.41
0
0
1.33.21
0
X
1.271.46*b
0.200.41 *
1.001.82*b
0.030.18*
2.101.52*a
0.070.25*


28
and nymph densities tended to be highest on bean alone. However no treatment showed a
clear advantage over bean alone in reducing densities of eggs or nymphs.
Eggs. While egg densities tended to be lowest in the two treatments containing
squash in 1995 (Table 2-1), these densities were significantly lower than those in bean
alone only during the second week of sampling. Egg densities tended to be highest in the
reflective mulch treatment in 1995, though mean egg counts in the reflective mulch
treatment were never significantly different from those on bean alone.
Egg counts in the reflective mulch treatment were 25% lower than bean alone
during week 1 in 1996 (Table 2-2), and 32% lower than bean alone during the first week
of sampling in 1997 (Table 2-3). There were no significant differences in egg counts
among treatments during the subsequent five weeks of sampling in 1996 or 1997.
Nymphs. In 1995, there were differences in nymph densities among treatments
only during the second week of sampling, when nymph densities in the mulch-and-squash
treatment were significantly lower than in bean alone (Table 2-4). Nymph densities
tended to be lowest in the mulch treatment when nymphs first appeared in 1996 and 1997
(Tables 5-6). However on the fifth week of sampling in 1996, nymph counts were
fourteen times higher in the mulch treatment than in the squash treatment, and twelve
times higher than in bean alone (Table 2-5). On the fifth week of sampling in 1997,
nymph counts were significantly higher in the mulch treatment than in the three other
treatments (Table 2-6).
Parasitized nymphs. No parasitism was recorded in 1995. Little parasitism was
observed in 1996, and was observed only in the lower stratum. During the final week of
sampling in 1996, parasitism was significantly higher in the squash treatment (0.12
0.23) than in the squash/mulch treatment (0; p < 0.05). Parasitism was intermediate in


116
A 1,0-m x 0.75 m plastic sheet (Olefinas, S.A, Guatemala) was spread out on a
wooden board at the base of the crop row. The plants were struck manually four times to
dislodge arthropods toward the sheet, which was then folded quickly into a ball and
sealed with masking tape. The samples were first refrigerated, then transported to the
Universidad del Valle in Guatemala City.
At the university, the samples were frozen to kill all arthropods, then opened for
identification. Arthropods were grouped as spiders, insect predators, hemipteran
herbivores, and Coleptera. Spiders were preserved in 80% ethanol and mailed to the
Division of Plant Industry, Gainesville, FL, where they were classified to family or genus.
Most insects were classified to family. Pentatomids were grouped as phytophagous or
predaceous based on buccal morphology (Slater and Baranowski 1978).
Statistical Analysis
Analysis of variance for a split-plot design (SAS 1996) was used to compare total
number of spiders, hemipteran herbivores, and Coleptera among tomato treatments.
Tukeys studentized range mean separation procedure was used to distinguish among
subplot treatments, when appropriate. Analysis of variance for randomized complete
block was used to compare total number of spiders, insect predators, hemipteran
herbivores, and Coleptera among cilantro, rosa de jamaica, velvetbean, and unsprayed,
intercropped tomato. Tukeys studentized range test was used to distinguish host
differences when appropriate.
Results
Insect predators consisted primarily of both adult and immature Geocoris
(Lygaeidae), assassin bugs (Reduviidae), and ladybird beetles (Coccinellidae) (Table 5-1).
The most common spiders were Misumenops sp. (Thomisidae), members of the
Oxyopidae, and Pardosa sp. (Lycosidae) (Table 5-2). Herbivorous hemiptera consisted


Table 4-8. Height and weight of tomato plants monocropped and intercropped with field corn, with and without imidacloprid.
Wk
Pesticide
Height (cm)
Weight (g)
Monocrop
Intercrop
mean
Monocrop
Intercrop
mean
2
Imidacloprid
21.914.94a'
21.22i7.21
21.56i6.09
.
_
Untreated
16.295.39b
24.69i6.66
21.09i7.38
-
-
-
mean
19.505.78
22.95i7.05
-
-
3
Imidacloprid
29.0310.29
33.06i7.06
31.05i8.92a
20.632.16
17.97i8.17
19.29 0.28a
Untreated
21.00i7.35
24.44i7.26
22.96i7.37b
5.46i4.16
6.94i6.28
6.30i5.43 b
mean
25.599.86
28.75i8.29
14.13il2.15
12.45i9.09
4
Imidacloprid
40.387.78
49.56i8.84
44.97i9.34a
31.06i9.34
48.06i23.18
39.56il9.19a
Untreated
26.585.16
35.00i9.83
31.39i8.99b
14.38i9.29
21.50i 14.11
18.45il2.39b
mean
34.46i9.64
42.28ill.75*2
23.91il2.39
34.78i23.06*
5
Imidacloprid
65.00i9.81
68.00i7.65
66.50i8.74a
125.36i62.63
127.50i36.82
126.48i49.62a
Untreated
55.89i8.48
57.92i7.33
57.05i7.70b
88.11i31.45
85.33i25.39
86.52i27.43 b
mean
61.090.15
62.96i8.96
108.60i53.32
106.42i37.69
'Means in the same column for a given week followed by a different letter are significantly different (p < 0.05) according to
analysis of variance. The absence of letters indicates no treatment differences (p > 0.1).
2* Intercrop mean is significantly different (p < 0.05) from corresponding monocrop mean according analysis of variance.
o


157
Rufino, J. 1998. Eficaz y funcional produccin de plantas de tomate. Agricultura
(Guatemala), p. 59-61.
Russell, E. P. 1989. Enemies hypothesis: a review of the effect of the vegetational
diversity on predatory insects and parasitoids. Environ. Entomol. 18: 590-599.
Russell, L. M. 1963. Hosts and distribution of five species of Trialeurodes. Ann. Ent.
Soc. Am. 56: 149-153.
Russell, L. M. 1977. Hosts and distribution of the greenhouse whitefly, Trialeurodes
vaporariorum. Cooperative Plant Pest Report 2, U. S. Department of Agriculture.
Washington, DC.
Salguero, V. E. 1993. Manejo de mosca blanca y acolochamiento en tomate. Ministerio
de Agricultura, Ganadera y Alimentacin, Guatemala City, Guatemala.
SAS Institute Inc. 1996. SAS/STAT Software: changes and enhancements through
release 6.11, SAS Institute, Cary, NC.
Scheiber, B. 1983. Principales enfermedades de frijol en Guatemala. Reunin del
Proyecto Cooperativo Centramericano de Mejoramiento de Frijol. 2a Reunin,
San Salvador, El Salvador.
Schultz, B. B. 1988. Reduced oviposition by green lacewings on cotton intercropped
with com, beans, or weeds in Nicaragua. Environ. Entomol. 17: 229-232.
Schuster, D. J. 1998. Intraplant distribution of immature lifestages of Bemisia
argentifolii (Homoptera: Aleyrodidae) on tomato. Environ. Entomol. 27: 1-9.
Schuster, D. J., and J. B. Kring. 1988. Management of insects on tomato with UV-
reflective mulches, Pages 5-19 in Report of tomato research supported by the
Florida Tomato Committee 1987-1988. IFAS, University of Florida, Gainesville,
FL.
Schuster, D. J., J. F. Price, J. B. Kring, and P. H. Everett. 1989. Integrated
management of the sweetpotato whitefly on commercial tomato. Citrus and
Vegetable Magazine (December): 11-12, 69-70, 72-75.
Schuster, D. J., P. A. Stansly, D. E. Dean, and J. E. Polston. 1996. Potential of
companion plantings for managing silverleaf whitefly and tomato mottle
geminivirus on tomato. Page 168 in T. J. Henneberry, N. C. Toscano, R. M.
Faust, and J. R. Coppedge, eds. Silverleaf whitefly: 1996 supplement to the five-
year national research and action plan. ARS 1996-01. United States Department
of Agriculture, Washington, DC.


Table 5-1. Insect predators recovered from samples collected on four crops, Baja
Verapaz, Guatemala.
119
Family
Individuals
Percent
Crops
Geocoris
46
38.3
Cilantro, rosa de jamaica, tomato,
(Lygaeidae)
velvetbean
Reduviidae
27
22.5
Cilantro, rosa de jamaica, tomato,
velvetbean
Coccinellidae
18
15.0
Cilantro, rosa de jamaica, tomato,
velvetbean
Cicindellidae
16
13.3
Velvetbean
Syrphidae
9
7.5
Cilantro, tomato,
Chrysopidae
2
1.7
Rosa de jamaica, tomato
Pentatomidae
2
1.7
Tomato, velvetbean
'Four times as many beat cloth samples were taken from tomato (48) as from each of the
other crops (12).
Table 5-2. Spiders recovered from samples collected on four crops, Baja Verapaz,
Guatemala
Family2
Genus
Individuals
Percent
Crops'
Thomisidae
Misumenops
34
31.2
Cilantro, tomato, velvetbean
Lycosidae
Pardosa
18
16.5
Cilantro, tomato, velvetbean
Oxyopidae
spp.
11
10.1
Tomato
Oxyopes
11
10.1
Cilantro, rosa de jamaica,
tomato
Tetragnathidae
Tetragnatha
8
7.3
Tomato
Theridiidae
Theridion
7
6.4
Rosa de jamaica, tomato,
velvetbean
Salticidae
6
5.5
Cilantro, tomato
Oxyopidae
Peucetia
4
3.6
Tomato
Araneidae
3
2.75
Cilantro, tomato
Philodromidae
2
1.8
Cilantro, tomato
Theridiidae
Coleosoma
2
1.8
Tomato
Corinnidae
Meriola
1
0.9
Velvetbean
Dictynidae
1
0.9
Tomato
Liniphiidae
Florenda
1
0.9
Tomato
'Four times as many beat cloth samples were taken from tomato (48) as from each of the
other crops (12).


CHAPTER 2
THE EFFECT OF SILVER REFLECTIVE MULCH AND A SUMMER SQUASH
0CUCURBITA PEPO L.) TRAP CROP ON DENSITIES OF IMMATURE BEMISIA
ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE) ON ORGANIC BEAN
(.PHASEOLUS VULGARISE.)
Introduction
Bemisia argentifolii Bellows & Perring, the silverleaf whitefly (also known as
Bemisia tabaci (Gennadius) strain B), has become a serious pest of horticultural and
agronomic crops throughout warm regions of the world (Brown et al. 1995). Since the
mid 1980s, the Florida vegetable industry has lost millions of dollars due to a variety of
diseases and disorders associated with B. argentifolii (Norman et al. 1993). These
include the tomato mottle and bean golden mosaic geminiviruses (Hiebert et al. 1996), as
well as irregular ripening of tomato and squash silverleaf (Maynard and Cantliffe 1989).
Bemisia has developed resistance to most classes of pesticides (Denholm et al. 1996,
Dittrich et al. 1990), forcing conventional growers to seek non-chemical alternatives to
Bemisia management. Synthetic pesticides are not an option for organic growers, who
face special challenges in the management of virus vectors.
The present study was undertaken to assess the efficacy of reflective plastic mulch
and yellow summer squash (Cucrbita pepo L.) as a trap crop for management of B.
argentifolii on snap bean (Phaseolus vulgaris L.) on an organic farm in north central
Florida. Florida is the foremost producer of snap bean in the United States (National
Agricultural Statistics Service 1998). In 1995, revenue from fresh market snap bean in
Florida exceeded $50 million (Florida Statistical Abstract 1996). Plastic mulches which
23


12
of high-value, labor-intensive fruits and vegetables in places like the United States
(Capinera et al. 1985, Risch et al. 1983).
Among the agronomic benefits attributed to some systems of polyculture is the
reduction of damage from arthropod pests (Altieri 1994, Altieri and Letourneau 1982,
Andow 1983, 1991a, Kass 1978. Litsinger and Moody 1976. Perrin 1977, Perrin and
Phillips 1978, Risch et al. 1983, Vandermeer 1989). This phenomenon was first
discussed extensively in Western scientific literature in the earlier part of the twentieth
century, based on observations of pest behavior in temperate and tropical silvicultural
systems (reviewed by Andow 1983. and Pimentel 1961). Additional information came
from traditional systems of polyculture in the tropics. Working in India, Aiyer (1949)
proposed three ways intercropping might reduce pest damage: 1) individual plants might
be more difficult to find because they are usually more dispersed in intercropped systems;
2) certain species might serve as trap crops, diverting pests from other crops; and 3) some
crops might have a repellent effect on herbivores.
Elton (1927, 1958) suggested that the ability of natural enemies to suppress
herbivores in naturally diverse agroecosystems was lost in simple systems, and promoted
the idea that diverse systems should be more stable than simple ones. Diverse
environments would offer a greater variety of habitats and victims to natural enemies
(Huffaker 1958), as well as alternate food sources such as pollen and nectar (van Emden
1963, 1965), enabling natural enemies to suppress herbivore populations more efficiently
than in simple environments. Drawing on Elton, Nicholson (1933), and his own work
with pests of Brassica olercea L. in simple and diverse systems, Pimentel (1961) refined
this idea. He proposed that the varied but limited resources of diversified cropping


71
Attempts to manage whiteflies with intercropping have produced variable results.
Al Musa (1982) and Schuster et al. (1996) reduced Bemisia-vectored geminivirus on
tomato by trap cropping with cucumber (Cucmis sativus L.) and squash (Cucrbita pepo
L.), respectively. However, efforts to reduce whitefly densities with trap crops have
generally been unsuccessful (Ellsworth et al. 1994, McAuslane et al. 1995, Puri 1996).
Barrier crops have been used to reduce densities of Bemisia (Morales et al. 1993) and
incidence of whitefly-transmitted virus on cowpea (Vigna unguiculata (L.) Walp.)
(Sharma and Varma 1984) and soybean (Glycine max (L.) Merrill) (Rataul et al. 1989).
Gold et al. (1990) found that densities of immature cassava whiteflies Aleurotrachelus
socialis Bondar and Trialeurodes variabilis (Quaintaince) were lower on cassava
(.Manihot esculent a Crantz) intercropped with cowpea than on monocropped cassava, but
attributed this in part to reduced host quality in intercropped treatments. Ahohuendo and
Sarkar (1995) reduced density of B. tabaci and incidence of cassava virus on cassava by
intercropping with maize (Zea mays L.) and cowpea. Fargette and Farquet (1988) found
that densities of B. tabaci and virus incidence were sometimes higher on cassava
intercropped with maize than on cassava grown alone.
The origin of the whitefly problem in Central America is associated with the
dense populations that developed on large-scale cotton (Gossypium hirsutum L.)
plantations along the regions Pacific coastal plain in the 1960s (Dardn 1992). Bean
golden mosaic geminivirus, vectored by B. tabaci (Costa 1975), was first described in
Guatemala in 1963 (Scheiber 1983). After peaking in the late 1970s, bean golden mosaic
declined in importance until 1989, when it decimated bean crops throughout Central
America (Rodriquez 1994). Devastating whitefly-transmitted geminiviruses spread


Table 2-9. Egg density (mean SD/cnr) of B. argentifolii by stratum on squash.
Year
Week
Treatment
Lower stratum
1995
2
Squash
Squash/mulch
1.00 1.67@
0.14 0.28
3
Squash
Squash/mulch
2.15 2.20@
0.83 0.81#
4
Squash
Squash/mulch
0.92 0.92#
0.60 0.74#
1996
1
Squash
Squash/mulch
0.96 0.72
0.07 0.21
2
Squash
Squash/mulch
0.02 0.08#
0.14 0.33#
3
Squash
Squash/mulch
1.98 2.11
2.36 2.21
4
Squash
Squash/mulch
2.07 4.23#
1.17 1.43@
5
Squash
Squash/mulch
0.31 0.67
0.36 0.70
6
Squash
Squash/mulch
0.64 0.72
1.50 2.45
Upper stratum
4.61 4.72@
3.05 3.40
0.33 0.48@
0.16 0.34#
0.10 0.20#
0.05 0.14#
10.36 10.35#
9.91 9.56#
5.35 . 8.41
10.33 9.52
7.93 7.88#
6.36 7.02@
0.72 0.92
0.91 1.07
0.67 0.84
1.10 1.15


120
Table 5-3. Hemipteran herbivores recovered from samples collected on four crops, Baja
Verapaz, Guatemala.
Family Individuals Percent Crops'
Miridae
657
92.0
Cilantro, rosa de jamaica, tomato,
velvetbean
Pentatomidae
19
2.7
Cilantro, tomato, velvetbean
Lygaeidae
14
2.0
Tomato, velvetbean
Pyrrhocoridae
13
1.8
Rosa de jamaica, velvetbean
Largidae
7
0.9
Cilantro, tomato
Coreidae
5
0.7
Tomato
'Four times as many beat cloth samples were taken from tomato (48) as from each of the
other crops (12).
Table 5-4. Beetles recovered from beat cloth samples collected on four crops, Baja
Verapaz, Guatemala.
Families
Individuals
Percent
Crops'
Chrysomelidae
49
43.4
Cilantro, rosa de jamaica, tomato,
velvetbean
Elateridae
28
24.8
Rosa de jamaica, velvetbean
Anthicidae
8
7.1
Cilantro, rosa de jamaica
Meloidae
7
6.2
Cilantro, velvetbean
Erotylidae
4
3.5
Rosa de jamaica, tomato, velvetbean
Nitidulidae
3
2.7
Cilantro, rosa de jamaica
Cleridae
3
2.7
Cilantro
Lampyridae
2
1.8
Tomato
7
2
1.8
Cilantro
?
2
1.8
Cilantro
Staphylinidae
1
0.9
Tomato
Tenebrionidae
1
0.9
Velvetbean
Histeridae
1
0.9
Cilantro
Mordellidae
1
0.9
Cilantro
?
1
0.9
Cilantro
'Four times as many beat cloth samples were taken from tomato (48) as from each of the
other crops (12).


Table 2-10. Nymph density (mean SD/cm2) on B. argentifolii by stratum on squash.
Year
Week
Treatment
Lower stratum Upper stratum
2
Squash
0.13 0.58
0.19 0.93
Squash/mulch
0
0
3
Squash
0.39 1.04
0.01 0.06
Squash/mulch
0.13 0.37
0
4
Squash
0.26 0.78
0
Squash/mulch
0.64 1.41
0
2
Squash
0
0
Squash/mulch
0
0.02 0.08
3
Squash
2.02 3.13#
0#
Squash/mulch
1.26 1.69#
0#
4
Squash
0.24 0.83
0
Squash/mulch
0.45 0.84
0.05 0.16
5
Squash
0.74 1.68
0
Squash/mulch
0.83 1.64
0
6
Squash
0.45 1.48
0.10 0.19
Squash/mulch
0.31 0.04
0


6
Bean 0.04 0.07 0.88 0.74 0.46 0.67
Mulch 0.02 0.05 1.80 3.14 0.91 2.34
Squash 0 0.82 1.25 0.41 0.96 17.05 16.74**
Squash/mulch 0.07 0.20 0,91 0.60 0.49 0.61 20.24 18.00**
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.
OO


97
Weather patterns in Guatemala were disrupted in 1998 by El Nio as they were
throughout the world. Precipitation in the Salam valley is typically highest in
September, and declines throughout October. In 1998, precipitation was sporadic in
September, and unseasonably heavy in October. Between 18 October, when bean was
first sampled, and 30 October, 39.8 mm precipitation was recorded at the San Jernimo
weather station. During 31 October and 1 November, as Hurricane Mitch passed through
the region to the south, 141 mm rain fell. From 2 November through 6 December, when
the final sample was taken, 44.9 mm precipitation fell (Instituto Nacional de Sismologa,
Vulcanologa, Meteorologa e Hidrologa, San Jernimo station).
The canopy provided by intercrop treatments in the rainy season may have offered
some shelter to whitefly adults during the rainy season. This may have contributed to the
higher whitefly levels observed on intercrop tomato in the mosaic and com/cilantro
studies on some dates.
Imidacloprid provided early season protection against whiteflies and other sucking
insects which was essential for growing bean in the dry season. Rainy-season tomato
treated with imidacloprid was more robust and had lower whitefly densities than
untreated tomato. The detergent and oil rotation used on bean in the dry season caused
phytotoxicity, but offered no protection from whiteflies and other sucking insects.
The failure of intercropping to reduce numbers of Trialeurodes and Bemisia may
be related to their wide host range, their host-finding mechanisms, and their mobility.
The disruptive crop hypothesis (Vandermeer 1989) is generally applied to specialist or
oligophagous herbivores rather than polyphages like Trialeurodes and Bemisia (Andow
1991a, Power 1990). Andow (1991a) reports that population densities of polyphagous


21
Research Objectives
The objective of the following research was to determine if intercropping could be
used to reduce densities of immature whiteflies compared to densities on crops grown in
monoculture. Intercropping studies were designed to test the reduction of whitefly
densities on bean and tomato. It was hoped that results from these crops would apply to
other economically-important crops. Population densities, and in some cases yield, were
measured to estimate whitefly incidence and damage under simple and mixed systems,
although damage was not measured directly.
Summarizing the literature, Vandermeer (1989) proposed three all-encompassing
hypotheses to explain how intercropping might reduce pest damage (trap crops, disruptive
crops, and increased natural enemies). The following field experiments focused on
testing two of these hypotheses, the trap crop hypothesis and the disruptive crop
hypothesis. The first set of experiments, carried out on an organic farm near Gainesville,
examined squash as a trap crop. The second set of experiments, carried out on a
University of Florida agricultural research farm near Gainesville, tested eggplant as a trap
crop and field corn as a barrier crop. The final set of experiments tested the potentially
disruptive effect of intercropping bean or tomato with poor or non-hosts of whitefly.
These last studies took place at a government agricultural research station in central
Guatemala. Data were gathered on parasitism in most of these studies, and on predators
in a few studies, but only the final experiment in Guatemala attempted to test the third
intercropping hypothesis, the enemies hypothesis, by intercropping with cilantro to
augment densities of generalist predators.


151
Kogan, M. 1996. Areawide management of major pests: is the concept applicable to the
Bemisia complex? Pages 643-657 in D. Gerling and R. Mayer, eds. Bemisia
1995: taxonomy, biology, damage, control, and management. Intercept Ltd,
Andover, Hants, UK.
Krug, I. 1993. Sistemas de produccin sustenables para Baja Verapaz. Programmade
Desarrollo Regional las Verapaces. Salam, Guatemala.
Laska, P., J. Betlach, and M. Harankoa. 1986. Variable resistance in sweet pepper to the
glasshouse whitefly. Acta Entomolgica Bohemsloaca 83: 347-353.
Legaspi, J. C., R. I. Carruthers, D. A. Nordlund, J. A. Correa, A. C. Cohen, D. Holbrook
and cooperators. 1994. Effect of inundative releases of the predator, Chrysoperla
rufilabris, to control sweetpotato whitefly in an organic field crop. Page 139 in T.
J. Henneberry, N. C. Toscano, R. M. Faust, and J. R. Coppedge, eds. Silverleaf
whitefly: 1994 supplement to the 5-year national research and action plan.
Agriculture Research Service No. 125, U. S. Department of Agriculture,
Washington, DC.
Lei, H., W. F. Tjallingii, and J. C. van Lenteren. 1998. Probing and feeding
characteristics of the greenhosue whitefly in association with host-plant
acceptance and whitefly strains. Ent. Exp. Appl. 88: 73-80.
Letourneau, D. K. 1987. The enemies hypothesis: tritrophic interaction and vegetational
diversity in tropical agroecosystems. Ecology 68: 1616-1622.
Litsinger, J. A., and K. Moody. 1976. Integrated pest management in multiple cropping
systems. Pages 293-316 in R. I. Papendick, P. A. Sanchez, and G. B. Triplett, eds.
Multiple cropping. American Society of Agronomy Special Publication 27,
Madison, WI.
Liu, H. Y., J. E. Duffus, and S. Cohen. 1993. Page 35 in T. J. Henneberry, N. C.
Toscano, R. M. Faust, and J. R. Coppedge, eds. Sweetpotato whitefly: 1993
supplement to the five-year national research and action plan. Agricultural
Research Service No. 112, U. S. Department of Agriculture, Washington, DC.
Lloyd, L. 1922. The control of the greenhouse whitefly with notes on its biology. Ann.
Appl. Bio. 9: 1-31.
Lynch, R. E., and A. M. Simmons. 1993. Distribution of immatures and monitoring
adult sweetpotato whitefly, Bemisia tabaci, in peanut. Environ. Entomol. 22:
375-380.
Martin, N. A., R. D. Ball, L. P. J. J. Noldus, and J. C. van Lenteren. 1991. Distribution
of greenhouse whitefly Trialeurodes vaporariorum and Encarsia formosa in a
greenhouse tomato crop: implications for sampling. New Z. J. Crop and Hort.
Science 19: 283-290.


B. argentifolii alone and in combination with the squash trap crop in Florida, and two
pesticides were tested as subplot treatments in two intercropping studies in Guatemala.
Counts from yellow sticky traps in the barrier test in Florida indicated that wind
direction was the primary factor determining movement of adult B. argentifolii, and that
the presence of a com barrier only marginally affected the penetration of adults into test
plots. None of the intercropping treatments consistently reduced densities of immature
whiteflies compared to densities on crops grown in monoculture. Some intercropping
treatments in the Guatemala studies reduced plant quality, making it difficult to interpret
results. The reflective aluminum mulch treatment significantly reduced egg counts during
the first week of sampling in two out of three years in Florida. Imidacloprid protected
bean from damage by whiteflies and other sucking insects during the dry season in
Guatemala, and reduced densities of immature whiteflies on tomato during the rainy
season. A detergent and oil spray rotation did not protect bean from whitefly or other
sucking insects during the dry season. Combining aluminum mulch or imidacloprid with
intercropping treatments did not provide any additional advantage over using them alone.
The lack of effect of intercropping on whitefly counts is discussed in relation to whitefly
host-finding mechanisms and mobility. Methods for sampling immature stages of
whiteflies on common bean are compared to determine the preferred sample unit and
location within the plant canopy for sampling.
vni


142
Location
Host
Whitefly species1
Population levels
Salam Valley,
Cajanus cajan
T vaporariorum
Moderate-high
Baja Verapaz
Canavalia sp.
?
Very low2
Elevation: 1000 m
Mucuna deeringiana
?
Very low2
Phaseolus vulgaris3
T. vaporariorum
B. tabaci
High
Low
Lycopersicon
T. vaporariorum
High
esculentum
B. tabaci
Low
Citrullus vulgaris
T. vaporariorum
Moderate
Cucrbita pepo
T. vaporariorum
High
Cucumis sativus
T. vaporariorum
High
Sechium edule
T. vaporariorum
High
Brassica olercea4
T. vaporariorum
Very low
Hibiscus sabdariffa
B. tabaci
Very low
Sanarate,
Phaseolus vulgaris
T. vaporariorum
High
El Progreso
Cucumis sativus
T. vaporariorum
High
Elevation: 750-850 m
Lycopersicon
esculentum
T. vaporariorum
High
Rio Hato,
Cucumis sativus
Bemisia argentifolii
High
El Progreso
Elevation: 300 m
Lycopersicon
esculentum
Bemisia argentifolii
High
'Dr. Andrew Jensen, formerly of the USDA, Beltsville. MD. identified T. vaporariorum
across elevations and Bemisia from Rio Hato. Dr. Avas Hamon of the Division of Plant
Industry, Gainesville, FL, identified Bemisia and additional T. vaporariorum from the
Salam valley. T. vaporariorum on C. cajan identified by H. A. Smith
2Only a few eggs and early instar nymphs found on young leaves.
The predominant whitefly species in the Salam valley is Trialeurodes vaporariorum.
Whitefly populations were highest at the end of the dry season (May), lowest early in the
rainy season (June-September), and increasing at the end of the rainy season (October-
November) in 1998. Observations of B. tabaci were low throughout the year. Based on
identifications of fourth-instar nymphs carried out by H. A. Smith. B. tabaci on bean was
estimated at 11% of the total population at the end of the dry season, 46% in the middle
of the rainy season, when overall populations were at their lowest, and 0.5% on tomato at
the end of the rainy season.
4cabbage


15
characterize arthropod behavior. Kareiva (1983) states the need for research that
identifies species specific traits... that govern the responses of herbivores to vegetation
texture. The ability of an herbivore to colonize a given cropping system, diverse or
simple, will depend largely on the range of its diet, the nature and sophistication of its
host-finding mechanisms, and its relative mobility (Kareiva 1983, Stanton 1983). The
same holds true for plant disease vectors (Power 1990) and natural enemies (Russell
1989, Sheehan 1986). The specific nature of vegetative diversity will also determine
arthropod response. Vegetative texture can vary in terms of density, patch size, spatial
array, and temporal overlap (Andow 1991a, Kareiva 1983), as well as the ratio of host to
non-host plants, which will have a greater influence on herbivore abundance than the
actual number of plant species (Power 1990, Stanton 1983).
Vegetative diversity can affect arthropod damage and densities by influencing the
rate at which an arthropod immigrates into a cropped area, its population dynamics once
it has entered, and the rate at which it emigrates from the area (Stanton 1983). The extent
to which immigration can be influenced depends on the host-finding mechanisms and
mobility of the arthropod. Intercropping with certain crops may interfere with the
olfactory cues certain insects rely on for host-finding (Perrin 1977, Stanton 1983). For
instance, Tahvanainen and Root (1972) demonstrated that tomato volatiles interfered with
host-finding by Phyllotreta cruciferae Goeze, a flea beetle, and led to reduced oviposition
on collards. Interference with visual host-finding cues has been suggested (Perrin 1977,
Stanton 1983). However, most examples of manipulation of visual perception concern
increased attraction of insects such as aphids to sparsely planted crops which stand out
against bare ground (Kennedy et al.l 961, Smith 1976).


Table 4-9. Whitefly eggs and nymphs (x SD/plant) on tomato monocropped and intercropped with corn, with and without
imidacloprid.
Wk Pesticide
Egg
Nymph
Monocrop
Intercrop
mean
Monocrop
Intercrop
Mean
1
Imidacloprid
10.569.81
8.19 11.78
9.380.73 a2
0.13i0.34
0
0.06i0.25
Untreated'
95.0895.76
33.1932.62
59.7172.80b
0
0
0
Mean
46.7974.86
20.6927.26
0.07i0.26
0
2
Imidacloprid
79.00i77.00
66.2559.02 a
72.6367.79
0.88.63
0.3 l0.87a
0.59.32
Untreated
139.75 174.49
601.3 l496.96b
403.50451.35
0.92.24
5.38i9.50b
3.46i7.47
Mean
105.04128.98
333.78441.67@3
0.89.45
2.84i7.12
3
Imidacloprid
188.19222.43
305.19i200.42
246.69216.56
4.13i5.35
12.134.68
8.13 1.60 a
Untreated
185.82303.86
403.88528.13
315.04456.45
66.00i93.13
142.00i234.75
111.04il91.25b
Mean
187.22253.09
354.53396.12
29.33i65.66
77.06i 167.42
4
Imidacloprid
825.63821.51
804.13497.59
814.88656.21
248.38i237.51a
134.50i 132.4 la
191.44i 194.85
Untreated
429.83441.12
1202.88844.78
871.57436.99
122.33 19.58b
491.75i415.42b
333.43i366.63
Mean
656.00692.49
1003.50 1321.40
194.36i200.16
313.13i350.36
54
Imidacloprid
66.83i220.28
1.00.59
33.92il56.00
260.67i213.49
81.25i69.40 a
170.96i 180.28
Untreated
4.758.39
0.250.87
2.05i5.6l
20I.3831.89
284.83 89.47b
251.4570.16
Mean
42.00i 170.56
0.62.31
236.95 83.53
183,04i 174.02
64
Imidacloprid
0.330.78
0
0.17i0.56 a
112.83i60.59
65.42i46.32 a
89.13i58.04
Untreated
78.78163.51
31.36 104.02
52.70i 132.43b
117.22i53.64
169.27 151.44b
145.85 18.27
Mean
33.95110.79
15.00i71.94
114.756.35
115.09 19.63
1 Untreated seedlings were produced in nurseries protected by fine mesh.
2 Means in the same column for a given week followed by a different letter are significantly different (p <0.05) according to analysis of
variance. The absence of letters indicates no treatment differences (p > 0.1).
3@ intercrop mean is significantly different (p < 0.1) from corresponding monocrop mean according to analysis of variance.
4Counts from the lower third of the plant only.


88
Incidence of Bemisia during the following two weeks was not high enough for
meaningful comparison.
Generalist predators collected from whole plant bean samples during week 4
included Geocoris sp. (Hemiptera: Lygaeidae), Coccinellidae (Coleptera), Thysanoptera.
Neuroptera, syrphid larvae (Dptera: Syrphidae), and spiders. Only Geocoris sp. was
present in sufficient quantities for statistical comparison. Levels of Geocoris sp. were
higher (p < 0.001) on imidacloprid-treated bean (0.60 0.87/plant) than on bean in the
detergent/oil treatment (0.05 0.22) and the control (0.25 0.16).
The parasitoids reared from bean and tomato in the diversity experiment were
almost entirely Encarsia pergandiella Howard (Hymenoptera: Aphelinidae), although a
few individuals from the Encarsia meritoria species complex were reared from the
second bean crop early in August. Sex ratios for E. pergandiella ranged from a low of
about 15% males in mid-May, when host and parasitoid populations were high, to 33%
males in July and August, when overall populations were low, to about 26% males in
November and December, when both populations were high again.
There were no statistical differences (p < 0.1) among treatments in levels of
parasitized nymphs during week 4 (12.33 16.79/plant). Parasitism was higher (p <
0.05) in the imidacloprid treatment than the other two treatments during week 5
(imidacloprid: 2.98 4.19/cm2, detergent/oil: 0.13 0.34, control: 0.78 1.26) and week
6 (imidacloprid: 29.50 21.23/plant, detergent/oil: 6.00 6.30, control: 4.75 6.86).
Percent parasitism, calculated as the percentage of parasitized nymphs to parasitized and
fourth-instar nymphs combined, ranged from about 33% during week four to 80% during
week 6. Parasitism and numbers of Geocoris were presumably highest on imidacloprid-


124
Populations of whitefly immature stages display high degrees of aggregation
within individual leaves and within the plant canopy (Naranjo 1996). This tends to
increase the sampling effort required to achieve acceptable population estimates for all
stages of whiteflies (van Lenteren and Noldus 1990). However, the distribution of
distinct immature stages throughout the plant canopy may permit the optimization of
sampling resources through stratified sampling plans (Cochran 1963).
Common bean (Phaseolus vulgaris L.) yields throughout Latin America and the
Caribbean Basin have been severely reduced by geminiviruses transmitted by Bemisia
(Brown and Bird 1992). Bean golden mosaic reached epidemic levels in south Florida for
the first time in 1993 (Blair et al. 1995). The development of whitefly management
programs for bean will require effective sampling methods, particularly for host plant
resistance studies (Ekbom and Rumei 1990). However there is little information
available for sampling Bemisia on bean.
The following study was carried out to develop a preliminary plan for sampling
whitefly immatures on bean. The purpose of the study was to determine the most
representative stratum from which to sample eggs, nymphs, parasitized nymphs, and red
eyed nymphs of B. argentifolii on bean, and to determine how well two distinct sample
units, a whole leaf or a disc punched from a leaf, served as predictors for whole plant
densities of these immature stages.
Materials and Methods
Research Design and Plot Management
The experiment was carried out at the University of Florida Green Acres
Agronomy Research Farm northwest of Gainesville, FL (2940'N, 8230'W). Espada


152
Martin, N. A., and J. R. Dale. 1989. Monitoring greenhouse whitefly puparia and
parasitism: a decision approach. New Z. J. Crop and Hort. Science 17: 115-124.
Maynard, D. N and D. J. Cantliffe. 1989. Squash silverleaf and tomato irregular
ripening: new vegetable disorders in Florida. Vegetable Crops Fact Sheet.
Florida Coop. Ext. Serv. VC-37. Institute of Food and Agricultural Sciences,
University of Florida, Gainesville, FL.
McAuslane, H. J. 1996. Influence of leaf pubescence on ovipositional preference of
Bemisia argentifolii on soybean. Environ. Entomol. 25: 834-841.
McAuslane, H. J., F.A. Johnson, D. L Colvin, and B. Sojack. 1995. Influence of foliar
pubescence on abundance and parasitism of Bemisia argentifolii on soybean and
peanut. Environ. Entomol. 24 : 1135-1143.
McAuslane, H. J., F. A. Johnson, and D. A. Knauft. 1994. Population levels and
parasitism of Bemisia tabaci on peanut cultivars. Environ. Entomol. 23: 1203-
1210.
McAuslane, H. J., F. A. Johnson, D. A. Knauft and D. L. Colvin. 1993. Seasonal
abundance and within-plant distribution of parasitoids of Bemisia tabaci in
peanuts. Environ. Entomol. 22: 1043-1050.
Meena, R. S., G. S. Rathore, B. S. Shekhawat, L. D. Yadav, and J. P. Agnihotri.
1984. Efficacy of sowing dates and trap crops in management of yellow mosaic
of moth (Vigna aconitifolia (Jacq.) Marechal). Indian J. Mycol. PI. Pathol. 14 :
304-309.
Moore, L., and T. F. Watson. 1990. Trap crop effectiveness in community boll weevil
control programs. J. Entomol. Sci. 25: 519-525.
Morales, F., and C. Cardona. 1998. El mosaico severo del frijol. Centro Internacional
de Agricultura Tropical, Cali, Colombia.
Morales, J. R.. D. E. Dardn, and V. E. Salguero. 1993. Parcela MIP de validacin y
transferencia en tomate. Page 130 in Manejo integrado de plagas en tomate; Fase
1: 1991-1992. Ministerio de Agricultura, Ganadera y Alimentacin, Guatemala
City, Guatemala.
Mound, L. A. 1962. Studies on the olfaction and colour sensitivity of Bemisia tabaci.
Entomol. Experiment. Appl. 5: 99-104.
Mound. L. A., and S. H. Halsey. 1978. Whitefly of the world. John Wiley and Sons,
Chichester, UK.
Murray, D. L. 1991. Export agriculture, ecological disruption, and social inequity: some
effects of pesticides in southern Honduras. Agrie. Human Values, Fall, p. 19-29.


113
Materials and Methods
In 1998, field studies were carried out in the Salam valley, a tomato-growing
region in central Guatemala, to determine if intercropping with non- and poor hosts
would reduce densities of immature whiteflies (Homoptera: Aleyrodidae) on tomato
[Lycopersicon esculentum Mill.) (see chapter 4). Three pesticide subplot treatments were
included to determine if the pesticide/intercrop combination offered an advantage over
either strategy alone. Cilantro (Coriandrum sativum L.) was included as an intercrop in
part because it was observed to support high densities of generalist predators, primarily
Geocoris spp. (Hemiptera: Lygaeidae) and Coccinellidae (Coleptera) during visits to
farms in the region. A few months into the study, predatory and parasitic Hymenoptera
and Coccinellidae were observed feeding on extra-floral nectaries at the base of the leaf
and on the corolla of rosa de jamaica (Hibiscus sabdariffa L.) (Malvaceae), another
species intercropped with tomato.
In order to determine if levels of generalist predators were higher on tomato
intercropped with cilantro, rosa de jamaica, and velvetbean (Mucuna deeringiana (Bort.)
Small) than on monocropped tomato, beat cloth samples were taken from all crops.
Location
The research was carried out at the Instituto de Ciencia y Tecnologa Agrcolas
(ICTA) field station in San Jernimo (15 03 N, 90 15' W), Baja Verapaz, Guatemala.
ICTA is the government agricultural research institute of Guatemala. The station is 1000
m above sea level. The area is classified as subtropical dry forest under the Holdridge
system (Holdridge 1967, de la Cruz 1982).


51
Materials and Methods
1996
Research design and plot management. The experiment was carried out at the
University of Florida Green Acres Agronomy Research Farm northwest of Gainesville,
FL (2940'N, 8230'W). Four treatments were compared: 1) bean planted in
monoculture, 2) bean intercropped with eggplant, 3) bean intercropped with field corn,
and 4) bean monoculture treated with imidacloprid (Provado 1.6F, Bayer, Kansas City,
MO), a systemic insecticide. The imidacloprid treatment was included for yield
comparison only. It was not sampled for whiteflies.
Crop varieties used were Espada garden bean (Harris Seed, Rochester, NY),
'Black Beauty eggplant (Ferry-Morse Seed, Fulton, KY), and the subtropical field corn
hybrid Howard HIST (Gallaher et al. 1998). Plant spacing within the row was 10 cm for
bean, 15 cm for corn and 46 cm for eggplant. Each plot contained 14 rows, 6.1 m in
length with 0.9 m between rows. Monoculture bean plots contained only beans.
Intercropped plots were planted in a 2:4:2:4:2 pattern, with corn or eggplant in the
outermost and central 2 rows, surrounding 2 four-row patches of bean. Each treatment
was replicated 5 times and arranged in a randomized complete block design.
Corn was planted 26 July and fertilized with 0.68 kg 15-0-14 (N-P205-K20) per
row. Com received 0.3 kg 15-0-14 per row on 9 August. Heavy Spodoptera frugiperda
(JE Smith) damage threatened the barrier crop treatment in August. Corn was treated
with 1.74 liter/ha methomyl (Lannate, DuPont Corp., Newark, DE) on 9 August and 29
August. Eggplant was transplanted 22 August when 3 wks old. Eggplant received 0.23
kg per row 15-0-14 fertilizer 27 August, and 0.8 kg on 27 September and 10 October.


50
plants, primarily in the family Gramineae, have been tested as barrier crops or intercrops
to reduce whitefly colonization and virus transmission among main crops. Results have
been mixed. Morales et al. (1993) reported that a sorghum {Sorghum bicolor (L.)
Moench) barrier reduced Bemisia densities, but not transmission of virus, on tomatos
{Lycopersicon esculentum Mill.). A pearl millet {Pennisetum typhoides (Burm. f.) Stapf
& Hubbard) barrier reduced whitefly virus transmission on cowpea (Vigna unguiculata
(L.) Walp.) (Sharma and Varma 1984) and soybean {Glycine max (L) Merrill) (Rataul et
al. 1989). Gold et al. (1990) found reduced densities of Aleurotrachelis socialis Bondar
and Trialeurodes variabilis (Quaintance) on cassava {Manihot esculent a Crantz)
intercropped with maize {Zea mays L.) and cowpea, but attributed this in part to reduced
host quality due to intercrop competition. Fargette and Farquet (1988), whose study
included the effect of wind direction, found densities of B. tabaci and virus incidence
were sometimes higher on cassava intercropped with maize than on monocropped
cassava.
These studies have been carried out primarily in the tropics, where safe,
inexpensive cultural control measures are a priority for low resource farmers. Extension
material from Central America promotes the use of crop barriers as a component of
whitefly management programs (Salguero 1993; Pan-American School of Agriculture
(Zamorano) poster: Reconozca y controle la mosca blanca). The present study was
undertaken in 1996 to test the usefulness of eggplant as a trap crop and field corn as a
barrier crop for management of B. argentifolii on snap bean {Phaseolus vulgaris L.). It
was continued in 1997 focusing only on the barrier crop treatment and including the
effects of wind direction and barrier row orientation.


161
van Lenteren, J. C., and L. P. J. J. Noldus. 1990. Whitefly-plant relationships: behavioral
and ecological aspects. Pages 47-87 in D. Gerling, ed. Whiteflies: their
bionomics, pest status and management. Intercept Ltd, Andover, Hants, UK.
van Lenteren, J. C. and J. Woets, 1977. Development and establishment of biological
control of some glasshouse pests in the Netherlands. Pages 81-87 in F.F. Smith
and R. E. Webb, eds. Pest management in protected culture crops. ARS-NE-85,
U. S. Department of Agriculture, Washington, DC.
van Sas, J., J. Woets, and J. C. van Lenteren. 1978. Determination of host-plant
quality of gherkin (Cucumis sativus L), melon (Cucumis mel L.), and gerbera
('Gerbera jamesonii Hook) for the green house whitefly Trialeurodes
vaporariorum (Westwood) (Homoptera: Aleyrodidae). Mededelingen van de
Faculteit Landbouwwetenschappen Rijkuniversiteit, Gent. 43: 409-420.
Vandermeer, J. 1989. The ecology of intercropping. Cambridge University Press,
Cambridge, UK.
Veierov, D. 1996. Physically and behaviorally active formulations for control of
Bemisia. Pages 557-576 in D. Gerling and R. Mayer, eds. Bemisia 1995:
taxonomy, biology, damage, control, and management. Intercept Ltd., Andover,
Hants, UK.
Verschoor-van der Poel, P. J. G. 1978. Host-plant selection by the greenhouse whitefly,
Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae). (in Dutch)
M.Sc. Thesis, University of Leiden, Leiden, Netherlands. 39 pp.
Verschoor-van der Poel, P. J. G., and J. C. van Lenteren. 1978. Host-plant selection by
the greenhouse whitefly. Med. Fac. Landbouw. Rijksuniv. Gent. 43: 387-396.
Vet, L. E. M., J. C. van Lenteren, and J. Woets. 1980. The parasite-host relationship
between Encarsia formosa and Trialeurodes vaporariorum. IX. A review of the
biological control of the greenhouse whitefly with suggestions for future research.
Z. Ang. Ent. 90: 26-51.
von Arx, R., J. Baumgartner, and V. Delucchi. 1984. Sampling Bemisia tabaci (Genn.)
in Sudanese cotton fields. J. Econ. Entomol. 77: 1130-1136.
Wang, K., and J. H. Tsai. 1996. Temperature effect on development and reproduction of
silverleaf whitefly. Ann. Entomol. Soc. Am. 89: 375-384.
Wilson, F. D., H. M. Flint, B. R. Stapp, and N. J. Parks. 1993. Evaluation of cultivars,
germplasm lines, and species of Gossypium for resistance to biotype "B" of
sweetpotato whitefly. J. Econ. Entomol. 86: 1857-1862.


61
movement of adults within the plot. However the overall trap counts in this study were
low. The contribution made by corn barriers to reducing whiteflies may depend on the
density of the whitefly population. Crop barriers such as com may be more effective
when used with other control measures. Short of employing manufactured barriers such
as floating row covers or fine mesh screens, whitefly adults probably cannot be excluded
from a cropped area (Norman et al. 1993).
Trap position had a significant effect on trap count (Table 3-3). The number of
whiteflies caught decreased as trap distance from the release point increased. The
interaction of treatment and trap position interaction was not significant, suggesting that
this decline was not different among treatments.
Data derived from attractive traps may be ambiguous. A gravid or hungry
whitefly adult which is surrounded by non-hosts, such as corn, may be more sensitive to a
distant patch of bright yellow than an adult in similar condition surrounded by acceptable
hosts, such as bean. It is conceivable that the whitefly adults in the corn treatments spent
more time searching and so were drawn from a greater area than the whitefly adults
trapped in the monocropped bean treatments. It is possible that fewer whitefly adults
entered the corn treatments than the monocropped bean, but that a higher proportion of
those entering the corn treatments were trapped. However, these considerations do not
alter the overall impression that where air currents can enter, whitefly adults can follow.
Conclusion
Eggplant, transplanted a few weeks before bean was planted, did not serve as a
trap crop for B. argentifolii. Wind direction was the overwhelming factor determining
movement of whitefly adults into experimental plots with or without barrier crops. In
downwind plots, corn rows planted perpendicular to the predominant wind direction


11
Intercropping
Intercropping is the agronomic practice of growing two or more crops
simultaneously in the same field (Andrews and Kassam 1976). Crops may be planted
without regard to rows (mixed intercropping), in alternating rows, or with different crops
alternating within the same row. Relay intercropping refers to planting of one intercrop
species before another so that their life cycles partially overlap (Kass 1978). The broader
term polyculture includes intercropping, but also encompasses intentionally combining
crops and weeds, and combining crops with beneficial non-crop plants, such as cover
crops or nursery crops (Andow 1991a). Perrin and Phillips (1978) include mixtures of
crop cultivars and multilines in their definition of intercropping, because such
combinations may possess some of the advantages associated with conventional
intercropping.
Traditional food-production systems in tropical Africa, Asia, and Latin America
are usually characterized by some degree of intercropping (Perrin and Phillips 1978). In
agricultural areas where labor is the primary resource and reduction of risk the primary
concern, polyculture systems have been developed which give higher and more secure
yields than monoculture (Perrin 1977). Successful intercropping systems are
characterized by greater efficiency in the use of solar radiation, nutrients, and soil
moisture, as well as higher yields, compared to monocropped production under the same
conditions (Andow 1991b, Kass 1978, Perrin 1977, Vandermeer 1989). In the first
decades of this century, intercropping was common in temperate regions (Andow 1983).
While generally considered inappropriate for the mechanized, chemical-intensive
agriculture of industrialized nations, intercropping methods might improve the production


140
Cilantro (Coriandrum sativum L.) and rosa de jamaica (Hibiscus sabdariffa L.)
supported high numbers of beneficial insects at the Guatemala site. However, tomato
intercropped with these crops did not have higher numbers of natural enemies than
tomato grown in monoculture. Predators found on tomato were predominantly spiders.
The failure to reduce densities of immature whiteflies through intercropping is
probably due to aspects of whitefly behavior discussed in the literature. Bemisia
argentifolii, Bemisia tabaci, and Trialeurodes vaporariorum are all highly polyphagous
species. They apparently do not rely on host-specific visual or olfactory cues for host
finding, responding rather to the broad range of greenish-yellow light spectra emitted by
most vegetation (Coombe 1981, 1982, Vaishampayan et al. 1975a, van Lenteren and
Noldus 1990). It is likely that these species must probe a crop in order to determine its
suitability as a host (Hunter et al. 1996, Lei et al. 1998). It may therefore be unlikely that
whitefly adults w ill be drawn away from one crop by the presence of another crop, even if
this second crop normally elicits greater rates of feeding or oviposition than the first crop.
Since the trap cropping mechanism relies largely on influencing choices made in the host
seeking stage, it may not be an effective method for managing polyphagous whiteflies.
While whiteflies are highly dispersive, they are weak fliers, relying on wind
currents to take them from crop to crop until an acceptable host is found (Byrne and
Bellows 1991). The presence of non-hosts may increase the movement of whitefly adults
within a cropped area, but may not encourage the emigration of adults from a cropped
area as has been demonstrated with more mobile herbivores in intercropping studies
(Bach 1980a, 1980b, Risch 1981). Whitefly adults may simply move short distances in
intercropping systems that contain non-hosts until acceptable hosts are found.


54
1 Release
point
Wind
Fipnre 3-1. Plot Plan, Green Acres 1997


19
The success and efficiency of natural enemies in polyculture relative to
monoculture is largely determined by the specifics of behavior, much as it is for
herbivores. The enemies hypothesis implicitly refers to generalist natural enemies, in that
it suggests polyculture will offer alternate prey or hosts, and alternate food sources, such
as pollen and nectar (Root 1973). Like specialist herbivores, specialist natural enemies
such as host-specific parasitoids may rely on sensitive visual, olfactory, and tactile cues to
find hosts. These cues are more likely to be obscured in polyculture than in monoculture
(Sheehan 1986). The disruption of plant patches may cause a specialist enemy to leave a
diverse area more quickly than a simple one. Host-feeding is essential for some
parasitoids, and alternate protein and carbohydrate sources such as nectar or pollen may
not serve as a substitute (Sheehan 1986).
There are many examples of predators achieving higher densities in monoculture
than polyculture (Corbett and Plant 1993). For instance, Schultz (1988) found
significantly fewer lacewing (Chrysopidae) eggs on cotton intercropped with corn or
weeds than on monocropped cotton. The assumption that predators will move from a
resource-rich intercrop to the main crop that the agriculturalist intends to protect may not
always be valid. Bugg et al. (1987) found that predators on knotweed (Polygnum
aviculare) did not tend to move from it to adjacent crops.
Few robust generalizations can be made to predict how polyculture will affect
arthropod density (Andow 1991a, Kareiva 1983). However, the literature suggests that
polyculture will reduce densities of monophagous herbivores more often than densities of
polyphagous herbivores (Andow 1991a). In addition, polyculture may favor some


96
Numbers of fourth-instar T. vaporariorum were lower (p < 0.05) in the
imidacloprid-treated intercrop (0.58 1.73/plant) than the untreated intercrop (52.33
45.92) during week 5. Numbers were not different between monocrop subplot treatments
(6.15 12.91) that week. During week 6, numbers of T. vaporariorum (20.61 30.61)
were not different among treatments. Numbers of fourth-instar B. tabaci (0.09 0.47)
were not statistically different (p < 0.1) during week 5. B. tabaci comprised 0.5% of the
fourth-instars observed that week.
There were fewer (p < 0.05) tomato plants in the untreated subplot treatments
(11.00 7.92) than in the imidacloprid-treated subplots (19.38 7.19).
The heavy rains associated with Hurricane Mitch presumably reduced arthropod
populations. Beat cloth samples of tomato yielded few arthropods and almost no
generalist predators.
Conclusions
The intercropping arrangements examined did not reduce densities of whitefly
immatures in a consistent manner. Whitefly counts were characterized by extremely high
variability, which may have obscured treatment differences. In the second bean crop of
the diversity study and the bean crop of the mosaic study, both conducted during the rainy
season, lower whitefly levels on intercropped plants resulted from reduced plant size and
health. Plant size and whitefly densities were greater on intercropped than on
monocropped tomato plants on some dates in the mosaic and com/cilantro studies.
Reduced plant quality among intercropping treatments is not uncommon in polyculture
studies (Kareiva 1983).


17
tomato. Trap cropping has been used to manage the cotton boll weevil (Anthomonus
granis Boheman) in Nicaragua (Swezey and Daxl 1988) and Arizona (Moore and
Watson 1990). The ability to support higher densities than a main crop does not make a
preferred crop a trap crop; the trap crop must actually reduce densities on the main crop
when the two are interplanted (Ali and Karim 1989). Trap crops are often treated with
pesticides to prevent damaging herbivores from building up and spilling over onto main
crops (Srinivasan and Moorthy 1991). Effective control often depends on the timing of
pesticide applications to the trap crop (Todd and Schumann 1988) or the timing of
planting for the trap crop in relation to the main crop (Kloen and Altieri 1990).
There are several ways herbivore density, damage, and growth may be affected by
vegetative diversity once an insect has entered a polyculture. Polycultures which support
high densities of natural enemies may increase predation and parasitism of herbivores
(Altieri and Letourneau 1982). For example, Letourneau (1987) found parasitism of
Diaphania hyalinata L. higher in squash intercropped with corn and bean (Phaseolus
vulgaris L.) than in monocropped squash. Intercropping may affect herbivore health by
affecting the suitability of individual plants, or by repelling certain insects because of
increased shading (Kareiva 1983). Hawkes and Coaker (1976) reported that Delia
brassicae (Wied.), the cabbage root fly, oviposited less on Brassica sp. intercropped with
clover (Trifolium sp.) than in pure stands. This was apparently due to higher rates of
departure from hosts within the patch rather than to increased difficulty finding them
(Coaker 1980).
The effect of polyculture on the transmission of arthropod-vectored pathogens
may vary according to the epidemiology of the pathogen. Incidence of non-persistent


60
1995, Puri et al. 1995, Schuster et al 1996). However, Al-Musa (1982) and Schuster et al.
(1996) reported a reduction in virus incidence on tomato (Lycopersicon esculentum Mill.)
using cucumber (Cucumis sativus L.) and squash, respectively, as trap crops.
Corn as a barrier crop. The corn did not grow well in 1996 due to insufficient
fertilizer. It attained a mean height of 1.18 m 0.34 (n = 150) and a density of 27 7
plants per 6.1m row (n = 30). We decided to re-evaluate the barrier effect in 1997 with
larger, properly fertilized plots. Eggplant did not appear to be a promising trap crop, and
so was not included in the field experiment the following year.
1997
Release of adult whiteflies. Average corn height was 2.45 1.97 m when
whitefly releases were made. The effect of treatment on trap count was not significant
(p < 0.10) on any of the four collection dates (Table 3-3). The block effect was highly
significant, and the interaction between treatment and block was significant or highly
significant on three of the collection dates. Wind direction was from the east or northeast
during the 4 days that collections were made (Table 3-4). Trap counts in plots to the west
of the release point were significantly higher than trap counts in plots to the east of the
release point for each treatment on each collection date (Table 3-4). When treatments
were compared on the basis of downwind plots only, counts were significantly lower in
the barrier treatment than in the other two treatments on two of the four collection dates.
Wind direction appeared to be the primary factor determining where whitefly adults were
trapped. This is consistent with observations that whitefly adults move passively with
wind currents as 'aerial plankton' (Byrne and Bellows 1991). Among downwind plots,
the barrier treatment tended to have the lowest counts, indicating that the arrangement of
corn rows perpendicular to the prevailing wind direction did have some effect on the


148
Duffus, J. E. 1996. Whitefly-borne viruses. Pages 255-263 in D. Gerling and R. Mayer,
eds. Bemisia 1995: taxonomy, biology, damage, control, and management.
Intercept Ltd, Andover, Hants, UK.
Ekbom, B. S., and X. Rumei. 1990. Sampling and spatial patterns of whiteflies.
Pages 107-121 in D. Gerling, ed. Whiteflies: their bionomics, pest status and
management. Intercept Ltd, Andover, Hants, UK.
Ellsworth, P. C., J. W. Diehl, and S. H. Husman. 1996. Establishment of integrated pest
mangement infrastructure: a community-based action program for Bemisia
management. Pages 681-693 in D. Gerling and R. Mayer, eds. Bemisia 1995:
taxonomy, biology, damage, control, and management. Intercept Ltd., Andover,
Hants, UK.
Ellsworth, P., D. Meade, D. Byrne, J. Chernicky, E. Draeger, and R. Gibson. 1994.
Progress on the use of trap crops for whitefly suppression. Page 160 in T. J.
Henneberry, N. C. Toscano, R. M. Faust, and J. R. Coppedge, eds. Silverleaf
whitefly: 1994 supplement to the five-year national research and action plan.
United States Department of Agriculture, ARS-125, Washington, DC.
Elton, C. S. 1927. Animal ecology. Sidgwick and Jackson, London, UK.
Elton, C. S. 1958. The ecology of invasions by animals and plants. Methuen, London,
UK.
Fargette, D. and C. Fauquet. 1988. A preliminary study on the influence of
intercropping maize and cassava on the spread of African cassava mosaic virus by
whiteflies. Aspects Appl. Biol. 17: 195-202.
Faust, R. M. 1992. Conference report and five-year national research and action plan for
development of management and control methodology for the sweetpotato
whitefly. Agricultural Research Service No. 107, U. S. Department of Agriculture,
Washington, DC.
Fehmy, M., A. H. Hegab, and G. W. Moawad. 1994. Evaluation of programs to
control cotton whitefly, Bemisia tabaci, in tomato and squash fields and reduce
spread of TYLCV in Egypt. Phytoparasitica 22: 348-349.
Flint, H. M., and N. J. Parks. 1990. Infestation of germplasm lines and cultivars of
cotton in Arizona by whitefly nymphs. J. Entomol. Sci. 25: 223-229.
Florida Statistical Abstract. 1996. University of Florida Press, Gainesville, FL.
Fransen, J. J. 1990. Natural enemies of whiteflies: fungi. Pages 187-210 in D. Gerling,
ed. Whiteflies: their bionomics, pest status, and management. Intercept, Ltd.,
Andover, Hants, UK.


56
Dust-and-release procedure. Byrne et al. (1996) developed a method of dusting
whitefly adults with a fluorescent pigment in the field and trapping them at a distance as a
means to monitor movement. We modified this method to distinguish the released
whitefly adults which were caught on the traps from trapped members of the naturally-
occurring field population.
Before dawn on 8 August the infested hibiscus plants were enclosed in 113.5 liter
plastic leaf litter bags. The nozzle of a Leseo technical duster (product 1964, Leseo Inc.,
Cleveland, OH) was forced through the plastic and approximately 8.5-14 g Day-Glo Fire
Orange fluorescent AX-14-N pigment (Day-Glo Color Corp., Cleveland. OH) was puffed
from the duster into the bag onto the infested plants. The hibiscus plants were transported
to the experimental area enclosed in plastic bags and arranged in 6 clusters of 6 plants
along the central path and between pairs of treatment plots. The plastic bags were
removed between 7:30 and 7:50 AM to allow a unified release of dyed whitefly adults.
The traps were removed and replaced at dusk. The second set of traps was removed at
dusk on 9 August. After removal, traps were kept refrigerated until examined.
On 10 August, the hibiscus plants were returned to the greenhouse. Traps were
placed in the plots from 8:00 AM to 5:00 PM on 14 August to determine that whitefly
adults from the first release were no longer measurably present in the area. On 24 August
the dust-and-release procedure was repeated. Traps were set out from 8:00 AM to 8:00
PM on 24 August, and replaced with traps that were recovered at dusk on 25 August.
Hibiscus plants were removed after the second set of traps had been retrieved. Traps
were examined using a Spectroline 365 nm black light (model B-14N, Spectronics Corp.,
Westbury, NY). The number of fluorescing whitefly adults on each trap was recorded.


131
coefficients calculated for eggs and nymphs are lower than those reported by Naranjo and
Flint (1994) and Schuster (1998), and suggest a distribution approaching random for disc
and whole leaf egg counts. The greater degree of aggregation among nymphs than eggs
indicated by lower b values for eggs is inconsistent with what is known about whitefly
biology (Ekbom and Rumei 1990). However the coefficients of variation observed in our
study are consistent with those reported by Naranjo and Flint (1994) and Schuster (1998)
for whitefly eggs and nymphs.
Time Costs and Conclusions
The time required to examine whole plants, feed leaves through the leaf area
meter, and record observations ranged from about 40 minutes per plant when plants were
three weeks old to about 70 minutes per plant when plants were five weeks or older.
About 15 minutes were needed to process two sets of whole leaf counts from three strata
each week. Four sets of disc counts from three strata consistently required 8-10 minutes.
Examination of the whole plant is too costly in terms of time for this to be used as
the sample unit for most sampling programs. However, some degree of whole plant
sampling may be required to determine the range of branches or strata in which the life
stage of interest will be found. Whole plant sampling is essential if an absolute estimate
is required. Therefore it will be impractical to obtain an absolute estimate in most
situations.
Stratified whole leaf samples gave better estimates than disc samples in that
coefficients of variation tended to be lower, regressions with whole plant counts were
significant more often, and the r values from these regressions tended to be higher.
However, the improved sampling parameters derived from whole leaf samples may not


4 THE ROLE OF CROP DIVERSITY IN THE MANAGEMENT OF A
WHITEFLY (HOMOPTERA: ALEYRODIDAE) SPECIES COMPLEX
ON BEAN CPHASEOLUS VULGARIS L.) AND TOMATO
(LYCOPERSICON ESCULENTUM MILL.) IN THE SALAM VALLEY,
BAJA VERAPAZ, GUATEMALA 68
Introduction 68
Materials and Methods 73
Results and Discussion 86
Conclusions 96
Summary 99
5 A COMPARISON OF SOME ARTHROPOD GROUPS ON
MONOCROPPED AND INTERCROPPED TOMATO (jLYCOPERSICON
ESCULENTUM MILL.) IN BAJA VERAPAZ, GUATEMALA 112
Introduction 112
Materials and Methods 113
Results 116
Discussion 118
6 METHODS FOR SAMPLING IMMATURE STAGES OF
BEMISIA ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE)
ON BEAN {.PHASEOLUS VULGARIS L.) 122
Introduction 122
Materials and Methods 124
Results and Discussion 127
Time Costs and Conclusions 131
Summary 132
7 SUMMARY AND CONCLUSIONS 139
APPENDIX SOME WHITEFLY HOSTS AT DIFFERENT ELEVATIONS
IN EASTERN GUATEMALA 142
REFERENCES 143
BIOGRAPHICAL SKETCH 163
vi


Fig. 6-1. Regression showing the relationship between whole plant egg counts/cm2 to upper stratum whole leaf egg counts/cnr on 15
bean plants during week 4 of sampling.
OO


114
Research Design
A split-plot design was used with two whole plot treatments (monocrop and
intercrop tomato) and three subplot pesticide treatments (imidacloprid, a detergent/oil
rotation, and control). Each treatment was replicated four times.
Whole Plot Treatments
Whole plots contained nine rows, 17 m in length. Between row spacing was 1.0
m. Monocrop plots consisted of eight rows of tomato. Intercrop plots consisted of nine
rows of a tomato/intercrop mix. Five intercrop species were planted in alternating rows
with tomato in the following order: velvetbean, rosa de jamaica, cilantro, cabbage
(.Brassica olercea L.), and corn (Zea mays L.). These crops were chosen to represent
diversity in plant architecture, plant chemistry, and uses. Aside from velvetbean, which is
used as a forage and green manure, each crop has domestic and market value.
The cultivar of tomato used was Elios (Petoseed, Saticoy, CA). Cultivar
information was not available for cilantro, velvet bean, and rosa de jamaica, which were
grown from locally-acquired seed. Corn and cabbage were not sampled for this study.
Spacing between plants was 20 cm for cilantro and velvetbean and 40 cm for
tomato and rosa de jamaica. Cilantro and rosa de jamaica were planted 25 March.
Velvetbean was planted 26 March. Tomato in the imidacloprid treatment was
transplanted 28 April. Tomato in the untreated and detergent/oil treatments was
transplanted 6 May.
Subplot Treatments
Each whole plot was divided into 3 sections of 5.67 m in length. These sections
were randomly assigned to the imidacloprid treatment, the detergent and oil treatment, or
the control.


10
transmission among main crops. Results have been mixed. Morales et al. (1993)
reported that a sorghum [Sorghum bicolor (L.) Moench] barrier slightly reduced Bemisia
densities and transmission of virus on tomato in the Motagua Valley, Guatemala. A pearl
millet [Pennisetum typhoides (Burm. f.) Stapf & Hubbard] barrier prevented whitefly
virus transmission on cowpea [Vigna unguiculata (L.) Walp.] (Sharma and Varma 1984)
and reduced it on soybean (Rataul et al. 1989) in India. In Colombia, Gold et al. (1990)
found reduced densities of Aleurotrachelis socialis Bondar and Trialeurodes variabilis
(Quaintance) on cassava intercropped with maize (Zea mays L.) and cowpea, but
attributed this in part to reduced host quality due to intercrop competition. Ahohuendo
and Sarkar (1995) reduced density of B. tabaci by more than 50% and incidence of
cassava virus by 40% on cassava by intercropping with maize and cowpea in Benin.
Fargette and Farquet (1988), whose study in the Ivory Coast included the effect of wind
direction, found densities of B. tabaci and virus incidence were sometimes higher on
cassava intercropped with maize than on monocropped cassava.
Successful management of Bemisia may require coordinated efforts throughout
agricultural regions, such as the government-imposed host-free periods attempted in the
Dominican Republic (Polston and Anderson 1997). Integrated pest management plans
for tomato growers have been developed in Central America (Hilje 1993), and attempts to
develop a collaborative model for whitefly management throughout the region are on
going (Hilje 1998). Ellsworth et al. (1996) describe efforts to develop a community-
based Bemisia management program in Arizona. Kogan (1996) discusses the difficulties
of adapting the integrated pest management for Bemisia to a region-wide management
program.


Table 4-10. Parasitoid complex by percent species from tomato monocropped and intercropped with com, with or without
imidacloprid.
Species
Encarsia pergcmdiella
Encarsia meritoria complex
A mi tus fuscipennis
Monocrop
Intercrop
Imidacloprid Untreated Imidacloprid Untreated
97.9(187)' 99.4(154) 92.2 (118) 96.2(435)
0.5 (1) 0 1.6 (2) 0.7 (3)
1.6 (3) 0.6 (1) 6.3 (8) 3.1 (14)
1 Number in parentheses represents total number of individuals from two sample dates (19 November 1998, 2 December 1998).


TABLE OF CONTENTS
page
ACKNOWLEDGMENTS iii
ABSTRACT vii
CHAPTERS
1 LITERATURE REVIEW AND RESEARCH GOALS 1
Whiteflies 1
Intercropping 11
Research Objectives 21
2 THE EFFECT OF SILVER REFLECTIVE MULCH AND A SUMMER
SQUASH (CUCURBITA PEPO L.) TRAP CROP ON DENSITIES OF
IMMATURE BEMISIA ARGENTIFOLII (HOMOPTERA:
ALEYRODIDAE) ON ORGANIC BEAN (PHASEOLUS VULGARIS L.) ... 23
Introduction 23
Material and Methods 24
Results 27
Discussion 30
Conclusion 33
3 POTENTIAL OF FIELD CORN (ZEA MA YS L.) AS A BARRIER CROP
AND EGGPLANT (SOLANUMMELONGENA L.) AS A TRAP CROP
FOR MANAGEMENT OF THE SILVERLEAF WHITEFLY, BEMISIA
ARGENTIFOLII (HOMOPTERA: ALEYRODIDAE) ON BEAN
(PHASEOLUS VULGARIS L.) IN NORTH FLORIDA 49
Introduction 49
Materials and Methods 51
Results and Discussion 57
Conclusion 61
v


Table 2-4. Nymph density of B. argentifolii (mean SD/cnr) on beans and squash, 1995
Bean Squash
Week
Treatment
Lower stratum
Upper stratum
Mean
Mean
1
Bean
0.20 0.59
0.11 0.29
0.15 0.46
Mulch
0.15 0.29
0.25 0.62
0.20 0.48
Squash
0.18 0.36
0.33 0.50
0.26 0.44
Squash/mulch
0.13 0.34
0.06 0.17
0.10 0.27
2
Bean
0.96 0.84a1
0
0.48 0.76a
Mulch
0.42 0.36ab
0.20 0.35
0.30 0.37ab
Squash
0.21 0.27b
0.07 0.17
0.14 0.24ab
0.16 0.77
Squash/mulch
0.21 0.40b
0.04 0.10
0.13 0.30b
Q**
3
Bean
0.27 0.472
02
0.14 0.35
Mulch
0.25 0.40
0
0.13 0.31
Squash
0.06 0.15
0
0.03 0.11
0.20 0.75
Squash/mulch
0.05 0.14
0.02 0.08
0.04 0.11
0.07 0.27
4
Bean
0.13 0.24
0
0.07 0.18
Mulch
0.20 0.57
0
0.10 0.41
Squash
0.01 0.06
0
0.01 0.04
0.13 0.56
Squash/mulch
0.01 0.06
0
0.01 0.04
0.32 1.04*
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means.2 Upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.10. *, **
indicate that nymph densities were significantly different between bean and squash at p < 0.05 and p < 0.01 according to the pairwise
t-test.
SO


16
The extent to which vegetative diversity will interfere with immigration also
depends on the range at which an insect detects the host, and whether this detection
mechanism is specific or general (Kareiva 1983, Stanton 1983). Host-specific orientation
cues tend to be characteristic of monophagous insects (Prokopy and Owen 1978), which
may in addition evolve sophisticated searching ability in order to find rare hosts.
Polyphagous insects such as certain whiteflies and aphids do not rely on host-specific
visual or olfactory cues, and respond generally to the spectra of yellowish light emitted by
most vegetation (van Lenteren and Noldus 1990, Power 1990). Whiteflies, aphids, and
thrips have limited ability to control their flight, and have been described as aerial
plankton (Byrne and Bellows 1991, Price 1976). The flypaper effect (Trenbath 1976)
suggests a mechanism by which weak fliers with unsophisticated host-finding
mechanisms such as whiteflies and aphids might be reduced in polyculture. Simply by
alighting on and probing diversionary intercrops, such insects may invest less time in
damaging main crops. However, this mechanism has not been demonstrated
scientifically.
Trap cropping is a method of pest suppression that relies on manipulating host
finding mechanisms. The herbivores decision-making must be influenced before it finds
and damages the main crop. Vandermeer (1989) writes that trap cropping should affect
generalist herbivores. However, the sensitive host-finding cues of monophagous
herbivores are presumably more vulnerable to manipulation than the general attraction to
most vegetation demonstrated by some polyphagous insects. Hunter and Whitfield
(1996) almost doubled yields and reduced densities of the Colorado potato beetle
(Leptinotarsa decemlineata (Say)) by more than half by using potato as a trap crop with


8
Host plant resistance to whiteflies is primarily derived from leaf characteristics
such as pubescence or the presence of glandular trichomes (Berlinger 1986). Some
degree of host plant resistance to Bemisia has been found in cotton (Flint and Parks 1990.
Wilson et al. 1993), soybean (McAuslane 1996) and tomato (Heinz and Zalom 1995).
Resistance to T. vaporariorum has been found in sweet pepper (Capsicum annuum L.)
(Laska et al. 1986) and melon (Cucumis mel L. var. cigrestis) (Soria et al. 1996).
Progress has been achieved recently in developing resistance to fte/wT/a-transmitted
geminiviruses in tomato (Scott et al. 1996, Nateshan et al. 1996).
The sessile habit of immature whiteflies renders them susceptible to many
pathogens (Fransen 1990), predators, and parasitoids (Gerling 1990). Successful
biological control programs have been developed to manage T. vaporariorum in
greenhouses, primarily with the parasitoid Encarsia formosa (Gahan) (Hymenoptera:
Aphelinidae) (Vet et al. 1980). In Florida, high rates of parasitism have been found on
weeds, organically grown vegetables (Stansly et al. 1997) and unsprayed peanuts
(McAuslane et al. 1994). However, the intensive use of broad-spectrum pesticides and the
rapid rate of increase of Bemisia prevents its suppression by natural enemies in most
agricultural systems (Hoelmer 1996). Exotic parasitoids have been introduced into
Arizona, California, Florida, and Texas to control Bemisia with little success (Hoelmer
1996, Nguyen 1996). Releases of predators in California (Brazzle et al. 1994, Legaspi et
al. 1994. Roltsch and Pickett 1994) and attempts to establish refugia for natural enemies
of Bemisia in the desert southwest have been similarly unfruitful (Roltsch and Pickett
1995, 1996). The arid conditions, heavy pesticide regimes, and continuous cropping
cycles that characterize agriculture in the southwestern United States may place biological


93
intercrop treatment, and 56.31 5.88 in the monocrop treatment. Only two bean plants
in the mosaic study showed symptoms of bean golden mosaic, both in the monocrop
treatment.
There were no treatment differences in egg density on any sampling date (Table 4-
6). Intercrop tomato was taller than monocrop tomato during weeks 2-4, resulting in
higher numbers of whitefly nymphs on intercrop tomato during those weeks (Table 4-6).
There were no treatment differences (p < 0.1) in numbers of fourth-instar T.
vaporariorum during week 5 (36.33 48.15/plant) or week 6 (48.92 71.19). Numbers
of parasitized nymphs (19.67 18.76/plant) were not different (p < 0.1) between
treatments during week 5. Numbers of parasitized nymphs were higher (p < 0.1) on
intercrop tomato (50.67 46.80) than monocrop tomato (23.00 26.01) during week 6.
Only one fourth-instar B. tabaci was found on tomato, comprising 0.08% of fourth-instar
nymphs observed during week 6.
Parasitoids reared from tomato in the mosaic experiment consisted of Encarsia
pergandiella, members of the Encarsia meritoria species complex, and Amitus
fuscipennis MacGown and Nebeker (Hymenoptera: Platygasteridae) (Table 4-7). A
Shannon-Weaver diversity index (H1) (Shannon and Weaver 1949) of 0.383 was
calculated for the parasitoid complex collected from tomato grown in monoculture, and
an index of 0.996 was calculated for parasitoids reared from tomato mixed with com and
rosa de jamaica. We speculate that the presence of extra-floral nectaries on rosa de
jamica favored the increased presence of the E. meritoria species complex and A.
fuscipennis.


149
Gallaher, R. N., R. McSorley, R. L. Stanley, and D. L. Wright. 1998. Howard IIIST and
Howard IIST subtropical corn hybrids. Agronomy research report AY-98-02.
Agronomy Department, Institute of Food and Agricultural Sciences, University of
Florida, Gainesville.
Gennadius, P. 1889. Disease of the tobacco plantations in the Trikonia. The aleurodid
of tobacco. Ellenike Ga. 5: 1-3.
Gerling, D. 1990. Natural enemies of whiteflies: predators and parasitoids. Pages 147-
185 in D. Gerling, ed. Whiteflies: their bionomics, pest status and management.
Intercept Ltd., Andover, Hants, UK.
Glover, D. and K. Kuterer. 1990. Small farmers, big business: contract farming and rural
development. New York, St. Martin's Press.
Gold, C. S., M. A. Altieri, and A. C. Bellotti. 1990. Direct and residual effects of short
duration intercrops on the cassava whiteflies Aleurotrachelus socialis and
Trialeurodes variabilis in Colombia. Agrie. Ecosys. Environ. 32: 57-68.
Gould, J. R., and S. E. Naranjo. 1999. Distribution and sampling of Bemisia argentifolii
and Eretmocerus eremicus on cantaloupe vines. J. Econ. Entomol. 92: 402-408.
Hawkes, C., and T. H. Coaker. 1976. Behavioral responses to host plant odours in adult
cabbage rootfly. Pages 85-89 in T. Jermy, ed. The host plant in relation to insect
behavior and reproduction. Plenum, New York.
Heinz, K. M., and F. G. Zalom. 1995. Variation in trichome-based resistance in Bemisia
argentifolii oviposition in tomato. J. Econ. Entomol. 88: 1494-1502.
Hendrix, D. L., T. L. Steele, and H. H. Perkins, Jr. 1996. Bemisia honeydew and sticky
cotton. Pages 189-199 in D. Gerling and R. Mayer, eds. Bemisia 1995:
taxonomy, biology, damage, control and management. Intercept Ltd., Andover,
Hants, UK.
Hiebert, E., A. M. Abouzid, and J. E. Polston. 1996. Whitefly-transmitted
geminiviruses. Pages 277-288 in D. Gerling and R. Mayer, eds. Bemisia 1995:
taxonomy, biology, damage, control, and management. Intercept Ltd, Andover,
Hants, UK.
Hilje, L. 1993. Un esquema conceptual para el manejo integrado de la mosca blanca en
el cultivo de tomate. Manejo Integrado de Plagas 29: 51-57.
Hilje, L. 1998. Un modelo de colaboracin agrcola internacional para el manejo de
moscas blancas y geminivirus en Amrica Latina y el Caribe. Manejo Integrado
de Plagas 49: 1-9.


Table 3-3. Analysis of variance for whitefly release data, 1997
August 8
August 9
August 24
August 25
Source
df
F
F
F
F
Block
3
10.74**
32.76**
56.24**
12.34**
Treatment
2
0.02
0.86
1.78
2.01
Trap position
4
4.67*
4.65*
2.99@
2.86@
Block* treatment
6
4.10**
2.54*
5.28**
1.81
Block*trap position
12
1.48
1.23
7.84**
0.80
Treatment*trap position
8
0.59
0.94
0.13
1.77
**p < 0.01; *p < 0.05; @p<0.1.
On


62
marginally reduced penetration of whitefly adults into plots on some dates when
compared to bean monoculture and corn rows planted parallel to the wind. Com barriers
planted perpendicular to the wind may be useful at certain whitefly population densities
when used with other control tactics.


128
densities to shift from the lower stratum to the middle stratum. Coefficients of variation
were consistently lower in the lower stratum. Significant (p <0.10) regression equations
were derived on only a few dates for mid or lower strata.
Parasitized nymphs were observed on the lower stratum from weeks 3-8. and on
the middle stratum from weeks 6-8. Parasitized nymphs were never observed in the
upper stratum disc counts. Coefficients of variation for parasitized nymph samples were
always very high (> 243%). However, the regressions of whole plant counts on leaf disc
counts from the lower stratum were significant (p < 0.05) each week (3-8) for parasitized
nymphs.
Red-eyed nymphs were observed on the lower stratum from weeks 3-8, and on the
middle stratum during weeks 5-7. Coefficients of variation were typically high (>200%).
Regressions of whole plant on disc counts were rarely significant.
The observation of highest egg densities in the upper stratum is consistent with
the tendency of Bemisia females to oviposit on young leaves (Ekbom and Rumei 1990).
Naranjo and Flint (1994) and Schuster (1998) reported higher coefficients of variation for
eggs in the uppermost branch than from the branches immediately below, as we observed
during the first weeks of sampling. This may be because fully expanded new leaves are
highly attractive for oviposition, while those still expanding are not. producing both very
high counts and zero counts in the upper stratum. The data suggest that estimates of egg
density should be taken from the upper stratum, where eggs are most prevalent, but that
intense sampling may be required because of high count variability.
Except for very late in the season, means tended to be highest and coefficients of
variation lowest for nymphs in the lowest stratum. It may be advisable to focus sampling


137
Table 6-4. Parameters for the Taylor power law for eggs and nymphs from three sample
units
Sample unit
!nM
b
7
r
n1
Egg
Disc (upper stratum)
0.056
1.12
0.867**2
7
Leaf (upper stratum)
-0.254
1.09
0.555+
7
Whole plant
-0.586
1.42
0.628*
7
Nymph
Disc (lower stratum)
-0.064
1.47
0.955**
7
Leaf (lower stratum)
-0.235
1.60
0.979**
7
Whole plant
-0.560
1.88
0.673*
7
1 Each sample point represents the mean of 60 disc counts, 30 whole leaf counts, or 15
whole plant counts from 7 weeks.
2**,*, and + indicate significance at p < 0.01, 0.05, and 0.1, respectively.


118
Some of the families of Coleptera collected contain both phytophagous and
predaceous members. The Cleridae, Histeridae, and Staphylinidae are primarily
predaceous, and the Mordellidae contains some predaceous groups (Borror et al. 1989).
Aside from one staphylinid, all of the Coleptera in these families were found on cilantro.
Their status as predators was not determined, but it would not have affected cilantro's
standing as the crop supporting the highest densities of insect predators (Table 5-6).
Lampyrids and meloids have predatory larvae, but only adults were collected.
Discussion
It is noteworthy that the arthropod groups on tomato were apparently unaffected
by the proximity of different crops supporting distinct arthropod communities. It is
possible that densities of generalist predators could be increased on tomato by
intercropping with higher densities of cilantro. However, the assumption that beneficial
insects will move from intercrops to main crops is not always valid (Bugg et al. 1987,
Corbett and Plant 1993). The data demonstrate the predominance of spiders as predators
on tomato, and indicate the negative effect of some pesticides on spider numbers. The
results suggest that cilantro may be a useful crop for augmenting levels of generalist
predators in some cropping systems, although movement to target crops could be
problematic.


ACKNOWLEDGMENTS
I am extremely grateful to Dr. Robert McSorley for serving as the chairman of my
committee. I feel very fortunate to have benefited from the depth of his knowledge and
his guidance. I am also very grateful to Dr. Heather McAuslane, who has helped with this
research project from its initial stages to the bitter end. I want to thank Debbie Boyd,
who gave me my first orientation in working with whiteflies, and my friend Dr. Rose
Koenig, without whom chapter 2 would not have happened. I would also like to thank
Dr. Jon Allen and Dr. Raymond Gallaher for serving on my committee.
I am grateful to Dr. Don Dickson for allowing his crew to help me with the
research at Green Acres. Without the help I received from Reggie Wilcox, the studies
described in chapters 3 and 5 would have been far more difficult, if not impossible, to
carry out. I am grateful to Dr. Jerry Stimac for taking the time to help me understand
sampling theory, one of my objectives when I started the PhD program. I am heavily
indebted to Jay Harrison, formerly of IFAS statistics, for many hours of assistance. I am
grateful to Dr. Greg Evans of the Division of Plant Industry for help with identification of
whitefly parasitoids and to Dr. Avas Hamon (also of DPI) and Dr. Andrew Jensen
(formerly of the USDA, Beltsville) for identification of whiteflies from Guatemala. I
want to thank John Frederick for all sorts of help. I thank Clay Scherer for his friendship.
As always, I am grateful to Dr. John Capinera for support and good advice.



PAGE 1

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PAGE 173

/' 6V 81,9(56,7< 2) )/25,'$


58
Whitefly adults were observed on eggplant the day following transplanting on 22
August, and eggs were observed in the 25 August sample (Table 3-2). When bean plants
were emerging, eggplants were quite large: they had an average of 7.0 1.3 branches, a
mean height of 17.33 0.28 cm and mean leaf area of 485 156 cm2 (n=5).
Bean vs. eggplant. Densities of eggs and nymphs peaked on eggplant 4 weeks
after transplanting and declined during the following weeks (Table 3-2). Egg densities
were one and a half times higher on eggplant than on bean during the first week that bean
was sampled (22 September). On all subsequent sampling dates, however, egg densities
were significantly higher on bean than on eggplant.
During the week that nymphs were first observed on bean, densities were
significantly lower on bean than on eggplant. During subsequent sampling dates, nymph
densities were either higher on bean or not statistically different. Observations of
parasitized and red-eyed nymphs were either higher on eggplant than on bean or not
significantly different on the two hosts.
Parasitoid species. All parasitoids reared from bean and eggplant were
hymenopterans from the family Aphelinidae.
Thirty-nine parasitoid individuals were recovered from bean leaves. Thirty-two of
these were Encarsia nigricephala Dozier (82%), 4 were Eretmocerus sp. (10.3%), and 3
were Encarsia pergandiella Howard (7.7%).
One hundred twenty-one parasitoid individuals were reared from eggplant leaves.
Fifty-one of these were Encarsia pergandiella (42.1%), 48 were Encarsia nigricephala
(39.7%), 13 were Eretmocerus sp. (10.7%), 6 were Encarsia transvena (Timberlake)
(5%), and 3 were Encarsia sp. (2.5%).


123
and Noldus 1990). Upon emergence, first instars tend to move a short distance from the
egg to find a feeding site (Byrne and Bellows 1991, Price and Taborsky 1992), although
they are capable of moving within and between plants to find healthy feeding sites
(Summers et al. 1996). Subsequent instars are sessile. Therefore nymphs usually
complete their development on the leaf where they were oviposited, and the age of the
nymph tends to correlate with the age of the plant leaf (Ekbom and Rumei 1990).
Estimates of egg density are usually taken from upper stratum leaves, and nymph
densities are estimated from the middle or lower canopy, depending on the host
(Ohnesorge et al. 1980, Naranjo and Flint 1994, Schuster 1998).
Whitefly nymphs are susceptible to parasitism in early instars (Gerling 1990), but
symptoms of parasitism are not obvious until the third or fourth instar. Mycetomes
become asymmetrical in parasitized nymphs, and in later stages the parasitoid larva or
fully-formed adult can be seen clearly through the cuticle (McAuslane et al. 1993).
Therefore, estimates of parasitism are usually taken from the portion of the plant canopy
containing the oldest nymphs.
In the final stage of healthy fourth-instar Bemisia nymphs, the red eyes of the
pharate adult become apparent (Byrne and Bellows 1991). Therefore, this stage can be
used to estimate the number of nymphs that have successfully completed development.
Several genera and species of whitefly can only be identified with the exuvia of this final
instar (the pupal case) (Byrne and Bellows 1991). Location of the most representative
stratum for final instar nymphs is necessary when species identification is required, such
as in areas with mixed whitefly populations.


146
Caldern, L. F., D. Dardn, and V. Salguero. 1993. Eficiencia de diferentes dosis de
aceite vegetal y detergente en el control de mosca blanca. In V. Salguero, D.
Dardn, and R. Fischer, eds. Manejo integrado de plagas en tomate, fase I: 1991-
1992. Proyecto MIP-ICTA-CATIE-ARF, Guatemala City, Guatemala.
Campbell, B. C., J. D. Steffen-Campbell, and R. J. Gill. 1996. Origin and radiation of
whiteflies: an initial molecular phylogenetic assessment. Pages 29-51 in D.
Gerling and R. Mayer, eds. Bemisia 1995: Taxonomy, biology, damage, control
and management. Intercept Ltd., Andover, Plants, UK.
Capinera, J. L., T. J. Weissling, and E. E. Schweizer. 1985. Compatibility of
intercropping with mechanized agriculture: effects of strip intercropping of pinto
beans and sweet corn on insect abundance in Colorado. J. Econ. Entomol. 78:
354-357.
Carnero, A., and J. L. Gonzlez-Andujar. 1994. Spatial and temporal distribution of
fourth-instar larvae of Trialeurodes vaporariorum and Bemisia tabaci in tomato
plants. Phytoparasitica 22: 317.
Chu, C., T. J. Henneberry, and A. C. Cohen. 1995. Bemisia argent ifolii: host preference
and factors affecting oviposition and feeding site preference. Environ. Entomol.
24: 354-360.
Coaker, T. H. 1980. Insect pest management in Brassica crops by intercropping. Integr.
Control Brassica Crops 3: 117-125.
Cochran, W. G. 1963. Sampling techniques. John Wiley and Sons, Inc. New York.
Cohen, S., J. E. Duffus, and H. Y. Liu. 1992. A new Bemisia biotype in the
southwestern United States and its role in silverleaf of squash and transmission of
lettuce infectious yellows virus. Phytopathology 82: 86-90.
Coombe, P. E. 1981. Wavelength specific behavior of the whitefly Trialeurodes
vaporariorum. J. Comp. Physiol. 144: 83-90.
Coombe, P. E. 1982. Visual behavior of the greenhouse whitefly. Physiol. Entomol. 7:
243-251.
Corbett, A., and R. E. Plant. 1993. Role of movement in the response of natural enemies
to agroecosystem diversification: a theoretical evaluation. Environ. Entomol. 22:
519-531.
Costa, A. S. 1975. Increase in the population density of Bemisia tabaci, a threat of
widespread virus infection of legume crops in Brazil. Pages 27-49 in J. Bird and
K. Maramorosch, eds. Tropical diseases of legumes. Academic Press, New York.


Table 4-6. Whitefly immatures (x SD/plant) and plant parameters of tomato monocropped and mix-intercropped with field corn and
rosa de jamaica. Mosaic study.
Egg Nymph Height (cm) Branees Weight (g)
Wk
Monocrop
Intercrop
Monocrop
Intercrop
Monocrop
Intercrop
Monocrop
Intercrop
Monocrop
Intercrop
1
0.25
0.77
0.56
1.03
0
0
13.13
3.42
15.50
5.45



'
2
147.81
171.18
113.06
71.56
0.13
0.34
3.44
4 46**i
13.81 =t
2.79
19.06
2.66**
4.88
0.72
4.88
0.62
-
-
3
364.13
347.75
13.25
41.63
20.09
26.88
5.25
4.88
5.08
5.00
505.88
244.57
15.58
35.77*'
4.94
4.46**
0.89
1.13
3.49
1.77
4
1351.25
1788.25
303.92
502.0
29.71
37.46
6.58
6.67
15.00
14.46
998.20
807.93
334.91
283.05@'
6.73
6.71**
0.67
0.65
7.62
6.31
52
0
0
160.58
90.62
206.75
135.47
52.67
10.62
50.67
12.36
7.25
1.29
8.83
1.03**
93.92
49.78
31,00
9.76**
62
0
0
115.92
91.69
136.42
187.32
-
-
-
-
-
-
**, *, @ indicate that intercrop mean is significantly different from corresponding monocrop mean at p < 0.01, p < 0.05, and p < 0.1
respectively.
2 Counts taken from lower third of plant only.
o
^0


87
During week 1, egg counts were lower (p < 0.05) in the imidacloprid-treated
intercrop than in the control intercrop (Table 4-1). During week 2, egg counts were lower
(p < 0.05) in the imidacloprid and detergent/oil treatments than the control. Nymph
counts during week 2 were different (p < 0.05) among all treatments, with the lowest
counts in the imidacloprid treatment and the highest in the detergent/oil treatment.
Three weeks after germination, bean plants treated with imidacloprid were clearly
larger and more robust than those in the detergent/oil treatment and control. Plants in the
detergent/oil treatment showed symptoms of phytotoxicity. In addition, plants in the
detergent/oil and control treatments were stunted, with shortened stems and petioles. A
chlorotic burn appeared along the leaf border and tip, typical of leafhopper damage.
Whole plant examinations during week 4 revealed high densities of thrips
(Thysanoptera) and leafhoppers (Homoptera: Cicadellidae) on plants in the detergent/oil
treatment and the control. Size differences between the imidacloprid-treated bean and the
other two treatments increased during subsequent weeks.
The imidacloprid-treated plants tended to have more eggs and nymphs than the
stunted plants in other treatments during weeks 3 and 4 (Table 4-2). There were no
subplot treatment differences during weeks 5 and 6 as plants senesced and whitefly
populations declined.
Fourth-instar B. tabaci nymphs were observed for the first time during whole
plant examinations on week 4. Densities of fourth-instar B. tabaci were lower (p < 0. 10)
in the imidacloprid treatment (0.13 0.35/plant) than in the control (7.62 12.22).
Densities in the detergent/oil treatment were intermediate (0.38 0.52). The ratio of
fourth-instar Bemisia to Trialeurodes from all treatments during week 4 was 65: 573.


LD
1780
1991
S(4s
UNIVERSITY OF FLORIDA
3 1262 08555 1348


160
Todd, J. W., and F. W. Schumann. 1988. Combination of insecticide applications with
trap crops of early maturing soybean and southern peas for population
management of Nezara viridula in soybean. J. Entomol. Sci. 23: 192-199.
Tonhasca, A., Jr., J. C. Palumbo, and D. N. Byrne. 1994a. Aggregation patterns of
Bemisia tabaci in response to insecticide applications. Entomol. Exp. Appl. 72:
265-272.
Tonhasca, A., Jr., J. C. Palumbo, and D. N. Byrne. 1994b. Distribution patterns of
Bemisia tabaci in cantaloupe fields in Arizona. Environ. Entomol. 23: 949-954.
Trenbath, B. R. 1976. Plant interactions in mixed crop communities. Pages 129-169 in
R. I. Papendick, P. A. Sanchez, and G. B. Triplett, eds. Multiple cropping.
American Society of Agronomy Special Publication 27, Madison, WI.
Trenbath, B. R. 1977. Interactions among diverse hosts and diverse parasites. Ann. N.Y.
Acad. Sci. 287: 124-150.
Tsai, J. H. and K. Wang. 1996. Development and reproduction of Bemisia argentifolii
on five host plants. Environ. Entomol. 25: 810-816.
Vaishampayan, S. M., M. Kogan, G. P. Waldbauer, and J. T. Woolley. 1975a. Spectral
specific responses in the visual behavior of the greenhouse whitefly. Ent. Exp.
Appl. 18: 344-356.
Vaishampayan, S. M., G. P. Waldbauer, and M. Kogan. 1975b. Visual and olfactory
responses in orientation to plants by the greenhouse whitefly. Ent. Exp. Appl. 18:
412-422.
van Boxtel, W., J. Woets, and J. C. van Lenteren. 1978. Determination of host-plant
quality of eggplant, cucumber, tomato and paprika for the greenhouse whitefly.
Med. Fac. Landbouw. Rijksuniv. Gent. 43: 397-408.
van de Meredonk, S., and J. C. van Lenteren. 1978. Determination of mortality of
greenhouse whitefly eggs, larvae and pupae on four host-plant species: eggplant,
cucumber, tomato and paprika. Med. Fac. Landbouw. Rijksuniv. Gent. 43: 421-
429.
van Emden, H. F. 1963. Observations on the effect of flowers on activity of parasitic
Hymenoptera. Entomol. Mon. Mag. 98: 255-259.
van Emden, H. F. 1965. The effect of uncultivated land on the distribution of cabbage
aphid on an adjacent crop. J. Appl. Ecol. 2: 171-196.


Table 4-3. Whitefly egg and nymphs (x SD)' on tomato under 2 cropping systems and 2 pesticide regimes. Diversity study.
Egg
Nymph
Wk
Pesticide
Monocrop
Intercrop
mean
Monocrop
Intercrop
mean
1
Detergent/oil
4.31 9.99
1.582.39
3.147.72 a2
1.313.40
0.421.16
0.932.68
Control
15.0025.03
14.7525.26
14.8824.74b
1.632.58
2.254.78
1.943.79
Mean
9.66 19.52
9.6U20.03
1.472.97
1.463.76
2
Detergent/oil
50.5041.35
65.33106.25
56.8668.42
31.2521.82
21.3335.23
27.0026.08
Control
578.00899.20
105.5056.95
341,75641.64
132.00 167.48
53.0047.23
92.5021.49
Mean
314.25653.27
88.2976.46
81.63 122.99
39.4342.61
3
Detergent/oil
3.585.38
9.2217.06
6.0011.85
7.007.07
21.0019.91
13.0015.37
Control
1.502.31
1.002.37
1.252.31
9.838.32
6.254.61
8.046.83
Mean
2.544.19
4.5211.69
8.427.69
12.57il5.04
4
Detergent/oil
_
_
-
0
0.671.41
0.290.96
Control
-
-
-
0.170.58
0.330.65
0.250.61
Mean
-
-
0.080.41
0.481.03
1 See text for sample units.
2 Means in columns for a given week followed by different letters are significantly different (p < 0.05) according to analysis of
variance. Absence of letters indicates no treatment differences (p >0.1).
o
4-


74
intercropped plants were compared with numbers on monocropped plants for each study.
These studies are referred to as the diversity, mosaic, and corn/cilantro studies. The
corn/cilantro study included a comparison of two methods of tomato production in the
nursery.
Diversity Study
This study was initiated in March toward the end of the dry season, when whitefly
populations are at their highest. Bean or tomato was intercropped in alternating rows
with corn, cabbage (Brassica olercea L.), cilantro (Coriandrum sativum L.), rosa de
jamaica {Hibiscus sabdariffa L.), and velvetbean {Mucuna deeringiana (Bort.) Small)
(Figure 4-1). These crops are either poor or non-hosts for whiteflies, and were chosen
from crops grown regionally to represent a diverse range of plant architecture and plant
chemistry. All have dietary and market value, except for velvetbean, which is primarily
used as a forage and green manure. The purpose of the study was to determine if the
presence of varied poor and non-hosts affected whitefly densities on bean and tomato
when compared to densities on bean and tomato grown in monoculture. This study
included subplots with pesticide treatments.
After the first bean crop had been harvested, a second bean crop was planted on
smaller scale. Whitefly numbers on monocropped and intercropped bean were compared
without pesticide subplot treatments.
The bean variety used was TCTA-Santa Gertudis, a cultivar developed and
promoted by ICTA as resistant to bean golden mosaic. 'Elios tomato seedlings
(Petoseed, Saticoy, CA) were purchased from Sal Vasquez, Estancia La Virgen, El
Progreso. The field corn hybrid used was TCTA HB-83 (ICTA 1993). Costanza


CHAPTER 1
LITERATURE REVIEW AND RESEARCH GOALS
Whiteflies
According to the system of classification commonly used in the United States,
whiteflies (family Aleyrodidae) belong to the order Homoptera (Borror et al. 1989). As
members of the suborder Sternorrhyncha, whiteflies are closely related to the psyllids,
aphids, and scale insects (Campbell et al. 1996). They are considered by some to be the
tropical equivalent of the aphids (Byrne and Bellows 1991). They occur throughout
warm regions of the world, and under certain conditions, in temperate regions (Bink-
Moenen and Mound 1990. Mound and Halsey 1978). The center of origin for aleyrodids
is unknown, although Pakistan is considered likely because of the diversity of whitefly
parasitoids in that region (Brown et al. 1995, Mound and Halsey 1978).
All known whiteflies are phloem-feeders (Byrne and Bellows 1991). Of the more
than 1,200 species described (Bink-Moenen and Mound 1990), the majority are
monophagous or oligophagous (Brown et al. 1995). However, polyphagy is common
among economically important species, of which there are probably fewer than 20 (Byrne
et al. 1990). Whiteflies cause crop losses by extracting water, amino acids and
carbohydrates from the phloem, and by the production of honeydew, a sugar-rich excreta
which accumulates on foliage and serves as a substrate for sooty molds (Hendrix et al.
1996). Sooty molds impede photosynthesis and reduce the quality of cotton (Gossypium
hirsutum L.) lint and fruit (Byrne et al. 1990). In addition to causing mechanical damage,


80
Imidacloprid-treated bean was harvested 29 June. Detergent/oil bean and
untreated bean was harvested 6 July. Tomato was harvested each week from 15 July
through 12 August and classified as large, medium, small, and reject.
On 10 July a second bean crop was planted in the former imidacloprid subplots.
A randomized complete block design with 4 replications was used to compare whitefly
immatures on bean grown under 2 treatments: monocropped and intercropped with the
five mature and senescing poor and non-host crops.
Spacing between bean plants was 20 cm. Bean was sampled weekly for 6 weeks
from 19 July through 23 August. Eight whole bean plants per plot were sampled during
week 1, four plants per plot on week 2, and two plants per plot for the remaining weeks.
The number of trifoliate leaves per plant was recorded each week. Bean was harvested 20
September.
Statistical Analysis
Treatments were compared using analysis of variance for split plot or randomized
complete block, followed by mean separation when appropriate (SAS Institute 1996).
Mosaic Experiment
This study was carried out toward the end of the rainy season. A mixed
intercropping pattern was used to evaluate corn and rosa de jamaica as crops which might
offer a cryptic environment for bean and tomato when intercropped in a mosaic pattern.
The same crop cultivars were used as in the diversity experiment. Tomato seedlings were
bought from Piloncito Verde, Chimaltenango.
Densities of immature whiteflies were compared on bean and tomato grown under
two treatments: bean and tomato grown in monoculture, and bean and tomato


132
justify the extra time required to gather them, or the necessity of possessing a leaf area
meter. Counts taken from a whole leaf must be divided by the leaf area prior to analysis,
which is time-consuming. Disc counts share a common density when they are recorded.
The time required to take whole leaf samples could be invested into improving the quality
of disc samples by taking more disc samples from the same leaf or from additional plants.
Additional studies to obtain more extensive data on Taylors power law parameters may
be useful for assessing and comparing the precision of specific sampling plans.
Our data indicate that the upper stratum of the bean plant offers the best area from
which to sample B. argentifolii eggs. The increase in egg density in the middle stratum
probably occurs too late in the season to be valuable for estimates of oviposition. The
best estimates of nymphs, parasitized nymphs, and red-eyed nymphs were found in the
lower stratum.
Summary
Eggs and nymphs of B. argentifolii were sampled on bean using whole plant
examinations and discs punched from leaves or whole leaf sample units taken from three
strata. Egg densities were highest in the upper stratum, but more variable than counts
from the middle stratum during the first weeks of sampling. The best estimates for
densities of nymphs, parasitized nymphs and red-eyed nymphs were taken from the
lowest stratum. Highest nymph densities shifted toward the middle stratum during final
weeks of sampling. Pooled counts of immature stages demonstrate the ability of the
whitefly to exploit the entire plant canopy as the bean crop grows. However, too much
information is lost for pooled counts to be useful in management programs.


156
Risch, S. J., D. Andow, and M. A. Altieri. 1983. Agroecosystem diversity and pest
control: data, tentative conclusions, and new research directions. Environ.
Entomol. 12: 625-9.
Rivas, G., Lastra, R., and Hilje, L. 1994. Manejo de las mosca blanca, B. tabaci, en
tomate, mediante semilleros cubiertos con mallas. Page 156 in M. de Mata, D. E.
Dardn, and V. E. Salguero, eds. Biologa y manejo del complejo mosca blanca-
virosis. III Taller Centroamericano y del Caribe sobre la Mosca Blanca, Antigua,
Guatemala.
Roditakis, N. E. 1990. Host plants of the greenhouse whitefly in Crete: attractiveness
and impact on whitefly life stages. Agrie. Ecosys. Env. 31: 217-224.
Rodriquez, R. 1994. Problemtica del compleja mosca blanca-virus del mosaico dorado
en frijol en Centroamerica. Pages 7-15 in M. de Mata, D. E. Dardn, and V. E.
Salguero, eds. Biologa y manejo del complejo mosca blanca-virosis. III Taller
Centroamericano y del Caribe sobre la Mosca Blanca. Antigua, Guatemala.
Rojas, M. R., R. L. Gilbertson, D. R. Russell, and D. P. Maxwell. 1993. Use of
degenerate primers in the polymerase chain reaction to detect whitefly-transmitted
geminivirus. Plant Dis. 77: 340-347.
Roltsch, W., and C. Pickett. 1994. Areawide establishment of Delphastus pusillus, a
predator of silverleaf whitefly, in the Imperial Valley. Page 147 in T. J.
Henneberry, N. C. Toscano, R. M. Faust, and J. R. Coppedge, eds. Silverleaf
whitefly: 1994 supplement to the 5-year national research and action plan.
Agricultural Research Service No. 125, U. S. Department of Agriculture,
Washington. DC.
Roltsch, W., and C. Pickett. 1995. Silverleaf whitefly natural enemy refuges in the
Imperial Valley. Page 135 in T. J. Henneberry, N. C. Toscano, R. M. Faust, and J.
R. Coppedge, eds. Silverleaf whitefly: 1995 supplement to the 5-year national
research and action plan. Agriculture Research Service No. 125, U. S.
Department of Agriculture, Washington, DC.
Roltsch, W., and C. Pickett. 1996. Evaluation of refuges and new refuge plants for
support of silverleaf whitefly natural enemies. Page 141 in T. J. Henneberry, N. C.
Toscano, R. M. Faust, and J. R. Coppedge, eds. Silverleaf whitefly: 1996
supplement to the 5-year national research and action plan. U.S. Department of
Agriculture, Washington, DC.
Root, R. 1973. Organization of a plant-arthropod association in simple and diverse
habitats. The fauna of collards (Brassica olercea). Ecologic. Monogr. 43: 95-
124.
Rosset, P. M. 1991. Sustainability, economies of scale, and social instability: Achilles
heal of non-traditional export agriculture? Agrie. Human Values, Fall 1991, p.
30-37.


32
A successful trap crop will draw a herbivore away from the main crop before the
herbivore has damaged the main crop by oviposition. feeding, or inoculation with a
pathogen. The limited success achieved managing Bemisia with trap crops may be due to
the mechanisms by which whiteflies find and accept hosts.
Whiteflies seeking hosts respond to the yellowish range of light spectra emitted by
most vegetation (Mound 1962, van Lenteren and Noldus 1990, Byrne and Bellows 1991).
Trialeurodes vaporariorum, B. tabaci and Aleurocanthus woglumi apparently do not
respond to crop-specific olfactory or visual cues (van Lenteren and Noldus 1990).
Trialeurodes vaporariorum must probe before accepting or rejecting a plant (van Sas et
al. 1978, Noldus et al. 1986a). Bemisia also seems to require gustatory information to
judge host suitability (Byrne and Bellows 1991). Examination of the precibarial and
cibarial chemosensillae by Hunter et al. (1996) indicates that B. tabaci can test plant sap
without ingesting it. This supports the notion that host discrimination by Bemisia occurs
after the host has been tasted.
Host 'preference by whiteflies among crops may not be apparent until after adults
have invested time in colonizing the less suitable crop. Trialeurodes vaporariorum will
leave certain acceptable hosts after a few hours, while spending days on other hosts (van
Sas et al. 1978, Verschoor-van der Poel and van Lenteren 1978). Similarly, T.
vaporariorum tends to accumulate in greater density on some hosts than others over a
given time period (Verschoor-van der Poel 1978 cited in van Lenteren and Noldus 1990).
If host preference for a given crop, such as a trap crop candidate, does not affect whitefly
behavior until after whitefly adults have oviposited and fed on the main crop, trap
cropping may have limited benefit for whitefly management.


Table 2-1. Egg density of B. argentifolii (mean SD /cm2) on beans and squash, 1995
Bean
Squash
Week
Treatment
Lower stratum
Upper stratum
Mean
Mean
1
Bean
Mulch
Squash
Squash/mulch
0.37 0.53
1.06 0.79
0.53 0.55
0.38 0.60
0.95 0.88ab'
0.96 0.92a
0.44 0.41 be
0.33 0.47c
0.66 0.78ab
1.01 0.85a
0.49 0.48ab
0.36 0.54b
2
Bean
Mulch
Squash
Squash/mulch
0.40 0.48
0.80 1.30
0.13 0.24
0.07 0.15
0.63 0.61ab
1.26 1.28a
0.13 0.22b
0.19 0.26b
0.52 0.56a
1.03 1.30a
0.13 0.23b
0.13 0.22b
2.80 3.95a**
1.60 2.80b**
3
Bean
Mulch
Squash
Squash/mulch
0.39 0.56b
0.81 0.75a
0.04 0.10c
0.19 0.34bc
0.40 0.85
0.57 0.73
0.11 0.20
0.15 0.38
0.40 0.71 ab
0.70 0.74a
0.07 0.16b
0.17 0.36b
1.24 1.83**
0.50 0.70**
4
Bean
Mulch
Squash
Squash/mulch
0.12 0.20
0.32 0.39
0.10 0.16
0.06 0.15
0.18 0.35
0.24 0.35
0.13 0.29
0.08 0.16
0.15 0.28
0.28 0.37
0.11 0.23
0.07 0.15
0.51 0.78**
0.32 0.59**
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experiment-wise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively.
U>
-P-


75
c
o
R
N
B
E
A
N
B
E
A :
N i
TWWWW


C
I
L
A!
Nl
Tj
Rl
0:



















B
E
A j
N
R I
0
S
d
J
EA
I
C !
A
i'
4
4
V
PQ
E
E
L
A
V
N
E
*

:
T
;
i
*
B
:
4
i
.
E
i
4
4
A
4
J
4
\
5 1
N
i
Figure 4-1. Intercrop Pattern: Diversity Experiment


Table 3-2. Immature B. argentifolii (mean SD/cnr) on bean and eggplant, 1996.
Date
Egg
Nyi
inpli
Parasitized nymph
Red-eyed nymph
Bean
Eggplant
Bean
Eggplant
Bean
Eggplant
Bean
Eggplant
Aug. 25
0.660.46
0
0
0
Sept. 1
0.891.02
1.3 ll .60
0
0
Sept. 8
1.030.65
0.520.33
0
0.0030.006
Sept. 16
3.530.72
2.390.33
0
0.0070.006
Sept.22
1.661.67@
2.741.72@
0*
1.84.72*
0
0
0
0
Sept.29
5.523.44*
l.681.72*
0.880.62*
2.131.78*
0
0
0@
0.0310.104@
Oct. 8
0.650.31*
0.240.41 *
1.590.83*
0.290.19*
0.0050.016*
0.0350.037*
0.0050.016
0.0120.015
Oct. 21
0.640.54*
0.230.22*
0.450.35
0.490.65
0.0090.014
0.0250.038
0.0030.009
0.0460.078
Nov. 4
0.260.26*
0.020.03*
0.280.19*
0.1U0.10*
0.0240.043(3)/
0.0690.067(2}
0.0060.012*
0.0480.043*
indicates that numbers on bean and eggplant are significantly different on a given date according to analysis of variance at a=0.05.
@ indicates a=0.1.
CT\


153
Naranjo, S. E. 1996. Sampling Bemisia for research and pest management applications.
Pages 209-224 in D. Gerling and R. Mayer, eds. Bemisia 1995: taxonomy,
biology, damage, control and management. Intercept, Ltd., Andover, Hants, UK.
Naranjo, S. E., and H. Flint. 1994. Spatial distribution of preimaginal Bemisia tabaci in
cotton and development of fixed-precision sequential sampling plans. Environ.
Entomol. 23: 254-266.
Naresh, J.S., and Y. L. Nene. 1980. Host range, host preference for oviposition and
development and the dispersal of Bemisia tabaci, a vector of several plant viruses.
Indian J. Agrie. Sci. 50: 620-623.
Nateshan, H. M., V. Muniyappa, S. H. Jalikop, and H. K. Ramappa. 1996. Resistance of
Lycopersicon species and hybrids to tomato leaf curl geminivirus. Pages 369-377
in D. Gerling and R. Mayer, eds. Bemisia 1995: taxonomy, biology, damage,
control and management. Intercept, Ltd., Andover, Hants, UK.
National Agricultural Statistics Service, 1998. United States Department of Agriculture,
Washington, DC.
Natwick, E. T., and K. S. Mayberry. 1994. Evaluation of repellents of silverleaf
whitefly on iceburg lettuce. Page 166 in T. J. Henneberry, N. C. Toscano, R. M.
Faust, and J. R. Coppedge, eds. Silverleaf whitefly: 1994 supplement to the five-
year national research and action plan. Agricultural Research Service No. 125, U.
S. Department of Agriculture, Washington, DC.
Nguyen, R. 1996. Importation and field release of parasites against the silverleaf
whitefly, Bemisia argentifolii, in Florida from 1990-1995. Page 134 in T. J.
Henneberry, N. C. Toscano, R. M. Faust, and J. R. Coppedge, eds. Silverleaf
whitefly: 1996 supplement to the 5-year national research and action plan. U.S.
Department of Agriculture, Washington, DC.
Nicholls, C. I. and Altieri, M. A. 1997. Conventional agricultural development models
and the persistence of the pesticide treadmill in Latin America. Int. J. Sustain.
Dev. World Ecol. 4: 93-111.
Nicholson, A. J. 1933. The balance of animal populations. J. Animal Ecol. 2: 132-178.
Noldus, L. P. J. J., R. Xu. M. H. E. R. Mansveld, and J. C. van Lenteren. 1986b. The
parasite-host relationship between Encarsia formosa and Trialeurodes
vaporariorum: analysis of the spatial distribution of greenhouse whiteflies in a
large glasshouse. J. Appl. Entomol. 102: 484-498.
Noldus, L. P. J. J., R. Xu, and J. C. van Lenteren. 1986a. The parasite-host relationship
between Encarsia formosa and Trialeurodes vaporariorum XIX. Feeding-site
selection by adult whiteflies. J. Appl. Ent. 101: 492-507.


108
Table 4-7. Parasitoid complex by percent species, Mosaic experiment.
Treatment
Species Monocrop Intercrop
Encarsia pergandiella 88.4(289)' 40.1(219)
Encarsia meritoria complex 0.6 (2) 13.7 (75)
Amitus fuscipennis 11.0 (36)46.2 (252)
1 Number in parentheses represents total number of
individuals from two sample dates (22 November 1998, 6 December 1998).


78
distinguished, but this is prohibitively time-consuming when high numbers of nymphs are
being counted.
In each study and for all crops, only the underside of leaves was examined for
whitefly immatures (Ekbom and Rumei 1990).
Bean was sampled on 6 occasions: 17 April (1 week after emergence), 25 April, 3
May, 12 May, 19 May, and 17 June. The sample unit on weeks 1 through 3 and week 5
was a 3.35 cm2 disc removed with a cork borer from upper and lower leaves (McAuslane
et al. 1995). The disc was removed from the underside of the central leaflet to the right of
the mid-vein. Five plants per plot were sampled on these weeks. The average of the 2
discs was used in treatment analysis. On weeks 4 and 6, one whole plant per subplot
replicate was sampled. Five plant heights per plot were measured on weeks 3, 5, and 6.
Five plants per plot were weighed on weeks 4, 5, and 6.
During week 4, five whole bean plants per plot were enclosed quickly in plastic
bags and refrigerated. These plants were sampled to estimate the number of generalist
predators on the bean plants as well as whitefly immatures.
Tomato was sampled on 4 occasions: 19 May, 1 June, 28 June, and 17 July. Disc
samples were taken from upper and lower strata on the first 2 sample dates. Whole
branches were examined for whitefly immatures from upper, middle and lower plant
strata on the latter two dates. Whole plants and branches were weighed to estimate the
percentage of the whole plant represented by the 3 strata. Five plants per plot were
sampled on the first two sampling dates, and one plant per plot was sampled during the
second two dates. Height and weight data on five plants per plot were gathered on weeks
2 and 3.


76
cabbage (Petoseed, Saticoy, CA) was used. Cultivar information was not available for
cilantro, velvet bean, and rosa de jamaica, which were grown from locally-acquired seed.
A tractor was used to cultivate the experimental area and form rows at the
beginning of the dry season (March 19) and rainy season (August 13) experiments.
Application of fertilizer, weeding and all other aspects of plot management were carried
out manually. Crops were fertilized according to local recommendations (ICTA 1993,
Superb 1997). Fungicides and pesticides were applied with a 16-liter Matabi Super 16
backpack sprayer (Goizper S. Coop., Guipzcoa, Spain). Fungicides were applied on a
weekly basis to tomato to control for foliar and root pathogens once the rains began in
May. Water from a furrow irrigation system was made available to the station every 6
days for 3 days during the dry season and upon request during the rainy season.
A split plot design was used with 2 whole plot treatments (monocrop, intercrop)
and 3 subplot pesticide treatments (imidacloprid, detergent/oil, control). Each treatment
was replicated 4 times.
Whole plots contained 17 rows, 17 m in length. Monocrop plots consisted of 8
rows of bean and 8 rows of tomato separated by one bare row. Intercrop plots consisted
of 8 rows of a bean/intercrop mix next to 8 rows of tomato/intercrop mix. A row of
velvetbean separated the bean and tomato sections in the intercrop plots. The other 4
intercrop species were planted in alternating rows with bean or tomato to either side of
the velvetbean in the following order: rosa de jamaica, cilantro, cabbage, corn.
Spacing between plants was 20 cm for bean, com, cilantro, and velvetbean and 40
cm for tomato and cabbage. Space between rows was 1.0 m. Rows were planted north to
south. Corn, cabbage, cilantro and rosa de jamaica were planted 25 March. Velvetbean


79
Fourth-instar whitefly nymphs were mounted in the laboratory of Lie. Margarita
Palmieri at the Universidad del Valle in Guatemala City and sent to Dr. Avas Hamon of
the Division of Plant Industry for identification. Dr. Andrew Jensen of the United States
Department of Agriculture in Beltsville, MD. kindly identified nymphs on dried plant
material. Leaves or whole plants with nymphs showing symptoms of parasitism were
placed in unwaxed cylindrical 0.95-liter cardboard cartons (Fonda Group Inc., Union, NJ,
USA) for parasitoid emergence. Several weeks later, dead parasitoids were placed on
cotton in gel capsules and sent to Dr. Greg Evans of the Division of Plant Industry,
Gainesville, FL, for identification.
Tissue from bean and tomato plants exhibiting symptoms of bean golden mosaic
or tomato leaf curl was analyzed using ELISA (Agdia Inc., Elkhart, IN) in the laboratory
of Lie. Margarita Palmieri. The total number of plants per row and number of plants with
bean golden mosaic symptoms was counted for all even-numbered rows in each bean
study. The total number of plants per row was counted in even-numbered rows for the
tomato treatments. Attempts to estimate percentage tomato leaf curl visually were
abandoned because virus symptoms are easily confused with other tomato disorders
(Polston and Anderson 1997).
Five velvetbean plants were examined for whitefly immatures on 3 May and 9
May. The leaves were traced onto paper, and this area was measured using a LI-COR
portable leaf area meter (model LI-3000A, LI-COR Inc., Lincoln, NE) in the United
States. Whole plant examinations were made of 12 cabbages on 6 June and 10 rosa de
jamaica plants on 8 June.


90
parasitized nymphs (week 3: 6.53 7.93/branch; week 4: 0.60 0.91/branch), plants per
row (11.16 1.87), or total yield per row (5.69 4.29 kg).
Seven tomato plants out of 10 showing geminivirus symptoms tested positive for
the presence of geminivirus.
Very few whitefly eggs or nymphs were found on cabbage, rosa de jamaica and
velvetbean. Cabbage plants were large (254.25 180.88 g) with well-formed heads when
sampled. Mean egg count was 0.17 0.58/plant and mean nymph count was 3.25 5.43.
Two fourth-instar T. vaporariorum nymphs were found. Rosa de jamaica plants weighed
164.67 150.92 g and were 49.33 12.14 cm tall. No whitefly eggs were found on the
rosa de jamaica. Average per plant count for nymphs and fourth-instar B. tabaci was 7.67
6.89 and 0.89 1.36 respectively. Velvetbean sampled on 3 May averaged 0.08 0.06
eggs and 0.03 0.04 nymphs/cnr. Velvetbean sampled on 9 May averaged 0.01 0.01
eggs and 0.05 0.06 nymphs/cnr.
Diversity Study: Second Bean Crop
Number of eggs was higher in the monocrop than the intercrop treatment during
weeks 3 (p < 0.05) and 4 (p < 0.01) (Table 4-4). Egg numbers did not differ by treatment
on other dates. However, intercrop plants had fewer trifoliates during week 5 (p < 0.05)
and week 6 (p < 0.01). Overall egg and nymph densities were therefore higher on the
intercrop plants during these weeks, since intercrop plants were smaller than monocrop
plants. The smaller size of intercrop beans was probably due to shading from intercrops,
particularly the rosa de jamaica, which was about 1.5 m tall in August.
There were no treatment differences (p < 0.1) on any sampling date for the second
bean crop between numbers of nymphs (Table 4-4), parasitized nymphs (week 4: 0.25


APPENDIX
SOME WHITEFLY HOSTS AT DIFFERENT ELEVATIONS
IN EASTERN GUATEMALA


Table 2-6. Nymph density of B. argentifolii (mean SD/cnr) on bean and squash, 1997
Bean
Squash
Week
Treatment
Lower stratum
Upper stratum
Mean
Mean
2
Bean
Mulch
Squash
Squash/mulch
8.29 4.81#
3.03 1.38
11.09 6.82#
5.98 3.62
0.68 1.26#
1.20 2.23
2.66 7.24#
0.39 0.69
4.48 5.19ab
2.12 2.03b
6.87 8.07a
3.19 3.83ab
12.47 24.15
6.25 10.40
3
Bean
Mulch
Squash
Squash/mulch
13.16 12.04
7.54 3.26
12.84 7.54
6.97 3.92
10.50 18.06
3.21 5.12
10.18 13.88
6.05 7.68
11.70 14.91
5.38 4.71
11.51 10.88
6.51 5.91
61.69 176.54
13.04 32.00
4
Bean
Mulch
Squash
Squash/mulch
3.59 2.77
6.12 7.10
3.34 2.77
2.46 1.55
4.32 4.75
2.50 4.91
6.70 12.52
0.02 0.05
3.96 3.78
4.30 6.19
5.02 8.93
1.24 1.65
14.98 29.31@
5.51 13.60
5
Bean
Mulch
Squash
Squash/mulch
0.95 0.91
7.89 5.52
1.93 1.99
2.55 2.78
3.29 3.41ab
11.29 9.09a
0.50 1.03b
1.84 2.3lab
2.12 2.69b
9.59 7.47a
1.21 1.70b
2.20 2.49b
3.35 6.14a
0.79 1.93b
6
Bean
Mulch
Squash
Squash/mulch
0.89 1.03
0.86 0.99
0.48 0.44
2.21 2.61
2.86 2.98
4.05 5.22
4.92 5.26
3.27 6.27
1.88 2.38
2.46 3.99
3.33 4.07
2.74 4.68
24.89 40.84**
16.49 29.18
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. *, ** indicate that mean densities in bean and squash are significantly different according to the pairwise t-test at p
< 0.05 and p < 0.01, respectively. # indicates that upper and lower stratum means are significantly different according to the pair-wise
t-test at p < 0.05.
4^


7
short distance from the egg to find a feeding site, (Byrne and Bellows 1991. Price and
Taborsky 1992), although they are capable of moving within and between plants to find
healthy feeding sites (Summers et al. 1996). Subsequent instars are sessile. For this
reason, nymph age tends to correlate with leaf age (Ekbom and Rumei 1990).
Researchers have taken advantage of this behavior to develop stratified sampling
plans for 'pupal and parasitized stages of T. vaporariorum in greenhouses (Martin and
Dale 1989, Martin et al. 1991, Noldus et al. 1986b) and egg and nymph stages of Bemisia
on cantaloupe (Cucumis mel L.) (Gould and Naranjo 1999, Tonhasca et al. 1994a,
1994b), cotton (Naranjo and Flint 1994, Ohnesorge and Rapp 1986a, von Arx et al.
1984), peanut (Arachis hypogea L.) (Lynch and Simmons 1993, McAuslane et al. 1993),
and tomato (Schuster 1998). Bemisia eggs and nymphs exhibit a highly aggregated
distribution on leaves and across plants (Naranjo 1996). Sampling plans have been
developed for white flies, primarily B. tabaci, to determine economic injury levels and to
compare the efficacy of control measures (Butler et al. 1986, Ekbom and Rumei 1990,
Naranjo 1996, Ohnesorge and Rapp 1986b).
Bemisia tabaci has demonstrated some degree of resistance to most classes of
broad-spectrum pesticides (Denholm et al. 1996. Dittrich et al. 1990), although novel
compounds (Horowitz and Ishaaya 1996) and biorational insecticides such as
detergents and oils (Stansly et al. 1996, Veierov 1996) continue to provide some measure
of control. One of the most effective and widely used compounds for whitefly control at
the time of writing is imidacloprid, a systemic pesticide which inhibits nicotinergic
acetylcholine receptors, produced by Bayer (Polston et al. 1994).


130
lower stratum. From weeks 5-7, densities in the middle and lower strata were not
significantly different. As was observed with the disc sample units, nymph densities were
highest in the middle stratum by week 8. Regression equations relating whole plant to
whole leaf counts were significant for both middle and lower strata on most sampling
dates.
Densities of parasitized nymphs were highest in the lower stratum from weeks 4-
6, but not significantly different from middle stratum densities during weeks 7-8.
Coefficients of variation were always lowest in the lower stratum. Regressions of whole
plant on whole leaf counts were significant in the lower stratum each week, but they were
not significant in other strata on any date.
Red-eyed nymphs were found in the lower stratum during weeks 3-8, and in the
middle stratum from weeks 6-8. Densities of red-eyed nymphs were significantly higher
in the lower stratum than the middle stratum on week 6, but not significantly different
between strata on weeks 7-8. Coefficients of variation were always lowest in the lower
stratum. Regression equations were significant in the lower stratum on weeks 4-6 and
week 8.
Pooled immature counts were highest in lower strata early in the study (weeks 3-
4), but even across strata for most of the remaining weeks. Coefficients of variation were
similar across strata for pooled immature counts, but tended to be lowest in the lower
stratum. Regressions of whole plant on whole leaf counts for pooled counts were
significant in two or more strata for each week of sampling.
The parameters for Taylors power law (Taylor 1961, 1984) for egg and nymph
counts from disc, whole leaf, and whole plant sample units are shown in Table 6-4. The b


155
Polston, J. E., and P. K. Anderson. 1997. The emergence of whitefly-transmitted
geminiviruses in tomato in the western hemisphere. Plant Dis. 81: 1358-1369.
Polston, J. E., P. Gilreath, D. J. Schuster, and D. O. Chellemi. 1994. Recent
developments in tomato geminiviruses: a new virus and a new pesticide. In C. S.
Vavrina, ed. Proc. Fla. Tomato Inst. University of Florida, IFAS, Vegetable
Crops Special Series, PRO-105, Gainesville, FL.
Powell, C. A., and P. J. Stoffella. 1993. Influence of endosulfan sprays and aluminum
mulch on sweetpotato whitefly disorders of zucchini squash and tomatoes. J.
Prod. Ag. 6: 118-121.
Power, A. G. 1990. Cropping systems, insect movement, and the spread of insect-
transmitted diseases in crops. Pages 47-69 in S. R. Gliessman, ed. Agroecology:
researching the ecological basis for sustainable agriculture, vol. 78. Springer-
Verlag, New York.
Price, P. 1976. Colonization of crops by arthropods: non-equilibrium communities in
soybean fields. Environ. Entomol. 5: 605-611.
Price, J. F., and D. Taborsky. 1992. Movement of immature Bemisia tabaci on poinsettia
leaves. Florida Entomologist 75: 151-153.
Prokopy, R. J., and E. D. Owens. 1978. Visual generalists and visual specialist
phytophagous insects: host selection behavior and application to management.
Entomol. Exp. Appl. 24: 609-620.
Prokopy, R. J., and E. D. Owens. 1983. Visual detection of plants by herbivorous
insects. Ann. Rev. Entomol. 28: 337-364.
Puri, S. N., B. B. Bhosle, P. S. Borikar, M. K. Fartade, R. N. Kolhal, M. Ilyas, B. R.
Kawthekar, G. D. Butler, and T. J. Henneberry. 1996. Wild brinjal Solarium
khasianum Clarke as a potential trap crop management tool for Bemisia in cotton.
Pages 237-240 in D. Gerling and R. Mayer, eds. Bemisia 1995: taxonomy,
biology, damage, control and management. Intercept, Ltd., Andover, Hants, UK.
Rataul, H. S., C. K. Gill, and S. Brar. 1989. Use of barrier crop and some cultural
measures in the management of yellow mosaic virus on soybean. J. Res. Punjab
Ag. Univ. 26: 227-230.
Risch, S. J. 1980. The population dynamics of several herbivorous beetles in a tropical
agroecosystem: the effect of intercropping corn, beans and squash in Costa Rica.
J. Appl. Ecol. 17: 593-612.
Risch, S. J. 1981. Insect herbivore abundance in tropical monocultures and polycultures:
an experimental test of two hypotheses. Ecology 62:1325-1340.


129
efforts for nymphs on the lower stratum, because this stratum provided the best
information on parasitized and red-eyed nymphs. It would therefore be possible to limit
disc sampling to the upper stratum for eggs and lower stratum for other stages. However,
a researcher primarily interested in estimating densities of nymphs rather than parasitized
or red-eyed nymphs should focus on the middle stratum as the bean crop ages.
Overall immature counts shifted from being highest in the lower stratum in weeks
2-3 to highest in the mid and upper strata on week 6. There was no difference in pooled
immature densities across strata on weeks 5, 7, or 8. The relatively even distribution of
immature forms throughout the plant indicates the ability of the whitefly to exploit the
entire plant habitat as leaf area increases. Coefficients of variation for pooled immatures
tended to be lower in the lower stratum. Significant regressions of whole plant on disc
counts occurred at times on all strata, but did not occur consistently on any given stratum.
Whole Leaf Samples
Patterns among whole leaf samples were similar to those found in disc samples.
Egg densities tended to be highest in the upper stratum, but were significantly higher (p <
0.05) than middle stratum densities only during weeks 5-7 (Table 6-3). Coefficients of
variation were lowest in the middle stratum during the first weeks of sampling, but tended
to be lowest in the upper stratum in later weeks. Regression equations of whole plant on
whole leaf egg counts were significant (p < 0.05) for upper and middle strata for most
weeks. For example, the regression of whole plant egg counts on upper stratum whole
leaf egg counts for week 4 is shown in Figure 6-1.
When plants were young (weeks 3-4), the highest (p < 0.05) nymph densities were
found in the lower stratum. Coefficients of variation were consistently lowest in the


77
was planted 26 March. Beans were planted 5 and 6 April. Tomatoes in the untreated and
detergent/oil plots were transplanted 6 May.
Each whole plot was divided into 3 sections of 5.67 m in length. These sections
were demarcated with nylon cord supported by stakes. Each section was randomly
assigned to the imidacloprid treatment, the detergent and oil treatment, or the control.
Imidacloprid (Confidor 70 WG) was prepared at a rate of 0.73 g/liter of water.
Approximately 10 cc of this mixture (73 mg imidacloprid) was applied to the base of
each plant at each application. Imidacloprid was applied to bean at emergence, 1 week
after emergence and 3 weeks after emergence. Imidacloprid is not registered for bean,
and was included for comparison only. Commercially-produced tomato seedlings
received 1 imidacloprid application in the nursery, and were treated 1 and 3 weeks after
transplanting.
Olmeca vegetable oil (Olmeca S.A.. Guatemala) and Unox laundry detergent
(Qumicas Lasser S.A., El Salvador) were applied at a rate of 1% or 16 cc/16 liter spray
tank (Caldern et al. 1993). An elbowed nozzle attachment was used to apply the
mixture to the lower surface of leaves. Detergent or oil was applied in rotation every 5
days.
Whiteflv Identification
Plants were examined under a dissecting microscope and the numbers of whitefly
eggs, nymphs, parasitized nymphs, and fourth-instar nymphs were recorded. The eyes of
the pharate adult become apparent in the final stage of fourth-instar Bemisia nymphs.
This stage was used to estimate the proportion of Bemisia relative to T. vaporariorum in
the nymph population. Earlier instars of Bemisia and T. vaporariorum can be


Aug. 25 1
2
3
4
5
x
3.331.97
l.OOil.lO
1.170.98
0.670.82
0.330.52
1.301.53*a
0.170.41
0
0
0
0
0.030.18*
0.330.52 0
l.OOil.lO 0
0.170.41 0
0.330.82 0
0.170.41 0
0.400.72b@ 0
1.171.17 0
1,000.89 0
0.500.55 0
0.831.17 0
1.000.89 0
0.900.92*a@ 0*
'Wind direction on release dates: Aug.8: 75; Aug.9: 97; Aug. 24: 61; Aug. 25: 55.
^indicates mean trap counts in the same treatment upwind and downwind of the release point are significantly different at p < 0.05
according to F-test for contrasts.
"Different letters indicate that mean trap counts in blocks downwind of release point are significantly different at p < 0.05 according to
F-test for contrasts.
@ indicates means are significantly different at p < 0.1 according to F-test for contrasts.
Row refers to trap location (l=nearest, 5- farthest from release point; see text), x = mean across all 5 row locations.
CO
^0


CHAPTER 7
SUMMARY AND CONCLUSIONS
A series of field experiments carried out in north central Florida and central
Guatemala failed to reduce densities of immature whiteflies (Homoptera: Aleyrodidae)
through intercropping. In Florida, attempts to reduce oviposition of Bemisia argentifolii
on common bean (Phaseolus vulgaris L.) by intercropping with more attractive" crops
(squash (Cucrbita pepo L.)and eggplant (Solarium melongena L.)) were not successful.
Attempts to reduce oviposition on bean in Florida, and on bean and tomato (Lycopersicon
esculentum Mill.) in Florida and Guatemala, by intercropping with poor and non-hosts
such as com (Zea mays L.) were also unsuccessful. Counts of adult B. argentifolii on
yellow sticky traps from a barrier crop study indicated that whitefly adults can enter
where air currents enter, and that the presence of a corn barrier only marginally reduced
penetration of adults into experimental plots.
A plastic mulch painted with a reflective aluminum strip reduced whitefly egg
densities on bean during the first week of sampling in 1996 and 1997 in Florida.
Imidacloprid protected bean from damage by whiteflies and other sucking insects during
the dry season, and reduced densities of immature whiteflies on tomato during the rainy
season at the Guatemala site. However neither reflective mulch combined with a squash
trap crop, nor imidacloprid combined with non- and poor-host intercrops, offered any
additional advantage in reducing whitefly densities over these control measures alone. A
spray rotation of vegetable oil and laundry detergent (1% mixed with water) did not
protect a dry-season bean crop from whiteflies or other sucking insects.
139


Table 2-3. Egg density of B. argentifolii (mean SD/cnr) on beans and squash. 1997
u>
Bean
Squash
Week
Treatment
Lower stratum
Upper stratum
Mean
Mean
1
Bean
Mulch
Squash
Squash/mulch
15.32 10.73a1
4.89 4.17b
9.18 4.14ab
5.93 3.48b
27.37 33.31 **
23.07 27.53 **
2
Bean
Mulch
Squash
Squash/mulch
16.77 10.44
11.25 8.18
14.48 10.27
13.29 16.62
27.11 13.47
29.86 15.15
34.95 22.13
26.46 18.91
21.94 12.81
20.55 15.19
24.71 19.74
19.88 18.50
123.71 137.66**
134.92 163.37**
3
Bean
Mulch
Squash
Squash/mulch
1.71 2.27#
0.30 0.54#
2.39 3.49
0.36 0.62#
8.50 5.09#
15.55 9.45#
7.36 6.80
12.87 11.16#
5.11 5.17
7.93 10.19
4.88 5.82
6.61 10.00
47.52 58.68**
45.70 44.85**
4
Bean
Mulch
Squash
Squash/mulch
0.25 0.27#
0.55 0.78#
0.39 0.85#
0.36 0.45#
7.73 9.04#
11.45 11.36#
6.29 4.01#
7.71 2.46#
3.99 7.28
6.00 9.60
3.34 4.14
4.04 4.17
36.22 33.11**
24.09 21.08**
5
Bean
Mulch
Squash
Squash/mulch
0.57 1.29
0.75 1.55#
0.09 0.20#
0.11 0.17#
3.45 3.59
10.39 13.21#
8.43 4.50#
7.52 7.57#
2.01 3.00
5.57 10.37
4.26 5.29
3.81 6.43
37.67 36.67**
46.50 37.98**


55
30.5 m, with the shorter side parallel to the central path. This design was used to allow
for a release of whitefly adults from points spaced evenly along the central path.
Com was planted 25 March. It was fertilized with 67 kg/ha 15-0-14 (N- P205-
K20) on 1 April, 26 April and 14 May. Bean was planted 1 July and fertilized with 33
kg/ha 15-0-14 (N-P205-K20) at planting, on 10 July and 20 July. Overhead irrigation was
used to supplement rainfall. Plots were weeded mechanically and by hand.
Mass-rearing of D. areentifolii. About 30 senescing broccoli (Brassica olerecea
L.) plants infested with B. argentifolii were removed from an organic farm near
Gainesville on 1-6 June. They were potted and placed with 36 flowering hibiscus
(Hibiscus rosa-sinensis L.) plants in a greenhouse at the Department of Entomology and
Nematology at the University of Florida. Hibiscus plants were watered regularly and
fertilized with Purcells Sta-Green plant food (18-6-12 N-P205-K20) (Purcell Industries,
Inc., Sylacauga, AL). By early August, the hibiscus plants were heavily infested with
whiteflies.
Trap preparation. Yellow sticky traps have been used in many instances to
monitor and sample whitefly adults (Ekbom and Rumei 1990). In the evening of 7
August, 180 plastic yellow 710-ml Solo Party cups (Solo Cup Company, Urbana, IL)
were coated with Tangle-Trap Insect Trap Coating (product 95010, Tanglefoot Company,
Grand Rapids, MI), an aerosol adhesive, for use as whitefly traps. The traps were
arranged in 5 rows within each plot at 1.5, 7.6, 14, 20, and 26 m from the edge of the plot
bordering the central path. Three traps were placed in each row. One trap was placed 3.8
m in from either side of the plot, and one was placed 7.6 m within the plot, at the center
of the row.


Lk>
^rt
Table 2-2. Egg density of B. argentifolii (mean SD/cm2) on beans and squash, 1996
Bean
Squash
Week Treatment Lower Stratum Upper stratum Mean
Mean
1
Bean
Mulch
Squash
Squash/mulch
4.16 3.28a1
0.97 1.14c
4.03 1.80ab
1.86 1.72bc
0.96 0.72**
0.07 0.21**
2
Bean
Mulch
Squash
Squash/mulch
0.69 0.78
1.71 2.02
0.36 0.50
1.31 1.59
1.59 0.60
1.40 0.95
1.38 0.77
1.19 1.01
1.14 1.06
1.56 1.55
0.87 0.82
1.24 1.31
5.19 8.89*
5.02 8.28*
3
Bean
Mulch
Squash
Squash/mulch
0.45 0.43
0.33 0.40
0.12 0.26
0.45 0.44
1.33 0.91
1.02 0.92
0.86 0.68
1.19 1.07
0.89 0.83
0.68 0.78
0.49 0.63
0.82 0.89
3.67 6.24*
6.34 7.74**
4
Bean
Mulch
Squash
Squash/mulch
0.02 0.08
0.17 0.41
0.02 0.08
0.05 0.11
0.43 0.66
0.88 0.98
0.48 1.05
0.43 0.52
0.23 0.51
0.52 0.82
0.25 0.77
0.24 0.42
5.00 6.87**
3.76 5.62**
5
Bean
Mulch
Squash
Squash/mulch
0.31 0.43
0.86 0.75
0.41 0.85
0.29 0.39
0.24 0.58
0.31 0.39
0.38 0.75
0.14 0.26
0.27 0.50
0.58 0.65
0.39 0.79
0.22 0.33
0.51 0.81
0.63 0.93*


53
placed in unwaxed cylindrical 0.95 liter cardboard cartons (Fonda Group Inc., Union, NJ)
to allow parasitoids to emerge.
Corn heiuht. The height of five corn plants per row was measured on 4 October to
assess the barrier effect.
Yield. Bean was harvested from two 2.0-m sections from each plot on 22
November. Fresh weight was recorded.
Statistical analysis. Densities of B. argentifolii eggs, nymphs, parasitized nymphs
and red-eyed nymphs were compared among bean treatments using analysis of variance
(PROC GLM, SAS version 6.11, SAS Institute 1996). Densities of whitefly immatures on
bean and eggplant in the trap crop test were compared using the same test, as was bean
yield. When appropriate, mean separation was carried out using Tukeys Studentized
Range test.
1997
Research desiun and plot management. In 1997 the com barrier treatment was
repeated on a larger scale. Three treatments were compared to evaluate the influence of
the barrier crop and the effect of barrier row orientation to wind direction on adult
whitefly movement. Prevailing winds in August in the area tend to be from the east. The
treatments were 1) bean planted in monoculture (bean alone), 2) alternating rows of
bean and com planted north to south (barrier) and 3) alternating rows of bean and com
planted east to west (open) (Figure 3-1).
Treatments were arranged in a randomized complete block strip split plot design.
Each treatment was replicated four times. The four blocks were arranged in pairs on
either side of a 12 m-wide path running north to south. Treatment plots were 15.25 m x


13
systems would support diverse but limited populations of both herbivores and natural
enemies. Competition over resources would dampen oscillations among all trophic
levels, creating a stable system, free from the pest outbreaks that characterized
monocultures.
Root (1973) found that herbivores were less dense in B. olereacea grown in
diverse than in simple stands, but determined that this could not be explained by
increased activity of natural enemies. Summarizing the literature. Root explicitly defined
the generally accepted "enemies hypothesis, and added to it the resource concentration
hypothesis to explain the reduction in herbivore load he had observed. According to the
resource concentration hypothesis, herbivores with a narrow host range are more likely to
find and remain on hosts grown in pure stands, and will attain higher relative densities in
simple environments (Root 1973). Trenbath (1976, 1977) outlined the fly-paper effect,
a variation on the resource concentration hypothesis, which states that the time spent
searching and probing diversionary intercrops will reduce the time and energy invested in
damaging main crops, and may increase mortality among potential pests before they
affect the main crop.
Vandermeer (1989) proposed three hypotheses to encompass all of the
mechanisms suggested by Aiyer (1949), Root (1973), and Trenbath (1976, 1977). The
disruptive crop hypothesis states that certain intercrop species will disrupt the ability of
a pest to attack the main crop. The trap crop hypothesis refers to the ability of a more
attractive intercrop to draw the pest away from the main crop. Intercrop systems which
reduce herbivore densities by attracting more natural enemies than monocrops are
examples of the enemies hypothesis.


I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Phil
Robert McSorley,
Professor of Entomology and Nematology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
/3ont- Allen
Vjhxiissor of Entomology and Nematology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
<"- rv\tA-*ja V
Heather J. McAuslane
Associate Professor of Entomology
and Nematology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.
Raymond N. Gallaher
Professor of Agronomy
This dissertation was submitted to the Graduate Faculty of the College of
Agriculture and to the Graduate School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy/
December 1999
Dean, College of Agriculture
Dean, Graduate School


22
An additional objective of the research was to determine if whitefly suppression
through intercropping could be enhanced by integration with other control strategies. In
the first set of studies, plastic mulch with a strip of reflective aluminum paint was tested
alone and in combination with the trap crop. Imidacloprid and a detergent/oil rotation
were tested as subplot pesticide treatments in some intercropping studies in Guatemala.
The final study in Guatemala included an initial evaluation of methods for protecting
tomato seedlings from whitefly damage in the nursery stage.


Table 6-3. Immature B. argentifolii on bean sampled on whole leaves from three strata mean numbers per cm2, coefficients of
variation, and r values'
Eggs
Nymphs2
Parasitized nymphs
Red-eyed nymphs
Total
Wk
Stratum
Mean
cv
2
r
Mean
CV
2
r
Mean
CV
2
r
Mean
CV
2
r
Mean
CV
2
r
1
first true
leaves
1.82
67
0.90**4
0
-
-
0
-
-
0
-

1.82
67
0.90**
3
upper
0.65a1
78
0.19+
0.23a
277
ns
0a
-
-
0a
-
-
0.88a
119
ns
middle
0.62a
70
0.56**
2.5b
60
0.52**
0.01a
403
0.45**
0.007a
381
ns
3.1b
59
0.63**
4
upper
0.65a
78
0.71**
0.1a
232
ns
0.001a
548
ns
0a
-
-
0.76a
84
0.52**
middle
0.47ab
68
0.40*
1.12b
110
0.56**
0a
-
-
0a
-
-
1.58b
84
0.77**
lower
0.31b
122
ns
1.98c
67
0.29*
0.07b
1 18
0.21*
0.01a
548
0.96**
2.36b
67
ns
5
upper
1.20a
103
ns
0.09a
283
ns
0a
-
-
0a
-
-
1.29a
98
ns
middle
0.39b
137
0.21 +
0.53b
101
0.50**
0.003a
548
ns
0a
-
-
0.92a
85
0.29*
lower
0.02c
344
ns
0.91b
93
0.41*
0.04b
168
0.60**
0.03a
288
0.64**
1.01a
86
0.28*
6
upper
1.36a
103
0.50**
0.17a
232
0.42**
0a
-
-
0a
-
-
1.55a
85
0.42**
middle
0.53b
182
ns
0.77b
85
ns
0.003a
547
ns
0.007a
548
ns
1.30a
95
0.33*
lower
0.06c
283
ns
0.83b
71
0.64**
0.05b
156
0.84**
0.07b
133
0.26+
1.00a
65
0.20+
7
upper
0.95a
130
0.58**
0.18a
180
ns
0.001a
395
ns
0a
-
-
1.13a
109
0.50**
middle
0.25b
130
0.33*
0.69b
139
0.33*
0.01a
429
ns
O.Olab
548
ns
0.95ab
117
0.30*
lower
0.03c
297
ns
0.40ab
93
ns
0.04a
232
0.58**
0.03b
182
ns
0.51b
86
ns
8
upper
0.24a
190
0.82**
0.23a
168
ns
0a
-
-
Oa
-
-
0.46a
118
0.61**
middle
0.12ab
251
0.43**
0.68b
136
0.40*
0.03b
213
ns
0.003ab
548
ns
0.82a
142
0.38+
lower
0.01b
207
ns
0.26a
96
0.46**
0.14b
203
0.84**
0.08b
209
0.44**
0.49a
121
0.74**
'r values for regression equation between whole plant data and disc punch data,
includes all nymphs except parasitized and red-eyed nymphs.
3means followed by letters are significantly different (p < 0.05) according to pair-wise t-test.
4**,*,+ indicate r significant at p < 0.01, p < 0.05, and p < 0.10, respectively; ns = not significant at p < 0.10.


Table 2-7. Parasitized nymph density (mean SD/cm2) of B. argentifolii on bean. 1997
Week
Treatment
Lower stratum
Upper stratum
Mean
3
Bean
0.32 0.36
0
0.16 0.29
Mulch
0.25 0.28
0
0.13 0.23
Squash
0.13 0.19
0
0.06 0.15
Squash/mulch
0.34 0.37
0
0.17 0.31
4
Bean
0.61 0.55
0.02 0.05
0.31 0.48
Mulch
0.61 0.70
0.02 0.05
0.31 0.57
Squash
0.89 0.86#
0.11 0.30#
0.50 0.74
Squash/mulch
0.34 0.42
0
0.17 0.33
5
Bean
0.41 0.67
0.02 0.05
0.21 0.50
Mulch
1.00 0.92#
0.09 0.25#
0.54 0.80
Squash
0.79 0.67#
0#
0.39 0.61
Squash/mulch
0.70 0.97
0
0.34 0.75
6
Bean
0.48 0.59
0.20 0.56
0.34 0.57a1
Mulch
0.25 0.50
0
0.13 0.37b
Squash
0.48 0.44
0
0.24 0.39ab
Squash/mulch
0.45 0.76
0.04 0.10
0.24 0.57ab
1 Means in the same column with the same letter are not significantly different according to Tukeys Studentized Range test with
controlled type 1 experimentwise error rate (a=0.05). The absence of letters in a column indicates lack of significant differences
among any means. # indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p <
0.05.


Table 4-2. Height and weight (x SD) of bean plants under 2 cropping systems and 3 pesticide regimes. Diversity study, first bean
crop.
Wk
Pesticide
Height (cm)
Weight (g)
Monocrop
Intercrop
mean
Monocrop
Intercrop
mean
3
hnidacloprid
8.192.76
8.702.75
8.442.73a'
-
-
-
Detergent/oil
5.85.33
5.72.29
5.79il.24b
-
-
-
Control
5.98.18
5.99.72
5.991.46b
-
-
-
Mean
6.672.15
6.802.40
-
-
4
hnidacloprid
26.61 9.05
28.556.86
27.607.95a
71.50i62.34
105.8058.94
88.6559.08a
Detergent/oil
13.393.36
11.85i2.95
12.603.21b
31.45i9.15
27.932.07
29.69 10.09b
Control
10.883.51
11.45i4.10
11.153.76b
38.6327.32
23.383.40
31.00 19.78b
Mean
16.859.04
17.38i9.41
47.1940.22
52.3750.51
5
Imidacloprid
_
_
-
158.50120.77
152.5072.98
155.5092.43a
Detergent/oil
-
-
-
25.95il4.68
36.757.79
31.35il6.16 b
Control
-
-
-
26.257.68
29.25ilO.69
27.758.76 b
Mean
-
-
70.2391.12
72.8371.01
6
Imidacloprid
46.7515.78
55.004.40
50.88 1.59a
160.2592.25
167.75 13.85
164.0096.01a
Detergent/oil
20.886.61
28.256.44
24.567.22 b
26.1320.22
37.5022.47
31.81i20.70 b
Control
25.504.43
21.00i5.23
23.255.09 b
24.8820.58
15.756.24
20.314.90 b
Mean
31.04il4.96
34.756.04
70.4283.37
73.6792.72
'Data are means of 5 replications. Means in columns for a given week followed by the same letter are not significantly different (p <
0.05) according to Tukeys Studentized Range test. No letters present indicate no differences for that week. No differences (p > 0.1)
between means in monocrop vs. intercrop treatments on any sampling date.


33
However, Al-Musa (1982) and Schuster et al. (1996) reduced the incidence of
virus in tomato (Lycopersicon esculentum Mill.) by trap cropping with cucumber
(Cucumis sativus L.) and squash, respectively. Meena et al. (1984) reported a reduction
in Bemisia-vectored yellow mosaic of moth bean (Vigna aconitifolia (Jacqu.) Marechal)
by trap cropping with guar (Cyanopsis tetragonoloba (Linn.) Taub), sesame (Sesamum
indicum L), millet (Pennisetum typhoides (Burm, F.) Stapf. and Hubb.) or sorghum
{Sorghum vulgare L). The latter two crops are not hosts of Bemisia, however, so it is
possible that a different mechanism was involved. These studies indicate that trap
cropping can be used to reduce transmission of virus by whiteflies.
Conclusion
In our study squash did not function as a trap crop either by reducing density of
whitefly or presence of virus on adjacent bean. Oviposition was consistently higher on
squash than on bean. Oviposition was significantly less on bean in plots with reflective
silver mulch during the first week of sampling in 2 of the 3 years of this study. Mulch
improved plant quality and increased yield compared to unmulched plants. Neither
squash, reflective mulch nor the combination of the 2 provided significantly greater
protection from B. argentifolii than bean planted alone on bare soil.


154
Norman, J. R., Jr., D. G. Riley, P. A. Stansly, P. C. Ellsworth, and N.C. Toscano. 1993.
Management of silverleaf whitefly: a comprehensive manual on the biology,
economic impact and control tactics. United States Department of Agriculture,
Washington, DC.
Ohnesorge. B., and G. Rapp. 1986a Methods for estimating the density of whitefly
nymphs (Bemisia tabaci Genn.) in cotton. Tropical Pest Management 32: 207-
211.
Ohnesorge, B., and G. Rapp. 1986b. Monitoring Bemisia tabaci: a review. Agrie.
Ecosys. Environ. 17: 21-27.
Ohnesorge, B., N. Sharaf, and T. Allawi. 1980. Population studies on the tobacco
whitefly Bemisia tabaci during the winter season. Zeitschrift fur Angewandte
Entomologie 90: 226-232.
Organic Materials Review Institute. 1998. Operating manual for review of brand name
products. Organic Materials Review Institute, Eugene, OR.
Pernezny, K., D. Schuster, P. Stansly, G. Simone, V. Waddill. J. Funderburk, F. Johnson,
R. Lentini, and J. Castner. 1995. Florida tomato scouting guide with insect and
disease identification keys. Cooperative Extension Service SP-22, Institute of
Food and Agricultural Sciences, University of Florida, Gainesville, FL.
Perrin, R. M. 1977. Pest management in multiple cropping systems. Agro-Ecosystems
3: 93-118.
Perrin, R. M., and M. L. Phillips. 1978. Some effects of mixed cropping on the
population dynamics of insect pests. Entomol. Exper. Appl. 24: 385-393.
Perring, T. M. 1996. Biological differences of two species of Bemisia that contribute to
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Perring, T. M., A. D. Cooper, R. J. Rodriguez, C. A. Farrar, and T. S. Bellows, Jr. 1993.
Identification of a whitefly species by genomic and behavioral studies. Science
259: 74-77.
Perring, T. M., K S. Mayberry, and E. T. Natwick. 1995. Silverleaf whitefly
management in cauliflower using a trap crop. Page 151 in T. J. Henneberry, N. C.
Toscano. R. M. Faust, and J. R. Coppedge, eds. Silverleaf whitefly: 1995
supplement to the five-year national research and action plan. ARS 1995-2. U. S.
Department of Agriculture, Washington, DC.
Pimentel, D. 1961. Species diversity and insect population outbreaks. Ann. Entomol.
Soc. Am. 54: 76-86.


83
recorded for weeks 2-5, and fresh plant weights were taken during weeks 3-5. On 2
December the number of tomato plants per row was recorded.
On 7 October one whole rosa de jamaica plant per block was examined for
whitefly immatures.
Statistical Analysis
Numbers of whitefly immatures and plant size characteristics were compared
between treatments using analysis of variance with SAS software (SAS 1996).
Nursery and Corn/Cilantro Study
In the final study, carried out toward the end of the rainy season, an attempt was
made to develop an overall management program for whitefly on tomato. Two methods
of tomato seedling production were compared in a nursery study. Seedlings were either
treated with imidacloprid or grown under protective mesh in covered nurseries. The
seedlings produced in this nursery study were then used in the corn/cilantro study.
Tomatoes in the corn/cilantro study were grown under four treatments: monocropped
with and without imidacloprid. and intercropped with and without imidacloprid. The
seedlings used in the imidacloprid treatments were those which had been treated with
imidacloprid in the nursery. The untreated seedlings were those which had been grown
under protective mesh.
The intercrop treatment consisted of tomato intercropped with corn and cilantro.
High numbers of generalist predators had been observed on flowering cilantro in the
diversity study, and an attempt was made to increase densities of predators on tomato by
intercropping with cilantro. In the intercrop treatment, corn was used to anchor the nylon
cord which supports growing tomato, replacing the wooden stakes which are normally


125
garden bean (Harris Seed, Rochester, NY) was used. Bean in some plots was
intercropped with rows of field corn (Zea mays L.) or eggplant {Solarium melongena L.)
to test for intercropping effects, the results of which are reported elsewhere (see
chapter 3). Bean was grown in a randomized complete block design of five blocks, each
containing three plots of bean with various intercrop treatments (corn, eggplant, control).
Whitefly densities on bean were not significantly affected by intercrop treatments,
allowing counts on bean from the intercrop treatments to be pooled for this study.
Therefore, whiteflies were sampled from bean in 15 plots.
Each plot contained 14 rows, 6.1 m in length with 0.9 m between rows. Spacing
between bean plants within each row was 10 cm. Beans were planted 15 September and
fertilized with 0.37 kg of 15-0-14 (N-P205-K20) per row on 23 September and 12
October. Overhead irrigation supplemented rainfall. Weeding was mechanical or by
hand. No pesticides were applied to the bean crop.
Sampling
Bean was sampled using three sample units: whole plant, whole leaf, and disc
punched from a leaf. For each sample unit, only the underside of the leaf was examined
(Ekbom and Rumei 1990). Whole plant sample units consisted of the examination of all
fully expanded leaves. Whole leaf sample units consisted of the examination of the
central leaflet of a trifoliate from the upper, middle, and lower plant canopy. Hereafter,
these three levels will be referred to as the upper, middle, and lower stratum. Disc punch
samples were taken from the same strata as whole leaf samples. They were made by
pressing a 3.35-cnr cork borer (McAuslane et al. 1995) into the lower right quadrant on
the underside of the leaf.


1
Squash
0.73 0.39#
54.00 27.50#
Squash/mulch
0.11 0.17#
46.03 20.47#
2
Squash
0.18 0.26#
247.25 75.65#
Squash/mulch
0.07 0.07#
269.77 125.02#
3
Squash
78.16 67.16#
16.88 26.85#
Squash/mulch
70.32 37.31#
21.09 39.17#
4
Squash
46.95 42.90
25.50 15.70
Squash/mulch
11.02 18.10
14.00 14.04
5
Squash
66.31 30.69#
9.04 8.14#
Squash/mulch
70.21 34.20
22.79 25.22
6
Squash
24.86 19.28
7.95 6.50
Squash/mulch
25.50 17.85
14.98 18.10
# indicates that upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.05. @ indicates that
upper and lower stratum means are significantly different according to the pair-wise t-test at p < 0.10.
-t-


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increasing world food supplies. Pages 1-10 in R. I. Papendick, P. A. Sanchez, and
G. B. Triplett, eds. Multiple cropping. American Society of Agronomy Special
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Bemisia tabaci. Pakistan J. Zool. 27: 269-272.
143


100
host, or in a some instances produced higher whitefly numbers than appeared in
monoculture.
Row and mixed intercropping with poor and non-hosts did not reduce densities of
whitefly immatures on bean or tomato compared to bean and tomato grown in
monoculture. Imidacloprid effectively reduced whitefly immatures in dry and rainy
seasons. Imidacloprid combined with intercropping offered no advantage over
imidacloprid applied in monoculture. A detergent and oil rotation was ineffective in
reducing damage from whitefly and other sucking insects in the dry season.
High numbers of beneficial insects were observed associated with flowering
cilantro and rosa de jamaica early in the rainy season. However, efforts to increase
beneficial insects in tomato with cilantro later in the rainy season failed because of heavy
precipitation and disease. Parasitoids reared from whitefly nymphs on bean from May
through July consisted entirely of Encarsia pergandiella, which was the only parasitoid
species present during the entire sampling period. Members of the Encarsia meritoria
species complex appeared in low numbers early in August, and were present throughout
the rainy season. Amitus fuscipennis was reared from whiteflies on tomato in November,
and achieved high levels in some treatments by early December. Parasitoid diversity was
increased in the rainy season on tomato intercropped with rosa de jamaica and corn when
compared to tomato grown in monoculture, or to tomato intercropped only with corn.


6
et al. 1996). Trialeurodes vaporariorum demonstrates similar orientation behavior to
these two wavelength ranges (Coombe 1981, 1982, Vaishampayan et al. 1975a).
Neither B. tabaci nor T. vaporariorum respond to host-specific visual or olfactory
cues (Mound 1962, van Lenteren and Noldus 1990, Vaishampayan et al. 1975a, 1975b).
Feeding behavior studies and examinations of precibarial and cibarial chemosensilla of B.
tabaci and T. vaporariorum indicate that the two species must probe a plant in order to
determine if it is an acceptable host (Hunter et al. 1996, Lei et al. 1998, van Lenteren and
Noldus 1990). Oviposition and longevity for each species vary on different crops. This
has led to rankings of host suitability for T. vaporariorum (van Boxtel et al. 1978, van
Lenteren and Noldus 1990, van de Meredonk and van Lenteren 1978), B. tabaci (Aslam
and Gebara 1995, Costa et al. 1991, Coudriet et al. 1985, Naresh and Nene 1980,
Simmons 1994), B. argentifolii (Chu et al. 1995, Tsai and Wang 1996, Wang and Tsai
1996), and to comparisons of host plant suitability for both species or biotypes of Bemisia
(Blua et al. 1995, Drost et al. 1998). Survival and host plant selection by a whitefly
female may be influenced by the plant species on which she was reared (Costa et al. 1991,
van Boxtel et al. 1978). Both B. tabaci and T. vaporariorum emigrate from some host
species more quickly than from others (Costa et al. 1991, van Lenteren and Noldus 1990,
Verschoor-van der Poel and van Lenteren 1978). This may influence host-specific rates
of oviposition.
Bemisia tabaci and T. vaporariorum females usually oviposit on the abaxial side
of young leaves (Noldus et al. 1986a, Simmons 1994). Bemisia tabaci females seem to
prefer a moderate degree of pubescence to either glabrous or extremely hairy leaf surfaces
for oviposition (Butler et al. 1986, McAuslane 1996). First-instar nymphs tend to move a