Intercropping and whitefly (Homoptera: Aleyrodidae) management

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Title:
Intercropping and whitefly (Homoptera: Aleyrodidae) management
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viii, 163 leaves : ; 29 cm.
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English
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Smith, Hugh Adam, 1963-
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Subjects / Keywords:
Aleyrodidae -- Control   ( lcsh )
Intercropping   ( lcsh )
Agricultural pests   ( lcsh )
Entomology and Nematology thesis, Ph. D   ( lcsh )
Dissertations, Academic -- Entomology and Nematology -- UF   ( lcsh )
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bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

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

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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oclc - 43707757
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Table of Contents
    Title Page
        Page i
    Dedication
        Page ii
    Acknowledgement
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
    Abstract
        Page vii
        Page viii
    Chapter 1. Literature review and research goals
        Page 1
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    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 (Phaseolus vulgaris L.)
        Page 23
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    Chapter 3. Potential of field corn (Zea mays L.) as a barrier crop and eggplant (Solanum melongena L.) as a trap crop for management of the silverleaf whitefly, Bemisia argentifolii (Homoptera:aleyrodidae) on bean (Phaseolus vulgaris L.) in North Florida
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    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 Salama Valley, Baja Verapaz, Guatemala
        Page 68
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    Chapter 5. A comparison of some arthropod groups on monocropped and intercropped tomato (Lycopersicon esculentum Mill.) in Baja Verapaz, Guatemala
        Page 112
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    Chapter 6. Methods for sampling immature stages of Bemisia argentifolii (Homoptera:aleyrodidae) on bean (Phaseolus vulgaris L.)
        Page 122
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    Chapter 7. Summary and conclusions
        Page 139
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    Appendix. Some whitefly hosts at different elevations in eastern Guatemala
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    References
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    Biographical sketch
        Page 163
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Full Text










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