The design of pest stable corn agroecosystems based on the manipulation of insect populations through weed management

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The design of pest stable corn agroecosystems based on the manipulation of insect populations through weed management
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Corn -- Diseases and pests -- Control   ( lcsh )
Pests -- Integrated control   ( lcsh )
Cropping systems   ( lcsh )
Corn -- Diseases and pests -- Control   ( fast )
Cropping systems   ( fast )
Pests -- Integrated control   ( fast )
Entomology and Nematology thesis Ph. D
Dissertations, Academic -- Entomology and Nematology -- UF
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Thesis--University of Florida.
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Includes bibliographical references (leaves 61-67).
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Also available online.
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Typescript.
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Vita.
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by Miguel Angel Altieri.

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THE DESIGN OF PEST STABLE CORN AGROECOSYSTEMS
BASED ON THE MANIPULATION OF INSECT POPULATIONS
THROUGH WEED MANAGEMENT







BY

MIGUEL ANGEL ALTIERI






























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


UNIVERSITY OF FLORIDA

1979







































To Grisell, Naraya, Joshua and Padre Ricardo
whose love and strength made this work possible,
and to mankind, for its evolution and survival

















ACKNOWLEDGEMENTS


My deep gratitude is expressed to the following persons who

contributed time, knowledge, resources, and friendship toward making

this work a reality.

To Dr. Willard H. Whitcomb for opening new horizons in my

knowledge of predators and biological control in general and for

serving as chairman of my committee.

To Dr. E.V. Komarek and Tall Timbers Research Station for

financial support and confidence in my research and ideas.

To Dr. S.H. Kerr for his continuous advice and support in

administrative and academic affairs.

To the Department of Entomology for financial support and for

the opportunity of obtaining higher education.

To the Soil and Health Foundation and Rodale Press for financial

and ideological support of my ecological approach to pest management.

To Dr. Ray William for his advice and friendship and for serving

as a philosophical catalyst.

To Drs. C.S. Barfield and D. Herzog for serving on my supervisory

committee and their cooperation, advice and encouragement through my

studies.

To Dr. Jerry Stimac for his advice and constructive criticisms.

To Maria I. Cruz for her help, patience and friendship.

iii












To Cesar Garcia, Rod Gillmore, Antonio Gonzalez, Fred Collins,

J. Ewell, R. Flammer, M. Palada, G. Wiser, R. Hudgens, for their

friendship and encouragement.

To Mr. Joe Quigg for providing me with an inspiring place to

write this dissertation.

To Grisell, Naraya, and Joshua for their love, patience,

understanding and encouragement.

To Fr. Richard Teall for his example of human and Christian

attitude.











































iv

















TABLE OF CONTENTS


ACKNOWLEDGEMENTS ............................................... iii

LIST OF TABLES ................................................. vii

LIST OF FIGURES ................................................ ix

ABSTRACT ....................................................... x

INTRODUCTION ................................................... 1

LITERATURE REVIEW .............................................. 4

Some Limitations for the Development of Pest
Management Systems......................................... 4

Ecological Theory and Pest Management....................... 6

Dynamics of Insect Populations in Complex Crop
Communities ............................................... 7

Effects of Crop Habitat Diversity on Natural
Enemies.................................................... 11

Parasitoids........................................... 11

Predators............................................. 14

Weeds as Sources of Predators............................... 1.7

MATERIALS AND METHODS........................................... 19

Tall Timbers Site.......................................... 19

Green Acres Site........................................... 22

RESULTS AND DISCUSSION.......................................... 24

Nature of the Weed Communities.............................. 24

Weeds of Tall Timbers Research Station................ 24

Weeds of Green Acres Farm............................. 26

Pest Incidence............................................. 26



v











Predator Colonization ..................................... 28

Predator Abundance and Diversity............................ 33

Trends of Individual Predator Species...................... 37

Predation Pressure ........................................ 39

Weeds and Number of Ground Predators........................ 41

Predator Dynamics in Weed Habitats.......................... 43

Crop Yields ............................................... 48

CONCLUSIONS .................................................... 53

APPENDIX ....................................................... 58

LITERATURE CITED ............................................... 61

BIOGRAPHICAL SKETCH.............................................. 68









































vi

















LIST OF TABLES


Table Page

1 Selected examples of multiple cropping systems
that effectively prevent insect pest outbreaks
(Altieri et al. 1978)..................................... 8

2 Selected examples of cropping systems in which the
presence of weeds enhanced the biological control of
specific crop pests....................................... 12

3 Selected examples of weeds that provide alternate
prey for general predators................................. 18

4 Densities and species numbers of weed communities
within corn fields at Tall Timbers Research
Station in north Florida (1978) ........................... 25

5 Relative abundance and species numbers of weed
communities within corn fields at Gainesville,
Florida (1979) ........................................... 27

6 Percent of damaged plants by fall armyworm,
Spodoptera frugiperda J.E. Smith, and percent
of ears damaged by corn earworm, Heliothis
zea (Boddie), in weed-free and weed-diversified
corn systems at two sites in north Florida............... 30

7 Colonization of the corn systems by predaceous
arthropods in north Florida................................ 31

8 Mean number of predator species throughout the
growing season in weed-free and weed-diversified
corn systems at two sites in north Florida............... 34

9 Total numbers and diversity of general predators
in different corn cropping systems at two sites
in north Florida.......................................... 35

10 Relative abundances of individual predator species in
different corn cropping systems at two sites in
north Florida............................................. 38






vii










11 Mean percent of Spodoptera eggs consumed by predators
over four sampling dates at two sites in
north Florida............................................ 40

12 Mean numbers of ground predators caught by
pitfall traps in different corn cropping systems
at two sites in north Florida........................... 42

13 Mean relative density and number of species of
predators associated with weed communities within
corn fields at two sites in north Florida................ 46

14 Mean numbers of individual predators species and
families collected on weed communities within corn
fields at two sites in north Florida..................... 49

15 Average corn yields under different cropping systems
at two sites in north Florida............................ 50

16 Common predaceous arthropods of north Florida corn
fields................................................... 58







































viii


















LIST OF FIGURES


Figure Page

1 Relationship (y = 104 0.2x) between weed density
and percent of corn plants damaged by fall
armyworm (Spodoptera frugiperda J.E. Smith)
in north Florida; r = -0.93, n = 15/mean................ 29

2 Comparison of mean abundance of Labidura riparia
(Pallas) in mulched corn plots and bare soil corn
plots in north Florida; confidence limits
(a = 0.05) indicated for each mean....................... 44

3 Relationship (y = 0 + 0.58x) between weed species
diversity and predator abundance in weed
communities of north Florida corn fields;
r = 0.84, n = 12/mean................................... 47


































ix

















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


THE DESIGN OF PEST STABLE CORN AGROECOSYSTEMS BASED ON
THE MANIPULATION OF INSECT POPULATIONS THROUGH WEED MANAGEMENT

By

MIGUEL ANGEL ALTIERI

December 1979

Chairman: W.H. Whitcomb
Major Department: Entomology and Nematology

Populations of insect pests and associated predaceous arthropods

were sampled by direct observations and other relative methods on

simple and diverse corn habitats at two sites in North Florida during

1978 and 1979. Through various cultural manipulations (direct sowing,

differential fertilization, early plowing, etc.) certain weed

communities were selectively established in alternate rows within corn

plots.

Fall armyworm (Spodoptera frugiperda J.E. Smith) incidence was

consistently higher in the weed-free habitats than in the corn habitats

containing natural weed complexes or selected weed associations. Corn

earworm (Heliothis zea Boddie) damage was similar in all weed-free and

weedy treatments suggesting that this insect is not greatly affected

by weed diversity. Only the diversification of corn with a strip of

soybean significantly reduced corn earworm damage.





x










In one of the sites distance between plots was deliberately

reduced. Because predators moved freely from one habitat to the other,

such movements made treatments more similar in their predator fauna

and it was hard to establish real population differences. Large

distances between plots minimized this complication and population

densities and diversity of common foliage insect predators were greater

in the weed diversified corn systems than in the weed-free plots.

Trophic relationships in the weedy habitats were very complex compared

to food webs in monocultures.

Differences in the abundance of ground predators between plots

could not be attributed to weed diversity. Predator pressure was

monitored using Spodoptera eggs as artificial prey. The numbers of

fall armyworm eggs taken were not shown to be related to numbers of

predators present nor to the type and density of the vegetation. Corn

systems mulched with rye straw had the lowest infestation of fall

armyworm and the highest numbers of predaceous earwigs.

The mean number of predator species and individuals was higher at

the site where corn plots were surrounded by complex vegetation than at

the site surrounded by annual crops, probably because diverse adjacent

areas provided refuge to predators, thus acting as colonization sources.

Corn-weed mixtures confer advantages in pest management and can

be agronomically acceptable, although corn yields were generally lower

in the weed diversified corn plots than in the weed-free plots. These

systems can be designed to be self-operating without much technological

inputs. Such an agroecosystem has obvious implications in an era of

energy crisis and environmental concern.






xi

















INTRODUCTION


The main motivation for this study is the concern that despite

numerous reviews and discussions concerning ecological theory and pest

management (e.g., Southwood and Way 1970, Price and Waldbauer 1975,

Levins and Wilson 1979), questions central to ecology like the

relationships among diversity, complexity and stability of organisms

in ecosystems have had no major impact on economic entomology. In

this study, stability is defined as restricted fluctuations in pest

population density through time and diversity as a measure of richness

of species in a habitat (Murdoch 1975). Little work has been done on

the design of crop systems based on ecological principles. Only a

few attempts have been made to modify monocultural systems in the

direction of diversity for the purpose of enhancing ecological features

vital for successful pest management (Litsinger and Moody 1976, Perrin

1977).

Most researchers concerned with the ecological basis for the

management of insect populations attribute outbreaks of insect pests

in agriculture to the extensive use of large crop monocultures

(Southwood and Way 1970, Price and Waldbauer 1975, Atsatt and O'Dowd

1976, Pimentel and Goodman 1978). Exposed fields and concentrations

of a single crop species open myriad possibilities for pest infestations

(Browning 1975). Pure crop stands provide concentrated resources and

uniform physical conditions that directly influence members of the crop




1







2



fauna (Root 1973). The abundance and effectiveness of general

predators are reduced because these simplified environments do not

provide adequate alternate sources of food, shelter, breeding sites,

and other environmental factors (Hagen et al. 1976). Herbivorous

insect pests are more likely to colonize and remain longer on crop

hosts that are concentrated because usually the pests' entire life

requirements are met in these simple environments (Root 1973). As a

result, the abundance of specialized pests attain economically

unacceptable levels.

Many crop plants, like their wild relatives, fare better in species

diverse, structurally complex communities (Wilken 1977). Crops grown

in floristically diverse habitats suffer a lower herbivore load than

conspecifics grown in monocultures. Many studies have shown that

reducing crop stand purity by interplanting different crop species

greatly reduces the density of herbivorous pests (Marcovitch 1935,

Stern 1969, DeLoach 1970, Tahvanainen and Root 1972, Raros 1973,

Dempster and Coaker 1974, Van Emden and Williams 1974, Litsinger and

Moody 1976, Perrin 1977, Altieri et al. 1978, Perrin and Phillips 1978,

Risch 1979). Outbreaks of certain types of crop pests are more likely

to occur in weed-free fields than in weed diversified crop systems

(Pimentel 1961, Adams and Drew 1965, Dempster 1969, Flaherty 1969,

Root 1973, Smith 1976a, Altieri et al. 1977). Crop fields with a dense

weed cover and high diversity usually have greater numbers of predaceous

arthropods than do weed-free fields (Pimentel 1961, Dempster 1969,

Flaherty 1969, Pollard 1971, Root 1973, Smith 1976b, Speight and Lawton

1976). Ground beetles (Dempster 1969, Speight and Lawton 1976, Thiele

1977), syrphids (Pollard 1971, Smith 1976b) and lady beetles (Bombosch







3


1966, Perrin 1975) have been reported to be greatly abundant in weed

diversified systems. The presence of certain weeds within crop habitats

offers many important requisites for natural enemies such as nectar and

pollen sources, alternate prey and hosts and microhabitats that may not

be available in a monoculture (Altieri and Whitcomb 1979).

Based on experimental results and on theoretical considerations

which imply that diversity of species and habitat complexity confer

stability to the insect community (Pimentel 1961, van Emden and Williams

1974, Murdoch 1975), some researchers have envisaged the maintenance of

specific weed associations in crop areas to provide subsidiary food

for entomophagous insects and thus, improve biological control of

certain pest species (van den Bosch and Telford 1964). Practical

implementation of this approach remains disappointingly low, however.

The present study describes some ways in which corn agroecosystems

could be designed to reduce the severity of insect pest problems.

Discussed herein are some of the changes in abundance that target pests

and associated predators undergo when certain weed associations are

selectively allowed to grow in corn fields to provide additional

safeguards against pest insects.

















LITERATURE REVIEW


Some Limitations for the Development of
Sound Pest Management Systems


The efficient integration of several methods of pest control to

suppress a complex of pests while achieving maximum yield and quality

on one hand and minimum damage to the environment on the other, have

been major goals of integrated pest management (IPM) (Rabb and Guthrie

1970, Metcalf and Luckman 1975). This desired balance has seldom been

attained, however. There are several reasons:

1. Contemporary pest management practice still has too narrow an

ecological knowledge base. Overemphasis on yield or market quality

almost invariably means shattering of the crop community homeostasis

(DeLoach 1970). Seldom can maximum stability and maximum productivity

be achieved simultaneously in the same ecosystem (Turnbull 1969).

Increasing agricultural production will always involve risk of serious

degradation of at least some agro-ecosystems (Loucks 1977). For this

reason, stabilization of yield rather than maximization of yield should

be a major goal of IPM.

2. The explosive expansion of the pesticide industry has inflicted

strong socioeconomic and philosophical obstacles to the development of

sound ecological pest management strategies. The present agricultural

epoch, clearly dominated by the petrochemical industry, envisions food

cultivation as a business enterprise to be operated strictly for the




4






5



purpose of generating profit in a market economy (Bookchin 1976, van

den Bosch 1978). Integrated pest management should be part of a

philosophy that views agriculture as the activity of a society whose

historic role is to maintain, within ecological limits, productive

land in order to sustain present and future generations of people.

3. Nearly all research in pest management is highly reductionist,

parochial and discipline-oriented. Few articles with truly holistic

approaches have emerged in the trade journals (Potts and Vickerman

1974). A common erroneous pattern of many pest management systems is

the combination of various cultural and biological methods of insect

control while simultaneously recommending chemicals for the control of

weeds and diseases. These programs can lead to intensification of pest

problems. For example, 2,4-D is a herbicide commonly used in corn for

the post emergence control of broadleaf weeds. When corn plots were
-i
treated with a regular dose of 0.55 kg, 2,4-D ha aphid numbers and

corn borer infestations increased significantly (Oka and Pimentel 1974).

Similarly, corn plants exposed to 2,4-D were significantly more

susceptible to corn smut disease and to southern corn leaf blight (Oka

and Pimentel 1974). Furthermore, sprays of 2,4-D at normally used rates

caused up to 50% mortality of the larvae stages of coccinellids (Adams

and Drew 1965). Part of this anti-holistic approach to pest management

resides in the lack of trained "generalists" who understand the principles

of managing resources as a part of a total interacting environmental

system (Pimentel 1970).







6


Ecological Theory and Pest Management


The ecological basis of pest management has been the topic of many

review and journal articles during the past decade (Southwood and Way

1970, Price and Waldbauer 1975, Pimentel and Goodman 1978, Levins and

Wilson 1979). Most approaches contrast the structure and function of

agroecosystems and natural, undisturbed ecosystems (Southwood and Way

1970, Price and Waldbauer 1975, Rabb et al. 1976, Pimentel and Goodman

1978). Browning (1975) has advised the study of natural ecosystems

from which knowledge can be gained that is readily applicable to

agroecosystems. According to Price and Waldbauer (1975), agricultural

ecosystems can be viewed in terms of two central concepts of ecology-

island biogeographical theory (Price 1976) and succession of communities.

Most ecologists agree that any pest management approach should try to

develop an agroecosystem that emulates later stages of succession (i.e.,

mature communities) as much as possible, for this is how stability can

be achieved (Root 1973). By adding selective diversity to crop systems,

it is hoped to capture for agroecosystems some of the stability properties

of natural communities (Murdoch 1975). The concept that increased

diversity leads to increased stability has been challenged on theoretical

grounds (van Emden and Williams 1974) but not by reliable studies in

agricultural communities.

For many researchers, the biogeographic region rather than the

single homogeneous field is the appropriate unit for pest management

research (Levins and Wilson 1979). An agroecosystem should be conceived

as an area large enough to include those uncultivated areas which

influence crops through intercommunity interchanges of organisms and

materials (Rabb 1978). Excellent studies on the role of uncultivated








7


land and crop field borders in the biology of crop pests and beneficial

insects have been made (Dambach 1948, Piemeisel 1951, van Emden 1965,

Pollard 1971, Hodek 1973, Thiele 1977). However, little is known yet

of the influence of adjacent habitats on pest incidence in cultivated

fields.


Dynamics of Insect Populations in
Complex Crop Communities


The predisposition of insect outbreaks to occur in monocultures is

well known (Pimentel 1961, Browning 1975). Studies of multiple cropping

systems have shown that populations of herbivorous pests reach higher

densities on crop plants grown in monocultural stands than on plants

grown associated with other plant species (Litsinger and Moody 1976,

Perrin 1977, Altieri et al. 1978, Perrin and Phillips 1978) (See Table

1 for additional examples). These patterns can be explained on the

basis of the following hypotheses:

1. Resource concentration: insect populations can be directly

influenced by the concentration or spatial dispersion of their food

plants. Many herbivores, particularly specialists, are more likely

to find and remain on hosts that are growing in dense or nearly pure

stands (Root 1973).

2. Associational resistance: ecosystems in which plant species are

intermingled possess an associational resistance to herbivores in

addition to the resistance of individual plant species (Root 1975).

Interplanting of host plants can drastically decrease colonization

efficiency and subsequent population density of crop pests (Tahvanainen

and Root 1972). In addition to their taxonomic diversity, polycultures














Table 1. Selected examples of multiple cropping systems that effectively prevent insect
pest outbreaks (Altieri et al. 1978).



::tltipl cropping system Post(s) regulated Factor(s) lovolved References


':IoLn intercropped with Iloll weevil (Anthonomun grandln) Population Increnme of patir!itic ltrcovitch (1935)
lf:';agec :uvpea wnaps (Euinyvt na sp.)

Pa.iclhs Intecrropped with Strawberry leafroller Popu.lntiio Increne of par islte Mnrcovitch (1935)
; '.rivbrres lC (Anryl.iA co~mtma) (Mi' ILcciit' nneiv 1. ri., M. i crobracon
Oriental fruit moth L echtuise nild LlxoplhnI vnrtliLlis)
(Crnpholioitn moltcta)
S:rip cropping of Plant bugs (l.ygu hesperus Prevention of emigration and synchrony van den Bosch and Stern
cotton and alfalfa and L. clisus) in ihe relntion between pc;ius and (1969)
natural enin ies.

:S.rlp cropping of cotton and Corn earworm (llcliothis zea) Increased abundance of pril:ators DeLoach (1970)
;1' i.lfa! on onei si!de and maize and cabbage looper (T.r__icholusia ni)
;:in so.ybvan on the other

!n!.ercropping cotton with sorghum Corn carworm Increased abundance of predators due Fye (1972); Burleigh
'r maize to an increase of alternute prey ct al. (1973)

".ile intercropped with canavalia Ptorachin danra and fall armyworm Not reported Cuevara (1962)
(Spi'diptera frugiperuln)

To:inao and Lobacco intercropped Flea beetloe (Phyllocttra cruclifrnn)) FecdJng inh.lbiltion by oldotri from Tahvanninen and Root (1972)
;l'.h cabbage nonhost planLs

T',; to inlterroppled with (nblrae Dlamondlhnck mothi (.Pl jtlla xyI. el).!a Chemicnlc r oi,'llency or ionrii lur, IRrno (1973)

:reinu I ntere ropped with mnlac Corn borer (O.ltrlit nrnl. fiumnulc.d ) Abundance of apiders (iL?: '.n'p.) Ranros (1973)

i'o-:mat intircropped with rotton Flea beetles (PonLagr_ :n up.) Cliemicnl r):iepllency Litsinger and Moody (1076)

cSname intercropped with cotton Hollothls app. Increase of beneficial in:;!cla and Laster and 'urr (1972)
trap crupping









00





















Table 1 continued



Cabbage intercropped with white Erioischia brassicae, cabbage aphids, Interference of colonization Dempster and Cooker (1974)
and red clover and inported cabbage butterfly and increase of ground beetles
(Pieris rapac)

Sesame intercropped with corn Webworms (Antigontra op.) Shading by the taller companion crop Litainger and Moody (1975)
or sorghum

Inturcropping cowpea and norgh.im Leaf beetle (Ootheca benni s.vni) Interference of air currents Litsinger and Moody (1975)

Cotton intercropped with okrn PodaEgica op. Trap cropping Litsin.ger and Moody (1975)

Corn intercropped with aweet Leaf beetles (Diabrotica spp.) Increase of parasiltj wasps Risch (1979)
potatoes and lenfhoppers (Agallia I.lniln)

Corn intercropped with beans Leafhoppers (Empoascn kraemeri) Increace of beneficial insects and Altierl et al. (1978)
leaf beetle (Diabrotica balteata) interference of colonization
and fall nrmyworn























U5







10


have a relatively complex physiognomy, chemical environment and

associated patterns of microclimates. This biotic, structural, chemical

and microclimatic complexity of mixed vegetation greatly ameliorates the

herbivore pressure on the crop systems as a whole (Tahvanainen and Root

1972).

3. Plant apparency: most crops are derived from early successional

herbs which largely escaped from herbivores in space and time (Feeny

1976). The effectiveness of natural crop plant defenses is reduced by

present agricultural methods. When planted in monocultures, crop plants

become more apparent to natural enemies than are their ancestors in

nature. The apparency of a crop plant is increased by close association

with conspecific individuals (Feeny 1977). Crop plants grown in mono-

culture are subjected to conditions for which their qualitative chemical

and physical defenses are inadequate (Feeny 1976).

4. Natural enemy hypothesis: this hypothesis predicts that there

will be a greater abundance and diversity of natural enemies of pest

insects in polycultures than in monocultures (Root 1973). Predators

tend to have broad diets and habitat requirements so they would be

expected to encounter a greater array of alternative prey and micro-

habitats in a heterogeneous environment (Root 1975). Annual crop

monocultures do not provide adequate alternate sources of food (pollen,

nectar, prey), shelter, breeding and nesting sites for effective

performance of natural enemies (Rabb et al. 1976).

The natural enemy hypothesis has been stated in the following way

(Root 1973):

a. A greater diversity of prey and microhabitats is available

within complex environments. As a result, relatively stable







11


populations of generalized predators can persist in these habitats

because they can exploit the wide variety of herbivores which

become available at different times or in different microhabitats.

b. Specialized predators are less likely to fluctuate widely

because the refuge provided by a complex environment enables their

prey to escape widespread annihilation.

c. Diverse habitats offer many important requisites for adult

predators, such as nectar and pollen sources, that are not

available in a monoculture.


Effects of Crop Habitat Diversity on
Natural Enemies


Parasitoids

Several authors have claimed that insect populations are more

stable in complex communities because a diverse habitat can maintain

an adequate population of the pest and its enemies at critical times

(van den Bosch and Telford 1964, DeLoach 1970). For example, parasitoids

are more effective in areas where there are abundant wildflowers that

provide nectar and pollen (van Emden 1962, Leius 1967, Syme 1975).

Also, since the life cycle of many parasitic insects is not synchronized

with that of their host's, some parasitoids must rely on alternate

hosts to maintain establishment within a community. In many cases,

weeds and other natural vegetation in and around crop fields harbor

alternate hosts for parasitic insects thus providing seasonal resources

to bridge the gaps in the life cycles of parasitoids and crop pests

(Peppers and Driggers 1934, van Emden 1965, Doutt and Nakata 1973, Syme

1975, Stern et al. 1976, Plakidas 1978). Additional examples can be

found in Table 2.













Table 2. Selected examples of cropping systems in which the presence of weeds enhanced the
biological control of specific crop pests.



Crip'iIng systems Weed species Pest(s) regulated Factor(a) involved References


Cabb.ige Crataegus rp. Diamondback moth Provision of altern:ate hosts for van Emden (1965)
(Plutella macullpennis) par.asiic wasps (Io.~enes sp.)
Cotton Ra.gwn ad and Rumex Ileliothis spp. Incrnased populatloun of predators Smith and Reynolds (1972)
cri;pus
Vineyards Wild blackberry Grape leafhopper (Erythroneura Incrcea.e of alternate hosts for Doutt and Nakata (1973)
(Th bu sp) ele~pnntula) the parasiticwasp AnTa gros cpo
Vineyards Johnsongrass Pacific mite (Eotetranychus Buildup of prcdaceous mites Flaherty (1969)
(Sor.hum li.ulep ense) wlllamettel) (MetanJlulus orcidelntnl tI)
!cans Cnosegrass (Fc.lusine Leafhoppers (Empoasca kraemeri) Chemlial repellency or maskinr Altieri et al. (1977)
indlic) and red spranglctop
(.Lfptochlno. filliformis)
Collards :Xagweed (Ambrosia nrtemi- Flea beetle (Phyllotreta Chemical repellency or masking Tohvanainen and Root
s ifolia) crucif rne) (1972)
Apple Natural weed complex Tent caterpillar (lalacosomn Increased .activity and Leius (1967)
nimrlcnnum) and codling moth abundance of paro;J tlc wasps
Carpocapsa pomonolla)
Veoetable crops Wild carrot (Daucus carota) Japanese beetle Incruesed activity of the parasitic King and Holloway (1930)
(Popillin Jnponlcn) wasp Tiphila p ll inwvera
Apple PIhcel ia sp. and Sun Jose scale (lundrn.plidiotus Incre.,ed abundance ndJ activityof Tflenga (1958)
IPrsnLm s". pernlciosus) and aphids para itic wasps (Ap!Ji!nus nal.
and g tis proci.)
Cruciferous crops Quick flowering mustards Cabbageworma (Plris app.) Increased activity of parasitic National Academy of
wasps (ApantrleR iL rr-tus) Sciences (1969)





---------------

















Table 2--continued


Corn Gannt ragweed European corn borer Provision of alternate hosts for the Syme (1975)
(Ostrinia nqhilnlis) tachinid parasite Lydolla prisesens

Cotton RIigwoed Doll weevil (Anthonomus Provision ".f altcrn.;te hosts for van den Bosch
nrndia) the parnalre Ecurvtori. tylodermattI and Telford (1964)

reacih l;an;wed Oriental fruit moth Provision of alternate hosts for van den Bosch
the parasite Microcontrus delicatus and Tclford (1964)

Alfalfa 1Natural blooming weed Alfalfa caterpillar Increased activity of theparasitic van den Bosch and
com:plex (Col ina eurytherni) wap Annt lcs mn dicig lnia Telford (1964)

Sweet potatoes Morninglory (Ipomoea Arnus tortoise beetle Provision of alternate hosts for Carroll (1978)
:. rI Jfoa I ) (CliIlyii rpha cas!itdea) the parasit.c i l1 rso1.la sp.

Sugar cane Iirrrin verticll. ta Crickut (;cinptrli:usv I'Proviion of nectar for the Wolcott (1942)
nol l.vy5tj atrorubcns viclinis) parasitne 1.a r amerlrana

.ungbeans Natural weed complex Beannfly (COpiLmyin Alteratiuon f colonization Litsinger and Moody
phaasoll) background (1976)

Brussel sprouts Natural weed complex Imported cabbage butterfly Alteration of colonization Smith (1976a, b)
(Pieris rapac) and aphids backgrolun and increase of
(Brevicry brassicac) predato'is

Sugar cane Razor gr as Dintraca saccharali. Provisin of nectar and pollen Hyers (1931)
(['apnLu spp.) and D. canelln for the ija.sitold Iobrcon app.


















1-'
l^







14


Peterson (1926) observed that uncultivated orchards were less

severly attacked by codling moth than thoroughly cultivated orchards.

Later, Peppers and Driggers (1934) and Allen and Smith (1958) showed

that percentage of fruit moth larval parasitism was always greater in

orchards with weeds than in clean cultivated orchards. Similarly,

Leius (1967) found that the presence of wild flowers in apple orchards

resulted in an 18-fold increase in parasitism of tent caterpillar pupae

over non-weedy orchards. Tent caterpillar egg parasitism increased

four times, and codling moth larval parasitism increased five times.


Predators

The replacement of natural communities or diversified agriculture

with large monocultures has caused general predator fauna impoverishment

in certain agricultural areas (van den Bosch and Telford 1964). As

far back as 1935, Marcovitch envisaged the diversification of cropping

systems as a means of increasing the efficacy of naturally occurring

predator populations. Later, Root (1973) proposed the "natural enemy

hypothesis" which states that predators are more effective and abundant

in diverse habitats than in simple ones. Results from several experi-

ments back up this hypothesis.

In the Solomon Islands, O'Connor (1950) recommended a cover crop

be used in coconut groves to improve the biological control of coreid

pests by the ant Oecophylla smaragdina subnitida Emery. In Ghana,

coconut gives light shade to cocoa and supports without apparent crop

loss, high populations of Oecophylla longinoda, keeping the latter free

from cocoa capsids (Leston 1973).






15



In the Canete Valley of Peru growing corn in conjunction with

cotton was ideal for the reproduction of predators that contributed to

the biological control of cotton leaf rollers Argyrotacnia sphaleropa

Meyrick and Platynota sp., and the bollworm Heliothis virescens (F.)

(Wille 1952, Beingolea 1957). Growing alfalfa strips within cotton

fields in California significantly increased numbers of predators

early in the season; these beneficials moved back and forth between the

alfalfa and the cotton (Stern 1969).

Intercropping systems of cotton with corn or sorghum presented

higher numbers of predaceous arthropods (primarily lady beetles and

lacewings) than cotton monocultural systems (DeLoach 1970, Fye 1972,

Burleigh et al. 1973, Stern et al. 1976). Similarly, cotton-sesame

interplantings had high populations of beneficial insects (Laster and

Furr 1972). Intercropping of corn and peanuts (Arachis hypogaea L.)

decreases the incidence of the corn borer [Ostrinia furnacalis (Guenee)]

probably because these habitats encourage the abundance of Lycosa sp.

spiders (Litsinger and Moody 1976). In Costa Rica, increasing resource

diversity by intercropping corn and sweet potatoes enhances the relative

abundance and diversity of predators (Risch 1979). Similarly, in

tropical Colombia, corn-bean polycultures had higher numbers of

predaceous Hemiptera and Dolichopodidae than corresponding monocultures

(Altieri et al. 1978). Larger numbers of ground beetles (i.e., Harpalus

rufipes) in mixed plots of cabbage and clover reduced survival of Pieris

caterpillars (Dempster and Coaker 1974).

Populations of many predator species seem to depend on general

abundance of hibernating sites and alternative hosts as well as flowers

in hedges and other habitats in the area, not just around the immediate






16


edge of the field (Pollard 1971). The management of habitats

surrounding crops could augment regional populations or predators if

widely practiced (Perrin 1975). Populations of arthropod predators

were higher in diverse permanent habitats than in simple habitats

(Pollard 1971, Fuchs and Harding 1976).

The importance to pest control of the presence of uncultivated

habitats adjacent to crops is inconclusive. More is known about the

influence of diversifying the crop habitat itself on insect populations.

The presence of certain weeds within a crop can greatly influence

the balance of members of the crop fauna. Reduced incidence of crop

pests in weedy crop systems compared to weed-free monocultures has

been demonstrated by Pimentel (1961), Dempster (1969), Tahvanainen and

Root (1972), Root (1973), Smith (1976b), and Altieri et al. (1977).

In many cases, the reduced pest numbers have been the result of an

increase of predator populations (Altieri and Whitcomb 1979).

Coccinellids, syrphids, Aphidoletes sp. and other predators were more

abundant and preyed more actively on aphids in cole plants grown among

diverse meadow vegetation than in cole monocultures (Pimentel 1961,

Root 1973). Ground beetles (Harpalus rufipes, Feronia melanaria and

others) and a harvest spider (Phalangium opilio) were more abundant in

weedy cabbage crops than in weed-free monocultures (Dempster 1969). In

England, Smith (1976a) found that oviposition of certain syrphid

predators and abundance of the anthocorid Anthocoris nemorum were

increased in brussel sprouts with a weedy background. Populations of

coccinellids were higher in weedy oat fields in New Brunswick than in

weed-free monocultures (Adams and Drew 1965). Similarly, areas of

dense weed cover in English cereal fields had more predatory ground







17


beetles (Carabidae and Staphylinidae) than did weed-free areas

(Speight and Lawson 1976).


Weed as Sources of Predators


Perrin (1975) and Altieri and Whitcomb (1979) have emphasized

the role of certain weeds as sources of alternate prey of important

predators of crop pests. To improve survival and reproduction of

predators within an agroecosystem, it is often desirable to have

subeconomic, fluctuating populations of alternate prey permanently

present in the crops (van den Bosch and Telford 1964). Specific

examples of weeds that provide alternate food resources for predaceous

arthropods are listed in Table 3. If widely encouraged, these plants

show potential in insuring a standing population of specific predators

in areas where these interactions occur consistently.













Table 3. Selected examples of weeds that provide alternate prey for general predators.



Weed Alternate prey Predators References


Urtica dioica Microlophium carnosum Coccinallidae and Perrin (1975)
Syrphidae

Pastinaca sp. and Aphids Coccinellidae and predaceous Bombosch (1966)
Achillea sp. Hymenoptera

Solanum carolinonse Lcptinotarsa decemlineata Lebia grandis Hemnnway and Whitcomb (1967)

Amaranthus sp. Disonycha glabrata Lebia analin "

Ocnothera Incinintaand Altica sp. Lelia viridi "
0. biennis

Heterotheca subaxilla- Zygogramma heterothecae Lebia atriventris, Perillus Altieri and Whitcomb (1979)
ris bioculntus, Plocrcc a viridans
and other spiders

Chenopodiumamnnroilosiolden Zy.orjmma nura]s, aplhids Callida decora, PI'rJil ui bioccn- Alticri and Whiccomb (1979)
and Cicadellidae tus, HlipLodclnmia cLnvcrgros, n :d
other Coccinellidinc; Lebin virid is_
Tctra.i natlhn p. aind other spidercs

Solidago altissima Uroleucon spp. '. convergcn' and other Coccinellidae, Altieri and Whitcomb (1979)
Chrysopa spp., Podalbrus sp. arn
Chauliognathus sp., Zelus cervicalis
and other Rcduviidae, Condyos;:tyJus
sp. Peucnt ri v f i inns and othcr' spiders,
Toxomerus sp. and other Syrphidae

Sorghum halocprn Non target mites Netaseiulus occid'1ntalis Flaherty (1969)

Cirs-um arvonse Altica carduorum Lebla v1idi\, Uliapalu Schaber ct al. (1975)
pennsylveakl::un



'-S
0,

















MATERIALS AND METHODS


The effects of weed diversity on the dynamics of corn insect pests

and associated predators were tested in experiments at two sites.


Tall Timbers Site


This experiment was conducted in a 5 ha field located at Tall

Timbers Research Station in northern Leon County, Florida. On April 5,
2
1978, the field was divided into eighteen 100 m plots (10 rows, each

10 m long) and planted with corn (Zea mays L., c.v. 'Kernel Greenwood

Hybrid'). The distance between rows was 0.9 m (36 in). Plots were

separated by 50 m to reduce variability due to immigration and emigration

of arthropods. The soil between the plots was kept free of vegetation

by frequent harrowing. Fertilizer (5-10-15) was applied to the corn

plots at a rate of 436.4 kg/ha (400 lb/a).

There were six treatments, each replicated three times:

1. Corn monoculture (weed-free).

2. Corn + weed mixture A. This mixture consisted of seeds of

Solidago altissima L. (golden rod), Amaranthus sp. (pigweed), and

Heterotheca subaxillaris (Lam.) (camphorweed), sowed when the corn was

planted.

3. Corn + mixture B. This mixture consisted of seeds of Ambrosia

artemisiifolia L. (ragweed), Chenopodium ambrosioides L. (mexican tea),

and Daucus carota L. (wild carrot) also sowed when the corn was planted.


19







20


4. Corn + natural weed complex, highly fertilized. The rows in

which the native weeds were allowed to grow were fertilized with 5-10-15

at a rate of 872.8 kg/ha (800 Ib/a).

5. Corn + natural weed complex, regularly fertilized at the usual

rate of 436.4 kg/ha.

6. Corn + natural weed complex determined by an early plowing

(the rows in which the weeds were allowed to grow were previously plowed

by the end of December 1977 and since then left undisturbed).

In all treatment plots, native weeds were allowed to grow freely

in the two middle rows and in the rows before the last on each side of

the plot. The plots in which weed mixtures were sowed had a background

of native weeds. Each plot had six rows of corn and four rows of weeds.

The area between the six rows of corn was kept weed-free by cultivation

and hoeing.

Pest incidence on corn by lepidopterous larvae (mainly Spodoptera

frugiperda J.E. Smith) was estimated by counting the number of plants

with damaged whorls in each plot. Thirty corn plants were randomly

selected every 7 days in each plot and their degree of damage was

evaluated visually. Similarly, numbers of predaceous arthropods on

corn were estimated by careful visual examination of the above-ground

parts of 30 corn plants at each plot every 7 days. The number and type

of predator species on each plant was recorded and, when possible,

prey items. Populations of predaceous arthropods present on the weeds

in the plots were evaluated by taking 20 sweepnet samples along the two

middle weedy rows of each plot. Sweepnet contents were analyzed

immediately by opening the sweepnet and counting the number of predator

species crawling along the bag.







21


Predator pressure in the various plots was assessed using fresh

2-day-old Spodoptera eggs. One hundred eggs were attached to pieces

of white paper towel on the rough side. These pieces were stapled to

white paper cards and then placed in the field (two cards per plot)

pinned to corn leaves at the center of each plot. Egg cards were left

for 24 hours, after which time they were collected and the numbers of

eggs attacked and removed were counted with the aid of a stereo micro-

scope. Fresh cards were placed again in the plots. This procedure was

repeated four times. In some instances the arthropods involved in egg

removal were specified by direct observation.

Weed density and species composition in each plot were estimated

on three occasions by using a 0.5 m2 quadrat. Each quadrat was randomly

thrown and examined twice in the two middle weedy rows of each plot.

Numbers of plant and species enclosed in each quadrat were recorded.

Relative abundance and species composition of soil arthropods were

monitored with pitfall traps filled with killing fluid (approx. 50 ml

of 95% alcohol). One trap per plot was used and was left in the ground

for 14 days. After this time, traps were removed, and the contents

were sorted to species and counted in the laboratory. This procedure

was repeated five times during the experiment.

The relative abundance and species composition of flying predaceous

insects (mainly Dolichopodidae and some wasps) were estimated by

placing a yellow pan in the middle of each plot. Each pan was filled

with water and a few drops of detergent were added to ensure that insects

caught sank to the bottom of the pans. The pans were left in the field

for 3 days. After this time, the pans were emptied and the contents






22


sorted according to species and counted in the field. This procedure

was repeated five times during the experiment.


Green Acres Site


This experiment was conducted in a 1 ha field at the University of

Florida, Green Acres Farm about 20 km west of Gainesville, Florida.

The entire field was planted solidly to corn ('Coker 71') on March 27,

1979, and afterwards divided into 18 randomly distributed plots of

100 m2 each (10 rows, each 10 m long). The distance between rows was

0.9 m (36 in.). Plots were separated by 8 m; however, corn was allowed

to grow around the plots. Those plants growing in the vicinity of the

plot edges were cut back regularly with a machete to a height of 20 cm.

Fertilizer (6-12-18) was applied to the corn plots at a rate of

436.4 kg/ha (400 lb/a).

There were six treatments, each replicated three times:

1. Corn monoculture (weed-free).

2. Corn + weed mixture A. This mixture consisted of seeds of

Amaranthus retroflexus L. (red pigweed), Xanthium pennsylvanicum

Wallroth (cocklebur), Oenothera biennis L. (evening primrose), and

Chenopodium ambrosioides L. (mexican tea), which were sowed simultaneously

with the corn.

3. Corn + weed mixture B. This mixture consisted of seeds of

Taraxacum officinale Wiggers (dandelion), Heterotheca subaxillaris

(Lam.) (camphorweed), Solidago altissima L. (goldenrod), and Bidens

pilosa L.(beggartick) which were simultaneously sowed with the corn.

4. Corn + soybean.

5. Corn + natural weed complex.







23


6. Corn + rye straw mulch.

In treatments 2, 3, and 5 native and sowed weeds were allowed to

grow freely between the two central corn rows of each plot. Selected

weeds were also grown in pots in the greenhouse and later transplanted

to the plots to assure a high population of desired weeds in each

treatment. In treatment 4, two rows of soybeans were planted between

the two central corn rows of each plot. The remaining area of the plots

was kept weed-free by cultivation and hoeing.

The incidence of Spodoptera frugiperda J.E. Smith, predator

population and predator pressure were estimated by the same methods

used at the Tall Timbers site.

Two pitfall traps were placed four times in all treatments to

estimate relative abundance of ground beetles and earwigs. No yellow

pans were used in this experiment.

Weed densities and species composition were estimated using a thin

(2 mm diameter) metal rod which was let down vertically to the ground

in 20 random places per plot, and all weed leaves (and soybean leaves

in the case of treatment 4) touched by the vertical middle were

recorded. When totalled up for all the intersections, this gave an

estimate of the percent cover and leaf area index of each weed species

and the total plant community (Wilson 1963).

In both experiments, corn yields were estimated by weighing corn
2
ears harvested from an area of 29 m2 in the center of each plot, once

the ears reached a moisture level of 15%.

















RESULTS AND DISCUSSION


Nature of the Weed Communities


Weeds of Tall Timbers Research Station

Weed communities within all corn plots were mainly composed of

associations of cocklebur, sicklepod (Cassia obtusifolia L.), Florida

purslane (Richardia scabra L.) and grass species. Because of dormancy

problems the weed seeds sowed in treatments 2 and 3 (weed mixture A

and B) failed to germinate adequately. Consequently, populations of

goldenrod, pigweed, camphorweed, wild carrot and mexicantea remained

at low densities during the whole study. However, the mere presence

of these weeds in the plots contributed to the background diversity of

each treatment making the weed communities different from each other.

The density of individual species within each weed community varied

considerably among treatment plots (Table 4). Treatments 2 and 4 had
2
the highest number of plants per 0.5 m and were dominated by grass

species such as Panicum sp. and Andropogon sp. Both Cassia and

grasses were stimulated by high fertilization. Richardia was

particularly abundant in treatment 2. The plot previously plowed in

December had a unique background of perennial weeds (e.g., Rubus sp.,

Phytolaca sp. etc.) and annuals such as rattlebox (Crotalaria

spectabilis Roth) and cypress vine (Ipomoea quamoclit L.).


24











Table 4. Densities and species numbers of weed communities within corn fields at Tall Timbers Research
Station in north Florida (1978).

Mean Mean number of 2
number plants per 0.5 m
Treatment Weed community Mean density of species per 0.5 m of weed in the weed
species community std.
deviation (n=9)


Xanthium Cassia Richardia Grasses*
2 Weed mixture A 2.0 17.0 77.0 56.0 11 82.5ab**12.6

3 Weed mixture B 3.0 36.0 58.0 23.0 7 66.8b 7.2

4 Natural weed complex 1.0 70.0 38.0 54.0 8 88.8a 18.1
highly fertilized

5 Natural weed complex 4.0 53.0 74.0 7.0 9 75.4ab 22.2
regularly fertilized

6 Natural weed complex 0.0 2.0 24.0 34.0 10 32.2c 12.9
determined by
December plowing


* Panicum sp., Andropogon sp. and Cyperus sp.
** Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).






26


Weeds of Green Acres Farm

Xanthium, Amaranthus and Bidens were the only weeds that germinated

heavily after being artificially sowed. All weed communities were

dominated by Richardia, although Xanthium and Bidens reached density

values high enough to make treatments 2 and 3 different from each other

and also from the natural weed complex (Table 5). Amaranthus germinated

slowly and was part of the weed community in treatment 2. Cassia and

grasses were not part of the natural background of weeds of the area.

Populations of goldenrod, camphorweed, evening primrose, dandelion and

mexican tea remained low during the study, but by the end of the corn's

cycle their densities started to increase in the different plots.


Pest Incidence


As shown in Table 6, the percent of corn plants with whorls

damaged by fall armyworm larvae was higher in the monocultures than in

any of the weed diversified corn plots, at both localities. At Tall

Timbers Research Station the incidence of fall armyworm was mostly

reduced when growing corn in association with a highly fertilized

natural weed complex. At Green Acres, fall armyworm incidence was

reduced mostly when growing corn with a rye straw mulch. It is

possible that a rye straw mulch may change the color or shape of the

corn background thus affecting the colonization of fall armyworm which

seems to respond to visual cues in locating a host (Southwood and Way

1970).

Fall armyworm incidence and weed density appeared to be inversely

correlated at Tall Timbers (r = -.93). As the density of plants in the

weed community increased, the percent of damaged whorls in the plots











Table 5. Relative abundance and species numbers of weed communities within corn fields at Gainesville,
Florida (1979).


Mean Mean number
Treatment Weed community % cover of weed
std. deviation species

2 Weed mixture A Xanthium 35.0 0.0 5b*

Amaranthus 11.6 3.8

Richardia 40.0 6.6
Community 85.0b 0.0

3 Weed mixture B Bidens 25.0 18.5 4b
(n=3) Richardia 66.6 18.3

Community 90.0b 3.3

5 Natural weed complex Richardia 62 20.3 2a
(n=3) Community 66.6a21.6


* Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).







28


decreased (Fig. 1). This suggests that the fall armyworm regulatory

mechanisms that emerge from growing weeds in corn fields are accentuated

with an increase in the population density of desired weeds.

At both localities, incidence of corn earworm was similar among

most treatments (Table 6). At Tall Timbers the lowest damage (non-

significant) occurred in the weed treatment 2 and at Green Acres less

damaged ears were observed where corn was grown with a strip of soybean.

Based on these results, it seems that Heliothis responds very

differently to habitat diversity than Spodoptera does. It is possible

that these species have different habitat colonization strategies.

Intercropping corn and soybean might be of aid in reducing corn earworm

infestations.

In general, the magnitude of the incidence of both pest complexes

varied according to the area. Fall armyworm attack was more severe at

Tall Timbers, whereas the incidence of corn earworm was more severe at

Green Acres. It should be noted, however, that damage evaluations were

made in different years, and these trends may change from year to year.


Predator Colonization


At the early stages of crop development only a few predaceous

arthropods colonized the corn fields. As food and habitat resources

became more available, numbers of predators and species richness at

each locality increased (Table 7). As the corn plants developed,

environmental conditions in both localities were ameliorated with time.

Weedy corn systems apparently became milder in microclimate and more

complex in trophic and habitat structure than the monocultures. These

changes affected the number and diversity of colonizing predator species.













110.


100


"1 90


80


4 70 *
o *
..
S60 .


50


S 40 -
*




20-


10



8 10 12 14 16 18 20 22 24 26 28 30 32

% of damaged plants

Figure 1. Relationship (y = 104 0.2x) between weed density and percent of corn plants damaged
by fall armyworm (Spodoptera frugiperda J.E. Smith) in north Florida; r = -0.93, n = 15/mean.











Table 6. Percent of damaged plants by fall armyworm, Spodoptera frugiperda J.E. Smith, and percent of ears
damaged by corn earworm, Heliothis zea (Boddie), in weed-free and weed-diversified corn systems
at two sites in north Florida.


Tall Timbers Research Station (1978) Green Acres Farm (1979)
Corn system
fall armyworm corn earworm fall armyworm corn earworm
(n=15) (n=9) (n=15) (n=6)

Monoculture 31.5a* 11.6** 32.7a26.6 15.1a7.9 65.0a 9.4
Weed mixture A 12.9b 10.2 28.3a20.3 7.3b3.6 56.6ab37.7
Weed mixture B 20.1b 11.3 35.6a26.5 7.7b3.0 59.2a 22.7
Natural weed complex 15.8b 11.8 32.2a23.7 5.5b3.6 69.2a 18.0
Natural weed complex 9.40c 8.5 35.6a22.7
highly fertilized
Natural weed complex 19.5b 11.8 38.3a22.2
determined by December
plowing
Soybean strip -- 9.7b3.9 53.3b 29.8
Rye straw mulch -- 1.3c0.9 60.5a 35.0


* Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).
** Means standard deviation.






o















Table 7. Colonization of corn systems by predaceous arthropods in north Florida. (Only dominant
predaceous species are listed for each census at the two sites).


site Year April May June July August

Tall Timbers (70) --- Doru, Leptotracholus Doru, Coccin:llidae Doru, Coccinellidae Doru, Goocoris
Ccc'inelTi-di-, Ple- uetia, Collida, Geocoris, spiders spiders,
Dolichopodidao 'EorI-s, Dolichopodidae
Corn I'onoculture Cealida, Podisus, Leitra chol;s Reduvidae
Peu.ceta
Green Acres (79) Spiders, Spiders, Geocorls, Notoxus, Doru, Spiders, ---
Cocclnellidae Coccinellidae O'rius,DTriiru', NOtoxus
Gc'ocoris, boiTchopod i dle,
cahli7a,] Doru Spiders
Orcel'i1unIm, Scymnus, Orc!el imum
P'ei-ctTiffa Orius
Iotoxulr

Tall Timbers (78) .--- Dolichopodidae, Spiders, Orchrlimum Doru, Geocoris Spiders,
Coccincllidae Dolichopodidae, Coccinellidac, Dolichopodidae
Doru, Spiders, Coccinell idar, Doru, spiders, Geocoris, Doru,
Cal Iida, Callida, 'o,jisus, Dolichopodidae, OrcheTnium
Corn and PIdTisuTs, CocoS Reduviidae
NlJtural Weed Tropiconabis Lei. l'..
C-';plcx Green Acres (79) Spiders, Coccinel ldae, Notoxus, O-1ius, Doru, Spiders, --
Coccinellidae, spiders Dri, PnucNtia, Geocoris,
Peucetia Cal ida, Geocoris, i.ebia Scymnus

CoEci nie'r dae, Scyinus 0"t:.nthus,
rPucetin l le- rui .

L.ept-o'rt'r. h e us
Notoxus, Dortn
b-rius



---i


















Table 7 continued


Site Year April May Jure July August

Tall Timbers (78) ------ .Pue.tia, e.olus., Leototrachelus, Doru, Geocoris Doru, Spiders
Tr. iic.nabis, Callida, Doru, LSeotrat ehiuI Do 1ichcpodidae
Coccinellidae, iolTihopod-i~ae, CoccTneli l e" Geocoris,
Dolichopodidae Coccinellidae, Spiders, Lebia, Leptotrachelus,
Spiders, Doru, IPo'isus, Sinea jIn~U OrchTiim",
Reduviidac-- i-- --- Do ichopodidae Coccinellidae
Leptotrachelus, Spiders, Reduviidae
Corn and Selected Callil-a, Orchol1 imum,
eleed Associations Orcli] 1 m r'oris, Lehia
C/ :_nthus
Green Acres (79) Spiders, Coccinellidae Doru, t s Peuc t Dr, Spiders,
Coccinell dae rPuceti i ids, ;oc riW Goonris,
--Or:coli mu, D-,-o.j, Coccirn llidae,
Spiders, Geocoris L
rus Cll ida Lobia, QcCAlULU
Orius, Notoxus, i -b
oni "I:ia ous7 Dol-ickoped i de ,
-a- Coccinellidae
I)Dc.nthus, Orchel iimum



















Lw
Na







33


At Tall Timbers, corn grown intermingled with natural weed complexes

and selected weed associations showed higher numbers of predator species

per unit area (Table 8) and greater numbers of predators (Table 9) than

monocultures. These trends were consistent throughout the growing

season. Later, when the corn reached harvest maturity, predator numbers

and diversity declined. Similar trends were observed by Price (1976)

and Mayse and Price (1978) in soybean fields when analyzing croplands

from the theory of island biogeography.

The above results suggest that predators in monocultures operate

in a more xeric environment which influence their colonization and

extinction rates dramatically. At Green Acres there were no differences

in predator diversity among the various plots. Only in June the

diversity of predator species was higher in the weed diversified systems

than in the monocultures. Because the habitats surrounding the Tall

Timbers plots were structurally more complex and probably provided more

local overwintering sites for predators, the mean number of predators

were higher there than in Green Acres. Species richness was similar

at both sites, however. It seems that by growing corn plants inter-

mingled with weeds or by retaining complex uncultivated borders

around the fields, colonization rates of predators are accelerated

and extinction rates are reduced (Price 1976).


Predator Abundance and Diversity


At Tall Timbers, predator abundance and diversity (number of

species per unit area) on corn plants were significantly higher in

weed-diversified systems than in weed-free systems. Arthropod

complexity seemed to parallel the trend in plant complexity. Conversely,










Table 8. Mean number of predator species throughout the growing season in weed-free and weed-diversified
corn systems at two sites in north Florida.


Mean number of predator species

Corn system Site Year April May June July August Total Mean +
Std. deviation


Corn monoculture Tall Timbers (78) --- 4.0 4.2 3.5 3.5 3.9a1.2*
(n=36)
Green Acres (79) 1.3 3.8 5.3 3.0 --- 3.4a1.5
(n=33)

Corn and natural Tall Timbers (78) --- 5.0 4.4 6.0 4.5 4.9b1.2
weed complex (n=36)

Green Acres (79) 0.5 3.3 7.0 3.0 --- 3.5a1.9
(n=33)

Corn and selected Tall Timbers (78) --- 4.3 4.5 4.6 4.3 4.2b0.9
weed associations (n=36)

Green Acres (79) 0.8 3.9 6.5 3.5 --- 3.6a1.9
(n=33)

* Mean in the same year, followed by the same letter, on any given column, are not significantly
different according to Duncan's multiple range test (p = 0.05).










Table 9. Total numbers and diversity of general predators in different corn cropping systems at two sites
in north Florida.


Number of individuals and species of predators per 30 corn plants std. deviation

Tall Timbers Research Station (1978) Green Acres Farm (1979)

Corn system Abundance Diversity Abundance Diversity
(n=36) (n=36) (n=33) (n=33)

Monoculture 7.2a*4.8 3.9a 1.2 4.9a 2.3 3.4a 1.5

Selected weed associations 8.8b 5.8 4.2b 0.9 4.3ab2.9 3.6a 1.9

Natural weed complex 8.6b 4.8 4.9b 1.1 3.7b 1.6 3.5a 1.8

Rye straw mulch -- --- 3.7b 2.8 3.1a 1.8

Soybean strip --- --- 4.0b 2.2 3.7a 1.5


* Mean followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).







36


at Green Acres total predator numbers on corn plants were slightly

greater on the monocultures than on the diverse corn systems. There

were no differences in the number of species of predators between weed-

free and weed-diversified corn systems at Green Acres. If mean numbers

of species and individuals per habitat space (predators on corn plus

predators on weeds in each plot) are considered, population densities

and species diversity or predators bypass significantly the levels

reached by predators in the monocultures at Green Acres. A quantitative

value of predators per habitat space cannot be given in the present

study because predator populations were estimated by absolute methods

in the corn plants and by a relative method on the weeds. The

qualitative assumption that predators were more abundant and diverse

in the weedy corn fields than in the monocultures seems valid because

predators moved back and forth between the weeds and the corn (Fye

1972).

It is possible that the high numbers of predators in the Green

Acres monocultures were due to the proximity of the experimental plots

(8 m apart) which could not prevent migration of predators from diverse

plots (Root 1973). This problem seemed to be minimized at the Tall

Timbers experiments mainly because of the greater distance between

plots.

In general, population densities of predators were greater in the

Tall Timbers plots than in those at Green Acres. These differences

were probably due to the nature of the surrounding habitats (Hodek 1973).

Tall Timbers plots were surrounded by annually burned pinelands, complex

early successional weedy fields and remanent tree forests which

apparently served as refugia providing a continuous influx of predators.






37


The Green Acres plots were surrounded by sorghum and corn fields, weedy

fields dominated by Linaria and Brassica and later by soybean fields.

The temporary nature of these habitats and the heavy pesticide treat-

ments that they go through make them poor predator reinforcement

resources (Fuchs and Harding 1976). The presence of two extra weed rows

in each plot at the Tall Timbers experiment might explain the differences

in predator abundance between weedy plots at both localities. Those

extra weed rows probably improved the habitat and resource base available

for predators at Tall Timbers.


Trends of Individual Predator Species


Each predator species, family or both responded differently to the

various treatments (Table 10). Population responses varied according

to species involved, weed diversity, year and locality. For example,

Geocoris spp. numbers were higher in weed-diversified systems (particu-

larly the natural weed complex) than in corn monocultures at Tall

Timbers in 1978, but Geocoris spp. showed no response to weed diversity

at Green Acres in 1979. Corn associated with the natural weed complex

had higher numbers of predaceous Coleoptera (Carabidae and Notoxus) than

any other system at Green Acres and more predaceous Hemiptera (Nabidae,

Orius and Zelus) and Coccinellidae than any other treatment at Tall

Timbers. Doru sp. had similar densities in all treatments at Tall

Timbers, but higher densities in the weed-free monocultures and corn-

soybean systems than weed-diverse plots at Green Acres. Spider densities

(including Peucetia viridans (Hentz)) were similar in all treatments at

both sites. Predaceous Orthoptera (Orchelimum sp. and Oecanthus sp.)

reached low densities in all treatments.


















Table 10. Relative abundances of individual predator species in different corn cropping systems
at two sites in north Florida. (Averages of 12 sampling dates).


Nuntbors of individuals per 30 corn plants std. deviation

Corn system Site Year Doru sp. Spiders Coccinellidae Peucetia G;eocoris Hemiptera Coleoptera Predaceous
Orthoptera

Monoculture Tall Timbers* (70) 5.2a3.7** 0.2 a0.2 0.7 a0.45 0.10a0.13 0.15.13 O.3 0.07a0.1 0.1 a0.13 0.0 a0.0
Green Acres*** (79) 1.9a1.6 0.6 a0.4 0.34a0.15 0.47a0.56 0.50a0.5 0.5 a0.9 0.3 a0.2 0.08a0.1

Selected weed Tall Timbers (70) 5.Ga3.2 0.32a0.25 0.30at0.5 C.05a0.05 0.35a-0.25 0.1 a0.1 0.24a0.2 0.05b0.04
asscociations
Green Acres (79) 0.8b0.5 0.4 a+0.2 0.4 a+0.2 0.50at0.4 0.53at0.4 0.51a0.2 0.34a0.2 0.0la0.01

Natural weed Tall Timbers (78) 4.7a2.8 0.2 a0.15 1.5 bl1.0 0.0 b0.0 0.89lo0.8 0.13a0.15 0.15a0.13 0.03b0.01
co!?plex
c x Green Acres (79) 0.6b0.6 0.4 a+0.2 0.3 a+0.15 0.50a0.4 0.17a+0.4 0.27a0.4 0.70b0.5 0.05at0.00

Soybean strip Tall Timbers (78) -- --- -- -- -- -- -
Green Acres (79) 1.3bl0.9 0.15a:t0.1 0.26a0.1 0.60a0.04 0.G7a0.4 0.41ai0.5 0.23a10.15 0.02al0.01

Rye r;traw Tall Timbers (70) -- -- -- -- ---
imulch
Grc.n Aeros (79) 0.9b.0.6 0.2 aJ:0.1 0.11 .O.1 0.50ac0.3 0.31an.0.1 0.99hi0.0 0.47a.0.2 0.15bi0.09

* n-3G
* tI';le in a sanm yaoir, followed by the same letter, on any qivon column, are notr nignificatl..ly different; accordlrn to Duncan's multiple
r.menj Lost (p = 0.05).
*** n=33










CO







39



Predation Pressure


Table 11, shows the mean percent of Spodoptera eggs consumed by

predators at both localities. There seemed to be no relationship

between predator abundance, weed diversity and predation pressure.

Rather, the percent of eggs consumed by predators was greater or equal

to some weedy treatments in the monocultures of Tall Timbers at all

sampling dates. At Green Acres predators consumed more eggs in the

monocultures and in the corn-soybean strip system than in any other

weed-diverse system. These results would imply that predators are more

efficient in consuming eggs in the monocultures. However, it seems

that eggs are more easily found by predators in monocultures. The lack

of other natural prey and the simplicity of the environment in a

monoculture facilitates the search of artificially placed eggs by the

few predators present. Conversely, in weed diversified systems physical

and trophic structures are more complex somehow diverting the attention

of predators from the eggs. In cereal fields of England, Speight and

Lawton (1976) found contrasting trends. The number of pupae taken by

predators increased with the density and frequency of weeds. Habitat

diversity provides adequate microclimates and enough food for predators

forcing them to slow down and remain longer in the complex habitat

(Root 1973). Predators never consumed less than 50% of the placed

eggs in the complex environments. The question remains whether this

rate of consumption would actually prevent an outbreak of fall army-

worm.

At Green Acres, the highest consumption of eggs occurred in the

monoculture and in the soybean strip corn systems. Because of the











Table 11. Mean percent of Spodoptera eggs consumed by predators over four sampling dates at two sites in
north Florida.


Mean % of eggs consumed std. deviation

Corn system Tall Timbers Research Station, 1978 Green Acres Farm, 1979
(n=24) (n=18)

Monoculture 79.2a*21.2 71.9a 2.7
Weed mixture A 67.5b 19.9 44.1c 25.0
Weed mixture B 73.9a 19.7 53.1b 25.5
Natural weed complex 80.1a 15.4 46.3bc15.1
Natural weed complex 64.8b 18.9 --
highly fertilized
Natural weed complex 68.7b 20.9 --
determined by December plowing
Soybean strip --- 70.la 3.5
Rye straw mulch --- 55.3b 11.1


* Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).









0f






41



proximity of the plots, it seems obvious to assume that predators were

moving from the diverse plots to rapidly consume the more "apparent"

eggs placed in the monocultures.

The high variance associated with the egg predation data probably

reflects the element of chance involved in the predators locating and

taking the prey offered to them in the small, discrete groups presented

by individual paper quadrats.

Predators actually observed taking eggs in the field included

Doru sp., Orchelimum sp., Peucetia viridans (Hentz), Hippodamia

convergens (Guerin-Men), Coleomegilla maculata DeGeer, Orius insidiosus

Say, Leptotrachelus dorsalis (Fab.), Callida decora (Fab.), various

spiders and ants.


Weeds and Number of Ground Predators


In Table 12, the data on the adult ground predator fauna (spiders,

earwigs, beetles and ants) caught in the set of pitfall traps over the

four sampling dates at both sites are summarized. Although pitfall

sampling has been criticized as a technique which poorly estimates

total numbers and may sample with varying effectiveness in different

habitats (Luff 1975), in this study pitfalls were considered a useful

method to obtain a relative estimate of the presence of ground predators

in the different plots.

Spider numbers were greater in the monocultures and in the corn

systems with weed mixture B at Green Acres. Earwigs were more actively

present in the corn systems with a rye straw mulch and monocultures

than in the corn systems with weeds and soybean. Figure 2 shows some

abundance trends of earwigs in the mulched and monoculture systems at










Table 12. Mean numbers of ground predators caught by pitfall traps in different corn cropping systems at
two sites in north Florida (Average of four sampling dates).


Mean numbers of predators std. deviation

Tall Timbers Research Station, 1978 Green Acres Farm, 1979

Corn system carabids ants Labidura riparia Pallas Spiders
(n=12) (n=12) (earwig) (n=24) (n=24)

Monoculture 3.2a*1.5 9.8b1.8 18.5b 4.9 8.4a4.4
Weed mixture A 3.4a 2.5 6.9c4.2 14.0bc2.4 6.1b2.7
Weed mixture B 1.6b 1.3 8.6b1.4 11.9c 1.4 7.6a3.8
Natural weed complex 3.0a 1.3 11.0al.6 11.4c 2.3 5.4b1.6
Natural weed complex 2.3a 2.2 9.1b3.8 -- --
highly fertilized
Natural weed complex 1.4b 1.4 5.6c1.8
determined by December
plowing
Soybean strip -- -- 13.1c 1.5 6.2b2.7
Rye straw mulch -- -- 23.5a 9.5 2.6c1.3


* Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).







43


Green Acres during the four sampling dates. Earwigs were more

abundant in the mulched plots. There is evidence which suggests that

earwigs are favored by soil covers and other similar cultural mani-

pulations (van den Bosch and Telford 1964). These evidences would

imply that earwigs reproduced in the mulched systems and later dispersed

to the other plots. During the second and third sampling dates earwig

numbers were similar in the mulched plots and monocultures. Possibly,

they concentrated more in the monocultures than in the weedy plots

because they did not find resistance to movement throughout the plots

otherwise imposed by the dense vegetation strips in the weedy plots.

At the last sampling date earwig populations were again higher in the

mulched plots than in the monoculture. Pitfall trapping apparently

extracts great part of the reproductive adult earwig population in the

mulched plots. If these plots are not sampled for a certain period of

time (12 days), earwig populations recover their original levels in

the mulch treatments bypassing in abundance the earwigs of the

monocultures (Fig. 2).

At Tall Timbers carabids presented similar abundance levels in the

monocultures and three weed-diversified corn systems. Ants were

significantly more abundant in the corn system with the natural weed

complex. There were no differences in the abundance of ants between

the monocultures and two weed-diversified plots.


Predator Dynamics in Weed Habitats


Several studies have shown that predator numbers and diversity

increase when the complexity of the plant community is enriched (Root

1973). In cereal fields of England, Speight and Lawton (1976) found











40
*





< \


30 /
o \
0
-4

O
H\


Ma 20 M
CN













( = 0.05) indicated for each mean.
-P-




o
0) \










May 26 May 28 May 30 July 12

Date of sampling

Figure 2. Comparison of mean abundance of Labidura rinaria (Pallas) in mulched corn plots (solid O
line) and bare soil corn plots (dotted line) in north Florida; confidence limits 9
(a = 0.05) indicated for each mean.







45


that the total number of ground beetles caught per pitfall traps

increased as a function of the density of weeds. Other researchers

have compared the predator fauna of croplands and fallow fields (Allan

et al. 1975) but no attempts have been made to compare the predator

fauna associated with different weed communities within crop fields.

The present study shows that through different manipulations

(direct sowing, differential fertilization, early plowing, etc.)

different weed communities can be established in corn fields. These

communities sustain predator complexes of various species diversities

and relative abundances (Table 13). Furthermore, because the predators

on the weeds move back and forth between the corn and weeds, the

predator species present on the corn also change. At Tall Timbers,

weed communities determined by December plowing, high fertilization

(872 kg/ha) and direct sowing of mixture A presented the highest

densities of predators. The predator fauna was significantly more

diverse in the highly fertilized weed community than in any other weed

community. Figure 3 suggests a direct relationship (r = .96) between

the number of weed species and the abundance of predators in the weed

communities at Tall Timbers.

Results from the Green Acres Farm show that relative abundances

of predators are similar between certain weed communities and also

between weed communities and the soybean strip. Also, there were no

significant differences in species richness among the various weed

communities and soybean strip. It should be noted that these trends

might be masked because of the proximity of the plots.

Total mean number of predators and number of species associated

with the different weed communities were significantly greater










Table 13. Mean relative density and number of species of predators associated with weed communities within
corn fields at two sites in north Florida.


Density and number of predator species std. deviation

Tall Timbers Research Station, 1978* Green Acres Farm, 1979**

Weed community Density (n=36) Number of species Density (n=21) Number of species
(n=36) (n=21)

Weed mixture A 9.3ab***2.8 5.8b0.73 ;4.3a0.8 3.8a 1.1
Weed mixture B 7.6b 2.0 6.1b0.86 4.4a0.6 4.6b 0.53
Natural weed complex 7.9b 2.1 5.6b0.93 3.;6a1.0 4.:3ab1.2
Natural weed complex 8.9ab 2.5 6.6a0.86 -- --
highly fertilized
Natural weed complex 10.3a 2.8 5.2cl.l -- --
determined by December
plowing
Soybean strip -- --- 4.1a1.2 4.1abl.l


Mean total 8.8 5.8 4.1 4.2.

* Average of 12 sampling dates. Numbers per 20 net sweeps per 10 m2 of weeds.

** Average of 7 sampling dates. Numbers per 20 net sweeps per 10 m of weeds.
*** Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).












12






10- *

















4--
2,
*





2 4 6 *
o /














Number of predators/20 sweeps/0 m weeds
S--






2 -* .












Figure 3. Relationship (y = 0 + 0.58x) between weed species diversity and predator abundance in
weed communities of north Florida corn fields; r = 0.84, n = 12/mean.






48



(Table 13) at Tall Timbers than at Green Acres. The structurally

more complex habitat surrounding the Tall Timbers plots possibly

accounted for these differences.

The mean numbers of individual predator species and families

collected on the different weed communities at both sites are summarized

in Table 14. In general most predators reached similar densities in

all communities. At Tall Timbers, Doru sp. was particularly abundant

in the weed mixture A community and predaceous Orthoptera (Orchelimum

sp. and Oecanthus sp.) reached the lowest densities in the natural

weed complexes. Cocinellidae were somewhat more abundant in the

highly fertilized weed complexes. Doru sp. was the only predator

species which was slightly more abundant in the soybean strip than on

the weed communities of Green Acres.


Crop Yields


The purpose of this study was to establish principles of corn

pest management under different ecological conditions and not to

develop methods to enhance crop production. Crop yields were

measured, however, in order to have a basis to evaluate the agronomic

potential of the proposed systems.

The mean corn yields of the different cropping systems at both

localities are shown in Table 15. In general, corn yields were lower

at Tall Timbers (mean for all treatments = 2.15 tons/ha) than yields

at Green Acres (mean for all treatments = 3.28 tons/ha) probably

because of a severe drought that affected Tallahassee during the 1978

summer. Both mean yield values do not fall under the average corn

yields of farmers in USA during 1970 (2.057 tons/ha) reported by















Table 14. Mean numbers of individual predator species and families collected on weed communities within
corn fields at two sites in north Florida.


Number of individuals per 20 not sweeps on 10 .m2 of weeds std. deviation

Wced co' munity Site Year Doru sp. Spiders Coccinollidae roPcetia G9,ocoric. HOmiptera Colcoptera Prodaceous
Orthoptera

Weed mixture A Tall Timbers* (78) 2.0 a1.3** 1.4a0.7 0.3 a0.3 1.3a:!:0.84 0.la10.13 0.6a0.4 0.3a0.1 1.9a0.9
Green Acres*** (79) 0.1 a0.15 0.9a0.5 0.04a0.02 1.la0.48 0.0a+0.56 0.2a0.1 .0.0.3 0.3a0.17
Weed mixture B Tall Timbers (70) 1.3 b0.8 1.3a0.7 0.4 a0.3 0.9:!:0.7 0.2':;0.1 0.6a0.4 0.5a0.2 1.4al.l
Grcon Acres (79) 0.08a0.07 0.5b!0.3 0.2 b0.15 0.7b.0.4 l.laO.8 0.4a0.2 0.4a+0.15 0.3at0.1
Natural weed Tall Timbers (70) 1.0 b0.7 l.4a0.8 0.5 aj:0.3 0.90.3 0.a 0.4a.3 0.4a. 0.5a:.0.2 O.Gb.0.2
coma- !.-x Green Acres (79) 0.3 b0.1 0.6b0.2 0.1 hb0.05 0.9an0.6 0.6bi0.3 0.2ai0.15 0.4a!0.1 0.4a0.3

Naturall weed Tall Timbers (70) 1.0 b0.6 1.3a0.0 0.9 b.:O.8 1.5.a0.9 0.4,40.2 1.0b0.0 0.5a10.2 1.4a;0.0
cow !..:> hlighly
cfe highly Green Acres (79) -- -- --- --- -- ~
fertilized
Natural weed Tall Timbers (78) 1.6abl.l 0.9a0.6 0.1 a0.08 0.6b!0.2 0.3a10.2 0.5a+0.3 0.4a0.1 0.2bt0.1
complaex Green Acres (79) --- -- -- --- -- -
dctcrmincd by
Dece(!;cer plowing
Soybeani: Strip Tall Tirmbers (78) --- -- -- --- "
Green Acres (79) 0.4 b;'0.1 0.3b0.l 0.2 bi0.1 0.9a.0..3 0.O9v0.4 O.la0.05 0.4a0.3 0.3a.t0.05

Average of 12 sampling dates (n=36).
** Means in a same year, followed by the same letter, on any given column, are not significantly different according to Duncan's multiple
range test (p = 0.05).
*** Average of 7 nampling dates (n-21).











Table 15. Average corn yields under different cropping systems at two sites in north Florida.


Tons per hectare std. deviation

Corn systems Tall Timbers Research Station, 1978 Green Acres Farm, 1979
(n=3) (n=3)


Monoculture 2.88a*1.38 4.86a0.38
Weed mixture A 3.46a 1.2 3.36b0.32
Weed mixture B 2.00b 0.85 3.22b0.7
Natural weed complex 1.32c 0.04 2.84b0.6
Natural weed complex 2.36b 1.34 --
highly fertilized
Natural weed complex 0.93c 0.68
determined by December
plowing
Soybean strip --- 4.03a0.63
Rye straw mulch --- 1.37c0.2


* Means followed by the same letter, on any given column, are not significantly different according to
Duncan's multiple range test (p = 0.05).









0






51



Pimentel et al. (1973). Considering the low energy inputs (no

pesticides or irrigation) invested in the management of the plots,

most yields seem economically and energetically acceptable.

Variances associated with yield means of some treatments were

high at both localities (especially treatments 1, 2 and 5 at Tall

Timbers and 3, 4 and 5 at Green Acres). In many cases this was due

apparently to gradients in moisture or nutrient levels in the soil.

In a few instances low yields in a replicate resulted from patchiness

in the attack of soil nematodes.

At Tall Timbers, corn monocultures and corn-weed mixture A systems

presented similar yields. All other treatments showed lower yields.

The lowest yields at Tall Timbers were observed in the corn systems

associated with the natural weed complex and with the natural vegeta-

tion resulting from an earlier December plowing. The latter system

was more mature than any of the other weed treatments and had a back-

ground of perennials (e.g., Rubus sp., Phytolaca sp.), aggressive

semi-annuals (Crotalaria sp., Eupatorium sp., Ipomoea sp.) and grasses

which tended to invade the corn rows on a regular basis. At Green

Acres, corn monocultures and corn-soybean strip systems showed the

highest yields. All other corn-weed associations had lower yields.

The lowest yields were observed in the corn systems with a rye straw

mulch. There are certain evidences which suggest that rye residues

might exert allelopathic effects on certain plants, including corn

(Rice 1974).

Although the selective presence of weeds within corn plots

reduces the incidence of fall armyworm, this reduction is unacceptable

from the yield point of view. This suggests that any advantage







52


offered by weeds to establish a pest equilibrium in corn fields is to

some extent offset by their interference (competition/allelopathy)

with crops, even if weeds are grown as alternate rows. A direct and

intense interference between weeds and crops grown in separate rows is

unlikely and has not been reported in the literature. Also, there

remains the question that no matter how much weeds reduce the

incidence of fall armyworm in corn, because corn can tolerate sizeable

populations of Spodoptera (Beingolea 1957) without yields being affected.

Thus, differences in yields between weed-free corn plots and corn

systems diversified with strips of weeds are explained by other factors

not considered in the present study.

















CONCLUSIONS


Present research suggests that it is possible to design corn agro-

ecosystems to reduce the incidence of fall armyworm, Spodoptera frugiperda.

By proper cultural manipulations (e.g., direct sowing, differential soil

fertilization, early plowing, etc.) certain weed communities can be

established within corn fields. The presence of these weed associations

addstrophic and structural diversity to the corn systems which result

in two main effects:

1. Act as plant defense guilds (Atsatt and O'Dowd 1976) with anti-

herbivore properties, reducing the incidence of Spodoptera frugiperda.

The biotic, structural, chemical and microclimatic complexity of corn-

weed mixed systems greatly ameliorates the noctuid pressure early in the

growing season. At Green Acres, Spodoptera damage was reduced most when

corn was grown in association with the natural weed complex. At Tall

Timbers, fall armyworm damage decreased as the density of the weed

community increased. Corn plants associated with a highly fertilized

natural weed complex were least damaged by fall armyworm. It is possible

that corn and natural weed complexes share coevolutionary links that

enhance the associational resistance of the crop community (Root 1973,

Murdoch 1975). It should be noted, however, that reduction of the

incidence of fall armyworm might prove unacceptable from the yield point

of view if weed interference is not effectively minimized.

2. Condition a continuously present set of natural enemies in the

fields. At Tall Timbers, foliage predator arthropod communities were


53







54



more diverse and abundant in weedy plots than in weed-free plots. At

Green Acres, predators were more abundant in the monocultures than in the

weed-diversified systems, and predator diversity was equal in all

treatment plots. An increase in the quantity of weeds in a given habitat

space generally was correlated, both with an increase in the abundance

and diversity of predaceous arthropods but not with an increase in the

disappearance of artificial prey.

Heliothis zea (Boddie) was not affected by weed diversity. Only the

diversification of corn systems with a strip of soybean decreased the

number of ears damaged by corn earworm, suggesting that successful

management of the corn earworm depends on introducing a select kind of

diversity (e.g., corn-soybean polycultures), rather than general habitat

diversity.

Results from crop-weed-insect predator interaction studies under

experimental field conditions are dictated by a number of factors such

as year, area, weed abundance and diversity, crop variety, etc., but more

so by the distance between experimental plots.

A complication in designing experiments with plots close to each

other is that the proximity of treatments permits insect predators to

move easily from one habitat to other. Such movements constitute

conservative errors tending to make predator fauna in the various

treatments similar. For this reason, the diversity of the predator fauna

was similar in all Green Acres plots and the densities of predators in the

monocultures was higher or equal than those in some weed-diversified corn

systems. A better approach to establish differences in predator levels

between simple and diverse crop habitats is the design of well distanced

experimental plots to prevent intercrop movements (like the Tall Timbers






55



experiments) or otherwise increase the size of the plots considerably. At

Tall Timbers predators were clearly more abundant and diverse in the weedy

corn plots than in the weed-free plots.

The data also suggest that the nature of the adjacent surrounding

habitats can dramatically influence predator complexes within corn

fields. Predator abundance and diversity were greater in corn plots

surrounded by mature, complex, natural vegetation (e.g., annually burned

pinelands at Tall Timbers Research Station) than when surrounded by

simple, annual crop fields. Encouraging plant diversity within and

outside corn fields increases the colonization rates of predators early

in the season and decreases extinction rates by providing shelter and

cover throughout the crop growth (Price 1976). It is tempting to

extrapolate these results to a between-field comparison and suggest that

corn fields adjacent to simple habitats will have fewer predators than

fields with complex borders, with the result that outbreaks of fall

armyworm are more likely in corn fields located in simple habitats. The

long series of assumptions cast the validity of this argument because

between field comparisons (like in this study) involve other factors such

as location, size of the fields, year, corn varieties, etc., which may

markedly influence pest and natural enemies dynamics.

Reduction of fall armyworm damage in corn grown with a rye straw

mulch and in corn grown in concert with a soybean strip suggests the

potential of both minimum tillage systems and intercropping systems of

corn and soybean as possible strategies to complement management of this

pest.

Elements of natural pest control undoubtedly exist in many mixed

cropping systems (e.g., corn-weed associations) and there are certain







56


ways in which these may be transferred into agronomically convenient

and economically acceptable monocultures. Based on these data corn

systems surrounded by a complex habitat, mulched with some cereal straw

and with a row of natural weeds between each 10 rows of corn might

effectively prevent outbreaks of Spodoptera. Many of these systems will

probably remain untried in the U.S. because of the potential for reduced

production or lower profits. Given economic and energetic constraints

and also due to the ecological impact of modern agricultural practices

(e.g., pesticide pollution), agroecological strategies will have to be

carefully evaluated on an environmental cost/benefit basis as well as on

an energetic basis. The challenge for pest managers will be the design

of a gentle technology which will be self operating with minimum external

inputs. Capitalizing on knowledge of beneficial plant associations will

provide a sound ecological basis to develop such technology.


















APPENDIX










Table 16. Common predaceous arthropods of north Florida corn fields*.


Collection sites

Tall Timbers Green Acres
Research Station, 1978 Farm, 1979

COLEOPTERA
Anthicidae
Notoxus monodon Fab. X X
Anthicas ephippium Laf. X

Cantharidae
Chauliognathus sp. X
Podabrus sp. X
Carabidae
Callida decora (Fab.) X X
Callida punctulata Le Conte X
Casnomia pennsylvanica (L.) X
Harpalus pennsylvanicus DeGeer X
Lebia analis Dejean X
Lebia viridis Say X X
Leptotrachelus dorsalis (Fab.) X X
Nemotarsus elegans LeConte X X
Pasimachus sublaevis Beavois X
Searites subterraneus Fab. X
Selenophorus palliatus Fab. X

Coccinellidae
Coleomegilla maculata DeGeer X X
Cycloneda sanguinea (L.) X X
Exochomus marginipennis Lec. X
Hippodamia convergens (Guerin-Men.) X X
Hyperaspis sp. X X
Olla abdominalis (Say) X X
Scymnus sp. X X
Malachiidae
Collops quadrimaculatus (Fab.) X

Mordellidae
Mordellistena sp. X X
Mordella sp. X X
Staphylinidae
Philonthus sp. X X
Pinophilus sp. X X

DIPTERA

Dolichopodidae
Condylostylus caudatus (Wied.) X X
C. sipho (Say) X X
Mesorhaga albiciliata (Aldrich) X



58






59



Table 16---continued.


Syrphidae
Toxomerus floralis (Fab.) X
T. marginatus (Say) X
T. politus (Say) X

DERMAPTERA

Forficulidae
Doru taeniatum (Dohrn) X X
Labiduridae
Labidura riparia (Pallas) X

HEMIPTERA

Anthocoridae
Orius insidiosus Say X X

Lygaeidae
Geocoris punctipes (Say) X X
G. uliginosus (Say) X X

Nabidae
Nabis roscipennis Reuter X X
Tropiconabis capsiformis Germar X X

Pentatomidae
Euthyrhynchus floridanus (L.) X X
Podisus maculiventris (Say) X

Reduviidae
Atrachelus cinereus (F.) X
Repipta taurus (F.) X
Sinea sp. X X
Sinea sanguisuga Stal. X X
Zelus cervicalis (Stal.) X X

HYMENOPTERA

Formicidae
Pheidole morrisi Forel X
Pheidole sp. X X
Conomyrma flavopecta (Smith) X X
Solenopsis invicta Buren X X
Sphecidae
Sphex sp. X
Tachytes sp. X

Vespidae
Polistes fuscatus (Fab.) X
P. annularis (Linn.) X X

NEUROPTERA

Chrysopidae
Chrysopa sp. X X







60


Table 16-continued.


Hemerobiidae
Micromus posticus (Walker) X

ORTHOPTERA
Gryllidae
Oecanthus sp. X X

Tettigoniidae
Orchelimum sp. X X

ARANEAE

Araneidae
Araneus juniperi (Emerton) X
Eriophora ravilla (Koch)
Tetragnatha laboriosa Hentz X X

Anyphaenidae
Aysha sp. X X

Clubionidae
Chiracanthium inclusum (Hentz) X X
Clubiona sp. X X

Lycosidae
Pardosa georgiae Chamberlin and Ivie X X
Pardosa milvina (Hentz) X X
Lycosa sp. X X

Oxyopidae
Oxyopes salticus Hentz X X
Peucetia viridans (Hentz) X X
Philodromidae
Philodromus placidus Banks X
Salticidae
Hentzia palmarum (Hentz) X X
Phiddippus regius (Koch) X X
Methaphiddippus galathea (Walck.) X X

Theridiidae
Latrodectus mactans (Fab.) X X
Theridion sp. X X
Thomisidae
Misumenops sp. X X
Xysticus fraternus Banks X
Xysticus texanus Banks X

* All specimens listed in this table were identified by Drs. R.E.
Woodruff (Coleoptera), F. Mead (Homoptera, Hemiptera), H. Weems
(Diptera) and G.B. Edwards (Araneae).

















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BIOGRAPHICAL SKETCH


Miguel A. Altieri was born in Santiago, Chile, on 3 September, 1950.

He received his high school certificate in 1967 from the Liceo Experimental

Manuel de Salas in Santiago. He also did his Junior Grade at the Notre

Dame High School in Los Angeles, California, in 1966. Later, he received

from the University of Chile his Bachelor of Science in Agronomy in May

of 1974.

In October of 1974 he entered the Universidad Nacional of Colombia

in Bogota. After conducting his research at the Centro Internacional de

Agricultura Tropical in Cali, he received the degree of Master of

Science in 1976.

From February to September 1977 he served as a research associate

at the Tall Timbers Research Station in Tallahassee. Since September

1977 he has been a graduate student of the Department of Entomology,

University of Florida.

During his residence in Gainesville, Miguel Altieri has traveled

to Colombia, Costa Rica and Mexico as a consultant in insect and weed

ecology. He is a member of the Sociedad Colombiana de Control de Malezas

(COMALFI), Entomological Society of America, Florida Entomological

Society and International Organization of Biological Control (IOBC).

Miguel Altieri is married to Grisell and has two children, Naraya

(6) and Joshua (3).




68










I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.





Willard H. Whitcomb, Chairman
Professor of Entomology & Nematology


I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.




Stratton Kerr
Professor of Entomology & Nematology


I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.





Carl S. Barfield
Assistant Professor of Entomology
& Nematology


I certify that I have read this study and that in my opinion
it conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.




Donald C. Herzog
Professor of Entomology & Nematology










I certify that I have read this study and that in my opinion
if conforms to acceptable standards of scholarly presentation and is
fully adequate, in scope and quality, as a dissertation for the
degree of Doctor of Philosophy.




Ray William
Associate Professor of Vegetable
Crops




This dissertation was submitted to the Graduate Faculty of the
College of Agriculture and to the Graduate Council, and was
accepted as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.



December, 1979
Dean, College of Agriculture




Dean, Graduate School




Full Text

PAGE 1

THE DESIGN OF PEST STABLE CORN AGROECOSYSTEMS BASED ON THE MANIPULATION OF INSECT POPULATIONS THROUGH TOED MANAGEMENT BY MIGUEL ANGEL ALTIERI A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1979

PAGE 2

To Grisell, Naraya, Joshua and Padre Ricardo whose love and strength made this work possible, and to mankind, for its evolution and survival

PAGE 3

ACKNOWLEDGEMENTS My deep gratitude is expressed to the following persons who contributed time, knowledge, resources, and friendship toward making this work a reality. To Dr. Willard H. Whitcomb for opening new horizons in my knowledge of predators and biological control in general and for serving as chairman of my committee. To Dr. E.V. Komarek and Tall Timbers Research Station for financial support and confidence in my research and ideas. To Dr. S.H. Kerr for his continuous advice and support in administrative and academic affairs. To the Department of Entomology for financial support and for the opportunity of obtaining higher education. To the Soil and Health Foundation and Rodale Press for financial and ideological support of my ecological approach to pest management. To Dr. Ray William for his advice and friendship and for serving as a philosophical catalyst. To Drs. C.S. Barfield and D. Herzog for serving on my supervisory committee and their cooperation, advice and encouragement through my studies. To Dr. Jerry Stimac for his advice and constructive criticisms. To Maria I. Cruz for her help, patience and friendship. iii

PAGE 4

To Cesar Garcia, Rod Gillmore, Antonio Gonzalez, Fred Collins, J. Ewell, R. Flammer, M. Palada, G. Wiser, R. Hudgens, for their friendship and encouragement. To Mr. Joe Quigg for providing me with an inspiring place to write this dissertation. To Grisell, Naraya, and Joshua for their love, patience, understanding and encouragement. To Fr. Richard Teall for his example of human and Christian attitude iv

PAGE 5

TABLE OF CONTENTS ACKNOWLEDGEMENTS iii LIST OF TABLES vii LIST OF FIGURES ix ABSTRACT x INTRODUCTION 1 LITERATURE REVIEW 4 Some Limitations for the Development of Pest Management Systems 4 Ecological Theory and Pest Management 6 Dynamics of Insect Populations in Complex Crop Communities 7 Effects of Crop Habitat Diversity on Natural Enemies 2.1 Parasitoids H Predators 14 Weeds as Sources of Predators 17 MATERIALS AND METHODS lg Tall Timbers Site ^ g Green Acres Site 22 RESULTS AND DISCUSSION 24 Nature of the Weed Communities 24 Weeds of Tall Timbers Research Station 24 Weeds of Green Acres Farm 26 Pest Incidence ~,c v

PAGE 6

Predator Colonization 28 Predator Abundance and Diversity 33 Trends of Individual Predator Species 37 Predation Pressure 39 Weeds and Number of Ground Predators 41 Predator Dynamics in Weed Habitats 43 Crop Yields 48 CONCLUSIONS 5j APPENDIX 58 LITERATURE CITED 61 BIOGRAPHICAL SKETCH 68 vi

PAGE 7

LIST OF TABLES Table Page 1 Selected examples of multiple cropping systems that effectively prevent insect pest outbreaks (Altieri et al. 1978) 8 2 Selected examples of cropping systems in which the presence of weeds enhanced the biological control of specific crop pests 12 3 Selected examples of weeds that provide alternate prey for general predators 18 4 Densities and species numbers of weed communities within corn fields at Tall Timbers Research Station in north Florida (1978) 25 5 Relative abundance and species numbers of weed communities within corn fields at Gainesville, Florida (1979) 27 6 Percent of damaged plants by fall arrayworm, Spodoptera frugiperda J.E. Smith, and percent of ears damaged by corn earworm, Heliothis zea (Boddie) in weed-free and weed-diversified corn systems at two sites in north Florida 30 7 Colonization of the corn systems by predaceous arthropods in north Florida 31 8 Mean number of predator species throughout the growing season in weed-free and weed-diversified corn systems at two sites in north Florida 34 9 Total numbers and diversity of general predators in different corn cropping systems at two sites in north Florida 35 10 Relative abundances of individual predator species in different corn cropping systems at two sites in north Florida 38 vii

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11 Mean percent of Spodoptera eggs consumed by predators over four sampling dates at two sites in north Florida 40 12 Mean numbers of ground predators caught by pitfall traps in different corn cropping systems at two sites in north Florida 42 13 Mean relative density and number of species of predators associated with weed communities within corn fields at two sites in north Florida 46 14 Mean numbers of individual predators species and families collected on weed communities within corn fields at two sites in north Florida 49 15 Average corn yields under different cropping systems at two sites in north Florida 50 16 Common predaceous arthropods of north Florida corn fields 58 viii

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LIST OF FIGURES Figure Page 1 Relationship (y = 104 0.2x) between weed density and percent of corn plants damaged by fall armyworm ( Spodoptera frugiperda J.E. Smith) in north Florida; r = -0.93, n = 15/mean 29 2 Comparison of mean abundance of Labidura riparia (Pallas) in mulched corn plots and bare soil corn plots in north Florida; confidence limits (a = 0.05) indicated for each mean 44 3 Relationship (y = 0 + 0.58x) between weed species diversity and predator abundance in weed communities of north Florida corn fields; r = 0.84, n = 12/mean 47 ix

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Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy THE DESIGN OF PEST STABLE CORN AGROECOSYSTEMS BASED ON THE MANIPULATION OF INSECT POPULATIONS THROUGH WEED MANAGEMENT By MIGUEL ANGEL ALTIERI December 1979 Chairman : W.H. Whi tcomb Major Department: Entomology and Nematology Populations of insect pests and associated predaceous arthropods were sampled by direct observations and other relative methods on simple and diverse corn habitats at two sites in North Florida during 1978 and 1979. Through various cultural manipulations (direct sowing, differential fertilization, early plowing, etc.) certain weed communities were selectively established in alternate rows within corn plots. Fall armyworm ( Spodoptera frugiperda J.E. Smith) incidence was consistently higher in the weed-free habitats than in the corn habitats containing natural weed complexes or selected weed associations. Corn earworm ( Heliothis zea Boddie) damage was similar in all weedfree and weedy treatments suggesting that this insect is not greatly affected by weed diversity. Only the diversification of corn with a strip of soybean significantly reduced corn earworm damage.

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In one of the sites distance between plots was deliberately reduced. Because predators moved freely from one habitat to the other, such movements made treatments more similar in their predator fauna and it was hard to establish real population differences. Large distances between plots minimized this complication and population densities and diversity of common foliage insect predators were greater in the weed diversified corn systems than in the weed-free plots. Trophic relationships in the weedy habitats were very complex compared to food webs in monocultures. Differences in the abundance of ground predators between plots could not be attributed to weed diversity. Predator pressure was monitored using Spodoptera eggs as artificial prey. The numbers of fall armyworm eggs taken were not shown to be related to numbers of predators present nor to the type and density of the vegetation. Corn systems mulched with rye straw had the lowest infestation of fall armyworm and the highest numbers of predaceous earwigs. The mean number of predator species and individuals was higher at the site where corn plots were surrounded by complex vegetation than at the site surrounded by annual crops, probably because diverse adjacent areas provided refuge to predators, thus acting as colonization sources. Corn-weed mixtures confer advantages in pest management and can be agronomically acceptable, although corn yields were generally lower in the weed diversified corn plots than in the weedfree plots. These systems can be designed to be self-operating without much technological inputs. Such an agroecosystem has obvious implications in an era of energy crisis and environmental concern. xi

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INTRODUCTION The main motivation for this study is the concern that despite numerous reviews and discussions concerning ecological theory and pest management (e.g., Southwood and Way 1970, Price and Waldbauer 1975, Levins and Wilson 1979) questions central to ecology like the relationships among diversity, complexity and stability of organisms in ecosystems have had no major impact on economic entomology. In this study, stability is defined as restricted fluctuations in pest population density through time and diversity as a measure of richness of species in a habitat (Murdoch 1975) Little work has been done on the design of crop systems based on ecological principles. Only a few attempts have been made to modify monocultural systems in the direction of diversity for the purpose of enhancing ecological features vital for successful pest management (Litsinger and Moody 1976, Perrin 1977) Most researchers concerned with the ecological basis for the management of insect populations attribute outbreaks of insect pests in agriculture to the extensive use of large crop monocultures (Southwood and Way 1970, Price and Waldbauer 1975, Atsatt and O'Dowd 1976, Pimentel and Goodman 1978). Exposed fields and concentrations of a single crop species open myriad possibilities for pest infestations (Browning 1975) Pure crop stands provide concentrated resources and uniform physical conditions that directly influence members of the crop 1

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2 fauna (Root 1973) The abundance and effectiveness of general predators are reduced because these simplified environments do not provide adequate alternate sources of food, shelter, breeding sites, and other environmental factors (Hagen et al. 1976) Herbivorous insect pests are more likely to colonize and remain longer on crop hosts that are concentrated because usually the pests* entire life requirements are met in these simple environments (Root 1973) As a result, the abundance of specialized pests attain economically unacceptable levels. Many crop plants, like their wild relatives, fare better in species diverse, structurally complex communities (Wilken 1977) Crops grown in f loristically diverse habitats suffer a lower herbivore load than conspecifics grown in monocultures. Many studies have shown that reducing crop stand purity by interplanting different crop species greatly reduces the density of herbivorous pests (Marcovitch 1935, Stern 1969, DeLoach 1970, Tahvanainen and Root 1972, Raros 1973, Dempster and Coaker 1974, Van Emden and Williams 1974, Litsinger and Moody 1976, Perrin 1977, Altieri et al. 1978, Perrin and Phillips 1978, Risch 1979) Outbreaks of certain types of crop pests are more likely to occur in weedfree fields than in weed diversified crop systems (Pimentel 1961, Adams and Drew 1965, Dempster 1969, Flaherty 1969, Root 1973, Smith 1976a, Altieri et al. 1977). Crop fields with a dense weed cover and high diversity usually have greater numbers of predaceous arthropods than do weedfree fields (Pimentel 1961, Dempster 1969, Flaherty 1969, Pollard 1971, Root 1973, Smith 1976b, Speight and Lawton 1976) Ground beetles (Dempster 1969, Speight and Lawton 1976, Thiele 1977) syrphids (Pollard 1971, Smith 1976b) and lady beetles (Bombosch

PAGE 14

3 1966, Perrin 1975) have been reported to be greatly abundant in weed diversified systems. The presence of certain weeds within crop habitats offers many important requisites for natural enemies such as nectar and pollen sources, alternate prey and hosts and microhabitats that may not be available in a monoculture (Altieri and Whitcomb 1979) Based on experimental results and on theoretical considerations which imply that diversity of species and habitat complexity confer stability to the insect community (Pimentel 1961, van Emden and Williams 1974, Murdoch 1975) some researchers have envisaged the maintenance of specific weed associations in crop areas to provide subsidiary food for entomophagous insects and thus, improve biological control of certain pest species (van den Bosch and Telford 1964) Practical implementation of this approach remains disappointingly low, however. The present study describes some ways in which corn agroecosystems could be designed to reduce the severity of insect pest problems. Discussed herein are some of the changes in abundance that target pests and associated predators undergo when certain weed associations are selectively allowed to grow in corn fields to provide additional safeguards against pest insects.

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LITERATURE REVIEW Some Limitations for the Development of Sound Pest Management Systems The efficient integration of several methods of pest control to suppress a complex of pests while achieving maximum yield and quality on one hand and minimum damage to the environment on the other, have been major goals of integrated pest management (IPM) (Rabb and Guthrie 1970, Metcalf and Luckman 1975). This desired balance has seldom been attained, however. There are several reasons: 1. Con temporary pest management practice still has too narrow an ecological knowledge base. Overemphasis on yield or market quality almost invariably means shattering of the crop community homeostasis (DeLoach 1970) Seldom can maximum stability and maximum productivity be achieved simultaneously in the same ecosystem (Turnbull 1969) Increasing agricultural production will always involve risk of serious degradation of at least some agro-ecosystems (Loucks 1977) For this reason, stabilization of yield rather than maximization of yield should be a major goal of IPM. 2. The explosive expansion of the pesticide industry has inflicted strong socioeconomic and philosophical obstacles to the development of sound ecological pest management strategies. The present agricultural epoch, clearly dominated by the petrochemical industry, envisions food cultivation as a business enterprise to be operated strictly for the 4

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5 purpose of generating profit in a market economy (Bookchin 1976, van den Bosch 1978) Integrated pest management should be part of a philosophy that views agriculture as the activity of a society whose historic role is to maintain, within ecological limits, productive land in order to sustain present and future generations of people. 3. Nearly all research in pest management is highly reductionist, parochial and discipline-oriented. Few articles with truly holistic approaches have emerged in the trade journals (Potts and Vickerman 1974) A common erroneous pattern of many pest management systems is the combination of various cultural and biological methods of insect control while simultaneously recommending chemicals for the control of weeds and diseases. These programs can lead to intensification of pest problems. For example, 2,4-D is a herbicide commonly used in corn for the post emergence control of broadleaf weeds. When corn plots were treated with a regular dose of 0.55 kg, 2,4-D ha" 1 aphid numbers and corn borer infestations increased significantly (Oka and Pimentel 1974) Similarly, corn plants exposed to 2,4-D were significantly more susceptible to corn smut disease and to southern corn leaf blight (Oka and Pimentel 1974). Furthermore, sprays of 2,4-D at normally used rates caused up to 50% mortality of the larvae stages of coccinellids (Adams and Drew 1965) Part of this anti-holistic approach to pest management resides in the lack of trained "generalists" who understand the principles of managing resources as a part of a total interacting environmental system (Pimentel 1970)

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6 Ecological Theory and Pest Management The ecological basis of pest management has been the topic of many review and journal articles during the past decade (Southwood and Way 1970, Price and Waldbauer 1975, Pimentel and Goodman 1978, Levins and Wilson 1979) Most approaches contrast the structure and function of agroecosystems and natural, undisturbed ecosystems (Southwood and Way 1970, Price and Waldbauer 1975, Rabb et al. 1976, Pimentel and Goodman 1978). Browning (1975) has advised the study of natural ecosystems from which knowledge can be gained that is readily applicable to agroecosystems. According to Price and Waldbauer (1975) agricultural ecosystems can be viewed in terms of two central concepts of ecology — island biogeographical theory (Price 1976) and succession of communities. Most ecologists agree that any pest management approach should try to develop an agroecosystem that emulates later stages of succession (i.e., mature communities) as much as possible, for this is how stability can be achieved (Root 1973). By adding selective diversity to crop systems, it is hoped to capture for agroecosystems some of the stability properties of natural communities (Murdoch 1975). The concept that increased diversity leads to increased stability has been challenged on theoretical grounds (van Emden and Williams 1974) but not by reliable studies in agricultural communities. For many researchers, the biogeographic region rather than the single homogeneous field is the appropriate unit for pest management research (Levins and Wilson 1979) An agroecosystem should be conceived as an area large enough to include those uncultivated areas which influence crops through intercommunity interchanges of organisms and materials (Rabb 1978) Excellent studies on the role of uncultivated

PAGE 18

land and crop field borders in the biology of crop pests and beneficial insects have been made (Dambach 1948, Piemeisel 1951, van Emden 1965, Pollard 1971, Hodek 1973, Thiele 1977). However, little is known yet of the influence of adjacent habitats on pest incidence in cultivated fields Dynamics of Insect Populations in Complex Crop Communities The predisposition of insect outbreaks to occur in monocultures is well known (Pimentel 1961, Browning 1975) Studies of multiple cropping systems have shown that populations of herbivorous pests reach higher densities on crop plants grown in monocultural stands than on plants grown associated with other plant species (Litsinger and Moody 1976, Perrin 1977, Altieri et al. 1978, Perrin and Phillips 1978) (See Table 1 for additional examples) These patterns can be explained on the basis of the following hypotheses: 1. Resource concentration: insect populations can be directly influenced by the concentration or spatial dispersion of their food plants. Many herbivores, particularly specialists, are more likely to find and remain on hosts that are growing in dense or nearly pure stands (Root 1973) 2. Associational resistance: ecosystems in which plant species are intermingled possess an associational resistance to herbivores in addition to the resistance of individual plant species (Root 1975) Interplanting of host plants can drastically decrease colonization efficiency and subsequent population density of crop pests (Tahvanainen and Root 1972) In addition to their taxonomic diversity, polycultures

PAGE 19

o o> u rH o -a K C eg B. jE. — t u i-j 31 O c c tcj u E ti o -3 CJ J* ? 4J B a o c — I as Cl O a •3 "3 C CS a u u O t; c_ Cc — < u c c O — O M a r_ CS. c u c o c o c£ c X> u V, VHI c" — 14 O u o 1* Cl T. c c (J -* u M C C C c I-i o CJ u C — • r-a c c: a 3 t? u b — u c c o z c jy U c5" 'J c r. xt n u U 'J d c_ a :> c U "H Q *J c c 1 _r: : CL O v. r: .J il u *S y n fj XT -i "3 Ii* c "f: • o c l. C — C cl V CJ b' i-

PAGE 20

g o u -a CJ rJ a a a c — t c M a c o D 01 cj o -H a ca C/l -i i_> u a c *-< (0 ifi — t rs ei U N c z cj C 11 — c f-* o v.O i-* O 1>-i a u to u cS go a U-l u 0) U-l U-l U-l C u-i o o a a a CI o j£ IX c JO G V CJ B 1 V 'n w lM 'J C C c 0 a c: a o ja a 1u k> 0 a a iCJ u =c u in (Q a *M c c rj U-< V* -H a a ^ o -b u u o *j -a o *j M u O i- c c c u c c m d cn #— • H jC a. e d s •5-4 q i-J CJ H a cj: ^ a u d M E o C i-i a d XI j-j C 01 a u C d u CJ a n J — d XI u ) — (J H < rZ o a C y u c u u o a cn G u a -J Zi H< n ixi tc XI E cn xi o u 3U e a u w (3 o •H c O o w (=! O a > x> tJ n c w £• E a c < o a cn a U c u c 2 VJ •—i tz H W u a CD ij O tn 'J ti CL a •~ "(J c cn £ o c a E -h U 1* G xi ez •H -H J~ o c £ >*O ftl U-l i— < -a rr -a r; n -a e p* CJ CJ a o — o a C w a — — : EU -4 C

PAGE 21

10 have a relatively complex physiognomy, chemical environment and associated patterns of microclimates. This biotic, structural, chemical and microclimatic complexity of mixed vegetation greatly ameliorates the herbivore pressure on the crop systems as a whole (Tahvanainen and Root 1972) 3. Plant apparency: most crops are derived from early successional herbs which largely escaped from herbivores in space and time (Feeny 1976) The effectiveness of natural crop plant defenses is reduced by present agricultural methods. When planted in monocultures, crop plants become more apparent to natural enemies than are their ancestors in nature. The apparency of a crop plant is increased by close association with conspecific individuals (Feeny 1977) Crop plants grown in monoculture are subjected to conditions for which their qualitative chemical and physical defenses are inadequate (Feeny 1976)". 4. Natural enemy hypothesis: this hypothesis predicts that there will be a greater abundance and diversity of natural enemies of pest insects in polycultures than in monocultures (Root 1973) Predators tend to have broad diets and habitat requirements so they would be expected to encounter a greater array of alternative prey and microhabitats in a heterogeneous environment (Root 1975) Annual crop monocultures do not provide adequate alternate sources of food (pollen, nectar, prey) shelter, breeding and nesting sites for effective performance of natural enemies (Rabb et al. 1976) The natural enemy hypothesis has been stated in the following way (Root 1973) : a. A greater diversity of prey and microhabitats is available within complex environments. As a result, relatively stable

PAGE 22

11 populations of generalized predators can persist in these habitats because they can exploit the wide variety of herbivores which become available at different times or in different microhabitats. b. Specialized predators are less likely to fluctuate widely because the refuge provided by a complex environment enables their prey to escape widespread annihilation. c. Diverse habitats offer many important requisites for adult predators, such as nectar and pollen sources, that are not available in a monoculture. Effects of Crop Habitat Diversity on Natural Enemies Parasitoids Several authors have claimed that insect populations are more stable in complex communities because a diverse habitat can maintain an adequate population of the pest and its enemies at critical times (van den Bosch and Telford 1964, DeLoach 1970). For example, parasitoids are more effective in areas where there are abundant wildflowers that provide nectar and pollen (van Emden 1962, Leius 1967, Syme 1975). Also, since the life cycle of many parasitic insects is not synchronized with that of their host's, some parasitoids must rely on alternate hosts to maintain establishment within a community. In many cases, weeds and other natural vegetation in and around crop fields harbor alternate hosts for parasitic insects thus providing seasonal resources to bridge the gaps in the life cycles of parasitoids and crop pests (Peppers and Driggers 1934, van Emden 1965, Doutt and Nakata 1973, Syme 1975, Stern et al. 1976, Plakidas 1978). Additional examples can be found in Table 2.

PAGE 23

12 h 0 *4 4J o H >M C 111 o. -j 5 I ~> 01 a td o u u o (A c VC a. co cr o nl u cr, tn s n e ff) a u ml to 3 C ii tJ -i U CJ 1 O 0 u !S w g c; a o e >. 3 5 o = C i| o c < 1 s >> \l -~ c a u a u -a c CJ > a s~ > j 4 CJ >CJ —i a -6 ti "S ? o c 3 CJ CJ t *o u a. •H O 5 J 1 S3 > H ev ca u u c a B *J> c U-i a o o. c a* a U -H o D. o — n W-l 3 CJ J-* M 0 g 1-4 J~ ^r; t> a U u u o t-i J CJ CJ U o -•) CJ 1— a o o a c C JS 3 CJ e C CJ — C CI C f3 to pp. M eq U M a M 3 9 a cm a t-i a. a cl a. CJ CI CJ CJ a o -J C c at a e c efl CJ c u w

PAGE 24

13 > -J o o c. 1 c-> 0 -C x: O Jo i r> c: o n 0 C3 Iri at o d —* C "3 4J e; o o a O U c -o ei X} o o 0 •H o tu o m rc -o e c V iJ o c; c c r: a E > C > > H U o y i-< xz c^ -r4 B) il X. 'J u *j E 3 c H T3 <— ~ c o c c *?{ c c c c H u ij u o o -I c c ^ rt ^ u V) XL c ^ GO J-. tc > > R3 O Jtf a ^ "3 > o 6 c iJ o O O U ~i jr. a r-< C r-i tT o u P. < .O < ^ 2(2 s m 1 81 =1 CLj f-4 w •H c o a I I •3 H

PAGE 25

14 Peterson (1926) observed that uncultivated orchards were less sever ly attacked by codling moth than thoroughly cultivated orchards. Later, Peppers and Driggers (1934) and Allen and Smith (1958) showed that percentage of fruit moth larval parasitism was always greater in orchards with weeds than in clean cultivated orchards. Similarly, Leius (1967) found that the presence of wild flowers in apple orchards resulted in an 18fold increase in parasitism of tent caterpillar pupae over non-weedy orchards. Tent caterpillar egg parasitism increased four times, and codling moth larval parasitism increased five times. Predators The replacement of natural communities or diversified agriculture with large monocultures has caused general predator fauna impoverishment in certain agricultural areas (van den Bosch and Telford 1964) As far back as 19 35, Marcovitch envisaged the diversification of cropping systems as a means of increasing the efficacy of naturally occurring predator populations. Later, Root (1973) proposed the "natural enemy hypothesis" which states that predators are more effective and abundant in diverse habitats than in simple ones. Results from several experiments back up this hypothesis. In the Solomon Islands, O'Connor (1950) recommended a cover crop be used in coconut groves to improve the biological control of coreid pests by the ant Oecophylla smaragdina subnitida Emery. In Ghana, coconut gives light shade to cocoa and supports without apparent crop loss, high populations of Oecophylla longinoda keeping the latter free from cocoa capsids (Leston 1973)

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15 In the Canete Valley of Peru growing corn in conjunction with cotton was ideal for the reproduction of predators that contributed to the biological control of cotton leaf rollers Ar gyro taenia sphaleropa Meyrick and Platynota sp., and the bollworm Heliothis virescens (F.) (Wille 1952, Beingolea 1957). Growing alfalfa strips within cotton fields in California significantly increased numbers of predators early in the season; these beneficials moved back and forth between the alfalfa and the cotton (Stern 1969) Intercropping systems of cotton with corn or sorghum presented higher numbers of predaceous arthropods (primarily lady beetles and lacewings) than cotton monocultural systems (DeLoach 1970, Fye 1972, Burleigh et al. 1973, Stern et al. 1976). Similarly, cotton-sesame interplantings had high populations of beneficial insects (Laster and Furr 1972). Intercropping of corn and peanuts ( Arachis hypogaea L.) decreases the incidence of the corn borer [ Ostrinia f urnacalis (Guenee) ] probably because these habitats encourage the abundance of Lycosa sp. spiders (Litsinger and Moody 1976) In Costa Rica, increasing resource diversity by intercropping corn and sweet potatoes enhances the relative abundance and diversity of predators (Risch 1979) Similarly, in tropical Colombia, corn-bean polycultures had higher numbers of predaceous Hemiptera and Dolichopodidae than corresponding monocultures (Altieri et al. 1978). Larger numbers of ground beetles (i.e., Harpalus rufipes ) in mixed plots of cabbage and clover reduced survival of Pieris caterpillars (Dempster and Coaker 1974) Populations of many predator species seem to depend on general abundance of hibernating sites and alternative hosts as well as flowers in hedges and other habitats in the area, not just around the immediate

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16 edge of the field (Pollard 1971) The management of habitats surrounding crops could augment regional populations or predators if widely practiced (Perrin 1975) Populations of arthropod predators were higher in diverse permanent habitats than in simple habitats (Pollard 1971, Fuchs and Harding 1976). The importance to pest control of the presence of uncultivated habitats adjacent to crops is inconclusive. More is known about the influence of diversifying the crop habitat itself on insect populations. The presence of certain weeds within a crop can greatly influence the balance of members of the crop fauna. Reduced incidence of crop pests in weedy crop systems compared to weedfree monocultures has been demonstrated by Pimentel (1961) Dempster (1969) Tahvanainen and Root (1972), Root (1973), Smith (1976b), and Altieri et al. (1977). In many cases, the reduced pest numbers have been the result of an increase of predator populations (Altieri and Whitcomb 1979) Coccinellids syrphids, Aphidoletes sp. and other predators were more abundant and preyed more actively on aphids in cole plants grown among diverse meadow vegetation than in cole monocultures (Pimentel 1961, Root 1973) Ground beetles ( Harpalus ruf ipes Feronia melanaria and others) and a harvest spider ( Phalangium opilio) were more abundant in weedy cabbage crops than in weedfree monocultures (Dempster 1969) In England, Smith (1976a) found that oviposition of certain syrphid predators and abundance of the anthocorid Anthocoris nemorum were increased in brussel sprouts with a weedy background. Populations of coccinellids were higher in weedy oat fields in New Brunswick than in weed-free monocultures (Adams and Drew 1965) Similarly, areas of dense weed cover in English cereal fields had more predatory ground

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17 beetles (Carabidae and Staphylinidae) than did weed-free areas (Speight and Lawson 1976) Weed as Sources of Predators Perrin (1975) and Altieri and Whitcomb (1979) have emphasized the role of certain weeds as sources of alternate prey of important predators of crop pests. To improve survival and reproduction of predators within an agroecosystem, it is often desirable to have subeconomic fluctuating populations of alternate prey permanently present in the crops (van den Bosch and Telford 1964) Specific examples of weeds that provide alternate food resources for predaceous arthropods are listed in Table 3. If widely encouraged, these plants show potential in insuring a standing population of specific predators in areas where these interactions occur consistently.

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18 CJ K u >£3 .rt \^ J2 U C (0 H O V. JO u E o o a a M a c ra •c c c <3 ra CJ CJ CO cr. ra ra •rt CO —t •a •a ra no -rt •c; •rt -rt M r. r-l to tM 0) i — 1 CJ ra ra M M t-J ra —1 4J u c rt O CI *a CJ c; to ra > u H e o ra H -H c ra rt a *D o Vcj CI •rt •rt CI a cj -E JO U o o 1 O a) P. o cc o J .J xt c < JO e o ~3 a ra ra CJ CI ra in CJ CJ r-i e ra u oi 4-1 o CJ ra l-l CJ u CJ •a ab u a ra .rt J= GQ tl u ra ra ra c u •C to 1 o CI ra C >1 ra -H c CI CC o •rt o rCO u cc o •rt > iJ o < t>0 •a an lSa ra i-H M u •rt >— ra x c 5 ra ra ra •H C JO ra f-l p. o 10 CI a. o CC c CO a V .rt ra c ra CO to CI ra o ra •rt 0J o cj ra c ra CJ g u CJ c 4J ra c M c .e a O u rt c ra u rt l-i a -H 4J •rt ra w o JO o U 10 •-i ra c IJ a VI ra o a o a 3 H. < CO < o o on cn B o ra CJ -rt u CJ -ra ra 4-J El < CJ < da la •-) -rt •rt w i s ra CI c CI >, ra a CJ ft K (0 i C c 0 CJ b -rt o Crt ra H u CI >• "CJ i | M o tx > •— H CI o u c CO rH '~ a LO •H •rt ca O (0 l-l l-l > r-i >-. •-5 o CJ tr. f-i CJ CJ w } g — < P-, ra > u i 5 jj o .* 0 t !m CO •3 i C-rt -c i-t C n i i | a ra > o •o t3 a 09 cPhi j-t c c J c JC CJ > to* ra CJ CO OB -ra "rt •rt CC tx CO ra •rt >j Vl c t-i to a IJ J-J a o ra CJ rt b! o I 3 i; s ra ra c > r_ c jC u *H H c -rt jj c CJ i> ra 0 — < o SB H c tc tl w a o >> a c ra jj u 4J p CJ H C i 'J c ro o z el .fl ci CJ o :-• seio o a CO H CO g o A- h •rt c E •a u v. 0J c: ta CJ M ra ra u CJ B u o ti <: ra E CI rt 10 SO c CJ -rt c rt c* CJ 1 — 1 •rt > ra ra u ra o CJJ e e ra 3 -o rt CO ^J l-l o o CO CO CJ

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MATERIALS AND METHODS The effects of weed diversity on the dynamics of corn insect pests and associated predators were tested in experiments at two sites. Tall Timbers Site This experiment was conducted in a 5 ha field located at Tall Timbers Research Station in northern Leon County, Florida. On April 5, 2 1978, the field was divided into eighteen 100 m plots (10 rows, each 10 m long) and planted with corn ( Zea mays L., c.v. 'Kernel Greenwood Hybrid 1 ). The distance between rows was 0.9 m (36 in). Plots were separated by 50 m to reduce variability due to immigration and emigration of arthropods. The soil between the plots was kept free of vegetation by frequent harrowing. Fertilizer (5-10-15) was applied to the corn plots at a rate of 436.4 kg/ha (400 lb/a). There were six treatments, each replicated three times: 1. Corn monoculture (weedfree). 2. Corn + weed mixture A. This mixture consisted of seeds of Solidago altissima L. (golden rod) Amaranthus sp. (pigweed) and Heterotheca subaxillaris (Lam. ) (camphorweed) sowed when the corn was planted. 3. Corn + mixture B. This mixture consisted of seeds of Ambrosia artemisii folia L. (ragweed) Chenopodium ambrosioides L. (mexican tea) and Daucus carota L. (wild carrot) also sowed when the corn was planted. 19

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20 4. Corn + natural weed complex, highly fertilized. The rows in which the native weeds were allowed to grow were fertilized with 5-10-15 at a rate of 872.8 kg/ha (800 lb/a). 5. Corn + natural weed complex, regularly fertilized at the usual rate of 436.4 kg/ha. 6. Corn + natural weed complex determined by an early plowing (the rows in which the weeds were allowed to grow were previously plowed by the end of December 1977 and since then left undisturbed) In all treatment plots, native weeds were allowed to grow freely in the two middle rows and in the rows before the last on each side of the plot. The plots in which weed mixtures were sowed had a background of native weeds. Each plot had six rows of corn and four rows of weeds. The area between the six rows of corn was kept weed-free by cultivation and hoeing. Pest incidence on corn by lepidopterous larvae (mainly Spodoptera frugiperda J.E. Smith) was estimated by counting the number of plants with damaged whorls in each plot. Thirty corn plants were randomly selected every 7 days in each plot and their degree of damage was evaluated visually. Similarly, numbers of predaceous arthropods on corn were estimated by careful visual examination of the above-ground parts of 30 corn plants at each plot every 7 days. The number and type of predator species on each plant was recorded and, when possible, prey items. Populations of predaceous arthropods present on the weeds in the plots were evaluated by taking 20 sweepnet samples along the two middle weedy rows of each plot. Sweepnet contents were analyzed immediately by opening the sweepnet and counting the number of predator species crawling along the bag.

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21 Predator pressure in the various plots was assessed using fresh 2-day-old Spodoptera eggs. One hundred eggs were attached to pieces of white paper towel on the rough side. These pieces were stapled to white paper cards and then placed in the field (two cards per plot) pinned to corn leaves at the center of each plot. Egg cards were left for 24 hours, after which time they were collected and the numbers of eggs attacked and removed were counted with the aid of a stereo microscope. Fresh cards were placed again in the plots. This procedure was repeated four times. In some instances the arthropods involved in egg removal were specified by direct observation. Weed density and species composition in each plot were estimated 2 on three occasions by using a 0.5 m quadrat. Each quadrat was randomly thrown and examined twice in the two middle weedy rows of each plot. Numbers of plant and species enclosed in each quadrat were recorded. Relative abundance and species composition of soil arthropods were monitored with pitfall traps filled with killing fluid (approx. 50 ml of 95% alcohol) One trap per plot was used and was left in the ground for 14 days. After this time, traps were removed, and the contents were sorted to species and counted in the laboratory. This procedure was repeated five times during the experiment. The relative abundance and species composition of flying predaceous insects (mainly Dolichopodidae and some wasps) were estimated by placing a yellow pan in the middle of each plot. Each pan was filled with water and a few drops of detergent were added to ensure that insects caught sank to the bottom of the pans. The pans were left in the field for 3 days. After this time, the pans were emptied and the contents

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22 sorted according to species and counted in the field. This procedure was repeated five times during the experiment. Green Acres Site This experiment was conducted in a 1 ha field at the University of Florida, Green Acres Farm about 20 km west of Gainesville, Florida. The entire field was planted solidly to corn ('Coker 71') on March 27, 1979, and afterwards divided into 18 randomly distributed plots of 100 m each (10 rows, each 10 m long). The distance between rows was 0.9 m (36 in.). Plots were separated by 8 m; however, corn was allowed to grow around the plots. Those plants growing in the vicinity of the plot edges were cut back regularly with a machete to a height of 20 cm. Fertilizer (6-12-18) was applied to the corn plots at a rate of 436.4 kg/ha (400 lb/a). There were six treatments, each replicated three times: 1. Corn monoculture (weedfree) 2. Corn + weed mixture A. This mixture consisted of seeds of Amaranthus retrof lexus L. (red pigweed) Xanthium pennsylvanicum Wallroth (cocklebur) Oenothera biennis L. (evening primrose) and Chenopodium ambrosioides L. (mexican tea) which were sowed simultaneously with the corn. 3. Corn + weed mixture B. This mixture consisted of seeds of Taraxacum officinale Wiggers (dandelion) Heterotheca subaxillaris (Lam.) (camphorweed) Solidago altissima L. (goldenrod) and Bidens pilosa L. (beggartick) which were simultaneously sowed with the corn. 4. Corn + soybean. 5. Corn + natural weed complex.

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23 6. Corn + rye straw mulch. In treatments 2, 3, and 5 native and sowed weeds were allowed to grow freely between the two central corn rows of each plot. Selected weeds were also grown in pots in the greenhouse and later transplanted to the plots to assure a high population of desired weeds in each treatment. In treatment 4, two rows of soybeans were planted between the two central corn rows of each plot. The remaining area of the plots was kept weedfree by cultivation and hoeing. The incidence of Spodoptera frugiperda J.E. Smith, predator population and predator pressure were estimated by the same methods used at the Tall Timbers site. Two pitfall traps were placed four times in all treatments to estimate relative abundance of ground beetles and earwigs. No yellow pans were used in this experiment. Weed densities and species composition were estimated using a thin (2 mm diameter) metal rod which was let down vertically to the ground in 20 random places per plot, and all weed leaves (and soybean leaves in the case of treatment 4) touched by the vertical middle were recorded. When totalled up for all the intersections, this gave an estimate of the percent cover and leaf area index of each weed species and the total plant community (Wilson 1963) In both experiments, corn yields were estimated by weighing corn 2 ears harvested from an area of 29 m in the center of each plot, once the ears reached a moisture level of 15%.

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RESULTS AND DISCUSSION Nature of the Weed Communities Weeds of Tall Timbers Research Station Weed communities within all corn plots were mainly composed of associations of cocklebur, sicklepod ( Cassia obtusifolia L.), Florida purslane ( Richardia scabra L.) and grass species. Because of dormancy problems the weed seeds sowed in treatments 2 and 3 (weed mixture A and B) failed to germinate adequately. Consequently, populations of goldenrod, pigweed, camphorweed, wild carrot and mexicantea remained at low densities during the whole study. However, the mere presence of these weeds in the plots contributed to the background diversity of each treatment making the weed communities different from each other. The density of individual species within each weed community varied considerably among treatment plots (Table 4) Treatments 2 and 4 had 2 the highest number of plants per 0.5 m and were dominated by grass species such as Panicum sp. and Andropogon sp. Both Cassia and grasses were stimulated by high fertilization. Richardia was particularly abundant in treatment 2. The plot previously plowed in December had a unique background of perennial weeds (e.g., Rubus sp. Phytolaca sp. etc.) and annuals such as rattlebox ( Crotalaria spectabilis Roth) and cypress vine ( Ipomoea quamoclit L.). 24

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rd 0 d M a M M (D (3 -P d XI rH O •H 8 c •H H •H o u t) G) a IH • O CO n 01 a H P (0 C T3 •w 10 H 0) o •H rH U fa (U ft £ M c id n ai •H +J •H CO PS o Q P n o c •H C o H -P id 4J CO G rH "I CO n a) a) o o c (d Scow m H cu to o •H 0 o ft IB 4-1 0 H to a a c id o S •P •H O o -a 3 O CM rH CN • CM CO CM rH rH CM + 1 +1 +1 +1 4 & d ab LO CO CO CM >jD CO in CO kO CO CO Ol CM rH + 1 0 CM CM co o o O O o CO r >* IT) CM LD CO o o o O O rCO CO <* co r> CM o o o o O ro o m CM rH co rLO o o o o O CM co rH <* O X X -a X 0) o G CD rH rH N rH H Cji B CU irH o N o •H o a < ra u H u +> u H rH Ei >1 o o H CD T3 A o H H 0 -P CD IW O rH a 3 o rH 0) CJ d ft 11 2: >1 CD x X U-l rH c u •H rH rH M rH •H CD s CO >1 d d d B fl in rH n rH !h C TJ rC 3 CD CD 0 0) +J tTi +J & -P P U o CJ cci •H d CD d ai a S S3 S3 rH S3 >0 Q 10 ft to CO H a u c id ft to c o & o ft o M 0 c < ft CO B 3 CD •rH c d fH CD rH d rH Q O C o > H 0> >1 C d in C o 0 • o H n a +1 ft -p CD rH p CO CD (U B •p d to a Qi CD d +J rH >1 O JQ rH ft -H CD -P & rH O rH B H O m MH c 01 d G 0 d G 3 S Q

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26 Weeds of G reen Acres Farm Xanthi urn, Amaranthus and Bidens were the only weeds that germinated heavily after being artificially sowed. All weed communities were dominated by Richardia although Xanthium and Bidens reached density values high enough to make treatments 2 and 3 different from each other and also from the natural weed complex (Table 5) Amaranthus germinated slowly and was part of the weed community in treatment 2. Cassia and grasses were not part of the natural background of weeds of the area. Populations of goldenrod, camphorweed, evening primrose, dandelion and mexican tea remained low during the study, but by the end of the corn's cycle their densities started to increase in the different plots. Pest Incidence As shown in Table 6, the percent of corn plants with whorls damaged by fall armyworm larvae was higher in the monocultures than in any of the weed diversified corn plots, at both localities. At Tall Timbers Research Station the incidence of fall armyworm was mostly reduced when growing corn in association with a highly fertilized natural weed complex. At Green Acres, fall armyworm incidence was reduced mostly when growing corn with a rye straw mulch. It is possible that a rye straw mulch may change the color or shape of the corn background thus affecting the colonization of fall armyworm which seems to respond to visual cues in locating a host (Southwood and Way 1970) Fall armyworm incidence and weed density appeared to be inversely correlated at Tall Timbers (r = -.93). As the density of plants in the weed community increased, the percent of damaged whorls in the plots

PAGE 38

u o I COO C IS O > c o • m + S c*> W >1 JJ •H B 3 o o a o u -p a I P id o M O CO O ro +1 +1 in O u p •H e d ro o II i g •H P •H p P T3 •H -a •H c C n ,C (d c rrj P ,c 1 a> Xi C o "d CJ (Q 1 0 •H X u ca 3 >1 p •H O U m p X •H E id CNJ ro o CM +1 +1 o +1 o 10 id -H •0 M id 0 -H >1 H O u o l-l w O u -a o a > rH id n — CO II -P II n5 C ro in 0 p CP H P. O 0 0 rj P G GJ U fc if rH P a Id 0 •rl M-l B & •H m p o ci o H id j g 3 rH o O > •H CP >, H • a O o o M II o +J a P — O rH n CJ
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28 decreased (Fig. 1). This suggests that the fall armyworm regulatory mechanisms that emerge from growing weeds in corn fields are accentuated with an increase in the population density of desired weeds. At both localities, incidence of corn earworm was similar among most treatments (Table 6) At Tall Timbers the lowest damage (nonsignificant) occurred in the weed treatment 2 and at Green Acres less damaged ears were observed where corn was grown with a strip of soybean. Based on these results, it seems that Heliothis responds very differently to habitat diversity than Spodoptera does. It is possible that these species have different habitat colonization strategies. Intercropping corn and soybean might be of aid in reducing corn earworm infestations. In general, the magnitude of the incidence of both pest complexes varied according to the area. Fall armyworm attack was more severe at Tall Timbers, whereas the incidence of corn earworm was more severe at Green Acres. It should be noted, however, that damage evaluations were made in different years, and these trends may change from year to year. Predator Colonization At the early stages of crop development only a few predaceous arthropods colonized the corn fields. As food and habitat resources became more available, numbers of predators and species richness at each locality increased (Table 7) As the corn plants developed, environmental conditions in both localities were ameliorated with time. Weedy corn systems apparently became milder in microclimate and more complex in trophic and habitat structure than the monocultures. These changes affected the number and diversity of colonizing predator species.

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29 G go -p id ft >d a a E S3 •H O in rH -a a *> en ca • •p o C I l-t II ft M C ~ O td O TJ -H H O 14 O -P C 0 a a ft c a -p +j -H -d W B a >0 01 •a 8 ft P Cn O II ft! U V P ft o — 1 — i 1 1 — — 1 — 1— o o o o o O ID in m CM H e >i u — o ft ll £ u c O -H •H t-l P (0 rfl W rH a >i K JQ (^ui 5'o/s^ubx<3 jo jaquinu) A^xsuap paaM

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33 At Tall Timbers, corn grown intermingled with natural weed complexes and selected weed associations showed higher numbers of predator species per unit area (Table 8) and greater numbers of predators (Table 9) than monocultures. These trends were consistent throughout the growing season. Later, when the corn reached harvest maturity, predator numbers and diversity declined. Similar trends were observed by Price (1976) and Mayse and Price (1978) in soybean fields when analyzing croplands from the theory of island biogeography. The above results suggest that predators in monocultures operate in a more xeric environment which influence their colonization and extinction rates dramatically. At Green Acres there were no differences in predator diversity among the various plots. Only in June the diversity of predator species was higher in the weed diversified systems than in the monocultures. Because the habitats surrounding the Tall Timbers plots were structurally more complex and probably provided more local overwintering sites for predators, the mean number of predators were higher there than in Green Acres. Species richness was similar at both sites, however. It seems that by growing corn plants intermingled with weeds or by retaining complex uncultivated borders around the fields, colonization rates of predators are accelerated and extinction rates are reduced (Price 1976) Predator Abundance and Diversity At Tall Timbers predator abundance and diversity (number of species per unit area) on corn plants were significantly higher in weed-diversified systems than in weedfree systems. Arthropod complexity seemed to parallel the trend in plant complexity. Conversely,

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35 CO c cd 0 p -H •rl P cn rd -rl LI o > >i in cn CO CO in CD P •P 'd •rl ^ rH rH rH rH rH CO ro +1 +i H + 1 + 1 -rl •P r> U ro rrl fCJ rrt U fd T3 cn 0 II fd fo fo fo M -p rH > C WD LT) rH 0 CO tn •rl • o 6 Q ro ro n CJ 0) +1 M -P H cn tn rd | >i P H to c fit cn O t H cn rH CU C ft li i •H o li i MH £ •H 5h rrt 0 O CD ro cn UD CO CN >h o (1) o *— •>1 o d) C fl CN CM i— 1 CN CN t — 1 o >h rd n +1 +i +1 +1 +1 -P c u T3 II rG c M c c rd d XI X! £) ns O >-) p — cn ro O u U H o -H 1) -p > cn 4H rd 4-1 T) — -P •H O co 0 T3 M c ft CT> >1 CN cn H H p cu -rl 4-1 •rl — rH o H 1 1 rl O CO \D +1 +1 +1 1 1 fd n C U CO 1 1 (0 o O II rrj ,Q -Q fi O •rl > c CN cn •p •rl p •rl — • • • fd U rd Q ro p 13 CD •P rH CD ft CO 0 >-l tn O ft 13 o c H c rl cu rd rd (0 > Vh CD -H Q) to co cn C r-l ty CD rd cc; CD 00 03 CO >i • cn u • • c — tn C CD LT> l rd lo 4h -r| rl nj n +1 +1 +i 1 o o > CD T) II 1 1 c • rl •9 c C rd rQ X) 0 o >i T3 CN CO p C •r| • II H •rl Eh CO CO u to CD ft •P — rl 4-1 rH CD 0 rH P > rd CD -P •H u Eh rH CO CD ,Q CD CD 4-> d • 6 6 C rd p to rd CD rd n3 c co cn H 0 C (0 M •H CD fd Sh O -P £ rl CD rH a •P 9 ^ R -r| O X 0) CD >1 rH 3 -C 0 rH jQ ft C -P w ft •rl rl to "O -P rH O rd O CD rH rd C u o g 3 p "O rH ft O 6 0 C 0) >d g •rl — j Eh -H teiti we wee i str rH CO 0 4H C CO •P fd rd cn >, rH CD rH rl c c o 0) tn 4J m P nS rd C O o u cn 0) CD 3 i-l c O CO Xt £ Q x> rl c rH -p 0) >1 rd O o 0) rd o Eh U s CO 2 CO He

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36 at Green Acres total predator numbers on corn plants were slightlygreater on the monocultures than on the diverse corn systems. There were no differences in the number of species of predators between weedfree and weed-diversified corn systems at Green Acres. If mean numbers of species and individuals per habitat space (predators on corn plus predators on weeds in each plot) are considered, population densities and species diversity or predators bypass significantly the levels reached by predators in the monocultures at Green Acres. A quantitative value of predators per habitat space cannot be given in the present study because predator populations were estimated by absolute methods in the corn plants and by a relative method on the weeds. The qualitative assumption that predators were more abundant and diverse in the weedy corn fields than in the monocultures seems valid because predators moved back and forth between the weeds and the corn (Fye 1972) It is possible that the high numbers of predators in the Green Acres monocultures were due to the proximity of the experimental plots (8 m apart) which could not prevent migration of predators from diverse plots (Root 1973) This problem seemed to be minimized at the Tall Timbers experiments mainly because of the greater distance between plots. In general, population densities of predators were greater in the Tall Timbers plots than in those at Green Acres. These differences were probably due to the nature of the surrounding habitats (Hodek 1973) Tall Timbers plots were surrounded by annually burned pinelands, complex early successional weedy fields and remanent tree forests which apparently served as refugia providing a continuous influx of predators.

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37 The Green Acres plots were surrounded by sorghum and corn fields, weedy fields dominated by Linaria and Brassica and later by soybean fields. The temporary nature of these habitats and the heavy pesticide treatments that they go through make them poor predator reinforcement resources (Fuchs and Harding 1976) The presence of two extra weed rows in each plot at the Tall Timbers experiment might explain the differences in predator abundance between weedy plots at both localities. Those extra weed rows probably improved the habitat and resource base available for predators at Tall Timbers. Trends of Individual Predator Species Each predator species, family or both responded differently to the various treatments (Table 10) Population responses varied according to species involved, weed diversity, year and locality. For example, Geocoris spp. numbers were higher in weed-diversified systems (particularly the natural weed complex) than in corn monocultures at Tall Timbers in 1978, but Geocoris spp. showed no response to weed diversity at Green Acres in 1979. Corn associated with the natural weed complex had higher numbers of predaceous Coleoptera ( Carabidae and Notoxus) than any other system at Green Acres and more predaceous Hemiptera (Nabidae, Orius and Zelus ) and Coccinellidae than any other treatment at Tall Timbers. Doru sp. had similar densities in all treatments at Tall Timbers, but higher densities in the weedfree monocultures and cornsoybean systems than weed-diverse plots at Green Acres. Spider densities (including Peucetia viridans (Hentz) ) were similar in all treatments at both sites. Predaceous Orthoptera ( Orchelimum sp. and Pecan thus sp.) reached low densities in all treatments.

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39 Predation Pressure Table 11, shows the mean percent of Spodoptera eggs consumed by predators at both localities. There seemed to be no relationship between predator abundance, weed diversity and predation pressure. Rather, the percent of eggs consumed by predators was greater or equal to some weedy treatments in the monocultures of Tall Timbers at all sampling dates. At Green Acres predators consumed more eggs in the monocultures and in the corn-soybean strip system than in any other weed-diverse system. These results would imply that predators are more efficient in consuming eggs in the monocultures. However, it seems that eggs are more easily found by predators in monocultures. The lack of other natural prey and the simplicity of the environment in a monoculture facilitates the search of artificially placed eggs by the few predators present. Conversely, in weed diversified systems physical and trophic structures are more complex somehow diverting the attention of predators from the eggs. In cereal fields of England, Speight and Lawton (1976) found contrasting trends. The number of pupae taken by predators increased with the density and frequency of weeds. Habitat diversity provides adequate microclimates and enough food for predators forcing them to slow down and remain longer in the complex habitat (Root 1973) Predators never consumed less than 50% of the placed eggs in the complex environments. The question remains whether this rate of consumption would actually prevent an outbreak of fall armyworm. At Green Acres, the highest consumption of eggs occurred in the monoculture and in the soybean strip corn systems. Because of the

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m c o 0 w CP tn 0 d M o p o 0 ft w 1-4 0 d p TS c H 0 H u O kH H a fa ft c P id H o o £ c o id -H > 0 P BO +1 U 0 n 3 o u eg CP & a u X} a II cn rH H ro in c 0 H ro WD < L0 c ai a U o co 01 c o -rH +J d tn d Eh E ai 4-1 m >i 0} c H 0 u -d CN cn r> <* 0i cn 0 Vi rH cn Ifl CO o d CN H H rH rH CN a CN +1 +1 +i + 1 +1 + to II a a aS fl d d jQ A & CN in cn rH CO W cn rro O CO t> CO kO cp § rH ft !h 0 i 0 u p +J rH 3 0 o c o s < cn a 0 H 3 +J X 1 -a n3 a 0 0 0 3 S in ro +1 d o + A ro IT) in X X 0 0 0 -a 0 0 rH rH 0 rH 0 N Qi a 1 •H B o 0 rH o >i 0 u •H 0 ,Q 0 ft rH TS t) u 0 H 3 0 0 0 0 0 H s 0 0 IH 0 C p & •rH to > >1 B d rH rH rH rH G d U d d A d o d P CP H 4J 0 to H 9 0 4J 4J G +j 0 ft 0 d d d 0 & 2 2 2 CO o p CP c • H •d 0 u u d +J a 0 yt 0 MH MH •H 'd >1 rH 4J C d 0 •H 4-1 iH ri CP H tO 4J o c 0 d 3 9 rH o o pj 0 > •H CP >1 c d in o o o Sh H 0 P 3 4-) 0 rH +j M 0 o 4J d m 0 CP 0 (J rrj P M >1 0 rQ rH Oh U •H 0 P rH o rH 1 rH 0 to in

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41 proximity of the plots, it seems obvious to assume that predators were moving from the diverse plots to rapidly consume the more "apparent" eggs placed in the monocultures. The high variance associated with the egg predation data probably reflects the element of chance involved in the predators locating and taking the prey offered to them in the small, discrete groups presented by individual paper quadrats. Predators actually observed taking eggs in the field included Doru sp. Orchelimum sp. Peucetia viridans (Hentz) Hippodamia convergens (Guerin-Men) Coleomegilla maculata DeGeer, Orius insidiosus Say, Leptotrachelus dorsal is (Fab.), Callida decora (Fab.), various spiders and ants. Weeds and Number of Ground Predators In Table 12, the data on the adult ground predator fauna (spiders, earwigs, beetles and ants) caught in the set of pitfall traps over the four sampling dates at both sites are summarized. Although pitfall sampling has been criticized as a technique which poorly estimates total numbers and may sample with varying effectiveness in different habitats (Luff 1975) in this study pitfalls were considered a useful method to obtain a relative estimate of the presence of ground predators in the different plots. Spider numbers were greater in the monocultures and in the corn systems with weed mixture B at Green Acres. Earwigs were more actively present in the corn systems with a rye straw mulch and monocultures than in the corn systems with weeds and soybean. Figure 2 shows some abundance trends of earwigs in the mulched and monoculture systems at

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42 a u o u X c co u 0) >H 4H •H id c •H CO CD CO -M & rd 13 H x 6> C H -rH H rH d X +J rd H n ft 3 ir o dH X s UH 0 3 id CD o Cn rd CO !H n CD o > +3 5 id id -d ft H SH ^d o rH fa o Cn X UH o 0 C CO c !h H co CO CO X a •H CO c a O CD IS a -P CM rH CJl 6 to id IX CO a u < CD CD M O CO r3 o H +J 5 -p cn x u u rd CD CO CD K ta u CD H Eh id Eh CO u — CD •a cn •H II ft C W ^ id rH id rd H M rd CN ft| II •rH M M 3 Ti H X CO CN X rH C II rd C CO rl CO VD +1 rd CM +1 X en +i rd H +1 CO <£> 01 +i X! CO CN +1 0 X o +1 o cn CM +1 O CO in m ro ro CN rH CM rH rH CN rH CM + 1 +1 + 1 + 1 +1 + 1 rH II id rd X rd rd X c CN CO O co 1 rH •H •H CO g E u c o 'd nrj u c a 0 0 a a u s S CO rH o 0 CD CD 2 rd H 3 -P rd 2 X CD TJ rH CD ft N g -H O rH U -H X -d rl CD CD CD 4H rH rH rd X +J X rd •z u CD X CD CD O rH CD ft Q O >i U X TJ -d CD CD CD C CM +1 X CM +1 0 CD <£> CN +1 O ro rH +i rd in ro CN co CN \r CD CO CO CD U rH rH rH ro H 1 1 td + 1 +1 +1 +1 +1 +1 1 1 X U X rd X CJ 1 1 CO cn \o O rH CD 01 CO CO rH cn in u rd CD U 3 cn C H o 4-> CD X -d ft rd x CJ o ft rH rH H rH 6 O X IP. CO CO 3 u a rd X rd CO CO CD X £ >1 CD o CO o p Cn a -H -a o 0 o rd X e a CD IM H •H u x rd U •H m -H C cn •H CO -p o G O 0 c CD > -H CP c • rd ^ in a o o • o !H II CD +J X CD rH 4-> CO O CD g X rd CO CD Cn CD a X in X T3 CO H ft •H -P rH g CJ S Q

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43 Green Acres during the four sampling dates. Earwigs were more abundant in the mulched plots. There is evidence which suggests that earwigs are favored by soil covers and other similar cultural manipulations (van den Bosch and Telford 1964) These evidences would imply that earwigs reproduced in the mulched systems and later dispersed to the other plots. During the second and third sampling dates earwig numbers were similar in the mulched plots and monocultures. Possibly, they concentrated more in the monocultures than in the weedy plots because they did not find resistance to movement throughout the plots otherwise imposed by the dense vegetation strips in the weedy plots. At the last sampling date earwig populations were again higher in the mulched plots than in the monoculture. Pitfall trapping apparently extracts great part of the reproductive adult earwig population in the mulched plots. If these plots are not sampled for a certain period of time (12 days) earwig populations recover their original levels in the mulch treatments bypassing in abundance the earwigs of the monocultures (Fig. 2) At Tall Timbers carabids presented similar abundance levels in the monocultures and three weed-diversified corn systems. Ants were significantly more abundant in the corn system with the natural weed complex. There were no differences in the abundance of ants between the monocultures and two weed-diversified plots. Predator Dynamics in Weed Habitats Several studies have shown that predator numbers and diversity increase when the complexity of the plant community is enriched (Root 1973) In cereal fields of England, Speight and Lawton (1976) found

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44 -k-= *o ro >i CO CM S O fin C •H H §• a H O CJ -p Q •H H 0 n n DO -p •rl o 6 rH •H ft rH C OJ o o C 0 Q 9 H 4-1 Pi 0 o O a g • c >d •H H Sh s o n rH Em rH rH rj p M — O c •H c !h H d Cj — •H U C •H (0 rH Sh "0 c> H -P J3 u CJ 0 0] m g 0 V) a jB
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45 that the total number of ground beetles caught per pitfall traps increased as a function of the density of weeds. Other researchers have compared the predator fauna of croplands and fallow fields (Allan et al. 1975) but no attempts have been made to compare the predator fauna associated with different weed communities within crop fields. The present study shows that through different manipulations (direct sowing, differential fertilization, early plowing, etc.) different weed communities can be established in corn fields. These communities sustain predator complexes of various species diversities and relative abundances (Table 13). Furthermore, because the predators on the weeds move back and forth between the corn and weeds, the predator species present on the corn also change. At Tall Timbers, weed communities determined by December plowing, high fertilization (872 kg/ha) and direct sowing of mixture A presented the highest densities of predators. The predator fauna was significantly more diverse in the highly fertilized weed community than in any other weed community. Figure 3 suggests a direct relationship (r = .96) between the number of weed species and the abundance of predators in the weed communities at Tall Timbers. Results from the Green Acres Farm show that relative abundances of predators are similar between certain weed communities and also between weed communities and the soybean strip. Also, there were no significant differences in species richness among the various weed communities and soybean strip. It should be noted that these trends might be masked because of the proximity of the plots. Total mean number of predators and number of species associated with the different weed communities were significantly greater

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46 to Q) •H O a ft — CO rH •K CN rH in CN 4-1 II c r*> O C i-H o rH 1 1 o CTl +1 +i +! i •H rH S-l X i 1 P cu (8 XI Cu H P CO 0 a S •a Ph • (0 a at — p u rH (0 o CN < II +i c CO o C! • • 10 +1 +i + 1 i 1 •H u p cd id rd 1 1 U o -H ro i 1 a> CO • • • ft c ro w cu Q o P rd CU P ft CO co r-~ CD >u Ol •H O O CD ft ro ro a) c CO rCO 01 CO rH rQ 0 • • • • • g •H Mh ro o o o o H P P O II +i +1 +t +1 +l c ft! a XI X! X Co U P CO rH (fl cTJ CN CO CD • • • • • C X in IT) in ftl g u P >i P 2 P cd -H OJ CO CO c CI) — CU cc; Q n CO o rH in co to II • • • • • P CN CN CN CN co +1 + 1 +1 +1 + I >1 •H p Eh H XI X co rd XI X rd cd H C ro 10 cn 01 ro rH cu ft) a Ci f r-> CO O Eh H X X 0 CD >a rH rH CD N i* •H o o rH < cq u o •H p a 1 •H •H rH rH H g g Co Co X! H M U a P P •H 0 0) p P X a cd Co 1 IS E3 a CD X CU cu u rH CD a q 0 >i o X o cu -H g c rH rH -H rd CO & rH P O P CU H P 13 ft CD rH + 1 X id rH + 1 Cd ft •rl M P to c cd CD X >, 0 CO CO in CO CO rd -p o p c cd cy to >s co CD IH O to CD CD 4H O g CN 01 CU -p TJ en C -H rH I 4 cd 01 UH O CD Cn rd M O > w CU +J id C •H rH cd to 4-1 o CO tn d rH 0 o p tn C •H •a u o tj 0 crj P C 0 rH 0 IJH •H CP >1 c • cd in C o 0 • o u II 0 p ft p 0 rH +J to 0 p cd 10 CO fl 0 a X rd p U >i cu X ft •H P & rH O D O to HH c co cd x c rd 0 S

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47 4 saToeds pea/A jo .laquinfi •H

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48 (Table 13) at Tall Timbers than at Green Acres. The structurally more complex habitat surrounding the Tall Timbers plots possibly accounted for these differences. The mean numbers of individual predator species and families collected on the different weed communities at both sites are summarized in Table 14. In general most predators reached similar densities in all communities. At Tall Timbers, Doru sp. was particularly abundant in the weed mixture A community and predaceous Orthoptera ( Orchelimum sp. and Oecanthus sp.) reached the lowest densities in the natural weed complexes. Cocinellidae were somewhat more abundant in the highly fertilized weed complexes. Doru sp. was the only predator species which was slightly more abundant in the soybean strip than on the weed communities of Green Acres. Crop Yields The purpose of this study was to establish principles of corn pest management under different ecological conditions and not to develop methods to enhance crop production. Crop yields were measured, however, in order to have a basis to evaluate the agronomic potential of the proposed systems. The mean corn yields of the different cropping systems at both localities are shown in Table 15. In general, corn yields were lower at Tall Timbers (mean for all treatments = 2.15 tons/ha) than yields at Green Acres (mean for all treatments = 3.28 tons/ha) probably because of a severe drought that affected Tallahassee during the 1978 summer. Both mean yield values do not fall under the average corn yields of farmers in USA during 1970 (2.057 tons/ha) reported by

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a rj 3 U O O > o 1 n IB a o c o V4 c ft id B 3 SS u a a VI c a u a E o o 0 SI a a — H O c -H o o u ra o rl rH rl CN n o O o ID o o -1 +! +t +1 +i a rj Rj irj .Q 13 n ri O •ST tH o o o i — 1 rn CN CM rH CN O o O o o o O -rl +1 +1 +i -H 13 rj a IS r;' r3 13 c*l in m • CO ? m O o o o O O o rH N" ri n i/i rH rH o o o o rH o •H a o +1 4 CN CO CO a n o m rH n o CO o o a o O o o O 1 o +1 +i h 4-i 4-1 +1 a 13 tu rQ A 1 rj *f n O *f CN L/1 rH CI rH o o O o o o o r m r CO CN CO CD o o o O o O O 1 O -H +1 t! -I H +1 +! 1 + 1 rj ns cd (3 JO 13 1 d cn If) ro CO rH o rH o rH O rH O V 1/1 n rH CO o rfH CO rH rH O o o o o O 1 H tI 4 1 +1 +1 4-1 4-1 +1 1 +1 4 t) XI a XI J3 1 CO U o rH rn o O m o CO d O rH o H o H rH CO 0) CO o CO o CO Gl co rr* xi o <3 -p ^-1 r, H a a •d 0 H -3 "J a ft •H c u H o o CI ;i 3 3 a o M 3 CJ <-> u 40 xi -> >: -H je X -H 0 .-.o X Ct rH o rH -1 -H r4 0 e a J J a r* in E Cj H ,i Ci H* jj Ci 51 •c a a 0 o o P O c .o o 0 u cJ U r4 ij O r G S >, o CI o rj 13 0 55 rQ I O I 4-1 I .3 fc 4: III *i n n CI 01 n n Vl o >4 n ci VI 1.1 M CI CJ 0 Cl a a o o CI a o o ,i 1 k, -S3 u '2 fj 10 u 1= o o fe u > -H < •H < -H < •H < -H < — i < tr< c £4 c Ec Eh c Eh C Eh rH o rH o rH a rH o rH ei ^4 rH Ci rj Vl -J Vl E< (J £4 CO o G tl 'J 0. 40 C rj O g Q O 40 01 c H •d u a o u a 40 e o >H a tH A9 A 4J CO ro >. rH 11 CN c -7 c; t> Cl s ri o ci JO rH to IC rl 4J O rj '-rH en c B H Vl 1/1 C rH r o -rl CO rH >i O n ID o 1 rj tl rj a ft rH n t~ M n 41 C4H O n O c a -r| 4J a Bl D> ri CI rj Vl c Bi Vl C o > c < rl Ht •

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50 a o •H a H > a a +j 03 +1 CD S id +J O cd u CD & tn C o o 6 u a h — ro to II a c u u < a a o M o CO 0\ a o •H +J d w i •H id w E cd +J tn >i tn C M o u 00 LO CO u m eg CO o ro vo id ro rH o o rH o a II +i + +1 +1 + +1 tn •X o (D (3 0 XJ 0 CO O o £) O o o O +1 +1 +i +1 a A rQ A O CN •3CO ro CN co ro ro cd 4 o o a u -p +J X X rH H 3 g 0 0 TS c CD CD c CD s 3 CO • o +1 cd ro O O +1 O r> CO x X X CD CD CD 0 0 rH rH 0) rH CD N a o ft •H E o O rH 0 >1 rC 0 0 ti u X) Pa lc n3 >a IH Tj H i CD a O CD CD H CD a MH CD C S >i ng tn aw rH rH rH rH u •H C Sh cd rd cd CD 2= cd !h +J O CD to •H CD rH -p +J a, >, CD Cd rd cd O & S3 2 23 CO 5 CP e 4 M o 0 0 rd P (3 CD M
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51 Pimentel et al. (1973). Considering the low energy inputs (no pesticides or irrigation) invested in the management of the plots, most yields seem economically and energetically acceptable. Variances associated with yield means of some treatments were high at both localities (especially treatments 1, 2 and 5 at Tall Timbers and 3, 4 and 5 at Green Acres) In many cases this was due apparently to gradients in moisture or nutrient levels in the soil. In a few instances low yields in a replicate resulted from patchiness in the attack of soil nematodes. At Tall Timbers, corn monocultures and corn-weed mixture A systems presented similar yields. All other treatments showed lower yields. The lowest yields at Tall Timbers were observed in the corn systems associated with the natural weed complex and with the natural vegetation resulting from an earlier December plowing. The latter system was more mature than any of the other weed treatments and had a background of perennials (e.g., Rubus sp. Phytolaca sp. ) aggressive semi-annuals ( Crotalaria sp. Eupatorium sp. Ipomoea sp. ) and grasses which tended to invade the corn rows on a regular basis. At Green Acres, corn monocultures and cornsoybean strip systems showed the highest yields. All other corn-weed associations had lower yields. The lowest yields were observed in the corn systems with a rye straw mulch. There are certain evidences which suggest that rye residues might exert allelopathic effects on certain plants, including corn (Rice 1974) Although the selective presence of weeds within corn plots reduces the incidence of fall armyworm, this reduction is unacceptable from the yield point of view. This suggests that any advantage

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52 offered by weeds to establish a pest equilibrium in corn fields is to some extent offset by their interference (competition/allelopathy) with crops, even if weeds are grown as alternate rows. A direct and intense interference between weeds and crops grown in separate rows is unlikely and has not been reported in the literature. Also, there remains the question that no matter how much weeds reduce the incidence of fall armyworm in corn, because corn can tolerate sizeable populations of Spodoptera (Beingolea 1957) without yields being affected. Thus, differences in yields between weedfree corn plots and corn systems diversified with strips of weeds are explained by other factors not considered in the present study.

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CONCLUSIONS Present research suggests that it is possible to design corn agroecosystems to reduce the incidence of fall armyworm, Spodoptera frugiperda By proper cultural manipulations (e.g., direct sowing, differential soil fertilization, early plowing, etc.) certain weed communities can be established within corn fields. The presence of these weed associations adds trophic and structural diversity to the corn systems which result in two main effects: 1. Act as plant defense guilds (Atsatt and O'Dowd 1976) with antiherbivore properties, reducing the incidence of Spodoptera frugiperda The biotic, structural, chemical and microclimatic complexity of cornweed mixed systems greatly ameliorates the noctuid pressure early in the growing season. At Green Acres, Spodoptera damage was reduced most when corn was grown in association with the natural weed complex. At Tall Timbers, fall armyworm damage decreased as the density of the weed community increased. Corn plants associated with a highly fertilized natural weed complex were least damaged by fall armyworm. It is possible that corn and natural weed complexes share coevolutionary links that enhance the associational resistance of the crop community (Root 1973, Murdoch 1975). It should be noted, however, that reduction of the incidence of fall armyworm might prove unacceptable from the yield point of view if weed interference is not effectively minimized. 2. Condition a continuously present set of natural enemies in the fields. At Tall Timbers, foliage predator arthropod communities were 53

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54 more diverse and abundant in weedy plots than in weedfree plots. At Green Acres, predators were more abundant in the monocultures than in the weed-diversified systems, and predator diversity was equal in all treatment plots. An increase in the quantity of weeds in a given habitat space generally was correlated, both with an increase in the abundance and diversity of predaceous arthropods but not with an increase in the disappearance of artificial prey. Heliothis zea (Boddie) was not affected by weed diversity. Only the diversification of corn systems with a strip of soybean decreased the number of ears damaged by corn earworm, suggesting that successful management of the corn earworm depends on introducing a select kind of diversity (e.g., corn-soybean polycultures) rather than general habitat diversity. Results from crop-weed-insect predator interaction studies under experimental field conditions are dictated by a number of factors such as year, area, weed abundance and diversity, crop variety, etc., but more so by the distance between experimental plots. A complication in designing experiments with plots close to each other is that the proximity of treatments permits insect predators to move easily from one habitat to other. Such movements constitute conservative errors tending to make predator fauna in the various treatments similar. For this reason, the diversity of the predator fauna was similar in all Green Acres plots and the densities of predators in the monocultures was higher or equal than those in some weed-diversified corn systems. A better approach to establish differences in predator levels between simple and diverse crop habitats is the design of well distanced experimental plots to prevent intercrop movements (like the Tall Timbers

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55 experiments) or otherwise increase the size of the plots considerably. At Tall Timbers predators were clearly more abundant and diverse in the weedy corn plots than in the weed-free plots. The data also suggest that the nature of the adjacent surrounding habitats can dramatically influence predator complexes within corn fields. Predator abundance and diversity were greater in corn plots surrounded by mature, complex, natural vegetation (e.g., annually burned pinelands at Tall Timbers Research Station) than when surrounded by simple, annual crop fields. Encouraging plant diversity within and outside corn fields increases the colonization rates of predators early in the season and decreases extinction rates by providing shelter and cover throughout the crop growth (Price 1976) It is tempting to extrapolate these results to a between-field comparison and suggest that corn fields adjacent to simple habitats will have fewer predators than fields with complex borders, with the result that outbreaks of fall armyworm are more likely in corn fields located in simple habitats. The long series of assumptions cast the validity of this argument because between field comparisons (like in this study) involve other factors such as location, size of the fields, year, corn varieties, etc., which may markedly influence pest and natural enemies dynamics. Reduction of fall armyworm damage in corn grown with a rye straw mulch and in corn grown in concert with a soybean strip suggests the potential of both minimum tillage systems and intercropping systems of corn and soybean as possible strategies to complement management of this pest. Elements of natural pest control undoubtedly exist in many mixed cropping systems (e.g., corn-weed associations) and there are certain

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56 ways in which these may be transferred into agronomical ly convenient and economically acceptable monocultures. Based on these data corn systems surrounded by a complex habitat, mulched with some cereal straw and with a row of natural weeds between each 10 rows of corn might effectively prevent outbreaks of Spodoptera Many of these systems will probably remain untried in the U.S. because of the potential for reduced production or lower profits. Given economic and energetic constraints and also due to the ecological impact of modern agricultural practices (e.g., pesticide pollution), agroecological strategies will have to be carefully evaluated on an environmental cost/benefit basis as well as on an energetic basis. The challenge for pest managers will be the design of a gentle technology which will be self operating with minimum external inputs. Capitalizing on knowledge of beneficial plant associations will provide a sound ecological basis to develop such technology.

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APPENDIX

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Table 16. Common predaceous arthropods of north Florida corn fields*. Collection sites Tall Timbers Green Acres Research Station, 1978 Farm, 1979 COLEOPTERA Anthicidae Notoxus mono don Fab. X X Anthicas ephippium Laf X Cantharidae Chauliognathus sp. X Podabrus sp. x Carabidae Callida decora (Fab.) x x Callida punctulata Le Conte X Casnomia pennsylvanica (L.) X Harpalus pennsylvanicus DeGeer X Lebia analis Dejean X Lebia viridis Say X X Leptotrachelus dorsalis (Fab.) X X Nemotarsus elegans LeConte X X Pasimachus sublaevis Beavois X Searites subterraneus Fab. X Selenophorus palliatus Fab. X Coccinellidae Coleomegilla maculata DeGeer X X Cycloneda sanguinea (L.) X X Exochomus marginipennis Lec. X Hippodamia convergens (Guerin-Men. ) X X Hyperaspis sp. X X 011a abdominalis (Say) X X Scymnus sp. X X Malachiidae Collops quadrimaculatus (Fab.) X Mordellidae Mordellistena sp. X X Mordella sp. X X Staphylinidae Philonthus sp. X X Pinophilus sp. X X DIPTERA Dol i chopodidae Condylostylus caudatus (Wied. ) X X C. sipho (Say) X X Mesorhaga albiciliata (Aldrich) X 58

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59 Table IS — continued. Syrphidae Toxomerus floralis (Fab.) X T. marginatus (Say) X T. politus (Say) X DERMAPTERA Forficulidae Doru taeniatum (Dohrn) X X Labiduridae Labidura riparia (Pallas) X HEMIPTERA Anthocoridae Orius insidiosus Say X X Lygaeidae Geocoris punctipes (Say) X X G. uliginosus (Say) X X Nabidae Nabis roscipennis Reuter X X Tropiconabis capsiformis Germar X X Pentatomidae Euthyrhynchus floridanus (L.) X X Podisus maculiventris (Say) X Reduviidae Atrachelus cinereus (F.) X Repipta taurus (F.) X Sinea sp. XX Sinea sanguisuga Stal. X X Zelus cervicalis (Stal.) X X HYMENOPTERA Formicidae Pheidole morrisi Forel X Pheidole sp. X X Conomyrma flavopecta (Smith) X X Solenopsis invicta Buren X X Sphecidae Sphex sp. X Tachytes sp. X Vespidae Polistes fuscatus (Fab.) X P. annularis (Linn. ) X X NEUROPTERA Chrysopidae Chrysopa sp. X X

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Table 16 — continued. Hemerobiidae Micromus posticus (Walker) X ORTHOPTERA Gryllidae Pecan thus sp. X X Tettigoniidae Orchelimum sp. X X ARANEAE Araneidae Araneus juniper i (Emerton) X Eriophora ravilla (Koch) Tetragnatha laboriosa Hentz X X Anyphaenidae Aysha sp. X X Clubionidae Chiracanthium inclusum (Hentz) X X Clubiona sp. X X Lycosidae Pardosa georgiae Chamberlin and Ivie X X Pardosa milvina (Hentz) X X Lycosa sp. XX Oxyopidae Oxyopes salticus Hentz X X Peucetia viridans (Hentz) X X Philodromidae Philodromus placidus Banks X Salticidae Hentzia palmarum (Hentz) X X Phiddippus regius (Koch) X X Methaphiddippus galathea (Walck.) X X Theridiidae Latrodectus mactans (Fab.) X X Theridion sp. X X Thomisidae Misumenops sp. X X Xysticus fraternus Banks X Xysticus texanus Banks X All specimens listed in this table were identified by Drs. R.E. Woodruff (Coleoptera) F. Mead (Homoptera, Hemiptera) H. Weems (Diptera) and G.B. Edwards (Araneae)

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LITERATURE CITED Adams, J.B. and M.E. Drew. 1965. Grain aphids in New Brunswick III. Aphid populations in herbicide-treated oatfields. Can. J. Zool. 43:789-794. Allan, D.A. J.H. Alexander and R. Greenberg. 1975. Foliage arthropod communities of crop and fallow fields. Oecologia 22:49-56. Allen, W.W. and R.F. Smith. 1958. Some factors influencing the efficiency of Apanteles medicaginis Muesebeck (Hymenoptera: Braconidae) as a parasite of the alfalfa caterpillar, Colias philodice eury theme Boisduvol. Hilgardia 28:1-43. Altieri, M.A., C.A. Francis, A. Schoonhoven and J. Doll. 1978. Insect prevalence in bean ( Phaseolus vulgaris ) and maize (Zea mays) polycultural systems. Field Crops Research 1:33-49. Altieri, M.A. A. Schoonhoven and J.D. Doll. 1977. The ecological role of weeds in insect pest management systems : A review illustrated with bean ( Phaseolus vulgaris L.) cropping systems. PANS 23:185-206. Altieri, M.A. and W.H. Whitcomb. 1979. The potential use of weeds in the manipulation of beneficial insects. Hort Science 14(1):12-18. Atsatt, P.R. and D.J. O'Dowd. 1976. Plant defense guilds. Science 193:24-29. Beingolea, 0. 1957. El sembrio del ma£z y la fauna benefica del algodonero. Estac. Exp. Agr. La Molina. Lima. Informe No. 104, 19pp. Bombosch, S. 1966. Occurrence of enemies on different weeds with aphids. p. 177-179. In: X. Hodek (Ed.), Ecology of Aphidophagous Insects. Academia Pub. House, Prague. Bookchin, M. 1976. Radical agriculture, p. 3-13. In: R. Merrill (Ed.), Radical Agriculture. Harper and Row Pub., New York. Browning, J. A. 1975. Relevance of knowledge about natural ecosystems to development of pest management programs for agroecosystems Proc. Amer. Phytopath. Soc. 1:191-194. 61

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62 Burleigh, J.G. J.H. Young and R.D. Morrison. 1973. Strip's cropping effects on beneficial insects and spiders associated with cotton in Oklahoma. Environ. Entomol. 2 (2) :281-285. Carroll, C.R. 1978, Beetles, parasitoids and tropical morning-glories: A study in host discrimination. Ecological Entomol. 3:79-85. Dambach, C.A. 1948. Ecology of crop field borders. Ohio State Univ. Press, Columbus. 203pp. DeLoach, C.J. 1970. The effect of habitat diversity on predation. Proc. Tall Timbers Conf. on Ecol. Anim. Control by Habitat Management. Tallahassee. 2:223-241. Dempster, J. P. 1969. Some effects of weed control on the numbers of the small cabbage white ( Pieris rapae L. ) on brussel sprouts. J. Appl. Ecol. 6(2) :339-405. Dempster, J. P. and T.H. Coaker. 1974. Diversification of crop ecosystems as a means of controlling pests, p. 106-114. In: D.P. Jones and M.E. Solomon (Eds.), Biology in Pest and Disease Control. Wiley, New York. Doutt, R.L. and J. Nakata. 1973. The Rubus leafhopper and its egg parasitoid: An endemic biotic system useful in grape pest management. Environ. Entomol. 2:381-386. Feeny, P. 1976. Plant apparency and chemical defense. Recent Adv. Phytochem. 10:1-49. Feeny, P. 1977. Defensive ecology of the Cruciferae. Ann. Missouri. Bot. Gard. 64:221-234. Flaherty, D. 1969. Ecosystem trophic complexity and Willamette mite Eotetranychus willametei (Acarina: Tetranychidae) densities. Ecol. 50:911-916. Fuchs, T.W. and J. A. Harding. 1976. Seasonal abundance of arthropod predators in various habitats in the lower Rio Grande, Valley of Texas. Environ. Entomol. 5 (2) :288-290. Fye, R.E. 1972. The interchange of insect parasites and predators between crops. PANS 18 (2) :143-146. Guevara, J.C. 1962. Efecto de las practicas de siembra y de cultivos sobre plagas en maiz y frijol. Fitotecnia Latinoamericana 1(1): 15-26. Hagen, K.S., S. Bombosch and J. A. McMurthy. 1976. The biology and impact of predators. In: C.B. Huffaker and P.S. Messenger (Eds.), Theory and Practice of Biological Control. Academic Press, New York.

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63 Hemenway, R. and W.H. Whitcomb. 1967. Ground beetles of the genus Lebia Latreille in Arkansas (Coleoptera: Carabidae) : Ecology and geographical distribution. Proc. Ark. Acad. Sci. 21:15-20. Hodek, I. 1973. Biology of Cocinellidae. Acad. Pub., Prague. 260pp. King, J.L. and J.K. Holloway. 1930. Tiphia popilliavora Rohwer, a parasite of the Japanese beetle. U.S. Dept. Agr. Cir. 145. Laster, M.L. and R.E. Furr. 1972. Heliothis populations in cottonsesame interplantings J. Econ. Entomol. 65 (5) :1524-1525. Leius, K. 1967. Influence of wild flowers on parasitism of tent caterpillar and codling moth. Can. Entomol. 99:444-446Leston, D. 1973. The ant mosaic tropical tree crops and the limiting of pests and diseases. PANS 19 (3) : 311-341. Levins, R. and M. Wilson. 1979. Ecological theory and pest management. Ann. Rev. of Entomol. (in press) Litsinger, J. A. and K. Moody. 1976. Integrated pest management in multiple corpping systems, p. 293-316. In: P. A. Sanchez (Ed.), Multiple Cropping. Amer. Soc. Agron. Special Pub. 27, Madison. Loucks, D.L. 1977. Emergence of research on agroecosystems. Ann. Rev. of Ecol. and System. 8:173-192. Luff, M.L. 1975. Some features influencing the efficiency of pitfall traps. Oecologia 19:345-347. Marcovitch, S. 1935. Experimental evidence on the value of strip cropping as a method for the natural control of injurious insects, with special reference to plant lice. J. Econ. Entomol. 28:62-70. Mayse, M.A. and P.W. Price. 1978. Seasonal development of soybean arthropod communities in east central Illinois. Agroecosystems 4:387-405. Metcalf, R.L. and W. Luckman. 1975. Introduction to Insect Pest Management. Wiley Interscience New York. 587pp. Murdoch, W.W. 1975. Diversity, stability, complexity and pest control. J. Appl. Ecol. 12:745-807. Myers, J.G. 1931. A preliminary report on an investigation into the biological control of West Indian insect pests. His Majesty's Stationery Office, London. 178pp. National Academy of Sciences. 1969. Principles of plant and animal control. Vol. 3. Insect pest management and control, p. 100-164.

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64 O'Connor, B.A. 1950. Premature nutfall of coconuts in the British Solomon Islands Protectorate. Agric. J. 21:1-22. Oka, I.N. and D. Pimentel. 1974. Corn susceptibility to corn leaf aphids and common corn smut after herbicide treatment. Environ. Entomol. 3(6) :911-915. Peppers, B.B. and B.F. Driggers. 1934. Non-economic insects as intermediate hosts of parasites of the oriental fruit moth. Ann. Entomol. Soc. Amer. 27:593-598. Perrin, R.M. 1975. The role of the perennial stinging nettle Urtica dioica as a reservoir of beneficial natural enemies. Ann. Appl. Biol. 81:289-297. Perrin, R.M. 1977. Pest management in multiple cropping systems. Agro-ecosystems. 3:93-118. Perrin, R.M. and M.L. Phillips. 1978. Some effects of mixed cropping, on the population dynamics of insect pests. Entomol. Exp. and Appl. 24:385-393. Peterson, A. 1926. Oriental fruit damage in cultivated and uncultivated orchards. Proc. Ann. Meeting, New Jersey State Hort. Soc. 3:83-86. Piemeisel, R.L. 1951. Causes affecting change and rate of change in vegetation of annuals in Idaho. Ecol. 32(l):53-86. Pimentel, D. 1961. Species diversity and insect population outbreaks. Ann. Entomol. Soc. Amer. 54:76-86. Pimentel, D. 1970. Training in pest management and the systems approach to control, p. 204-226. In: R.L. Rabb and F.E. Guthrie (Eds.), Concepts of Pest Management. North Carolina State Univ., Raleigh. Pimentel, D. and N. Goodman. 1978. Ecological basis for the management of insects populations. Oikos 30:422-437. Pimentel, D, L.E. Hurd, A.C. Belloti, M.J. Foster, I.N. Oka, O.D. Sholes and R.J. Whitman. 1973. Food production and energy crisis. Science 182:443-449. Pitre, H.N. and F.J. Boyd. 1970. A study of the role of weeds in corn fields in the epidemiology of corn stunt disease. J. Econ. Entomol. 63(1) :195-197. Plakidas, J.D. 1978. Epiblema scudderiana (Clemens) (Lepidoptera: Olethreutidae) a winter host reservoir for parasitic insects in southwestern Pennsylvania. New York Entomol. Soc. 86 (3) :220-223. Pollard, E. 19 71. Hedges VI: Habitat diversity and crop pests: A study of Brevicoryne brassicae and its syrphid predators. J. Appl. Ecol. 8:751-780. Potts, G.R. and G.P. Vickerman. 1974. Studies on the cereal ecosystem. Adv. Ecol. Res. 8:107-147.

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65 Price, P.W. 1976. Colonization of crops by arthropods: Nonequilibrium communities in soybean fields. Environ. Entomol. 4: 8-10. Price, P.W. and G.P. Waldbauer. 1975. Ecological aspects of pest management, p. 37-73. In: R.L. Metcalf and W.H. Luckman (Eds.), Introduction to Insect Pest Management. Wiley Inter science, New York. Rabb, R.L. 1978. A sharp focus on insect populations and pest management from a wide-area view. Bull. Entomol. Soc. Amer. 24(1): 55-61. Rabb, R.L. and F.E. Guthrie. 1970. Concepts of pest management. North Carolina State University, Raleigh. 242pp. Rabb, R.L., R.E. Stinner and R. van den Bosch. 1976. Conservation and augmentation of natural enemies, p. 233-254. Iri: C.B. Huf faker and P.S. Messenger (Eds.), Theory and Practice of Biological Control. Academic Press, New York. Raros, R.S. 1973. Prospects and problems of integrated pest control in multiple cropping. IRRI Saturday Seminar, Los Banos, Philippines. 20pp. Rice, E.L. 1974. Allelopathy. Academic Press, New York. 345pp. Risch, S. 1979. A comparison by sweep sampling, of the insect fauna from corn and sweet potato monocultures and polycultures in Costa Rica (Unpublished data) Root, R.B. 1973. Organization of a plant-arthropod association in simple and diverse habitats: The fauna of collards ( Brassica oleracea ) Ecol. Monogr. 43:95-124. Root, R.B. 1975. Some consequences of ecosystem texture, p. 83-97. In : S.A. Levin (Ed.), Ecosystem Analysis and Prediction. Soc. Ind. Appl. Math., Philadelphia. ~;; Shaber, B.D. E.U. Balsbauch and B.H. Kantack. 1975. Biology of the flea beetle, Altica carduorum (Col: Chrysomelidae) on Canada thistle ( Cirsium arvense ) in South Dakota. Entomophaga 20(4): 325-335. Smith, J.G. 1976a. Influence of crop background on natural enemies of aphids on brussel sprouts. Ann. Appl. Biol. 83:15-29. Smith, J.G. 1976b. Influence of crop background on aphids and other phytophagous insects on brussel sprouts. Ann. Appl. Biol. 83: 1-13. Smith, R.F. and H.T. Reynolds. 1972. Effects of manipulation of cotton agroecosys terns on insect pest populations, p. 373-390. In: M.T. Farvar and J. P. Milton (Eds.), The Careless Technology. Natural History Press, New York.

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67 van Emden, H.F. 1965. The role of uncultivated land in the biology of crop pests and beneficial insects. Sci. Hort. 17:121-136. van Emden, H.F. and G.F. Williams. 1974. Insect stability and diversity in agro-ecosystems. Ann. Rev. Entomol. 19:455-475. Whitcomb, W.H. and K. Bell. 1964. Predaceous insects, spiders and mites of Arkansas cotton fields. Agr. Expt. Sta. Univ. Arkansas Bull. 690. Wilken, G.C. 1977. Integrating forest and small-scale farm systems in middle America. Agro-ecosystems 3:291-302. Wille, J.E. 1952. Entomologia Agricola del Peru. 2nd Ed. Est. Esp. Agr. La Molina, Lima. 543pp. Wilson, J.W. 1963. Estimation of foliage denseness and foliage angle by incline point quadrats. Austr. J. Bot. 11:95-105. Wolcott, G.N. 1942. The requirements of parasites for more than host. Science 96:317-323.

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BIOGRAPHICAL SKETCH Miguel A. Altieri was born in Santiago, Chile, on 3 September, 1950. He received his high school certificate in 1967 from the Liceo Experimental Manuel de Salas in Santiago. He also did his Junior Grade at the Notre Dame High School in Los Angeles, California, in 1966. Later, he received from the University of Chile his Bachelor of Science in Agronomy in May of 1974. In October of 1974 he entered the Universidad Nacional of Colombia in Bogota. After conducting his research at the Centro Internacional de Agricultura Tropical in Cali, he received the degree of Master of Science in 1976. From February to September 1977 he served as a research associate at the Tall Timbers Research Station in Tallahassee. Since September 1977 he has been a graduate student of the Department of Entomology, University of Florida. During his residence in Gainesville, Miguel Altieri has traveled to Colombia, Costa Rica and Mexico as a consultant in insect and weed ecology. He is a member of the Sociedad Colombiana de Control de Malezas (COMALFI) Entomological Society of America, Florida Entomological Society and International Organization of Biological Control (IOBC) Miguel Altieri is married to Grisell and has two children, Naraya (6) and Joshua (3). 68

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Willard H. Whit comb, Chairman Professor of Entomology S Nematology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Stratton Kerr Professor of Entomology & Nematology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Carl S. Barfield Assistant Professor of Entomology & Nematology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Donald C. Herzog Professor of Entomology & Nematology

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I certify that I have read this study and that in my opinion if conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Ray William Associate Professor of Vegetable Crops This dissertation was submitted to the Graduate Faculty of the College of Agriculture and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December, 1979 Dean, College of Agriculture Dean, Graduate School