Interactions of resistant corn cultivars, Spodoptera frugiperda (J.E. Smith) and Archytas marmoratus (Townsend)

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Interactions of resistant corn cultivars, Spodoptera frugiperda (J.E. Smith) and Archytas marmoratus (Townsend)
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Thesis (Ph. D.)--University of Florida, 1990.
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Includes bibliographical references (leaves 126-135).
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by Maude F. Christian-Meier.
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Vita.

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INTERACTIONS OF RESISTANT CORN CULTIVARS,
SPODOPTERA FRUGIPERDA (J.E. SMITH) AND
ARCHYTAS MARMORATUS (TOWNSEND)


















By

MADE F. CHRISTIAN-MEIER


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


UNIVERSITY OF FLORIDA


UNIVERSITY OF FLO."10A UarAkS


1990













ACKNOWLEDGEMENTS


I gratefully acknowledge all members of my supervisory

committee for their guidance, advice, and encouragement:

Drs. Carl Barfield, Gary R. Buckingham, B.R. Wiseman, Frank

Slansky, Dale Habeck and Rachel Shireman. Their support has

enabled me to carry out this research. They were always

prepared to take time out of their ever busy schedules to

assist in various aspects of my academic endeavors.

Dr. Barfield gave me invaluable academic advisement and

was helpful in suggesting sources of financial aid. I

benefited from working with Dr. Buckingham at the biological

control laboratory; it was a professional experience that

will serve me well in my career. Dr. Wiseman was the

inspiration behind my pursuit of a host plant resistance

project. Dr. Slansky was responsible for my interest in

insect nutritional ecology. Dr. Shireman and Dr. Habeck

represent much of what I hope to be. I have been fortunate

in having a committee that has shown genuine concern for my

wellbeing, as well as being a source of enlightenment.

I wish to acknowledge the American Association of

University Women for providing a grant that made this

research a reality. I would like to thank Dr. Strayer for







his continual support. Dr. Yu was kind enough to let me use

his laboratory for all the detoxication enzyme work and

reviewed the results.

My sincere gratitude goes to Dr. Sandra Davis, a good

friend and more, and Christine Bennett, Paul Ruppert, Dr.

Lim Nong, May Buckingham and Glinda Burnett for being all

they are. I would also like to thank my fellow graduate

student friends for all we have learned from each other.

Special thanks go to my mother Grace Christian, my

husband Henry Meier and children Fernanda, Jack, Anthea and

Rachel for their personal sacrifice and love. Finally, I

wish to thank all the good friends I made in Gainesville,

Florida, for enriching my life in Gainesville.


iii














TABLE OF CONTENTS



Page



ACKNOWLEDGEMENTS................... ....................ii

LIST OF TABLES .......................................vi

LIST OF FIGURES.... .................................viii

ABSTRACT.................................................xi

CHAPTER

I. INTRODUCTION..... ............................1

Biology and Life Cycle of the Fall Armyworm...3
Host Plant Resistance.........................6
Chemical Control.............................11
Tri-trophic Interactions.....................13

II. CONSUMPTION, DEVELOPMENT AND FECUNDITY OF
FAW ON DIETS CONTAINING SILKS OF THE
RESISTANT CORN CULTIVAR IZAPALOTE
CHICO' ........................................ 16

Introduction................................. 16
Materials and Methods.........................18
Results and Discussion.......................23
General Discussion...........................58

III. FIELD PERFORMANCE OF ARCHYTAS MARMORATUS
AGAINST FALL ARMYWORM SPODOPTERA FRUGIPERDA
ON STOWELL'S EVERGREEN AND MpSWCB-4.......... 61

Introduction................................. 61
Materials and Methods......................... 68
Results and Discussion.......................71
















IV. CONSUMPTION DEVELOPMENT AND FECUNDITY OF
FALL ARMYWORM ON STOWELL'S EVERGREEN AND
MpSWCB-4 AS AFFECTED BY ARCHYTAS
MARMORATUS................................... 85

Introduction................................85
Materials and Methods........................86
Results and Discussion........................ 90

V. EFFECT OF RESISTANT AND SUSCEPTIBLE CORN
CULTIVARS ON MICROSOMAL MONOOXYGENASES AND
INSECTICIDE SUSCEPTIBILITY OF THE FALL
ARMYWORM......................................... 103

Description of Microsomal Monooxygenases.... 103
Materials and Methods.......................109
Results and Discussion......................114

VI. SUMMARY AND CONCLUSIONS.....................121

REFERENCES CITED.....................................126

APPENDIX................................................136

BIOGRAPHICAL SKETCH...................................143













LIST OF TABLES


Table Page



1. Developmental parameters of FAW 5th and
6th larval instars feeding on diets con-
taining silks of a resistant corn cultivar
Zapalote Chico and a susceptible cultivar
Stowell's Evergreen. 24

2. Fifth instar till pupation feeding effi-
ciencies, and relative consumption and
growths rates for fall armyworm larvae
reared on diets containing silks of a
resistant corn cultivar Zapalote Chico (R)
and a susceptible corn cultivar Stowell's
Evergreen (S). 25

3. Developmental parameters (Consumption,
growth and biomass)of 6th instar fall
armyworm larvae feeding on diets contain-
ing silks of a resistant corn cultivar
Zapalote Chico and a susceptible corn cul-
tivar Stowell's Evergreen. (24hrs). 56

4. Rates of consumption, rates of growth,
and efficiency of conversion of ingested
food for 6th instar FAW larvae reared
on diets containing silks of resistant
corn cultivar Zapalote Chico and a
susceptible corn cultivar Stowell's
Evergreen. (24 hrs). 57

5. Parasites reared from fall armyworm larvae
collected in Tifton, Georgia, June 1988 and
June 1989. 72













6. Developmental parameters of fall armyworm
6th instar larvae reared on leaves of a
susceptible corn cultivar (Stowell's Ever-
green) and a resistant cultivar (MpSWCB-4). 91

7. Developmental parameters of fall armyworm
6th instar larvae reared on leaves of a
susceptible corn cultivar (Stowell's Ever-
green) and a resistant cultivar MpSWCB-4. 92

8. Effect of parasitism by A. marmoratus on
FAW larvae. Consumption, growth and
biomass parameters. 99

9. Effect of parasitism by A. marmoratus on
last instar fall armyworm feeding
efficiencies, consumption and growth rates. 100

10. Aldrin epoxidase activities of fall
armyworm Larvae fed various diets of
resistant and susceptible corn cultivar
leaves. 115

11. Microsomal oxidase activities of fall
armyworm larvae fed a meridic diet
containing various concentrations of
silks of a resistant corn cultivar
(Zapalote Chico) and a susceptible
corn cultivar (Stowell's Evergreen). 116

12. Toxicity of insecticides applied topically
to the Fall armyworm reared on a suscep-
tible and resistant corn cultivars. 118

A-1. Parameters for determination of performance
values. 136

A-2. Larval duration and mortality of fall
armyworm reared on diets containing silks
of a resistant corn cultivar Zapalote Chico
and a susceptible corn cultivar Stowell's
Evergreen. 137

A-3. Mean number of eggs laid by fall armyworm
females reared on a pinto bean check diet
(CK) and diets containing silks of


vii













Stowell's Evergreen corn (SEG) a susceptible
corn cultivar, and Zapalote Chico corn a
resistant corn cultivar. 138

A-4. Mean percentage parasitization of fall
armyworm larvae on whorl stage resistant
corn (MpSWCB-4) and susceptible corn
(Stowell's Evergreen) by Archytas marmoratus
in Tifton Georgia. 139

A-5. Mean percentage parasitization of fall
armyworm larvae on whorl stage resistant
corn (MpSWCB-4) and susceptible corn
(Stowell's Evergreen) by Ophion flavidus
in Tifton Georgia. 140

A-6. Mean percentage parasitization of fall
armyworm larvae on whorl stage resistant
corn (MpSWCB-4) and susceptible corn
(Stowell's Evergreen) by other parasites
and total percentage parasitization in
Tifton Georgia. 114

A-7. Mean pupal weights and eggs laid by fall
armyworm reared on whorl stage resistant
(MpSWCB-4) and susceptible (Stowell's
Evergreeen) corn. 142


viii













LIST OF FIGURES


Figure _Pae

1. Larval duration for fall armyworm reared on
a check (CK) pinto bean diet, and diets
containing silks of Stowell's Evergreen
corn (SEG), a susceptible cultivar, and
Zapalote Chico corn (ZC), a resistant cul-
tivar. ZC5, ZC10, and ZC20 = 5 g, 10 g,
and 20 g of ZC silk in 300 ml of check
diet respectively. 29

2. Developmental time for fall armyworm larval
instars 5 to pupation, on diets containing
silks of Stowell's Evergreen corn (SEG), a
susceptible cultivar, and Zapalote Chico
corn (ZC), a resistant cultivar. ZC5, ZC10
and ZC20 = 5 g, 10 g, and 20 g of ZC silk
in 300 ml of check diet respectively. 31

3. Fresh weights (mg) of newly molted fall
armyworm 5th instar larvae reared on a
check (CK) pinto bean diet, and diets con-
taining silks of Stowell's Evergreen corn
(SEG),a susceptible cultivar, and Zapalote
Chico corn (ZC), a resistant cultivar.
ZC5, ZC10, ZC20 = 5 g, 10 g, 20 g of ZC
silk in 300 ml of check diet respectively. 33

4. Percent dry weight of 5th instar fall
armyworm larvae reared on a check (CK)
pinto bean diet, and diets containing silks
of Stowell's Evergreen corn (SEG), suscep-
tible cultivar, and Zapalote Chico corn
(ZC), a resistant cultivar. ZC5, ZC10,
and ZC20 = 5 g, 10 g, and 20 g of ZC silk
in 300 ml of check diet respectively. 35


5. Pupal fresh weights of fall armyworm
reared on a pinto bean diet (CK)
and diets containing silks of Stowell's
Evergreen corn (SEG), a susceptible
cultivar, and Zapalote Chico corn (ZC),
a resistant cultivar. ZC5, ZC10, and













ZC20 = 5 g, 10 g, and 20 g of ZC silk
in 300 ml of check diet respectively. 39

6. Pupal dry weights of fall armyworm reared
on a meridic pinto bean diet (ck), and
diets containing silks of Stowell's
Evergreen corn (SEG), a susceptible corn,
(S) and Zapalote Chico corn (ZC), a
resistant cultivar. ZC5, ZC10, and ZC20
= 5 g, 10 g, and 20 g of ZC silk in 300
ml of check diet respectively. 41

7. Percentage dry weight of fall armyworm
reared on a pinto bean check diets con-
taining silks of Stowell's Evergreen (S)
and Zapalote Chico (R) corn. ZC5, ZC10,
and ZC20 = 5 g, 10 g and 20 g of ZC silk
in 300 ml of check diet respectively. 43

8. Mean number of eggs laid by fall armyworm
females reared on a pinto bean check diet
(CK) and diets containing silks of
Stowell's Evergreen corn (SEG) a suscep-
tible cultivar, and Zapalote Chico corn
(ZC) a resistant cultivar. ZC5, ZC10,
and ZC20 = 5 g, 10 g and 20 g of ZC silk
in 300 ml of check diet respectively. 45

9. Calibration curve for 5th instar fall
armyworm feeding on a pinto bean check
diet. Estimated regression line for
relative growth rate (RGR) against
relative consumption rate (RCR) for 24
hour feeding. 48

10. Calibration curve for fall armyworm
feeding on a meridic diet, with plots of
growth rates against consumption rates
for larvae feeding on diets containing
5 g each of silks of Stowell's Evergreen
and Zapalote Chico corn. 50


11. Calibration curve for FAW feeding on a
meridic diet, with plots of growth rates
against consumption rates for larvae feed-







ing on diets containing 10 g and 20 g of
Zapalote Chico corn silks. 53

12. Calibration curve for fall armyworm feed-
ing on a pinto bean check diet, with
plots of growth rates (RGR)m against con-
sumption rate (RCR) for larvae feeding on
test diets containing silks of resistant
and susceptible corn cultivars. (5 g of
Stowell's Evergreen corn silk and 5 g, 10
g, and 20 g of Zapalote Chico corn silks). 55

13. The kinds of interactions, direct and
indirect, in a typical food web, showing
the relationships between intrinsic and
extrinsic defense of plants and the trade-
offs between them. +, -, 0 indicate
positive, negative or noeffects, the plant
may impact on the herbivore-natural
enemy relationship. From Price (1986). 65

14. Percentage parasitization by artificially
applied Archytas mormoratus, Ophion
flavidus and other parasities, of fall
armyworm collected from a susceptible
corn cultivar, Stowell's evergreen and a
resistant corn cultivar MpSWCB-4 in Tifton,
Georgia. 76

15. Percentages of total parasitization of
FAW collected from Stowell's Evergreen
(S) and MpSWCB-4 (R) corn in Tifton,
Georgia, 1988. 78

16. Percentage parasitization of fall armyworm
collected from Stowell's Evergreen (S)
and MpSWCB-4 (R) corn plots in Tifton,
Georgia, 1989. 80

17. Percentages of total parasitization of
fall armyworm larvae collected from
Stowell's Evergreen (susceptible) corn and
MpSWCB-4 (resistant) corn in Tifton,
Georgia, 1989. 82


18. Fresh weight and dry weight of pupae of
fall armyworm reared on leaves of
Stowell's Evergreen corn (susceptible
cultivar) and MpSWCB-4 corn (resistant
cultivar). 94

19. Mean number of eggs laid by fall armyworm







females reared on leaves of Stowell's
Evergreen corn (susceptible cultivar)
and MpSWCB-4 corn (resistant cultivar). 96

20. Fresh weights of 6th instar fall armyworm
larvae parasitized by Archytas marmoratus,
and unparasitized larvae. 98

21. Schematic representation of the cytochrome
P-450-dependent microsomal MFO system. 105


xii













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

INTERACTIONS OF RESISTANT CORN CULTIVARS, SPODOPTERA
FRUGIPERDA (J.E. SMITH) AND ARCHYTAS MARMORATUS (TOWNSEND)

By

MADE CHRISTIAN-MEIER

August 1990

Chairperson: Carl S. Barfield
Cochairperson: Gary R. Buckingham
Major Department: Entomology and Nematology

Measures of consumption, development and fecundity of

Spodoptera frugiperda (J.E. Smith) were obtained on diets

containing silks of the resistant corn cultivar 'Zapalote

Chico'. Field performance of Archytas marmoratus Townsend

against S. frugiperda in whorls of Stowell's Evergreen

(susceptible corn cultivar) and MpSWCB-4 (resistant corn

cultivar) was investigated. Effects of resistant and

susceptible corn cultivars on the detoxication enzyme aldrin

epoxidase and insecticide susceptibility of S. frugiperda to

three classes of insecticides were determined.

Diets containing Z. Chico corn silks inhibited growth

of S. frugiperda larvae. Pupal weight and fecundity were

reduced as Z. Chico silk content of the diet was increased.

There was a significant increase in larval developmental


xiii










time from 15 days to 33 days on the control diet and diet

containing 20 g of Z. Chico silks, respectively.

Present investigations represent an attempt to study

three trophic levels in an agricultural system with a

resistant corn cultivar as the producer, S. frugiperda the

primary consumer, and A. marmoratus the secondary consumer.

Compatibility of a parasitoid as a control method, in

combination with resistant varieties, may be erroneously

assumed. The host diet may have a potentially deleterious

effect on a parasitoid's performance. Performance of A.

marmoratus was not affected adversely by S. frugiperda

feeding on the resistant corn cultivar. Detoxication enzyme

levels increased slightly when larvae were reared on MpSWCB-

4, compared to those reared on Stowell's Evergreen. Z.

Chico silk diets caused inhibition of the detoxication

enzyme. Relative potency of three insecticide classes

toward S. frugiperda was pyrethroid > carbamate >

organophosphate. S. frugiperda reared on the susceptible

cultivar were more susceptible to methomyl and bifenthrin,

with LD50's of 2.39 ug/g larva and 0.45 ug/g larva,

respectively, than larvae reared on MpSWCB-4 with LD50's of

4.19 ug/g larva and 1.29 ug/g larva, respectively. The

organophosphate chlorpyrifos was more toxic to larvae reared

on MpSWCB-4 than larvae reared on Stowell's Evergreen.


xiv













This work is dedicated to my parents Dr. and Mrs. F.
Christian Sr., my brothers, Dr. F. Christian Jr. and E.
Christian, my nieces and nephews, my husband Henry Meier, my
children Fernanda, Jack, Anthea and Rachel.














CHAPTER I

INTRODUCTION


Pest control is recognized as an integral
part of modern agriculture, and necessarily
requires the discovery of new information and
technology to address evolving pest problems. The
fervor for routine applications of pesticides has
subsided because of socio-environmental and,
recently, economic concerns. These concerns have
brought about renewed interest and greater in-
depth research on insect resistance in plants and
in biological control. While it is generally
conceded that these two pest control methods are
compatible and among the most important integrated
pest management tactics, most of the research in
these two fields has been done independently of
the other. (Verbatim from the preface, Boethel
and Eikenbary 1986)


The above statement holds true for the management of

the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith)

[Lepidoptera: Noctuidae], on corn, Zea mays (L.), in the

United States. The more that is understood about the pest

regarding its ecology, biology, natural enemies,

biochemistry etc., the better the chance of knowing the

ramifications of a control method and successfully

controlling the pest. The purpose of this project was to

investigate some insect-plant interactions among FAW and two

resistant cultivars of corn, Zapalote Chico (ZC) and MpSWCB-

4. Plant resistance to FAW was reviewed by Wiseman and

Davis (1979). Since then, sources of resistance to FAW have






2

been found in Zapalote Chico and MpSWCB-4 corn. An attempt

was made in the present study to evaluate the compatibility

of the resistant cultivar MpSWCB-4 and a FAW natural enemy,

Archytas marmoratus (Townsend) [Diptera: Tachinidae], in

multitactic control of FAW. The effect of FAW feeding on

the resistant corn cultivars ZC and MpSWCB-4 and on a

detoxification enzyme and subsequent insecticide

susceptibility of FAW was studied.

Integrated pest management (IPM) principles serve as

guidelines in efforts to manage insect pests (see Barfield

and O'Neil 1984; Bottrell 1979).

1. POTENTIALLY HARMFUL SPECIES WILL CONTINUE
TO EXIST AT TOLERABLE LEVELS OF ABUNDANCE.

The objective of IPM is to lower pest populations

below economically important levels; eradication

is not the objective.

2. THE ECOSYSTEM IS THE MANAGEMENT UNIT.

The boundaries of and the couplings among the

components of the system must be identified before

design and implementation of an IPM program.

3. THE USE OF NATURAL ENEMIES IS MAXIMIZED.

An understanding of how natural enemies work in

the system must be acquired so that optimal use

can be made of their impact on target pest

populations.

4. ANY CONTROL PROCEDURE MAY PRODUCE
UNEXPECTED AND UNDESIRABLE CONSEQUENCES.

An ecologically based management strategy is less






3

likely to result in "negative effects" within the

system being managed.

5. AN INTERDISCIPLINARY SYSTEMS APPROACH IS
ESSENTIAL.

The assumption is that information

collected by various scientists can and will be

integrated. (1-5 Verbatim from Barfield and

O'Neil 1984, p. 43-45)


The present project was undertaken as an attempt to

make a contribution to the management of FAW in corn, one of

the world's most widely distributed food plants and the most

important crop in the United States.



The Fall Armyworm

The fall armyworm (FAW), Spodoptera frugiperda (J.E.

Smith), is one of the most important pests of corn in the

United States. It is polyphagous, mobile, and annually

attacks corn and other crops, especially gramineous species,

throughout the southeastern United States. FAW populations

can be found during the entire year in south Florida and

Texas; this pest migrates from these areas into uninfested

regions farther north each spring (Rabb and Kennedy 1979).

Luginbill (1928) lists more than 60 host plant species for

FAW, but states that the larvae would probably confine their

feeding to corn (Zea mays L.), sorghum (Sorghum bicolor

(L.)), and grasses such as Bermudagrass (Cynodon dactylon

(L.)) if those plants were available. Corn presently is






4
implicated (Kennedy and Margolies 1985, Wiseman 1985) as the

crop that contributes devastating numbers of these migratory

moths to infest other major agronomic crops.

Sparks (1979) reviewed the life cycle of FAW. Adult

FAW are nocturnal. They move toward host plants suitable

for feeding and oviposition at dusk. Eggs are laid in

clusters and protected by a dense covering of scales.

Masses contain from a few to hundreds of eggs which hatch in

2-4 days at temperatures around 21.1 26.7 "C. Larvae

consume egg shells as they hatch and feed on host plants

until completion of 6 instars. The sixth instar drops to

the ground and pupates in the soil at a depth of about

2.54 7.62 cm, depending on soil texture, moisture, and

temperature. The first 3 instars are small and require less

than 2% of the total foliage consumed. The life cycle

requires about 30 days at 80 *F (Barfield et al. 1978)

(June-August) throughout the southeastern U.S. and around

the Gulf Coast states. At colder temperatures, a FAW

generation may take 80-90 days.

FAW does not diapause and, therefore, its geographical

range in the U.S. in winter is reduced to south Florida and

the southern coastal areas of Georgia, Alabama, Mississippi,

Louisiana and Texas (Hinds and Dew 1915). In early spring,

FAW populations begin to spread northward and westward

within the continental U.S. at rates estimated in some years

to be about 300 miles per generation. Southerly winds are

believed to be an important factor in this annual dispersal.









Single year loss estimates due to FAW in the southeastern

U.S. have ranged between $ 30 and 60 million (Sparks 1979).

Pitre (1986) indicates that FAW populations in the

southeastern United States are resistant to carbaryl

[Sevin], methyl parathion [Metacide], trichlorfon

[Dylox], and diazinon [Spectracide]. Methomyl [Lannate],

seemed to be effective in all areas, but susceptibility in

Florida was declining. Tactics such as chemigation (the

application of insecticides through irrigation water), as

opposed to aerial broadcasting, and the use of synthetic

pyrethroids are new methods being adopted to improve the

efficiency of chemical control of FAW. As numerous attempts

at FAW control fail, a feasible alternative management

strategy is sought. Resistant host plants and natural

enemies are considered to be premier alternatives to

chemical control.

A single tactic (e.g., chemical control) or a

combination of tactics (e.g., resistant varieties and

natural enemies together) may be adopted in an attempt to

manage FAW on corn. Dependence on chemicals alone

historically leads to resistance, resurgence and adverse

effects on non-target organisms among other problems. Each

year in the United States, despite the use of all pesticides

and other controls, pests destroy about 37% of all potential

crops (Pimental et al. 1978). Sole dependence on chemical

control is not a sustainable strategy for pest control.





6

Resistant varieties and natural enemies, on the other hand,

are sustainable strategies that do not pose any of the

above-mentioned problems. Understanding the nutritional

ecology of the FAW on a host plant, and how this might

impinge on natural enemies or affect the efficacy of

pesticides, is useful in designing pest control strategies.

Tri-trophic interactions occur among corn (the primary

producer), FAW (the consumer), and natural enemies at the

third trophic level. The role of resistant host plants on

FAW feeding, natural enemy attack of FAW, and insecticide

susceptibility of FAW will be discussed.



Host Plant Resistance and Natural Enemies

Painter (1951, p.2) defined plant resistance to insects

as ". the relative amount of heritable qualities

possessed by the plant which influence the ultimate degree

of damage done by the insect". He also points out that in

order to be a useful character, resistance must be

inherited. Resistance is relative, and various degrees of

resistance can be recognized (e.g., immunity, high

resistance, low resistance). The mechanisms generally

accepted and used frequently by entomologists studying plant

resistance to insects are those proposed by Painter (1951):

non-preference, antibiosis and tolerance. These three

mechanisms may be of a physical or chemical nature, and

there are many factors that 'condition' all of the

mechanisms: (1) how the insect utilizes the plant or plant






7

parts for food, shelter or oviposition; (2) whether adults

and larvae use the same plant for food; (3) features of the

environment such as temperature; and many others. A review

of the state-of-the-art of plant resistance to FAW was

presented by Wiseman and Davis (1979).

The mechanisms of resistance can have a direct effect

on the feeding behavior of an insect pest. Antibiosis

includes those adverse effects on the insect's life history

which result when a resistant variety is used for food

(Painter, 1951). Measured effects of resistance may be in

the form of death in the early instars, small size or low

weight, abnormal longevity, low food reserves, reduced

fecundity, death in the prepupal stage, and abnormal

behavior (Owens, 1975). A resistant variety may possess

adequate food supply and provide all necessary nutrients,

while a susceptible variety may be lacking in a necessary

diet substance. An insect may therefore gorge itself,

destroying or consuming the susceptible host plant, to meet

its dietary needs. This statement is somewhat counter-

intuitive, but explains to an extent differences in

consumption rates for insects feeding on resistant, versus

susceptible, varieties of plants. Furthermore, a resistant

variety may lack some qualities which provide the attractive

stimulus in susceptible varieties, and resistant varieties

may possess repellant qualities which can compete with or

mask an attractant (Wiseman et al. 1983). Wiseman et al.

(1983) state that these characteristics make plant





8

resistance to insects the most useful of all integrated

controls. Wiseman (1985) reviews the mechanisms and types

of plant resistance. Several corn genotypes have been

identified as having resistance to the FAW (Wiseman et al.

1981, Williams et al. 1983). 'Zapalote Chico' 2451 # (PC3)

is a dent corn resistant to silk and ear feeding in the

field. A dent corn is the commercial classification of corn

characterized by a depression in the crown of the kernel

caused by unequal drying of the hard and soft starch making

up the kernel. Some of the highest levels of resistance

known in corn have been found in the silks of 'Zapalote

Chico' (ZC) corn (Straub and Fairchild 1970; Wiseman et al.

1976,1977, 1978). Investigation into the feeding and

developmental responses, which will define the phytochemical

nature of the resistance, has been attempted. Wiseman et

al. (1984) developed and used a laboratory bioassay to

evaluate feeding responses of FAW to ZC. Wiseman and

Widstrom (1986) reported the mechanisms of resistance of ZC

corn silks to FAW larvae, indicating that antibiotic factors

present in the silks result in the production of small

larvae, small pupae and longer life cycle compared with

those fed silks of the susceptible sweet corn 'Stowell's

Evergreen'. An attempt was made to investigate the effect

of the resistant cultivar 'Zapalote Chico' on FAW feeding,

growth, development and fecundity.

Biological control, abbreviated biocontroll" involves

importation, conservation, and/or encouragement of






9

parasites, parasitoids and predators to reduce pest

densities to a level below the economic injury level, and

(ideally) maintain them there. Applied biocontrol implies

active intervention with biotic components of agroecosystems

(Horn 1988). Successful biocontrol is relatively safe,

permanent and economical after the initial investment.

The concurrent use of plant resistance and biological

control is compatible in principle since both aim at

suppression of the insect pest population. However such

plant-pest-natural enemy interactions are rarely

investigated. In the present study a second resistant

cultivar, also thought to be resistant via antibiosis,

MpSWCB-4, was used to study the effect of a resistant

cultivar on a natural enemy of FAW, Archytas marmoratus

(Townsend) (Diptera: Tachinidae), in the field. Archytas

marmoratus (AM) is a primary larviporous, larval-pupal

parasitoid of FAW and other noctuids in North and South

America and in the West Indies (Sabrosky 1978, Ashley 1979).

Findings by Gross et al. (1976) and Pair et al. (1986a),

from collections of 5th and 6th instars of the corn earworm,

Helicoverpa zea (Boddie), and FAW larvae on corn, revealed

that AM is a major parasitoid of these species, especially

in south Georgia and north Florida. It has since been

reared successfully on a large scale using maggots extracted

mechanically from fecund females (Gross and Johnson, 1985).

Some research has been done on possible detrimental effects

of plant antibiosis on biological control agents of various





10

insect pests including the FAW. Helicoverpa zea has an

ichneumonid parasite, Hyposoter exiquae (Viereck). Campbell

and Duffy (1979) found this parasite could be poisoned by an

antibiotic (a-tomatine) from resistant tomato plants. Even

though a-tomatine is useful in controlling tomato pests, it

has an adverse effect on the natural enemy, H. exiguae.

preventing the parasitoid from exerting an additional pest

population regulation effect. This is an example of

potential incompatibility of a resistant variety and

biological control. The antibiotic in the resistant plant

protects the insect from its natural enemy by serving as a

prophalactic against the parasitoid. Yanes and Boethel

(1983) found that the introduced parasitoid, Microplitis

demolitor Wilkinson, in the soybean looper, Pseudoplusia

includes, greatly reduced larval weight and leaf

consumption for loopers reared on soybean resistant variety

PI 227687. Here, the resistant host plant synergizes the

effect of the natural enemy and proves to be compatible with

the biological control agent. Isenhour and Wiseman (1989)

studied parasitism of FAW by Campoletis sonorensis, affected

by host feeding on silks of ZC. Wiseman et al. (1983)

looked at the influence of resistant and susceptible corn

silks on selected developmental parameters of the corn

earworm. Isenhour et al (1989) reported enhanced predation

by Orius insidiosus (Say) on larvae of FAW and corn earworm

caused by prey feeding on the resistant corn genotypes

MpSWCB-4 and ZC. The influence of resistant plants on






11

interactions between insect herbivores and natural enemies

must be studied to determine compatibility.



Chemical Control

Most insecticides exert a density-independent effect on

insect populations. Application of a chemical control

usually results in populations below an economic injury

level (EIL), the lowest pest population density which will

cause economic damage; however, population density is not

regulated about a mean, as is (ideally) the case with

biological control. If the population is not reduced

sufficiently, re-application of insecticides becomes

necessary to achieve low densities of pests.

Insecticides presently are the chief weapon against

insect pests. Chemical control is the most widely used

single technique to reduce densities of insect pests. An

advantage of using insecticides is that the proper

insecticide, properly applied at the right time, nearly

always causes swift death to insects in the treated area.

They are useful in emergencies, when a rapid reduction in

insect population density is necessary to prevent serious

economic loss.

There is an ongoing search for alternative methods of

pest control in corn and other crops, but predictably,

insecticides will be used on a large scale, world wide, for

some time to come (Kumar, 1984). It is therefore necessary





12

to understand interactions of pesticides with other pest

management tactics.

Mixed function oxidases (MFO), located in the

endoplasmic reticulum of cells, play an important role in

the metabolism of xenobiotics such as insecticides. A

detailed review of the functional role of MFO in insects has

been published by Hodgson (1983a). This system is thought

to be partially responsible for the selective toxicity of

insecticides, the development of resistance and the degree

of herbivore polyphagy. The effects of host plants on the

MFO has been researched by Yu (1982a). Host plants have

been reported to affect both the insect's detoxification

system and its susceptibility to pesticides. Yu (1982a)

found corn-fed FAW to be more tolerant of certain pesticides

than soybean-fed FAW. The present study investigates the

effect of two resistant corn cultivars on the MFO of FAW and

on susceptibility of FAW to three groups of insecticides a

carbamate, an organophosphate and a synthetic pyrethroid.

Pesticides frequently are components of pest management

programs in corn, so knowing the factors that influence

efficacy, which can vary as a function of host plant and

nutritional status (Kea et al. 1978, Berry et al. 1980, Yu

1982a) is useful.





13

Interactions Among Three Trophic Levels

Price et al. (1980) described the influence of plants

on interactions between insect herbivores and natural

enemies. Terrestrial communities are composed of at least

three interacting trophic levels: plants, herbivores, and

natural enemies of herbivores. Price et al. (1980) argue

that theory on insect-plant interactions cannot progress

realistically without consideration of the third trophic

level. Plants have many effects, direct and indirect,

positive and negative, not only on herbivores but also on

the enemies of herbivores. The third trophic level must be

considered as part of a plant's battery of defenses against

herbivores.

There are several theories on interactions among

plants, herbivores and natural enemies of herbivores;

however these theories are rarely viewed in the same context

or even in the context of how they apply in agroecosystems.

The theory of plant chemical defense, as developed by Feeny

(1975, 1976), permits some predictions about the efficacy of

the herbivores' enemies as influenced by the life history of

the plant. Feeny (1976) points out that any plant condition

that lowers the growth rate of an insect herbivore makes it

available to natural enemies for a longer period and raises

the probability of mortality. For example, where insects

fed on plants containing tannins and other digestibility

reducers; lowered digestibility of food was compensated

partially by prolonged feeding. He argues that plants with





14

digestibility reducers, should support herbivores that are

more heavily attacked by enemies, than herbivores on plants

without digestibility reducers. Agroecosystems have made

previously non-apparent plants more apparent to insects and

have provided habitats with high resource availability.

Increased herbivory has been the predictable result.

Aspects of interaction between plant genotypes and

biological control were discussed by Bergman and Tingey

(1979). They emphasize that, as resistant cultivars become

more widely used in pest management, their compatibility

with biological control agents must be given serious

consideration. The concurrent use of resistant cultivars

and natural enemies can provide density-independent

mortality in times of low pest density and dynamic density-

dependent mortality in times of pest increase (resistant

cultivars can exert density-independent mortality on the

pest while natural enemies exert dynamic density-dependent

mortality). Numerous studies indicate that predator and

parasite performance may be altered by the host plant of the

prey (e.g., Flanders 1942, Smith 1957). Painter (1951)

discussed 2 ways in which plant resistance can influence

natural enemies. A reduction in prey population may affect

the success of some predators and parasites if prey density

falls below the optimum searching capacity of the natural

enemy. Secondly, host plant-induced changes in prey

physiology and behavior may modify the success of natural

enemies. The purpose of this study was to investigate






15

specifically, relationships among FAW, resistant cultivars

of corn and a natural enemy of FAW, in the context of pest

management.














CHAPTER II


CONSUMPTION, DEVELOPMENT AND FECUNDITY OF FAW ON DIETS
CONTAINING SILKS OF THE RESISTANT
CORN CULTIVAR 'ZAPALOTE CHICO'


Introduction

The mechanisms of plant resistance generally accepted

and used by entomologists in insect plant resistance studies

are those proposed by Painter (1951): preference/

nonpreference, antibiosis and tolerance. Antibiosis effects

may take the form of death in the early instars, small size

or reduced weight, abnormal longevity, low food reserves,

less fecundity, death in the prepupal stage, and abnormal

behavior (Owens, 1975). Antixenosis is a term used for

nonpreference, by which feeding or oviposition may be

delayed or prevented (Wiseman, 1985) by either absence of a

stimulant or the presence of a deterrent.

A review of the history and the state-of-the-art of

plant resistance to FAW was presented by Wiseman and Davis

(1979). Wiseman and Widstrom (1986) reported that both

nonpreference and antibiosis resistance to feeding are

manifested in the silks of Zapalote Chico (ZC) corn to

Helicoverpa zea (Boddie). Silks of ZC 2451 have been

demonstrated to cause high mortality of FAW by the 10th day













of larval feeding. Wiseman and Widstrom (1986) reported

that antibiotic factors in the silks result in production of

small larvae, small pupae, and longer life cycle compared

with those fed silks of Stowell's Evergreen (SEG) sweet

corn. They found that larvae fed a diet of 80 g (fresh

weight) susceptible silks weighed 246 mg compared with only

4 mg for those fed ZC silks. A high degree of nonpreference

also was indicated. Waiss et al. (1979) found high levels

of a flavone glycoside, maysin, in ZC.

Slansky (1982) describes the paradigm where insects may

alter performance to reach maximum possible fitness.

Alterations in performance may involve compensatory

responses; for example, altering consumption rate and/or

food utilization to obtain sufficient nutrients to reach

some minimum weight required to stimulate ecdysis (Slansky

and Scriber 1985). Antibiosis, as caused by feeding on ZC,

may stimulate some compensatory response in FAW.

Reported here are experiments to determine the effects

of silks of the resistant corn cultivar ZC on fall armyworm

larval growth, development and fecundity. The method of

Blau et al. (1978) was used to separate the effects of

feeding inhibition and toxicity leading to antibiosis, the

suspected mechanism of resistance.






18

Materials and Methods

Insects

FAW eggs were obtained from a colony maintained on a

pinto bean meridic diet (Perkins, 1979) at the Insect

Biology and Population Management Research Laboratory,

Tifton, Georgia. Neonate larvae were reared on experimental

diets. One neonate larva was placed in each cup, and the

cup was sealed with a paper lid. Thirty cups of each diet

were set up. The cups were maintained at 26.7 + 2 *C, at

least 75% RH and 14L:10D. Daily observation allowed records

to be kept of the duration of each instar and of any

mortality. Fresh weights of newly molted fifth instars were

taken, and fresh food fed to them was weighed to allow

measurement of food consumption. Percent dry weight of the

initial diet fed was determined by noting the fresh weight

of 5 samples of each of the five diets, drying for 48 hours

in the oven at 60 + 1 *C and re-weighing. Pupae were

removed within 8 hours of pupation, dried and weighed.

Feces and uneaten diet were collected, dried and weighed to

obtain their respective dry weights. A Mettler H35AR

balance, accurate to 0.1 mg was used for all weights. All

material was dried at 60 + 1 *C for at least 48 hours.



Experimental Diet

Two corn genotypes were selected: cv. Stowell's

Evergreen, a sweet corn susceptible to silk and ear feeding

in the field by Helicoverpa zea (Boddie) larvae, and cv.






19

Zapalote Chico 2451# (PC3), a dent corn resistant to silk

and ear feeding in the field. The corn used in this

experiment was grown in, single-row-plot plantings, 6.1 m

long and 0.76 m apart, at Tifton in 1987 according to

agronomic practices common to the area.

Diet prepared as follows was provided by Dr. B. Wiseman

of the USDA laboratory in Tifton Georgia: Silks of each

genotype were harvested 2 days after emergence from the husk

leaves. They were excised at the tip of the ear, bulked,

and oven dried at 41 +30 C for about 10 days, then ground to

a fine powder (1.0 mm screen) using a Cyclotec TC1093

(Fisher Scientific, Atlanta) sample mill. Dry silks were

stored in a freezer at -10 + 00 C until prepared for use.

One concentration of the SEG silk diet was prepared to serve

as a susceptible diet check. This contained 5 g of SEG

silks in diluted pinto bean diet (300 ml diet : 100 ml

distilled water) (Burton 1967). The control diet contained

no silk. Three concentrations of ZC silk diet were

prepared: 5 g, 10 g, and 20 g. The silk-pinto bean diets

were dispensed into 30 ml plastic diet cups, 10 ml per cup,

and allowed to solidify for about 2 hours.



Quantitative Performance Studies

Larval food consumption, growth and food utilization

indices were calculated, using gravimetric techniques based

on dry weights (Slansky and Scriber 1985) in 1988.

Performance indices included: larval period (days), day 1





20

pupal dry weight, dry weight gained during the last instar,

food utilization efficiencies (AD = Approximate

digestibility, ECD = Efficiency of conversion of digested

food, and ECI = Efficiency of conversion of ingested food),

relative growth rate (RGR) and relative consumption rate

(RCR). These parameters and their interrelationships are

defined in the appendix (after Finke 1977). Experimental

results were analyzed using the GLM procedure and Tukey's

studentized range test (p=0.05) to separate means (SAS

Institute Inc. 1989).



Toxicity/Feeding Inhibition Test

Unequivocal demonstration of toxicity is often hampered

by the behavioral responses of experimental insects, and

slow growth does not necessarily indicate adverse effects on

metabolism, but possibly a result of behavioral inhibition

of feeding (Waldbauer 1962). Blau et al. (1978) describe a

technique that permits clearer distinction between feeding

inhibition and toxicity. This method was used to separate

the effects of feeding inhibition and toxicity leading to

antibiosis, the suspected mechanism of resistance for ZC

silks.

Twelve newly molted 6th instar FAW larvae reared from

eggs were weighed individually, placed in individual petri

dishes and fed a quantity of pinto bean check diet to

determine an approximate fresh weight consumed in 24 hrs. A

calibration curve for final instar larvae reared from eggs






21

on the check pinto bean meridic diet was obtained by

offering groups of 6 newly molted 6th instar larvae

different amounts of diet, ranging from none at all,

approximately 25%, 50%, 75%, and more than the amount

previously determined to be consumed within 24 hours. All

of the larvae were set up individually in petri dishes.

(Percentage fresh weight of both diet and larvae were

determined by obtaining fresh weights of 5 samples of diet

and 10 individual larvae and oven drying these as previously

described). Larvae were left to feed and then both they and

any remaining diet weighed after 24 hrs. Resultant data

were used to determine the regression slope of growth rate

on consumption rate for the check diet.

Groups of 12 newly molted 6th instar larvae, reared

from eggs on the check diet, were then individually weighed

and fed the check diets and test diets containing SEG and ZC

silks for 24 hrs., in individual petri dishes (1 group per

diet). Relative growth and relative consumption rates were

calculated and plotted relative to the calibration curve for

the check diet.



Fecundity Studies

Thirty FAW larvae were reared individually, in 30 ml

plastic diet cups until pupation on a pinto bean diet and on

SEG and ZC silk diets as previously described. Pupae were

removed and sexed. Separate sexes were placed in

PlexiglassTM cages (45x40x40 cm). Upon emergence, males and





22

females were paired, (10 pairs were set up per diet) and

placed in oviposition cages (screen cylinders 20 cm x 9 cm)

and supplied 'BountyTM' hand towel paper as an oviposition

substrate. The paper towel was secured by a rubber band

over the top of the cage. Moths were provided a 10% sucrose

solution. All oviposition cages were held in a rearing

chamber (PercivalTM incubator model I-35LLs) at 26.0 + 1o C

and 90% RH. Oviposition substrates were removed every

second day, and replaced. All surfaces, the oviposition

substrate, and the sides of the cage were checked for eggs

and all eggs counted under the microscope.

Eggs were counted under the microscope with the aid

of a fine forceps and a hand held dissecting pin to separate

eggs within a mass. Experimental results were analyzed

using the GLM procedure of SAS (SAS Institute Inc. 1989).



Pupal Weight Study

FAW larvae were reared on the five diets as described,

individually, until pupation. Thirty-six pupae were removed

from cups within 24 hours of pupation, sexed and weighed.

Percent dry weight of pupae was determined by obtaining

fresh weights and oven drying a sample of 10 pupae of each

sex from each diet for 48 hrs at 60o C. Experimental

results were analyzed using the GLM procedure and Tukey's

studentized range test to separate means, since this test is

moderately conservative (SAS Institute Inc. 1989).






23

Results and Discussion

Tables 1 and 2 summarize the developmental parameters

for FAW reared on the pinto bean check (CK) diet, the SEG

silk and the ZC silk diets. By incorporating three

different quantities of ZC silks into the diet, a gradient

of resistance was created, to facilitate the observation of

the effects on larval mortality and developmental time.

Effects of a "low dose" or "high dose" of any allelochemical

or antibiotic factor should have been observed at the two

extremes, ZC-5 g and ZC-20 g.



Larval Mortality and Developmental Time

Developmental time for FAW larvae (1st instar to

pupation) varied on the diets containing silks of SEG and

ZC. Larvae reared on the pinto bean check diet (which did

not contain any silks) had the shortest developmental time

of about 15 days (Figure 1). Larval duration was about 17,

21, 23, and 33 days on the SEG, ZC-5, ZC-10, and the ZC-20

diets respectively. Larvae fed the SEG and the ZC-5 g silk

diets had progressively longer developmental times; the

differences were statistically significant. There was a

significant increase in developmental time to about 23 days

and 33 days when larvae fed on ZC-10 g and ZC-20 g silk

diets, respectively. There was no significant increase in

larval duration for larvae fed ZC-5 and the ZC-10 diet. The

ZC-20 g diet caused a twofold increase in developmental time

when compared to the check diet.









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Comparing the time to develop from the 5th instar to

pupation on the silk and CK diets, larvae fed on the ZC-20 g

silk diet took 16 days to pupate compared with 6, 7, or 8

and 9 days respectively on the CK, SEG and ZC-5 and ZC-10

diets, respectively (Figure 2). This was consistent with

the pattern observed for overall developmental time for the

different diets. There was a significant difference in time

to develop from 5th instar to pupation for larvae fed the CK

and the SEG diet. Development time from 5th to pupation for

larvae fed ZC-5 and ZC-10 diets were not significantly

different using Tukey's studentized range test to separate

means. Table 1 summarizes the data for the developmental

parameters of fall armyworm larval instars feeding on diets

containing silks of Zapalote Chico and Stowell's Evergreen.

Sample sizes and results of Tukey's studentized range test

are indicated on all tables.

Mortality was not significantly different for the

different diets. Percent mortality was 3%, 6%, 3%, 6% and

6% for larvae fed the CK, SEG, ZC-5, ZC-10, and ZC-20 silk

diets respectively. Some of the larvae died from drowning

in water that had condensed in the growth chamber as a

result of using a humidifier within the chamber. Percent

mortality on the different diets is presented in Table A-2.

Differences in sample sizes in the materials and methods and

sample sizes used in statistical analyses were due to deaths

from drowning, and larvae sacrificed by being weighed and

dried to obtain dry weight percentages.








Larval Weights

Figure 3 indicates that there was a progressive

decrease in fresh weight of 5th instar FAW larvae reared on

the various ZC silk diets; however, these were not

significantly different.

Percentage dry weights for 5th instar larvae reared on

the different diets are presented in Figure 4. There was no

indication of a significant difference in larval percent

dryweight between the CK and the various ZC silk diets.

Fifth instar larvae fed the SEG silk diet had a

significantly higher percent dry weight than larvae reared

on all the other diets.



Pupal Weights

Figure 5 shows the mean pupal fresh weights of FAW

reared on diets containing Zapalote Chico silks. Pupal

fresh weight decreased significantly as the quantity of ZC

silk increased in the diet (Tukey's studentized range test).

There were significant differences found in pupal dry

weights for the different diets (Figure 6). Pupae from the

CK diet had the highest dry weight. Dry weight of the CK,

SEG and ZC-5 g pupae were similar (no significant

differences). Dry weight of pupae from the ZC-10 g diet was

significantly lower than the dry weights of pupae from the

CK, SEG and the ZC-5 g diets, and significantly higher than

dry weight of pupae from the ZC-20 g diet. As ZC silk

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36

weight decreased to a significantly lower weight of 44.20 mg

compared to 76.26 mg on the CK diet.

Diet Dry Weights

Significant differences were found among the dry

weight percent of the different diets (Figure 7). This may

have been due to the quantity of silk in the different

diets. The ZC-20 g diet had the highest dry weight content

at 38%. The maximum difference in percentage dry weight

between diets was 11 percentage points (i.e. 38-27%).



Relative growth rate

The relative growth rate (RGR = RCR x AD x ECD)

[biomass dry weight gain (mg)/average caterpillar dry weight

(mg)/developmental time (days)] also decreased with an

increase of ZC silk in the diet consumed (Table 2). The FAW

larvae reared on the ZC-10 g diet had an RGR of 0.26

mg/mg/day while larvae reared on the ZC-20 g silk diet had

an RGR of 0.13 mg/mg/day. A doubling of silk concentration

in this instance gave rise to a 50% reduction in growth

rate. This trend was not observed between ZC-5 and ZC-10.



Relative consumption

The relative consumption rate of dry weight (dw) (RCR =

mg dw consumed/mg mean body dw/day) decreased as the silk

concentration of Zapalote Chico in the diet increased. The

lowest RCR was with the ZC-20 g diet, and the highest on the

silk free diet. There was a significant difference in RCR






37

between the larvae reared on the CK diet and on all the test

diets (Table 2).



Approximate Digestibility

There were some significant differences in approximate

digestibility (AD = [mg dw consumed mg dw feces]/mg dw

consumed) for larvae feeding on the silk diets, (Table 2).

Larvae fed the ZC-5 diet had an AD that was significantly

higher than the other diets. The approximate digestibility

(AD) estimates the proportion of ingested food (dry weight)

that is consumed and assimilated.



Efficiency of Conversion of Digested Food

Efficiency of conversion of digested food into insect

tissue (ECD) [biomass gain/(ingestion-feces) all dry

weights] varied significantly on the different diets. No

progressive trend was observed with and increase in silk

concentration. The ECD for larvae fed the SEG diet was

significantly higher than the ECD of for all diets except

ZC-10, (Table 2).



Efficiency of Conversion of Ingested Food

The efficiency of conversion of ingested food into

insect tissue (ECI) [biomass gain/ingestion, all dry weight

mg], decreased on the ZC silk diet. ZC-20, the diet with

the highest concentration of silk resulted in the lowest

ECI, compared to the highest on the control diet. The ECI










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46

for larvae fed the CK, SEG and ZC-10 diets did not differ

significantly. FAW larvae feeding on the SEG silk diet had

a higher ECI than FAW feeding on the ZC-5 diet.



Fecundity

Lynch et al. (1983) described a way of estimating

fecundity by using the regression of egg mass weight on

number of eggs and larvae to determine the number of eggs

per mass and total fecundity of the fall armyworm. The FAW

egg mass consists of several layers of eggs, the bottom

layer containing the largest number of eggs. Determination

of FAW fecundity is, therefore, laborious when eggs are

counted individually, but in order to be accurate, this most

laborious method was employed. The mean numbers of eggs

laid by FAW females reared from 1st instar to pupation on

the pinto bean check diet and diets containing silks of ZC

are shown in Figure 8. An increase in ZC silk resulted in a

decrease in eggs laid. FAW females fed the ZC-20 g diet

laid significantly fewer eggs than females reared on the CK

and other test diets (Scheffe's test; Table A-3).



Toxicity/Feeding Inhibition Study

Figure 9 illustrates the estimated regression line for

FAW consumption and growth rate when feeding on a pinto bean

meridic diet with plots of growth rates against consumption

rates for FAW larvae feeding on the check diet (no silks)

for 24 hrs.. The r2 for the regression line was 0.8876.









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The equation obtained was:

RGR=(RCR x 0.2458) 0.2191

This is the calibration curve indicating the effect of

varying consumption rate on larval growth rate in the

absence of corn silks. Figure 10 shows plots of growth

rates against consumption rates for FAW larvae feeding on

diets containing 5 g of SEG and ZC silks for 24 hrs. These

plots are close tothe calibration curve. They are neither

below the curve, which would have been indicative of

toxicity, nor do they fall in the low consumption range.

These diets, SEG and ZC-5 are neither toxic nor do they

inhibit feeding. Figure 11 shows plots of growth rates

against consumption rates for FAW larvae feeding on diets

containing 10 and 20 g of ZC silks for 24 hrs. These points

fall below the calibration curve, giving an indication that

these diets may prove to be toxic to FAW. If feeding had

been inhibited, the points would have fallen on the

calibration curves, but in the range of low consumption

rate. This leads to the conclusion that the antibiosis

resistance observed for Zapalote Chico corn against FAW

larvae is a direct result of an antibiotic agent that is

toxic to the FAW. Tables 3 and 4 show biomass gained,

feeding and growth rates, as well as efficiency parameters

for FAW feeding for 24 hrs on the different silk diets.

Figure 12 shows all plots of growth rates and consumption









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rates for all the test diets, plotted against the

calibration curve.


General Discussion

Using the method of Blau et al. 1978, the mechanism of

resistance of Zapalote Chico corn was determined to be one

of antibiosis resulting from toxicity. This supports

observations made by Wiseman and Widstrom (1986) on the

mechanisms of resistance in Zapalote Chico corn silks to FAW

larvae, but further specifies the basis of antibiosis

observed. The toxic effect of ZC corn silks may explain

some results obtained for FAW developmental parameters

measured during the 5th and 6th instars for FAW reared on

control check and silk diets such as reduced final body

weight (Bf), reduced relative growth rate (RGR), reduced

relative consumption rate (RCR).

Increased food consumption in response to reduced

nutrient levels is well documented (Slansky and Wheeler

1989; Scriber and Feeny 1979; Timmins et al., 1988).

However increasing the quantity of an allelochemical or

toxin as occurred here warrants a different set of

responses. Allelochemicals have been implicated as factors

responsible for reduced feeding rates and efficiencies

(Slansky and Scriber 1985), and for inhibiting and

prolonging insect growth (Stipanovic 1983). There was no





59

significant difference in body weight attained by the 5th

instar (Bi) by larvae reared on the check and ZC silk diets.

However final body weight (Bf), biomass gained (B) and mean

body weight (MB) decreased with an increase in ZC silk.

Over the fifth and sixth instar, the decline in biomass

gained (B) with increase in ZC silk was directly associated

with the decrease in food consumption (I), RCR, RGR, and

ECI. The toxicity of the ZC silk may have been the cause of

mortality observed in larvae fed the ZC-20 g diet.

The increase in percentage dry weight of the ZC-20 g

diet may have been responsible for the decline in

consumption rate, and may have been a confounding factor in

the interpretation of a xenobiotic causing the decline in

consumption rate. There was no progressive increase in

percentage dry weights of the Z. Chico silk diets that can

be directly correlated to the progressive decrease in

consumption rate observed with an increase in Z. Chico silk

concentration.

Pupal weight and fecundity declined significantly as ZC

silk content of FAW diet increased. This is an indication

of an inability to fully compensate for the effects of the

toxicity of the xenobiotic in the silks of ZC. This

reduction in fecundity will have ecological consequences.

Reduced fecundity in one generation implies a decrease in

population size of the subsequent generation. Increased

mortality in the larval stage has similar consequences. In

the context of pest management, this is an ideal situation,






60

as reduction in FAW populations would greatly reduce crop

damage and alleviate movement to alternate crops.

Prolonged developmental time makes larvae vulnerable to

a number of mortality agents for a longer period e.g.

parasites and predators (Price et al., 1980). The resistant

variety may have effects on natural enemies of herbivores

that are indirect in nature. A study of the effects a

resistant corn cultivar has on an economically important

pest such as the FAW, results in a better understanding of

interactions between the pest and host plant. Toxic

compounds in plants can be sequestered or cycled in

herbivores and used as defenses against the third trophic

level (Price 1986). Toxic compounds and allelochemicals

have an effect on the detoxication enzymes of FAW (Yu 1982).

The detoxication enzymes may be stimulated or induced,

increasing or decreasing the insects ability to detoxify

other xenobiotics such as pesticides. Understanding the

mechanisms of a resistant corn cultivar at the biochemical

level would constitute useful information that could be

utilized in the design of pest control strategies.












CHAPTER III

FIELD PERFORMANCE OF ARCHYTAS MARMORATUS AGAINST FALL
ARMYWORM SPODOPTERA FRUGIPERDA ON STOWELL'S EVERGREEN,
A SUSCEPTIBLE CORN CULTIVAR AND MpSWCB-4
A RESISTANT CORN CULTIVAR


Luginbill (1969), among others, considers the use of

resistant varieties an ideal method for controlling insects.

A fundamental requisite for development of IPM strategies in

corn is a precise understanding of the relationships between

corn genotypes and beneficial insects. It is important to

understand the relationships among host larvae, their diets,

and natural enemies, to determine the effectiveness of these

natural enemies as biological control agents.

Resistant varieties and natural biological control

agents may ultimately prove invaluable in developing

management programs for corn pests. Resistant varieties may

enhance natural control organisms, because insects feeding

on resistant plants generally require more time for

development and may be in a weakened condition (Maxwell et

al. 1972, Turnipseed and Sullivan 1976). Information on the

influence of resistant corn varieties upon beneficial insect

populations is lacking. Isenhour et al. (1989a) researched

enhanced predation by the bug Orius insidiosus (Say) on

larvae of Helicoverpa zea (Boddie) and Spodoptera frugiperda







(J.E.Smith) (FAW) caused by prey feeding on a resistant corn

genotype "MpSWCB-4". FAW larvae fed the resistant MpSWCB-4

had significantly higher rates of predation by adult Q.

insidiosus than did armyworm fed "Cacahuancintle", a

susceptible genotype. Studies on the European corn borer,

Ostrinia nubilalis (Hibner), and a tachinid parasitoid,

Lydella grisescens Robineau-Desvoidy indicated that corn

variety influenced the rate of parasitization (Franklin and

Holdaway 1966). In cotton, glabrous phenotypes are less

attractive to H. zea for oviposition than are hirsute

phenotypes; however, rates of egg parasitization by

Trichogramma pretiosum (Riley) were greater in glabrous

types (Treacy et al. 1985). On the other hand, resistant

host plants may be indirectly detrimental to parasitoids by

modifying the rate of reproduction and nutrition of

developing host larvae (Powell and Lambert 1984, Orr and

Boethel 1985).

Parasites, predators and parasitoids represent three

different carnivorous, interspecific interactions between

animals. In the parasites, many generations may occur on or

in a host, and there is a tendency towards the evolution of

host specificity and a more complex interrelationship.

Predators, on the other hand must locate a large number of

prey (= host) in order to grow and reproduce. Parasitoids

fall between these two extremes. Only one generation is

produced per host, and only the immature stage is parasitic,






63

while the adults are free living. A parasitoid requires the

entire host and kills it, thus eliminating the potential for

the evolution of a mutualistic relationship after the host

has been attacked. The host becomes a container for the

developing parasitoid, enabling the parasitoid to modify the

host behavior for its benefit.

The term "parasitoid" was first used by Reuter in 1913

(Hassel and Waage, 1984) to describe insects that develop as

larvae on the tissues of other arthropods, which they

eventually kill. Parasitoids have been described as

"predator-like" parasites.

Fall armyworm is attacked by a diverse complex of

natural enemies including 53 known species of parasitoids

(Ashley 1979, Gardener and Fuxa 1980). Several laboratory

studies have been carried out to assess the effectiveness of

selected parasitoids as biotic control agents of FAW

(Mitchell et al. 1984, Isenhour 1985, Pair et al. 1986a).

Archytas marmoratus (Townsend) is a tachinid parasitoid

(larval-pupal) of the Noctuidae in North and South America

and in the West Indies (Sabrosky 1978, Ashley 1979).

Numerous authors have reported its occurrence throughout the

southern United States (Quaintance and Brues 1905, Luginbill

1928, Vickery 1929, Bibby 1942, Parencia 1964, Shepard and

Sterling 1972). Findings by Gross et al. (1976) from

collections of 5th and 6th-instar larvae of H. zea and S.

frugiperda, respectively, on corn indicated that A.

marmoratus is a major parasitoid of these species,





64

particularly in south Georgia and north Florida.

Gross and Johnson (1985) discussed advances in large-

scale rearing and biological studies of A. marmoratus.

Gross et al. (1985) evaluated the performance of

mechanically extracted maggots of A. marmoratus against

larval populations of H. zea and FAW on whorl and tassel-

stage corn.

Evaluating the effects of a variety of corn on a

parasitoid of fall armyworm represents a tri-trophic

interaction, with corn as the producer, fall armyworm the

primary consumer and the parasitoid a secondary consumer.

Insect parasitoid-host relationships are often considered to

be simple, and compatibility of a parasitoid as a control

method with resistant varieties has been assumed.

Figure 13 illustrates interactions that occur in three

trophic levels. Price (1986) describes two kinds of plant

defense or plant resistance: INTRINSIC DEFENSE, where the

plant alone produces the defense through production of

chemicals such as toxins or digestibility reducers, or

through physical defense by trichomes or toughness and

EXTRINSIC DEFENSE of plants, when the plant benefits from

the natural or applied enemies of herbivores. He states

that plant breeders have emphasized the study of intrinsic

defense mechanisms; whereas, those studying biological

control of herbivores have emphasized the need for extrinsic

defense of plants. A dichotomy results because each of the































THIRD TROPHIC LEVEL


Extrinsic defense
of plant



SECOND TROPHIC LEVEL



FIRST TROPHIC LEVEL


FIRST TROPHIC LEVEL


Intrinsic defense
of plant


Figure 13. The kinds of interactions, direct and indirect, in a
typical food web, showing the relationships between intrinsic and
extrinsic defense of plants and the trade-offs between them. +, -,
0 indicate positive, negative or noeffects, the plants may impact
on the herbivore-natural enemy relationship. From Price (1986).


10





66

above mentioned independent disciplines is interested in

two trophic level systems. The two groups appear to be

melding at present and, during the last decade, the details

of direct and indirect effects of plants on herbivores and

their natural enemies at the chemical level are receiving

much attention (e.g. Bergman and Tingey 1979, Bell and Carde

1984, Barbosa and Letourneau 1988).

Host diet influences suitability of a host for a

parasitoid. Slansky (1986) discussed the nutritional

ecology of endoparasitic insects and their hosts. Campbell

and Duffey (1979) described a case of potential

incompatibility of plant antibiosis with biological control.

Effects of pest-resistant soybeans on the development of

hymenopterous parasitoids of Lepidoptera have been studied

(Yanes and Boethel, 1983; Powell and Lambert, 1984; Orr and

Boethel, 1985; and Beach and Todd, 1986). Rogers and

Sullivan (1986) studied the effects of two plant

introductions (PIs) on the lygaeid predator Geocoris

punctipes (Say). All of these except Slansky (1986) (where

nutritional ecology is discussed), document some adverse

effects on the development of beneficial species when hosts

or prey fed on pest resistant soybean genotypes.

A host may be suitable when feeding on one plant

species, but not another. The parasitoid Hyposoter exiguae

(Viereck) could not survive in H. zea larvae fed on

artificial diet containing even small amounts of the

alkaloid a-tomatine, but if cholesterol was added to the





67

diet, this deleterious effect was eliminated (Greaney et al.

1984). Bergman and Tingey (1979) reviewed various types of

interactions that occur between host plants, insect pests,

and their natural enemies. There is a direct influence of

plant hosts on natural enemies; volatiles, plant growth

characteristics, toxic and nutritional factors, and foliage

morphology all directly affect natural enemies. Doutt

(1964) discussed the directed orientation of predators and

parasites to host plants of their prey and the differential

attraction that occurs among crop varieties. Franklin and

Holdaway (1966) reported differential attraction of Lydella

grisescens Robineau-Desvoidy, a parasite of the European

corn borer, Ostrinia nubilalis (Hubner) to different maize

hybrids. Many predators and parasites use plant juices,

nectar, and pollen as sources of food and water. Q.

insidiosus a predator of H. zea, consumes juices and pollen

of corn and cotton (Dicke and Jarvis 1962).

Bergman and Tingey (1979) also pointed out that the

nutritional substrate offered by a host plant can indirectly

influence predator and parasite fitness in several ways;

prey confined to resistant hosts commonly experience reduced

growth rates, greater developmental time and mortality, and

decreased fecundity. Such dramatic alterations of

physiological processes may change the nutritional quality

of the prey as food/host for predators and parasites.

Gross and Pair (1986) reviewed the impact of endemic

parasitoids and predators as regulators of fall armyworm





68

populations and identified areas of research and development

that must be addressed before significant advances can be

made in importation and augmentation. Pair et al. (1986)

discussed the influence of four corn cultivars on FAW

establishment and parasitization. Gross (1988) studied

field survival and performance of mechanically extracted

maggots of A. marmoratus on FAW and indicated that good

field survival occurred.

Isenhour and Wiseman (1988) reported effects of

parasitism of the FAW by Campoletis sonorensis (Cameron) as

affected by host feeding on resistant Zea mays L. cv.

Zapalote Chico. The research described in this chapter

examined the use of mechanically extracted larvae of A.

marmoratus (Townsend), as a biological control agent against

FAW, is compatible with the use of a leaf-feeding resistant

cultivar of corn (MpSWCB-4).



Materials and Methods

Two varieties of corn, "Stowell's Evergreen" (SEG) and

MpSWCB-4 were planted on an experimental farm near Tifton,

Georgia and maintained according to recommended cultural

practices. Test plots (2 each year) were planted in May

1988 and 1989. There were ten replicates for each cultivar.

Each replicate contained paired rows of each cultivar.

Plants in test plot 1 were used for the A. marmoratus

performance study, by apply FAW neonate larvae to whorls of





69

the plants when they reached the 6-8 leaf stage. Plants in

test plot 2 were used at the 10-12 leaf stage.

The A. marmoratus (AM) colony used in the study was

maintained at the USDA Biology and Population Management

Research Laboratory. It had been established in 1981 from

fifth-instar CEW larvae occurring in mid-green tassel stages

of "Silver Queen" sweet corn near Tifton, Georgia, USA.

Additional AM adults, closing from pupae of CEW and FAW

that were collected in southern Georgia and northern

Florida, had been added intermittently to the culture.

Laboratory cultures were maintained on CEW larvae from the

Tifton laboratory colony (Young et al. 1976) reared by the

method of Burton (1969) on corn-soy-milk-solid diet (Burton

1970). AM adults were held in plyboard and screened cages

as described by Gross and Young (1984). They were provided

with sugar cubes and free water from saturated paper towels

in a 1.5 by 10 cm Petri dish during the 10 day pre-

larviposition period. A. marmoratus maggots were extracted

mechanically from fecund females as described by Gross and

Johnson (1985) and suspended in the desired volume of a

0.35% solution of hydroxyethylcellulose (Minidrift)

(Soilserv, Inc., Salinas, California.

The FAW larvae were also obtained from a colony

maintained at the USDA Biology and Population Management

Research Laboratory in Tifton, Georgia. Corn plants at the

6-8 and 10-12 leaf stage were used. Neonate FAW were

applied directly into the whorl using a mechanical larval






70

dispenser developed by Wiseman (Wiseman et al. 1980). FAW

eggs deposited on paper towels were placed in plastic bags

and held at 32oC until hatch. Larvae were removed by

inverting the bags and gently shaking the towel while it

remained in the bag, to dislodge larvae. Corn-cob grits

(grits-o'cobs No. 2040, Anderson's Cob Division, Maumee,

Ohio 43537) were mixed with the FAW larvae in a concentrated

form. The FAW-corn cob grits mixture was shaken and diluted

until about 20 FAW larvae per delivery were obtained. They

were then transported to the field for immediate dispensing

into the whorls.

All plants in both the 6-8 and 10-12 leaf stage of both

MpSWCB-4 and SEG were infested at the same time. The FAW-

grits mixture in the dispensing bottle was thoroughly mixed

and repeatedly agitated during the application process to

ensure standardized delivery of about 20 FAW neonates per

plant. Infestations were made in the morning, after most of

the dew had evaporated (ca. 9:00 am).

After 10 days, freshly prepared AM maggots in a mini

drift suspension were diluted until an eye dropper delivered

larvae at the rate of 25 larvae per drop. One drop of the

maggot suspension was applied directly into the centers of

the whorls of plants in each "test row" for each replicate,

12 plants per row, 10 replicates per variety. Host larvae

(FAW) within the whorl were exposed to AM maggots for 72

hrs, then the FAW were retrieved. The second row of each

replicate for each cultivar was left for rating leaf damage





71

at 14 days. Thirty FAW larvae were collected from each

replicate (1 tray) and placed individually on a modified

pinto bean diet in 30 ml plastic diet cups. Control FAW

larvae were obtained by harvesting 2 plants for each (AM)

untreated row of each replicate.

Larvae collected from the field were placed in Percival

Growth Chambers at 26.0 10 C, 14:10 L:D photoperiod, and

85 5% RH. Emergence of FAW and all parasites was noted,

and percent parasitization calculated. Data were analyzed

by PROC GLM (SAS) and percentage parasitization values were

changed via arcsin transformation prior to analyses (SAS

Institute 1989). Scheffe's method was used (p=0.05) to

separate means (SAS Institute 1989). This method of

separating means is considered to be conservative, and was

selected for that reason.


Results and Discussion

Table 5 lists parasites reared from fall armyworm

larvae collected from Stowell's Evergreen and MpSWCB-4 corn

in Tifton, Georgia during the month of June in 1988 and

1989. The tachinid A. marmoratus was applied after

mechanical extraction from fecund females, at the rate of 25

larvae per plant but all other parasites emerging from the

FAW larvae represent natural infestations.

In 1988, the naturally occurring ichneumonid parasitoid

Ophion flavidus Brulle occurred at much higher rates of

parasitization of FAW, than the artificially applied











TABLE 5. Parasites reared from fall armyworm larvae collected
in Tifton, Georgia, June 1988 and June 1989.



HOST PLANT: Corn

VARIETIES: Stowell's Evergreen and MpSWCB-4


Braconidae
Apanteles sp.
Cotesia marginiventris (Cresson)
Rogas laphygmae Viereck

Eulophidae
Euplectrus comstockii Howard

Ichneumonidae
Ophion flavidus Brulle

Tachinidae
Archytas marmoratus Townsend






73

parasitoid A. marmoratus, (Figure 14). There were higher

rates of parasitization by AM on the resistant variety

(MpSWCB-4) than on the susceptible variety SEG, on both

plots, but the differences were not statistically

significant (p= 0.05). This trend was reversed with Ophion,

which had significantly higher rates of parasitization on

the susceptible variety than on the resistant. It has been

suggested that Ophion may be in direct competition with

Archytas in the field (Gross, 1988). In addition, the FAW

may have been in the more desired developmental stage for

the maggots on the SEG than those on MpSWCB-4 since MpSWCB-4

causes a reduction in growth and development of FAW. There

were no significant differences between MpSWCB-4 and SEG at

the 6-8 leaf stage, when the total number of larvae

parasitized were analyzed. At the 10-12 leaf stage,

however, the differences were significant, and the trend of

parasite occurrence was higher overall on the susceptible

cultivar. Ophion flavidus numbers were higher in the

susceptible cultivar and contributed to making overall

parasitization rates in 1988 highest in SEG (S), (Figure

14).

Ophion flavidus parasitized 20% of FAW larvae collected

from SEG at the 10-12 leaf stage. This was the highest rate

of parasitism found in 1988 for a single parasitoid of FAW

larvae in the study. Archytas marmoratus had a high of 8%.

The highest total percentage parasitism was 31%. This

represented parasitization that occurred for only 3 days,






74

while FAW in the whorl had been exposed to parasitization by

other naturally occurring parasites prior to the application

of AM.

Figure 15 shows the percentage of total parasitization

by the parasitoids in the 1988 field study. Ophion flavidus

was responsible for as much as 73% of total parasitization,

and this occurred at the 10-12 leaf stage on SEG, while the

maximum contribution to total parasitization by AM was 37%

which occurred in plot 2, on MpSWCB-4. Other parasites,

namely Cotesia marginiventris (Cresson), Rogas laphvymae

Viereck, and Apanteles sp. together were responsible for as

much as 30% of parasitism at the 6-8 leaf stage on both SEG

and MpSWCB-4.

In 1989, Ophion was less prevalent than in 1988,

(Figure 16). Levels of parasitization by AM were higher

perhaps due to reduced competition by the absence of Ophion.

Parasitization by AM was highest on the 10-12 leaf stage of

SEG, and lowest on SEG at the 6-8 leaf stage. The trends

observed in 1988, with higher levels of parasitism on

MpSWCB-4 were not consistent in 1989. AM did significantly

better at the 6-8 leaf stage on MpSWCB-4 than on SEG,

parasitizing 13% and 8%, respectively, of FAW larvae. At

the 10-12 leaf stage, AM did better on SEG but not

significantly better.

A. marmoratus was responsible for over 95% of total

parasitism found on SEG at both stages of plant development

and 65-80% of parasitism on MpSWCB-4, Figure 17. The












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83

highest level of parasitism by Ophion in 1989 was 11%,

attained from FAW on MpSWCB-4 at the 10-12 leaf stage.

Compatibility of AM with the resistant corn variety

MpSWCB-4 is important. Performance of AM against FAW was

not significantly reduced at any time. In 1988 AM performed

consistently better on MpSWCB-4 than on SEG (but

statistically not significant). There can be a number of

explanations for the observed trend of improved performance

of AM on MpSWCB-4. AM maggots are not mobile. When applied

to the whorl, FAW larvae moving within the whorl due to non-

preference factors of resistance permit AM larvae

opportunity to parasitize FAW more easily than if the

resistance in the plants were not manifested. Thus as the

FAW larvae move more within the whorl of the resistant corn,

MpSWCB-4, than in the susceptible SEG, there is an increased

probability of attack by AM.

Rohlfs and Mack (1985) reported from field studies in

Alabama that AM was recovered from all sizes of FAW larvae

and the frequency of parasitism of large larvae was not

significantly greater than for small larvae during a 2-yr

period. They found that rates of adult emergence of the

competitor parasitoid 0. flavidus generally exceeded those

of AM by about 2-fold, but did not differ significantly

among the FAW larval developmental stages evaluated.

Furthermore, they added that 0. flavidus oviposited in FAW

larvae of all sizes, but large larvae were most frequently

parasitized.






84

Ophion and other parasitoids prevalent in 1988, were

less common in 1989. Rates of parasitism by Ophion were

approximately 2-fold higher than A. marmoratus in 1988,

while this was not the case in 1989. Archytas parasitized

20% of FAW larvae collected from SEG, at the 10-12 leaf

stage, while Ophion had its highest level of attack (11%) on

FAW on the 10-12 leaf stage of MpSWCB-4. The results of

this study indicate that both A. marmoratus and 0. flavidus

are capable of impacting populations of fall armyworm in

whorl stage corn. This study further indicates that the use

of the resistant corn cultivar MpSWCB-4 may enhance their

performance. There is the possibility that the use of a

resistant cultivar, such as MpSWCB-4, against the FAW which

has both non-preference and antibiotic mechanisms of

resistance could be antagonistic to enhancement of

biocontrol agents that attack later larval instars such as

AM or Ophion. Results of this study indicate compatibility

of the resistant cultivar and AM. Nonpreference factors may

stimulate the FAW to move about within the whorl, while

antibiosis tends to retard growth and development. A delay

in FAW development may not coincide with the most

susceptible stage of the larvae for parasitism as compared

to FAW developing at a faster rate on SEG. For parasites

that attack eggs or small larvae, the use of nonpreference

and antibiotic resistance such as that manifested by MpSWCB-

4 would be more compatible in combining these two control

tactics.












CHAPTER IV

CONSUMPTION, DEVELOPMENT AND FECUNDITY OF FALL ARMYWORM ON
STOWELL'S EVERGREEN A SUSCEPTIBLE CORN CULTIVAR AND
MpSWCB-4 A RESISTANT CULTIVAR AS AFFECTED
BY THE PARASITOID ARCHYTAS MARMORATUS

Slansky (1986) reviewed the impact of insect

parasitoids on the physiology and behavior of their hosts

within the context of the nutritional ecology of parasitoids

and their hosts. An insect host such as the FAW becomes the

living space for parasitoids and active defensive responses

against parasitoids occur at both behavioral and

physiological levels. Nutrient composition of the host

hemolymph (e.g., amino acids, proteins and carbohydrates)

and fat body (e.g., glycogen) are often altered after

parasitization, as are host hormones and metabolic rates

(Vinson and Iwantsch 1980).

Slansky (1986) stated "because the genetic fitness of a

parasitoid within a host is directly dependent on host

activities, natural selection will undoubtedly commonly

result in evolution of parasitoid-influenced changes in host

physiology and behavior that improve the parasitoids

fitness". Also mentioned is the fact that if sufficient

nutrients for complete parasitoid development are lacking,

then continued feeding by the host will be permitted and

perhaps stimulated above the unparasitized level by the