Measurement of and selection for insecticideresistance in various populations of beetarmyworm Spodoptera Exigua (Hubner)

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Authors Aldosari, Saleh, 1964-

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Measurement of and selection for insecticide resistance in various populations of beet armyworm Spodoptera Extgua (Hubner)

Aldosari, Saleh Abdullah, M.S.

The University of Arizona, 1990

U M I 300 N. Zeeb Rd. Ann Aibor, MI 48106

MEASUREMENT OF AND SELECTION FOR INSECTICIDE RESISTANCE

IN VARIOUS POPULATIONS OF BEET ARMYWORM

SPODOPTERA EXIGUA (HUBNER)

by

Saleh Aldosari

A Thesis Submitted to the Faculty of the

DEPARTMENT OF PLANT PROTECTION

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 9 0

2

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements of an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: flLO 0 S R X

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

T. F. WATSON 7, / J £-0

DATE

3

ACKNOWLEDGMENT

I would like to express my sincere thanks to Dr. T. F. Watson for granting me the opportunity of studying under him. His continued guidance, understanding, and encouragement helped to complete this study. I am also grateful to him for use of materials and equipment at his laboratory for this study. It has truly been a pleasure to work under his guidance.

I would like to express my special thanks *to Drs. Leon Moore and Paul Bartels for serving on my committee, and for their review of the manuscript. I am also grateful to Dr. Merritt R. Nelson, the Head of the Department of Plant Pathology for his help, guidance, suggestions, and encouragement during my study.

A special thanks to Dr. Sakuntala Sivasupramaniam for valuable suggestions on the design of my experiment and also for her assistance in setting up the laboratory techniques. Thanks are also due to Dr. Abdelgadir A. Osman for his help and suggestions during my study. I would like to thank Suzanne Kelly for offering and providing assistance in using the computer for graphs and probit analysis of the results. My special thanks are also extended to Justine Collins for providing the most needed last-minute help in printing the manuscript.

I would like to express thanks to the faculty, staff and students of the Entomology Department for their assistance in making my stay here enjoyable as well as interesting. I would like to offer my appreciation to my Government the Kingdom of Saudi Arabia for the financial support throughout my study. I would like to thank my family for their continual love, patience, encouragement, and for praying for me throughout my study at The University of Arizona. Finally, I thank God for making all of this possible and making my life enjoyable.

4

TABLE OF CONTENTS

PAGE

LIST OF ILLUSTRATIONS 6

LIST OF TABLES 7

ABSTRACT 8

INTRODUCTION 9

Biology of BAW 9 Cultural Control 10 Biological Control 10 Chemical Control 12

CHEMICAL INSECTICIDES 13

Organophosphate Insecticides 13 Carbamate Insecticides 14

The Mode of Action of Organophosphate and Carbamate Insecticides 14

A-Oxidation 15 B-Hydrolysis 15 C-Conjugation 15

Pyrethroid Insecticides 16 Synthetic Pyrethroids 16 The Mode of Action of Pyrethroids .... 17

RESISTANCE 18

Metabolic Resistance 20 Non-metabolic Resistance 21

A-Reduced penetration 21 B-Reduced target insensitivity 22

Monitoring-Survey and Selection for Resistance . .22

MATERIALS AND METHODS 25

Chemicals Used 25 Insect Strains 25 Rearing of Beet Armyworm 26 Bioassay Procedures 27 Evaluation of Cyfluthrin, Profenofos, and Methomyl Resistance in Field Populations . . . .28

Selection for Cyfluthrin Resistance 28

5

TABLE OF CONTENTS

PAGE

Cross-Resistance to Methomyl and Profenofos ... 29 Analysis of Data 30 Dosage Mortality 30

RESULTS AND DISCUSSION 31

Susceptibility to Cyfluthrin, Profenofos, and methomyl in Field Populations 31

Selection for cyfluthrin resistance 42 Cross-Resistance to profenofos and methomyl ... 48

CONCLUSION 53

REFERENCES 54

6

LIST OF ILLUSTRATIONS

Figures Page

1 Dosage-mortality lines for cyfluthrin on strains of beet armyworm collected from Yuma and Marana, relative to a standard laboratory strain 33

2 Dosage-mortality lines for profenofos on strains of beet armyworm collected from Yuma and Marana, relative to a standard laboratory strain 36

3 Dosage-mortality lines for methomyl on strains of beet armyworm collected from Yuma and Marana, relative to a standard laboratory strain 38

4 Dosage-mortality lines for cyfluthrin, profenofos, and methomyl on Marana strain of beet armyworms, relative to a standard laboratory strain 39

5 Dosage-mortality lines for cyfluthrin, profenofos, and methomyl on Yuma strain of beet armyworm, relative to a standard laboratory strain 40

6 Dosage-mortality lines for cyfluthrin on larvae of beet armyworm subjected to LDS0 pressure for 10 generations 45

7 Dosage-mortality lines for cyfluthrin, profenofos, and methomyl before and after 9 generations of selection pressure with cyfluthrin on BAW (Marana strain) 51

7

LIST OF TABLES

Table Page

1 Toxicity of cyfluthrin to two beet armyworm strains collected from different locations relative to a standard laboratory susceptible strain 32

2 Toxicity of profenofos to two beet armyworm strains collected from different locations relative to a standard laboratory susceptible strain 35

3 Toxicity of methomyl to two beet armyworm strains collected from different locations relative to a standard laboratory susceptible strain 37

4 Dosage-mortality Data cf cyfluthrin on the beet armyworm (Marana strain) subjected to LDfl0 pressure for several generations . 44

5 Relative Cross-Resistance to profenofos and methomyl in cyfluthrin-selected (Cs) larvae of beet armyworm (Marana strain) 50

ABSTRACT

8

A comparative study was performed to investigate the

tolerance levels of beet armyworm -to three insecticides,

cyfluthrin, profenofos, and methomyl. The field strains were

collected from Yuma and Marana,AZ whereas the susceptible

laboratory strain was obtained from California. Dosage-

mortality data were obtained by topical application on third

instar larvae. Compared to the susceptible strain, both Yuma

and Marana strains exhibited an increase in the LD50 to

cyfluthrin by 15.65 and 5.45-fold, respectively. Both strains

also exhibited an increase in the LD50 to profenofos and

methomyl by 14.10, 17.77 and 2.95, 8.07-fold, respectively.

The cyfluthrin-selected strain (Marana strain) tested for

cross resistance to profenofos and methomyl and exhibited an

increase in LDJ0 by 24.68 and 3.32-fold,respectively.

INTRODUCTION

9

The beet armyworm (BAW), Spodoptera exigua (Hubner), is an

important insect attacking many of the principal crops grown

in southern Arizona. The importance of this insect lies in its

occurrence on a wide variety of economic crops throughout the

year (Forst, 1954). Such crops include cotton, alfalfa, and

lettuce. It is believed that this insect was introduced from

southern Europe to the California Coast where it was first

described by L. F. Harvey (1976). The first reported

appearance of beet armyworm in Arizona occurred in 1916 at

Phoenix where larvae were found feeding on turnips (Campbell

and Duran, 1929). High populations often occur in Arizona in

the Spring and Fall months (Hills, 1968).

Biology of BAW:

According to Forst 1954, the four developmental stages of

BAW are egg, larva, pupa, and adult,each of which possesses a

unique characteristic(s). The eggs are ivory colored when

first laid, and within a day the color changes to light yellow

and later to black when the larvae are about to hatch. As a

larva develops the color changes from green to brown and

finally to dark-olive black. The larval stage can be divided

into four instars and its development largely depends on

climatic conditions, especially temperature and the

availability of food. During midsummer the larva may take 16

10

days to complete development, whereas in the winter it may

take 76 days (Campbell and Duran, 1929).

The Pupa of the BAW is fusiform in shape. The average

length is about 15 mm depending on the size and vigor of the

last larval stage. The color is light brown or chestnut, with

the margins of the abdominal segments dark red. The color is

almost black before the time of emergence to the adult stage.

The adult is a moth typical of most of the phalaenids. It

is slightly smaller, however, than most of the cutworm moths

that are so commonly seen and associated with this group. The

wing spread varies from 25 to 33 mm. The general appearance is

that of an inconspicuously marked grayish moth. The life cycle

of the BAW can be completed in 20 days or less depending on

the availability of food and optimum temperature.

Cultural Control:

Cultural Control is a major part of the integrated pest

management program to control BAW (Forst 1954). Sanitation

measures and elimination of weeds in the field may reduce many

breeding sites which serve as alternative hosts for the adult.

Biological Control:

Biological Control is another significant method

providing control of this insect. Many studies have been

conducted to evaluate control of beet armyworm by using

different biological control agents such as nematodes,

viruses, and parasites. The purpose of these studies was to

11

prove the effectiveness of these agents. Hara and Kaya, (1982)

reported the ability of Neoaplectan carpocapsae to invade and

reproduce in insecticide-killed beet armyworm larvae. They

first used N carpocapsae to infect the insect, then treated

them with trichlorfon, mevinphos, methomyl, fenvalerate, and

permethrin. The susceptibility of the BAW pupae to infection

by the nematode N carpocapsae was moderate compared with

Galleria mellonella and Pseudaletia unipuncta. The age of the

pupa and the magnitude of the dosage of the nematode did not

significantly affect mortality of both S exiaua and P

unipuncta (Kaya and Hara,1980). Kamionek (1979) demonstrated

that an organophosphate nematicide adversely affected the

development and reproduction of N carpocapsae when insects

were infected with the nematode prior to their treatment with

the chemical.

Nuclear Polyhedrosis Viruses, at present, are the most

prevalent of all virus types known. In 1988, Smith et al.

performed a study to compare the control of beet armyworm

larvae in chrysanthemum, Chrysanthemum morifolium (Ramatuelle)

in crops of different heights by application of comparable

virus rates with different types of application systems. The

authors concluded that both high-volume hand spraying and low-

volume spinning disk application systems are advisable for

Nuclear Polyhedrosis application to control S exiaua in

greenhouse chrysanthemums, with the latter system giving

12

marginally better results. Kaya and Grieve (1982) tested the

nematode Neoaplectana carpocapsae as a control agent of BAW.

They found that the prepupal and adult stages are the most

susceptible to nematode, though some pupae were infected. N

carpocasae appears to be a promising control agent for S

exiqua and other soil-pupating Lepidopterous insects.

Chemical Control:

Historically, many insecticides applied either as sprays

or dusts have been used in Arizona to control the beet

armyworm. Formulations such as paris green, sodium

fluosilicate, cryolite, DDT ,toxaphene, aldrin, dieldrin,

heptachlor, chlordane, and parathion, gave good control in the

past (Forst 1954). Due to the adverse effect on the

environment, organochlorine compounds have been removed from

the agricultural insecticide market. Consequently, control of

BAW has shifted to other insecticides, mostly carbamate and

organophosphate compounds and some pyrethroids.

Shorey (1963) obtained good control of beet armyworm on

cabbage and cauliflower with low rates of several

organophosphorus and carbamate insecticides. Bisabri-Ershadi

and Hejazi (1982) performed toxicological parameters of

Benzoylphenyl Ureas with beet armyworm larvae. They tested

these chemicals against BAW larvae in laboratory bioassays and

obtained LC50 values of 0.33 ppm with UC 62644, 0.27 ppm with

penfluron, 1.1 ppm with EL 1215, 1.4 ppm with diflubenzuron,

13

8.1 ppm with Bay Str 8514. Topical bioassays against fourth

instar larvae using penfluron, diflubenzuron, and Bay Str

8514, proved that penfluron has the highest toxicity.

CHEMICAL INSECTICIDES

Organophosphate insecticides:

In 1911, it was discovered that a combination of a

phosphorus atom with four dissimilar substituents results in

the formation of a chiral phosphorous center (Ohkawa, 1982).

Consequently , the organophosphates (OPs) replaced the

persistent Organochlorine compounds. The insecticidal action

of OPs was observed in Germany during World War II in a study

of materials closely related to nerve gases such as sarin,

soman, and tabun (Ware, 1989). Organophosphate is usually used

as a generic term to include all the insecticides containing

phosphorus. OPs have several common names such as organic

phosphates, phosphorus insecticides, nerve gas relatives,

phosphate, phosphate insecticides, and phosphorus ester or

phosphoric acid esters (Ware,1982). All of them are derived

from phosphoric acid and generally are the most toxic of all

pesticides to vertebrate animals (Ware, 1989). The most

commonly used organophosphate insecticides in agriculture are

the trialkylphosphates, diaklyphosphates, phosphorothiates,

and phosphorodithioates (Matsumura, 1975). O'Brien, 1978

14

categorized OPs into P(s) compounds and P(o) compounds.

Carbamate Insecticides:

Carbamate compounds were originally derived from natural

compounds. A botanical drug model that led to the development

of these insecticides was Physostigmene which was derived from

the Calabar bean Phvsosticrma venenosum. The first commercial

carbamate was N,N,dimethyl carbamate by CIBA-GEIGY in 1950.

Due to its extremely low toxicity, it had very little impact

on the agricultural industry. The first successful carbamate,

carbaryl, was introduced in 1956 (Ware, 1989) .

Mode of Action of organophosphate and carbeunate insecticides:

Both organophosphate and carbamate insecticides produce their

acute toxic actions by inhibiting acetylcholinesterase (ACHE) ,

and thus lead to an accumulation of acetylcholine at the

effector sites (Murphy, 1975; Matsumura, 1980; Magee; 1982) .

The organophosphorus compounds mimic the gross molecular shape

of their target enzyme acetylcholinesterase, and this

structural similarity is their most conspicuous feature

(Matsumura, 1975) . The acetylcholinesterase has two sites, the

"anionic site" and "esteric site". Ohkawa (1982) reported that

compounds having similar structure are subject to common

bitransformation reaction mediated largely by mixed-function

oxidases, arylesterases, and glutathione S-transferases.

There are various metabolic reactions in living organisms

that are caused by OPs.

15

A- Oxidation;

This reaction is the metabolic transformation of

thiophosphoryl (P=S) esters to the corresponding phosphoryl

(P=0) esters in many organisms (Ohkawa et al.,1976).

Unexpectedly, this reaction enhances the toxicity of the

chemical (Fest and Schmidt, 1973). The P=S esters being poor

anti-cholinesterases, mixed-function oxidase-mediated

activation is necessary for intoxication by thiono-phosphates

(Ohkawa, 1982).

B - Hydrolysis:

The organophosphorus compounds undergo enzymatic

hydrolysis at the ester or acid anhydride bonds, this reaction

is mediated by anylesterase, which hydrolyze P-nitrophenyl

acetate at a faster rate than the butylate and is not

inhibited by Paraoxon (Ohkawa, 1982).

C - Conjugation:

Conjugation reactions are energy dependent by natural or

foreign compounds or their metabolites combine with readily

available, endogeneous conjugating agents to form conjugants.

Examples of conjugation agents include glucuronic acid,

sulfate, acetyl, methyl, and glycine (Wilkinson, 1976).

Glutathione -S- hydrolases (GSH) dependent transferase

reaction is characterized by its direct enzymatic attack on

the substrate. Working with a rat liver and an insect tissue,

Fukami and Schishido (1966) demonstrated the importance of the

16

role played by GSH in the dealkylation of methyl and ethyl

parathion by a soluble enzyme. A characteristic feature of

organophosphate and carbamate insecticides is their structural

"fit" with the CHE enzyme. Carbamate and OPs inhibit (CHE) and

behave in a similar manner in biological systems. Yet, there

are some significant differences. For instance, some

carbamates are excellent inhibitors of aliesterase

(miscellaneous aliphatic esterases whose exact functions are

not known), and their selectivity against the CHE of a

different species is more pronounced. Also, CHE inhibition by

carbamate is apparently reversible. When CHE is inhibited by

a carbamate insecticide, it is described as carbamylated

whereas in organophosphate poisoning it is referred to as

phosphorylated (Ware, 1989). In 1977, Suksayretrup and Plapp

demonstrated that when flies were poisoned with methamidophos

(0,S- dimethyl phosphoramido thioate) and methyl parathion

(0,0- dimethyl-O-P-nitrophenyl phosphorothioate) in vivo, they

underwent the typical range of symptoms associated with ACHE

inhibition.

PYRETHROID INSECTICIDES

Synthetic Pyrethroids:

Pyrethrum is made from the dried flowers of Chrysanthemum

cinerariaefolium which belong to the family Compositae.

Pyrethroids are synthetic analogues of pyrethrum which include

Pyrethrin, Cinerins and Jasmolin (Matsumura, 1975). The

17

insecticidal principals are called Pyrethrin. All pyrethroids

are esters and are thus composed of an alcohol and an acid

moiety. Head (1973) stated that the natural pyrethrin is made

up of six components derived from a combination of two

different acids and three different alcohols. The synthetic

pyrethroids are more stable than the natural pyrethrin due to

the lower reactivity of the side chains. In the last decade,

many of synthetic pyrethroids have been discovered by various

workers (Elliott, Janes, and Potter, 1978)

Mode of Action of Pyrethroids :

Pyrethroids are known to cause temporary paralysis.

According to Sawicki (1962), the term "knockdown" was

indicated to describe the fast and normally reversible

paralysis caused by application of low doses of pyrethroids to

flies. The primary studies on the mode of action of

pyrethroids were performed by Narahashi (1962a, 1962b, 1971).

He demonstrated the direct reaction of pyrethroids on the

nerve membrane producing symptoms of hyperexcitation. He

reported that allethrin, (RS) -3-allyl-2-methyl-4-oxocyclopent-

2-enyl (RS)-cis/trans chrysanthemate, interferes with the

normal axonal conduction to the central nervous system of

arthropods by altering the permeability of the nerve membrane

to sodium and potassium ions.

Miller and Adams (1982) demonstrated that the

neurophsiological responses vary with the type of nerve

18

element assayed, temperature, and the insecticide applied.

Burt and Goodchild (1971) studied the molecular structure of

pyrethroids and its corresponding toxicity to whole

cockroaches and to their giant fiber axons. They concluded

that the nerve axon may not be the major site of action for

pyrethroids, and that additional processes seem to occur

outside the nervous system.

RESISTANCE

In the last several decades, many arthropod species have

developed resistance to insecticides. Today, insects display

resistance to every class of insecticide, one of the most

challenging problems facing entomologists. Insect species

reported to be resistant have nearly doubled during the last

decade, rising from 224 species in 1970 to 428 in 1980

(Georghiou and Mellon, 1983). In 1986, Georghiou stated that

97% of the resistant species were of agricultural or medical

importance due to the very high reliance on chemical

control.Resistance is frequently seen among orders of insects

including major pests such as Diptera (35% of the total),

Lepidoptera (15%), Homoptera (10%) and Heteroptera (4%).

The term "resistance" has been variously defined by

different authors. Roush (1980) stated that resistance is a

genetic phenomenon which usually results from pesticide

selection pressure. Resistance is also defined as a

19

significant increase in insensitivity of a population towards

an insecticide leading to diminished control (Georghiou and

Mellon, 1983). The pattern of resistance in field populations

can be observed and understood by documenting the local and

regional histories of pesticide-use, (Georghiou, 1972, 1986).

Roush and Mckenzie (1987) suggested that the rate of

development of resistance in the field is dictated by the

initial frequency, dominance of resistance alleles, relative

fitness of the genotype and the population structure.

Considering the different classes of insecticides, 62% of

resistant species reported are resistant to cyclodiene

insecticides, 52% to DDT, 47% to organophosphates and lower

percentages in more recently introduced pyrethroids and

carbamates (Georghiou, 1986). A thorough understanding of the

basis of differential toxicity, development of effective

synergists and knowledge of mechanisms of resistance is

essential to undertake this problem (Sawicki and Lord, 1970).

It is generally accepted that the development of resistance to

insecticides in insects may be influenced by many factors such

as decreased penetration of the toxicant, increased

detoxication, and increased insensitivity at the site of

action (Farnham, 1971; Briggs et al. 1976; Devries and

Georghiou, 1981; Bull, 1981). Joint action of resistance

factors has been reported by many workers. The

phosphorotriester hydrolases and glutathione S-transferases

20

responsible for organophosphorus insecticide resistance in

certain insect species detoxify the insecticide and often

function along with other resistance mechanisms such as mixed

function oxidase, altered cholinesterases and penetration. The

significance of each mechanism varies with the structure of

the insecticide and the stage of development of the insect

(Dauterman, 1983) . Farnham (1971) performed experiments to

elucidate the genetics of a resistant strain of house fly by

isolating the individual resistant chromosome. He concluded

that pyrethroid resistance was attributable to four factors:

(A) Pen, which reduced the rate of penetration, but gave no

resistance to DDT, (B) Kdr-NPR. showed a great delay in

knockdown by natural pyrethrin and DDT. (C) PY-ses. gave

resistance to natural pyrethrin, and (D) PY-ex, gave slight

resistance to natural pyrethrin but was found to give strong

resistance when synergized.

Metabolic Resistance :

Metabolic resistance to insecticides is most important

with biodegradable insecticides such as organophosphates and

carbamates, and probably with synthetic pyrethroids (Plapp and

Wang, 1983). Because metabolic factors generally play the most

significant role in the overall biochemical defense system for

developing resistance in insects, it is the most studied

aspect of resistance mechanisms (Oppenoorth and Welling,

1976). Plapp and Wang (1983) stated that the detoxication

21

mechanisms involved include aliesterases, carboxylesterases,

mixed function oxidases and 6SH transferases.

Non-Metabolic Resistance:

According to Matsumura (1983) non-metabolic mechanism of

resistance may lead to high levels of resistance especially

when combined with metabolism processes. There are two non-

metabolic factors which lead to the development of resistance

: reduced penetration and reduced insensitivity of the target

site.

A - Reduced penetration:

The idea that reduced penetration of insecticides through

the cuticle of insects may be a cause of resistance is not

new. Sanchez and Sherman (1966) reported that resistance to

DDT in a strain of tobacco budworm is partially attributed to

reduced penetration. In fact, insecticides within the same

group may show significant variation in their capacity to

penetrate through the cuticle of the same insect species.

Holden (1979) confirmed this phenomenon by reporting that the

penetration (approximately 60% trans, 40% cis isomers)

through the cuticle of the American, Cockroach Periplanta

americana L, was significantly greater than that of

cypermethrin. Szeicz et al. (1973) demonstrated that rates of

penetration of carbaryl, Malathion, Endrin, and DDT were rapid

and reached higher concentrations in internal tissues of

tobacco budworm larvae of the susceptible strain. Matsumura

22

(1983) reported that penetration at the target site appears to

be more important as a determinant of resistance with stable

insecticides (eg. Dieldrin) whereas cuticular penetration

would play a significant role in conferring resistance to

easily metabolized insecticides such as Malathion.

B - Reduced target insensitivitv:

Matsumura (1983) stated that target or site insensitivity

is the resistance caused by reduction in the amount of the

toxicant reaching the "target" and reducing the sensitivity of

this site to the toxicant. The term "site insensitivity" was

coined to describe the knockdown resistance (KDR) factor which

renders the nervous system insensitive to DDT and pyrethroids

(Wilkinson, 1983; Farnham, 1977). Matsumura (1983) mentioned

that site insensitivity as a mechanism of resistance is

effective and important since it protects the sensory system

for a period of time that is long enough for the poison to be

eliminated through metabolism. Matsumura (1983) reported that

organophosphates and carbamates may cause the development of

target insensitivity due to modifications of ACHE and the

target enzyme of these insecticides.

Monitoring survey and Selection for resistance:

Meinke and Ware (1978) evaluated tolerance levels of the

beet armyworm to methomyl in three strains obtained from

different locations in Arizona (Safford, Mesa, Yuma). They

found that Safford and Mesa strains were 3 times as tolerant

23

as the Yuma strain. Cobb and Bass (1975) compared two strains

from California and Florida and found that the California

larval strain was more, susceptible to carbaryl, methyl

parathion and EPN than the Florida strain. Larvae from both

cultures were equally susceptibile to methomyl. They also

reported that the most toxic insecticide to the resistant

Florida strain was a mixture of methyl parathion and methomyl.

Michael and Trumble (1989) developed a pheromone trap

technique (ATT) for monitoring insecticide resistance in field

populations of beet armyworm. It allows monitoring of

susceptibility levels over time, as well as in different

locations that have different histories of insecticide

selection pressure. Yoshida and Parrela (1987) detected

resistance to methomyl in beet armyworm collected from

floricultural crops in Florida and consequently, the use of

this insecticide was discontinued. Brewer and Trumble (1989)

detected resistance to methomyl in beet armyworm collected

from a tomato field in Orange County (California) in Fall

1986, but the resistance level declined four fold by the

following spring. Chaufaux and Ferron (1986) reported

resistance to deltamethrin in Central America. In selection

experiments, Alava and Lagunes (1976) found that beet armyworm

developed resistance to Malathion, Endrin, and methomyl. A

recent study (Brewer and Trumble 1990) revealed that

resistance to fenvalerate, permethrin and methomyl has

24

developed among adult male of BAW populations in California

(U.S.A), Baja California Norte and Sinaloa (Mexico). The last

study regarding the tolerance levels of beet armyworms to

insecticides in Arizona was done by Meinke in 1977. Since that

time little information has been added regarding insecticide

resistance in Spodoptera species. The use of insecticides

including cyfluthrin, profenofos, and methomyl is increasing

in Arizona. This extensive use of insecticides may lead to

development of resistance rendering these insecticides

ineffective. Consequently, this study was initiated to :

evaluate current tolerance levels of the beet armyworm to

cyfluthrin, profenofos, and methomyl in Yuma and Marana

strains in Arizona; investigate the capacity of a field strain

of BAW to develop resistance to cyfluthrin with selection

pressure in the laboratory; and, evaluate cross-resistance to

profenofos and methomyl in cyfluthrin-selected strain.

MATERIALS AND METHODS

25

Chemicals used

Three insecticides were used in this study :

1) Technical grade profenofos [0-(4-bromo-2-chlorophenyl) 0-

ethyl S-propyl phosphorothioate; Curacron, 90.9%, CIBA- GEIGY

CORP., Agricultural Division, Greensboro, NC 27409].

2) Technical grade methomyl [S-methyl N-[(methylcarbamoyl)oxy

]-thioacetimidate; Lannate or Nudrin, 98.7%, E. I. Du pont De

Nemours Co].

3) Technical grade cyfluthrin, Cyano (4-fluro-3-phenoxyphenyl)

methyl 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropane-

carboxylate; Baythroid, 94 %, Mobay Chemical Corp,

Agricultural Chemical Division, Kansas City, MO 46120].

All chemicals were prepared in redistilled acetone and

refrigerated at 5°C. Fresh preparations were made at three-

monthly intervals.

Insect Strains

Field collections of beet armyworm from two agricultural

areas in southern Arizona were made during October and

November 1989 to establish initial laboratory cultures. The

Yuma Agricultural Experiment Station is an area of high

insecticide usage. The Marana Agricultural Experiment Station

26

has a lower use rate of insecticides. Hence, both areas were

chosen as collection sites.

A susceptible beet armyworm strain used in baseline

studies was obtained from Sandoz Crop Protection Corp., Wasco,

California. This strain had been maintained on artificial

pinto bean diet for 6 years and had not been exposed to

insecticides during that period.

Rearing of beet armyworm strains

The rearing procedure adopted in this study was adapted

from the method used in rearing the pink bollworm. Pectinophora

aossvpiella (saund) (Osman, 1989). The rearing method was

modified to suit BAW laboratory culturing.

Adult moths were held in 230 ml paper ice cream cups (S

308 Sweetheart) and fed with 10% sugar solution, diffused

through cotton plugs (30 mm length, Johnson and Johnson)

inserted through an opening in the mesh-lined cover. These

containers were also used to hold pupae. The container lids

covered with squares of coarse white papers which served as

oviposition sites were weighted down with metal rings. The wax

coating on the inside of the cups helped prevent oviposition

on container walls. The paper squares used for egg sheets were

washed in 7.0% formalin solution for 5 min to reduce virus

infection and subsequently washed with tap water. After

drying, the egg sheets were transferred into plastic cartons

to permit egg development and hatch. Neonate larvae were

27

transferred with a camel's hair brush into 29.6 ml (1 oz)

plastic creamer cups, half filled with a modified lima bean

diet (Patana 1969), and capped with paper lids. Two to four

larvae were placed in each cup. For the first three days,

these larvae were placed in an environator at 30±1°C, with a

light: dark cycle of 16:8. Pupae were collected from these

cups, placed in 230 ml paper ice cream cups, covered with the

mesh-lined lids, and allowed to develop into adults. When a

large number of larvae were needed to perform the dosage-

mortality studies, 8 to 12 larvae were placed in each cup.

Most of the growth stages of beet armyworm were held at 27±1

°C and 30 to 60% relative humidity.

Bioassay Procedures

Third-instar larvae weighing 18±3 mg were used in all

bioassays. To determine the average weight, 15 to 20 percent

of larvae designated for testing that day were weighed prior

to treatment. Twenty to thirty larvae per each of three to

four replicates were used at each of 5 to 6 concentrations;

controls were treated with acetone.

Dosing was done by topical treatment of 1 fil of the

insecticide solution to the dorsal surface of the larval

throax, and was made with a model M ISCO Micro-applicator

equipped with a 0.25 CC syringe and a 27 gauge needle that was

blunted and polished. This was accomplished without removal of

the larva from its diet cup. Treated larvae were held in these

28

cups at 27±1°C. Preliminary range finding experiments were

conducted to determine those dilutions that would give a 10 to

90% mortality. This ensured good distribution for log probit

analysis.

Evaluation of Profenofos, Methorny1, and Cyfluthrin Resistance

in Field Populations

Two strains of beet armyworm collected from natural

populations were employed to determine levels of

susceptibility to profenofos, methomyl, and cyfluthrin.

Between October and November 1989, a large number

(approximately 500) of larvae was collected from each of the

locations mentioned above. The larvae were transferred into

plastic storage boxes (31.8 x 17.1 x 9.5 cm) with

approximately 1 cm of diet on the bottom; when returned to the

laboratory, they were transferred to the plastic ice cream

cups as mentioned earlier.

Pupae of the different strains were placed in separate

bioassay cups and held at 27±1°C. Five to six-day old larvae

(averaging 18±3 mg) were used in all the bioassay tests. The

resistance in field populations was evaluated in the second

generation of both strains because of an insufficient number

of larvae in the F1 generation. The field and laboratory

strains were treated with profenofos, methomyl, and

cyfluthrin.

Selection For Cyfluthrin Resistance

29

This experiment was conducted to investigate the

potential for cyfluthrin resistance in a field population of

BAW. The strain used in this study was obtained from Marana.

This strain was maintained in the laboratory for two months

prior to the initiation of any chemical treatment. Insecticide

applications on larvae of the Marana strain were made by the

method previously described. Development of resistance was

monitored by calculating a dosage-mortality regression (ld-pm)

line at every generation except F5. Selection pressure was

applied every generation. Larvae were exposed to an LD80 dose

to maintain pressure towards resistance; only those

individuals surviving a dose giving approximately 70-90

percent mortality were used in the selection pressure. Slopes,

LD50 and LD95 values, derived from the ld-pm lines, were

compared between generations to follow the development of

resistance. This test was conducted for nine generations.

Cross-resistance to methomyl and profenofos

Since selection with a chemical can result in increasing

tolerance to other chemicals. A test was performed against the

F9 generation of the Marana resistant strain of BAW. Dosing

with methomyl and profenofos and LD50 determinations were

followed as previously described. Data were compared to LDS0

values from a 1989 field population of BAW from Marana,

Arizona. The purpose of this study was to determine if there

was any cross-resistance between methomyl and profenofos.

30

Analysis of Data

Dosage Mortality:

The results obtained from the dosage mortality

experiments were evaluated using log dosage-probit mortality

(ld-pm) regression lines described by Finney (1971). LDS0 and

LD95 values and their respective 95% confidence intervals,

slopes, and standard errors were derived from the analysis .

Separation of the 95% fiducial limits at the LD50 values was

used to indicate significant differences (P < 0.05) .

Resistance Ratios (RR):

RR's were calculated by dividing the LD50 value obtained

in the test population by that of either susceptible or F2

population .

RESULTS AMD DISCUSSION

31

Susceptibility TO Cyfluthrin, Profenofos, and Methomyl in

Field Populations.

Results of probit analysis on response of BAW to

cyfluthrin are summarized in Table 1 and graphically presented

in Fig. 1. Based on 95% non-overlapping confidence intervals

for LD50s, both field-collected populations were significantly

different from the Laboratory susceptible strain. The

resistance ratios of the field strains ranged from 5.45 to

15.65 and the populations tested demonstrated different levels

of susceptibility to cyfluthrin. As shown in Table 1, the Yuma

strain had a significantly higher LD50 value compared to the

Marana strain. This may be due to the difference in use of

insecticides between the two areas (T.F. Watson, Univ. of

Arizona, personal communication, 1990). The susceptible strain

had the lowest LDS0, 0.20.

The regression lines of all strains, including the

susceptible strain demonstrated relatively low slope values of

1.09 to 2.13. This indicates that a high degree of genetic

heterogeneity exists among these strains.

The field populations also exhibited variable degrees of

resistance to profenofos (Table 2; Fig.2). The resistance

ratios (RR's) were high between the two strains and the

laboratory susceptible strain. At the LDS0, the populations

32

Table 1. Toxicity of cyfluthrin to two Beet Armyworm strains

collected from different locations relative to a standard

Laboratory susceptible strain!/.

Strain No. of Slope±SE2/ LDS03/(95% CI) LD95 RR4/

larvae

Yuma 600 2.13±0.21 3.13(2.22-3.33) 18.50 15.65

Marana 360 1.12±0.14 1.09(0.55-1.11) 31.32 5.45

Suscept 420 1.09±0.14 0.20(0.11-0.22) 6.53 1.0

1/ Data analyzed by computer probit analysis (Finney, 1971).

2/ SE standard error.

3/ Lethal dosage n q / q .

4/ RR : resistance ratio = LDS0 of Fx / LD50 of susceptible

strain.

33

-2 -1 LOG 10 DOSE (ug/lorvo)

Figure 1. Dosage mortality lines for cyfluthrin on strains of

beet armyworms collected from Yuma and Marana, relative to a

standard laboratory strain.

34

relative to the Laboratory strain were 14.10 and 17.77-fold

for Yuma and Marana, respectively. The slopes of ld-pm lines

were flat and similar which suggests that they are

heterogenous strains. The LD 5 0 of the Yuma strain, 27.93 n g / g ,

was less than that of the Marana strain, 35.19 ng/g, both of

which were significantly higher than the susceptible strain,

1.98 n g / g .

Table 3 and Figure 3 present results of the toxicity test

with methomyl against the two field and the laboratory

susceptible strains. These data showed large differences in

the LD50 values of all strains. The Marana strain showed the

highest resistance to methomyl (LDS0 = 143.80), followed by

Yuma strain (LD50 = 52.57). The susceptible strain exhibited

an LD50 of 17.81/xg/g. The resistance ratios vary from 2.95 to

8.07-fold. The slopes were 1.72, 1.23 and 1.62 for the Yuma,

Marana and laboratory strains, respectively, suggesting

heterogeneity among all.

The data collected from these studies verified that

resistance to cyfluthrin, profenofos, and methomyl existed in

both field populations of the BAW surveyed. Because resistance

is mainly a consequence of insecticide use, comparisons of

resistance levels in different locations is valuable for

decision-making in insect control. In both areas, Yuma and

Marana,the LD50s were higher than those detected by Meinke and

Ware (1978). It seems that the continuous selection pressure

35

Table 2. Toxicity of profenofos to two Beet Armyworm strains

collected from different locations relative to a standard

laboratory susceptible strain!/.

Strain No. of Slope±SE2/ LD503/(95% CI) LD95 RR4/

larvae

Yuma 600 1.75±0.21 27.93(21.11-33.88) 242.36 14.10

Marana 360 1.72±0.18 35.19(23.88-52.22) 315.78 17.77

Suscept 480 2.96±0.28 1.98(1.66-2.22) 7.12 1.0

1/ Data analyzed by computer probit analysis (Finney, 1971).

2/ SE standard error.

3/ Lethal dosage in ng/g*

4/ RR : resistance ratio=LD50 of Fx / LD50 of susceptible

strain.

36

-1 0 LOG 10 DOSE (ug/lorvo)

Figure 2. Dosage mortality lines for profenofus on strains of

beet armyworms collected from Yuma and Harana, relative to a

standard laboratory strain.

37

Table 3. Toxicity of methomyl to two Beet Armyworm strains

collected from different locations relative to a standard

Laboratory susceptible strainl/ .

Strain No. of Slope±SE2/ LD503/(95 % CI) LDW RR*

larvae

Yuma 600 1.72±0.26 52.57(41.11-61.66) 474.65 2.95

Marana 360 1.23+0.16 143.8(106.66-208.8) 3068.78 8.07

Suscept 360 1.62±0.24 17.81(14.44-21.11) 183.04 1.0

1/ Data analyzed by computer probit analysis (Finney, 1971).

2/ SE standard error.

3/ Lethal dosage in n g / g .

4/ RR : resistance ratio= LD50 of Fx / LDS0 of susceptible

strain.

38

-1.0 •0.5 0.0 0.5 LOG 10 DOSE (ug/lorvo)

Figure 3. Dosage mortality lines for methomyl on strains of

beet armyworms collected from Yuma and Marana, relative to a

standard laboratory strain.

39

98

am.

MCTH

3 2 0 1 1 LOG 10 DOSE (ug/larva)

Figure 4. Dosage mortality lines for cyfluthrin, profenofos,

and methomyl on Marana strains of beet armyworms, relative to

a standard laboratory strain.

-3 -2 -1 0 LOG 10 DOSE (ug/larvo)

1

Figure 5. Dosage mortality for cyfluthrin, profenofos, and

methomyl on Yuma strains of beet armyworms, relative to a

standard laboratory strain.

41

in the field has resulted in the high level of resistance

detected in this study. The geographic and temporal variation

of beet armyworm resistance to these insecticides suggests the

need for proper insecticide management to avoid higher levels

of resistance which may lead to economic crop loss. The

resistance management strategies that should be considered

include the use of economic thresholds, proper timing of

insecticidal applications and the proper chemical group(s) of

insecticides.

The slope of the ld-pm lines is a measure of the

variation in response to a toxic chemical (Hoskins, 1960). In

many insect species resistance is due to just one or a closely

linked cluster of genes. Initially selection causes the ld-pm

lines to flatten out but gradually steepens as the population

approaches homozygosity for resistance. (Hoskins and Gordon,

1956) .

When resistance is polyfactorial, many gene combinations

are selected simultaneously so that population heterogeneity,

and thus slope, do not change noticeably under selection

pressure (Brown, 1959) .

This study (Tables 1, 2 and 3) showed similar slopes for

all beet armyworm strains studies. The response of the

California strain to methomyl, profenofos and cyfluthrin

suggest that this population is highly heterogeneous. The flat

curves seem to be a normal response of beet armyworm

42

population to methomyl which attacks several enzymes in the

insect at one time (Meinke, 1977). In the present study the

data obtained suggest that a substantial level of resistance

has developed among the BAW populations to cyfluthrin,

methomyl and profenofos. The results suggest that resistance

appears to be developing more rapidly in populations that,

historically, have been exposed to a high level of intensive

insecticidal pressure.

Selection For Cyfluthrin Resistance

Selection of BAW larvae with cyfluthrin at LDao level

produced strong tolerance towards this compound. After nine

generations of continuous selection pressure, the LD50 in the

F9 generation had increased 70.72-fold compared to the LD50 of

the F2 generation (Table 4). Within the course of this study,

ld-pm lines were calculated during all the generations except

F5. These lines are presented in Figure 6. The LD50 and LD95

levels of the F2 generation were 1.09 and 31.32 Mg/g*

respectively, and the slope was 1.12. These values established

a base-line or reference point by which future changes in

cyfluthrin susceptibility can be compared. The median lethal

dose increase from 1.09 nq/q in the F2 to 2.25 M9/9 in the F3

generation. The LD95 in the F3 was 14.65 nq/q, a 2.13-fold

decrease compared to the LD95 of the F2 (31.32). The slope

increased from 1.12 in F2 to 2.02 in the F3 generation. By F4

generation the LD50 increased by 3.66 fold and the LD95

43

decreased by 1.8-fold compared with the F2 generation. The

slope increased from 2.02 in the F3 to 2.66 in the F4. The ld-

pm line for the F6 generation exhibited marked increases in

the LDS0 and LD95, 19.26 and 575.22, respectively, relative to

prior generations. The slope for the F6 (1.11) had decreased

slightly from the F4 (2.66). The RR (Resistance Ratio) had

increased by 17-fold compared with the F2 generation. Table 4

shows slight changes in the slope of succeeding generations

inspite of selection pressure as explained by Meinke and Ware

(1977). The same authors suggested that a flat curve seems to

be the normal response of a beet armyworm population. By the

F7 generation, however, tolerance to cyfluthrin was greatly

increased and the LD50 reached 63.27 pg/q which represent a

more than 58.05-fold gain as compared to LDS0 of F2 generation.

Considering the LD95, the F7 generation exhibited a 23.52-fold

increase compared to that of F2 generation. Similarly, the LD50

decreased from 63.27 in F7 to 57.97 /itg/g in the F8 generation.

The LD95 decreased from 736.89 to 560.72 ng/q in the F8

generation and the slope increased from 1.54 to 1.66. The

highest LD50 level occurred in generation 9, at the termination

of the study. When compared to the F2 generation, the LDS0 rose

from 1.09 to 77.09 fig/g, a 70.72-fold increase. By F9 the

slope increased to 2.37 and the LD95 decreased to 379.18. The

response by BAW larvae to cyfluthrin selection progressed

through a series of changes that closely resemble a population

Table 4. Dosgae-mortality Data of cyfluthrin on the Beet Armyworm (Marana) subjected

to LDg0 pressure for several generations!/.

Generation No. of Slope±SE2/ LD503/(95%CI) LD.5 RR4/

larvae

F2 360 1.12±0.14 1.09(0.55-1.11) 31.32 1.0

F3 480 2.02±0.21 2.25(1.66-2.22) 14.65 2.06

F4 400 2.66±0.27 3.99(3.33-4.44) 16.56 3.66

F6 600 1.1110.11 19.26(15-23.88) 575.22 17.67

F7 480 1.54±0.17 63.27(48.0-76.66) 736.89 58.05

F8 480 1.66±0.21 57.97(43.33-70.55) 560.72 53.18

F9 480 2.37±0.24 77.09(65.55-90) 379.18 70.72

F10 480 1.4510.19 59.06(42.77-73.33) 795.53 54.1

1/ Data analyzed by computer probit anslysis (Finney, 1971).

2/ SE standard error.

3/ Lethal dosage in /xg/g.

4/ RR: resistance ratio=LD50 of Fx / LD50 of susceptible strain. ̂

45

•2 -1 0 LOG 10 DOSE (ug/lorvo)

Figure 6. Dosage mortality lines for cyfluthrin on larvae of

the beet armyworm subjected to LDM pressure for 9 generations.

46

demonstrating resistance to the selecting agent. The F3

generation showed a 2.06-fold increase in tolerance to

cyfluthrin and an LD50 of 2.25 ng/g which is still well within

the range of susceptibility. Brown and Pal (1971) stated that

at the beginning of the selection process, a slight increase

in LD50s may be independent of specific genes for resistance.

The term " vigor tolerance " has been applied to describe this

phenomenon by Hoskins and Gordon (1956). This expression

implies that weaker individuals become eliminated in the early

generation of selection while the stronger individuals, being

more fit and showing increased vigor, survive. This effect

(vigor tolerance ) was exhibited by the F6 and was especially

evident at the LD95 level.

Since the slope of the ld-pm line measures the variance

in response to the toxin (Hoskins 1960), a steep slope is the

reaction from a homogeneous population, whereas a shallow or

flat slope shows a broad range in response, or heterogeneity.

The latter effect was seen in the F2 and F6 generations. The

F3 and F4 showed a sharp increase towards homogeneity as the

slope doubled compared to the F2 generation. There was a 3.66-

fold increase in the LD50s as evidenced by a marked shift of

the ld-pm line to the right. These changes in LDSOs and slopes

may be indicative of a population developing specific

resistance. When this occurs, ld-pm lines become flatter as

the LD50 increases (exhibited by the F2), then become steeper

47

as the population becomes homogeneous for resistant

individuals (Hoskins and Gordon, 1956). The steep line in the

F4 generation probably represents the elimination of the very

susceptible insects. When the population becomes more

homogeneous for resistant individuals, the LDS0 level should

increase. It was noted that during selection pressure in the

F7, new dilutions containing higher concentrations of

cyfluthrin were needed to attain 80 percent mortality. This

was manifest in the F9 generation which has been a common

occurrence in other resistance studies (Brown and Pal, 1971).

Georghiou and Taylor (1976) stated that in most cases of

laboratory selection, resistance evolves gradually at first

and then develops at a faster rate, depending on the

phenotypic expression of the R gene (s) in the resistant

homozygote. Although the slope in the F6 had decreased, the

population showed that the development of resistance to

cyfluthrin was occurring. An explanation for the flattened

slope may be that the population has not become pure for the

resistance factor, thus more selection was needed. This is

evidenced by the significant increase in the LD50 demonstrated

by the F7 generation compared to the F6. In fact, the level of

tolerance increased 58.05-fold from the base-line level

established by the F2 generation. When selection for

resistance was stopped, the LDS0 decreased from 77.09 (F9) to

59.06 (F10) . The slope decreased from 2.37 to 1.45 whereas the

48

LD95 increased from 379.18 in F9 to 795.53 f J tg /g . The RR dropped

from 70.72 to 54.1-fold.

If the cyfluthrin selected strain from the present study

(LD50 77.09 nq/q) is compared to a standard susceptible

laboratory strain (LD50 0.20 ng/g) from California, it would

exhibit >385-fold increase. This further demonstrates the high

level of resistance to cyfluthrin presently found in the Cs

strain, and provides evidence that the mechanism for

resistance is a heritable one. Resistance to pyrethroids and

DDT, has been linked in insects possessing "target site

insensitivity" or Kdr-like mechanisms (Plapp 1976, Elliott, et

al.1978, Beeman 1982). In contrast, Meinke and Ware (1977)

stated their failure in developing resistance to methomyl upon

selection pressure in the laboratory in Marana, Yuma, and Mesa

strains. In the present study, it was found that the Marana

strain has become highly resistant to cyfluthrin under

laboratory selection pressure. When a cross-resistance study

with Cs strain was made, it showed high levels of resistance

to methomyl and profenofos.

Cross-Resistance To Profenofos and Methomyl

Cross-resistance to profenofos and methomyl was induced

in BAW larvae by selection in the laboratory with cyfluthrin

for ten generations to a level of 54.1-fold cyfluthrin

resistance. When compared to a susceptible field strain, the

differences in tolerance at the LDS0 level to profenofos and

49

methomyl were 3.32 and 24.68-fold, respectively. The Cs strain

which became resistant during selection, also became highly

resistant to other insecticides such as profenofos and

methomyl. The values of LDS0 for both profenofos and methomyl

were 117.06 and 3549.87 nq/q, respectively. The Cs strain was

treated with profenofos and methomyl. The slope of ld-pm line

of profenofos increased from 1.72 in F2 generation to 2.99 in

the F10 generation whereas the slope of ld-pm line of methomyl

decreased from 1.23 to 1.16.

To evaluate the status of cyfluthrin, profenofos, and

methomyl as control agents of beet armyworm in Arizona,

several variables should be taken into consideration. The fact

that the BAW is an active insect pest and has several host

plants provide this pest with a wide range of movement and

exposure to a variety of habitats with varying amounts of

insecticide pressure. Georghiou (1972) states "under field

conditions, it is well known that species with alternate hosts

are slower in developing resistance than strictly one-host

species". Hills (1968) found that wild hosts play an

insignificant role in maintaining the major population numbers

in southern Arizona. He also found that most of the population

comes from irrigated areas under cultivation. Since most BAW

come from the relatively small area under cultivation,

cultural practices and rational use of insecticides may be of

great importance in determining the speed at which resistance

Table 5. Relative cross-resistance to profenofos and methomyl in cyfluthrin-selected

(Cs)l/ larvae of Beet Armyworm (Marana strain)2/.

Gen Insecticide No. of Slope±SE3/ LDS04/(95% C.I) LD̂ SR5/

larvae

F2 Cyfluthrin 360 1. 12±0. 14 1. 09(0.55-1.11) 31. 32 1. 0

F10 Cyfluthrin 480 1. 45±0. 19 59. 06(42.77-73.33) 795. 33 54. 18

F2 Profenofos 360 1. 72±0. 18 35. 19(23.88-52.22) 315. 78 1. 0

F10 Profenofos 480 2. 99±0. 28 117. 06(105-130) 414. 00 3. 32

F2 Methomyl 360 1. 23±0. 16 143. 8(106.66-208.8) 3068. 78 1. 0

F10 Methomyl 480 1. 16±0. 15 3549. 87(2694.44-4473.3) 91916. 87 24. 68

1/ Selected at LD0O pressure during 9 generations.

2/ Data analyzed by computer probit analysis (Finney, 1971).

3/ SE standard error.

4/ Lethal dosage in jtxg/g.

5/ RR: resistance ratio=LD50 of Fx / LD50 of susceptible strain. ui

51

mn

s MCTHP2

-1 0 1 LOG 10 DOSE (ug/lorvo)

Figure 7. Dosage mortality lines for cyfluthrin, profenofos,

and methomyl before and after 9 generations of selection

pressure with cyfluthrin on BAW (Marana strain).

52

develops. Many economic crops are good refuges for BAW larvae.

Since the BAW is generally a secondary pest in crops such as

cotton and alfalfa (Eveleens, van den Bosch, and Ehler, 1973) ,

little research has been performed on this insect. Moreover no

economic thresholds have been established. The potential use

of parasites for partial BAW control seems promising.

According to Fullerton (1977), fifty to sixty percent

parasitization of BAW larvae on mid-season sugar beets was

observed in central Arizona in 1976. The significance of

establishing economic threshold levels and using natural

control to its fullest extent can not be over emphasized. Such

a trend will lead to a substantial reduction in using chemical

control and consequently slow down development of resistance

to cyfluthrin, profenofos, and methomyl in BAW populations.

53

CONCLUSION

The BAW can cause major economic loss to a variety of

different crops. Presently, the insecticides, cyfluthrin,

profenofos, and methomyl, are all used in Arizona to control

this pest. The two populations tested seem to have a high

degree of heterogeneity as indicated by the relatively flat

dosage-mortality curves. A 70.7-fold increase in tolerance to

cyfluthin was developed after nine generations compared to the

level obtained at the second generation. This investigation

demonstrates that the BAW has the ability to become resistant

to pyrethroids (cyfluthrin) in a relatively short period of

time provided that the population is exposed to a high degree

of selection pressure.

Cross-resistance to both profenofos and methomyl was

detected in the cyfluthrin selected strain. This investigation

demonstrated that increasing levels of resistance to

cyfluthrin appears to accompany an increase in resistance to

profenofos and methomyl. It is evident that field selection

with a pyrethroid insecticide could ultimately lead to higher

levels of resistance in other organophosphate or carbamate

insecticides. These findings solidify previous thoughts that

a good pest management program should always resort to

pesticides only when economic thresholds are approached.

54

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