btcottoncov1

United States

Department of

Agriculture

Agricultural

Research

Service

ARS–154

January 2001

Bt Cotton

Management of the

Tobacco Budworm-Bollworm

Complex

United States

Department of

Agriculture

Agricultural

Research

Service

ARS–154

January 2001

Bt Cotton

Management of the

Tobacco Budworm-Bollworm

Complex

D.D. Hardee, J.W. Van Duyn,

M.B. Layton, and R.D. Bagwell

Abstract

Hardee, D.D., J.W. Van Duyn, M.B. Layton, and R.D.

Bagwell. 2000. Bt Cotton & Management of the

Tobacco Budworm-Bollworm Complex. U.S. Depart-

ment of Agriculture, Agricultural Research Service,

ARS–154. 40 pp.

Preservation of Bt technology is critical for cotton

producers across the U.S. Cotton Belt because of

increasing insecticide resistance and production costs.

Frequent introduction of new transgenic cotton

varieties creates a need to continuously evaluate their

cost-effectiveness and develop efficient plans for their

deployment. This publication explains how Bt cotton

is developed, how it controls insect pests, and how it

can most effectively be used in insect pest manage-

ment. Restrictions and limitations to the use of Bt

cotton are discussed, such as insects’ development of

resistance to it and approaches to preserving the

technology for long-term profits.

Audiences for the publication consist of research and

extension entomologists in the public and private

sectors, consultants, and cotton producers.

Keywords: bollworm, Bt cotton, budworm, cotton,

Heliothis virescens, Helicoverpa zea, refuge, resistance

monitoring, transgenic cotton

Mention of trade names, commercial products, or

companies in this publication is solely for the purpose

of providing specific information and does not imply

recommendation or endorsement by the U.S. Depart-

ment of Agriculture over others not mentioned.

This publication reports research involving pesticides.

It does not contain recommendations for their use nor

does it imply that uses discussed here have been

registered. All uses of pesticides must be registered by

appropriate state or Federal agencies or both before

they can be recommended.

While supplies last, copies of this publication may be

obtained at no cost from D.D. Hardee, USDA–ARS,

P.O. Box 346, Stoneville, MS 38776, e-mail:

[email protected]. Copies also available from F.L.

Carter, National Cotton Council, P.O. Box 820285,

Memphis, TN 38182.

Copies of this publication may be purchased from the

National Technical Information Service, 5285 Port

Royal Road, Springfield, VA 22161; telephone (703)

605–6000.

Issued January 2001

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the

basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital

or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require

alternative means for communication of program information (Braille, large print, audiotape, etc.) should

contact USDA’s TARGET Center at (202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Office of Civil Rights, Room 326–W, Whitten Building,

1400 Independence Avenue, SW, Washington, D.C. 20250–9410 or call (202) 720–5964 (voice and TDD).

USDA is an equal opportunity provider and employer.

Acknowledgments

Advisory panel on resistance management plans:

C.T. Allen, University of Arkansas; J.S. Bacheler, North

Carolina State University; J.H. Benedict, Texas A&M

University; J.R. Bradley, Jr., North Carolina State

University; M.A. Caprio, Mississippi State University;

T. W. Fuchs, Texas A&M University; D.D. Hardee,

USDA–ARS; M.B. Layton, Mississippi State Univer-

sity; B.R. Leonard, Louisiana State University; P.M.

Roberts, University of Georgia; R.H. Smith, Auburn

University; and J.W. Van Duyn, North Carolina State

University.

The following individuals provided invaluable input

during multiple reviews of this publication’s content

and technical accuracy:

J.J. Adamczyk, Jr., USDA–ARS; C.T. Allen, University

of Arkansas; G.L. Andrews, Mississippi State Univer-

sity; J.S. Bacheler, North Carolina State University; J.H.

Benedict, Texas A&M University; M.L. Boyd, Univer-

sity of Missouri; J.R. Bradley, Jr., North Carolina State

University; E. Burris, Louisiana State University; M.A.

Caprio, Mississippi State University; F.L. Carter,

National Cotton Council; B.L. Freeman, Auburn

University; T.W. Fuchs, Texas A&M University; L.D.

Godfrey, University of California at Davis; F.L. Gould,

North Carolina State University; M.A. Karner, Okla-

homa State University; G.L. Lentz, University of

Tennessee; B.R. Leonard, Louisiana State University;

S.R. Matten, U.S. Environmental Protection Agency;

W.J. Moar, Auburn University; J.W. Mullins, Monsanto

Corporation; R.D. Parker, Texas A&M University; P.M.

Roberts, University of Georgia; M.E. Roof, Clemson

University; C.G. Sansone, Texas A&M University; R.W.

Seward, University of Tennessee; J.E. Slosser, Texas

A&M University; R.H. Smith, Auburn University; S.D.

Stewart, Mississippi State University; N.P. Storer,

North Carolina State University; and D.V. Sumerford,

USDA–ARS.

The authors also express their appreciation to K.R.

Ostlie, W.D. Hutchison, and R.L. Hellmich, authors of

“Bt Corn & European Corn Borer.” 1997. University of

Minnesota, St. Paul, NCR Publication 602, for allowing

use of ideas and text.

This publication arose from a collaborative multistate

and multiagency effort to provide timely guidelines

for cotton producers, crop consultants, extension and

industry personnel, and agricultural agency adminis-

trators on recommended ways to deploy Bt cotton

technology. Since this technology is changing rapidly,

the publication will be updated as needed.

All Bt cotton plants contain one or more foreign

genes derived from the soil-dwelling bacterium,

Bacillus thuringiensis; thus, they are transgenic

plants.

The insertion of the genes from B. thuringiensis

causes cotton plant cells to produce crystal

insecticidal proteins, often referred to as Cry-

proteins. These insecticidal proteins are effective

in killing some of the most injurious caterpillar

pests of cotton, such as the larvae of tobacco

budworms and bollworms. This new technology

for managing insect pests was approved for

commercialization in the United States by the

U.S. Environmental Protection Agency (EPA) in

October 1995 and is now available from several

seed companies in this country, as well as in

many other cotton-growing countries around the

world.

Cotton varieties containing the Cry1Ac Bt protein

provide protection against three major U.S.

Bt Cotton

Management of the

Tobacco Budworm-Bollworm

Complex

Tobacco budworm,

Heliothis virescens

(F.), larva

cotton is one of the first crop

protection products from biotechnology.

cotton pests—tobacco bud-

worms, bollworms, and pink

bollworms. Bt cotton also

reduces survival of other

caterpillar pests such as beet

armyworms, cabbage loopers,

cotton leafperforators, fall

armyworms, southern army-

worms, and soybean loopers.

The protection it provides

against tobacco budworms,

pink bollworms, and European

corn borers is greater than the

protection provided by the

most effective foliar insecti-

cides. Unfortunately the

protection it affords cotton

against bollworms is generally

proper use and long-term

preservation of this valuable

technology.

Frequent introduction of

new transgenic cotton

varieties creates a need to

continuously evaluate their

cost-effectiveness and

develop efficient plans for

their deployment. A goal of

this publication is to answer

questions about the technol-

ogy by explaining how Bt

cotton is developed, how it

controls insect pests, and

how it can be most effec-

tively used in insect pest

management. Restrictions

on and limitations to the use

of Bt cotton are discussed,

such as insects’ develop-

ment of resistance to it and

approaches to preserving

the technology for long-

term profits.

Tobacco budworm, Heliothis

virescens (F.), and bollworm,

Helicoverpa zea (Boddie),

cause more damage to

cotton than any other insect

pest in the U.S. Cotton Belt.

The combined cost of

controlling these pests and

the losses they inflict on

cotton production exceeds

$300 million a year. Insecti-

cides used against tobacco

budworms and bollworms

Tobacco budworm,

Heliothis virescens

(F.), moth

less than that provided by

registered insecticides.

The preservation of the Bt

technology is critical because (1)

bollworms, tobacco budworms,

and pink bollworms continue to

develop resistance to foliar-

applied insecticides, (2) use of

insecticides is tied to ecological

concerns, and (3) Bt genes are

valuable resources. This publi-

cation focuses on how Bt cotton

affects the tobacco budworm

and bollworm. It is intended to

provide information that will

guide producers, research and

extension entomologists, con-

sultants, and industry in the

he insect-disease-causing organism

Bacillus thuringiensis (Bt) is a naturally

occurring soilborne bacterium found

worldwide. A unique feature is its produc-

tion of crystal-like proteins that selectively

kill specific groups of insects and other

organisms. When the insect eats these Cry-

proteins, its own digestive enzymes

activate the toxic form of the protein. Cry-

proteins bind to specific receptors on the

intestinal walls and rupture midgut cells.

Susceptible insects stop feeding within a

few hours after taking their first bite, and,

if they have eaten enough toxin, die within

2 or 3 days.

Different Bt strains produce different Cry-

proteins, and there are hundreds of known

strains. Scientists have identified more

than 60 types of Cry-proteins that affect a

wide variety of insects. Most Cry-proteins

are active against specific groups of in-

often create other problems, such as higher

populations of beet armyworms and cotton

aphids and an increased pesticide load in the

environment. Frequent exposure of insect pests to

insecticides results in the development of insecti-

cide resistance, which reduces the overall effec-

tiveness of available insecticides, increases crop

losses, and leads to higher pest control costs and

lower farm profits.

The severity of tobacco budworm and bollworm

infestations and resistance to synthetic insecti-

Bollworm larva

feeding in boll

cides vary across the Cotton Belt, both between

and within the states. Because of this variation

and the price of the technology, not all areas of

the Cotton Belt are able to economically justify

the use of Bt cotton. However, where insect

infestations are severe, Bt cotton offers a new

management tool for producers, helps ensure

against yield loss in the presence of heavy infesta-

tions of insecticide-resistant tobacco budworms,

and aids in reducing bollworm damage.

Bt—What Is It?

sects, such as the larvae of certain kinds of flies,

beetles, and moths. For example, Colorado potato

beetle larvae are affected by Cry3A proteins;

Cry1Ac is used against tobacco budworms; and

European corn borers can be killed with Cry1Ab,

Cry1F, Cry1Ac, and Cry9c proteins. Other Cry-

proteins are active against mosquito larvae, flies,

or even nematodes. Some Cry-proteins have been

used for more than 30 years in various liquid and

granular formulations of natural Bt insecticides,

mainly to control caterpillars on a variety of

crops. The Bt cotton varieties presently used

against tobacco budworms, bollworms, and

certain other caterpillars produce the Cry1Ac

protein.

overcomes most of the aforementioned limita-

tions. The plant-produced Bt proteins are pro-

tected from rapid environmental degradation

since they are not directly exposed to the environ-

ment. Incomplete coverage is not usually a

problem because the plants produce the proteins

in all tissues where larvae feed, thus ensuring

that the larvae will eat the Cry-protein. The

protein is always present whenever newly

hatched larvae feed, eliminating the timing

problem associated with foliar application. The

result is that Bt cotton has a built-in system that

efficiently and consistently delivers Cry-toxins to

the target pests from the time a newly hatched

larva takes its first bite (fig. 1).

Bt cotton offers a vastly improved method for

delivering Cry-insecticides to target insects,

compared to traditional Bt sprays. Bt cotton may

also be considered a form of host plant resistance,

in that the Cry-protein trait is carried in the

plant’s genes, as is traditional plant resistance to

insects.

The Why’s and How’s of Creating

Bt Cotton

ioinsecticides like Bt that are sprayed on

crops may perform as well as synthetic

insecticides in very limited situations, but the

performance of Bt insecticides has been inconsis-

tent in many instances.

The erratic performance in cotton is attributed to

four reasons:

• The toxin is rapidly degraded by ultraviolet

light, heat, high leaf pH, or desiccation.

• Caterpillars must eat enough treated plant

tissue to get a lethal dose of the toxin, since

the toxin has no contact effect.

• The sites where tobacco budworms and

bollworms feed are difficult to cover with the

foliar-applied sprays.

•

Cry-proteins are less toxic to older larvae.

A cotton plant modified to produce Cry-protein

within the plant tissues that caterpillars eat

Figure 1. Mode of action for

toxin after eaten by a tobacco budworm larva.

Modified with permission from Ostlie et al. 1997.

Biotechnologists created Bt cotton by inserting

selected exotic DNA, from a Bt bacterium, into

the cotton plant’s own DNA. DNA is the genetic

material that controls expression of a plant’s or an

animal’s traits. Following the insertion of modi-

fied Bt DNA into the cotton plant’s DNA, seed

companies moved the Cry-protein trait into high-

performance cotton varieties by traditional plant

breeding methods. Agronomic qualities for yield,

harvestability, fiber quality, and other important

characteristics were preserved at the same time

the Cry-protein gene was added to commercial

varieties.

The three primary components of the genetic

package inserted into cotton DNA include:

Protein gene. The

gene, modified for im-

proved expression in cotton, enables the cotton

plant to produce Cry-protein. The first varieties of

cotton produced in the United States contained

one Cry-protein gene—Cry1Ac. Other varieties

contain a “stacked” gene complex, for example—

one gene for insect control (Cry1Ac) and one

gene to protect the cotton from application of the

herbicide glyphosate. Future cotton varieties may

include these genes, other genes that allow the

plant to produce different Cry-proteins, or insecti-

cidal proteins from sources other than

Bt.

There

are many possible combinations for crop im-

provement traits.

Promoter. A promoter is a DNA segment that

controls the amount of Cry-protein produced and

the plant parts where it is produced. Some pro-

moters limit protein production to specific parts of

the plant, such as leaves, green tissue, or pollen.

Others, including those used in

cotton and

certain

corn varieties, cause the plant to

produce Cry-protein throughout the plant. Pro-

moters can also be used to turn on and turn off

protein production. Current varieties of

cotton

produce some

protein throughout the growing

season.

Genetic marker. A genetic marker allows re-

searchers to identify successful insertion of a

gene into the plant’s DNA. It also assists plant

breeders in identifying and developing new cotton

lines with the

gene. A common marker is an

herbicide tolerance gene linked to the

gene.

Following a transformation attempt to place the

and marker gene into the plant’s DNA, plants are

treated with herbicide. Plants that were success-

fully transformed have the

gene and the

herbicide resistance gene and will survive herbi-

cide treatment; plants without the marker gene,

and hence without the linked

gene, will be

killed by the herbicide.

This genetic package—a Bt gene plus a promoter

and marker—can be inserted into cotton plant

DNA through a variety of plant transformation

techniques. Transformed plants may be affected

by the genetic package, as well as the location of

the new genes in the plant DNA. The insertion

site may affect Bt protein production and other

plant functions as well. So biotechnology compa-

nies carefully scrutinize each transformation to

ensure adequate production of Bt protein and to

limit possible negative effects on agronomic

traits.

Following a successful transformation, plants are

entered into a traditional backcross breeding

program with the variety chosen to receive the

foreign Bt gene package. The final product, a Bt

cotton variety, is developed after four or five

backcross generations. Even though the new

transgenic Bt cotton variety may be named after

the parent variety, agronomic qualities can be

considerably different.

The Safety of Bt and Bt Cotton

Controlling Tobacco Budworms and

Bollworms: Does Bt Cotton Do the Job?

hen compared with other

insecticide management

practices, Bt cotton dramatically

improves the control of tobacco

budworms and, to a lesser extent, the

control of bollworms. For example,

chemical insecticides, including some

new chemical classes, often control

from 70 to 95 percent of a susceptible

tobacco budworm population. As

indicated in table 1, the level of

tobacco budworm control achieved

with Bt cotton can be very dramatic.

Bt cotton varieties may provide more

than 98 percent control of tobacco

budworm throughout the growing

season. For growers whose cotton is

plagued by high densities of insecti-

cide-resistant tobacco budworms, Bt

cotton is a very welcome technology.

Bollworm/budworm

eggs on a leaf. Courtesy

of S. Stewart.

efore registering Bt cotton, EPA reviewed

data on Bt insecticides that had been accu-

mulated for decades. Bt Cry-proteins were found

to be toxic only to certain insect groups and to

have no known negative effects to humans,

domestic animals, fish, wildlife, or other organ-

isms. EPA exempted Bt-produced Cry-proteins

from the requirement of tolerance in food because

of their history of safety and because they de-

grade rapidly in the environment. These Cry-

proteins are considered among the safest and

most environmentally friendly insecticides

known.

Bt cotton is less effective against bollworms. Still,

it can eliminate as many as 60 to 90⫹ percent of

the bollworms infesting a cotton field (table 1).

When there are high numbers of bollworms on Bt

cotton during the bloom stage, growers may need

to apply one or more supplemental insecticide

treatments to prevent economic damage. This has

been well documented through field research.

Fortunately, the bollworm can still be managed

more effectively and inexpensively with currently

available insecticides—except perhaps in South

Carolina where resistance to insecticides has been

detected.

Bollworm moths lay their eggs on cotton plant

terminals, leaves, buds, and flowers. As the eggs

hatch, the larvae may move into open blooms

where they feed on flower parts, including

pollen, that are known to have a lower level of

Cry-protein than other plant parts. This feeding

on less toxic parts may result in lower mortality.

The bollworm in addition is naturally more

tolerant of Cry1Ac Bt protein than the tobacco

budworm. These two factors help explain why

more bollworms than budworms survive on Bt

cotton. Moreover, Cry-protein expression in Bt

cotton decreases about 80 days postplanting,

which may allow higher survival of bollworms.

This late decline may also reduce Cry-protein

effectiveness against other insect pests that occur

later in the growing season and feed on mature

leaves.

Table 1. Survival of tobacco budworms, bollworms, and fall armyworms on

and non-

cotton genotypes

Percent survival

Insect 1994 1995

Tobacco budworm

cotton leaf 1 0

cotton square 2 0

on non-

cotton leaf 86 84

on non-

cotton square 69 67

Bollworm

cotton leaf 7 23

cotton square 5 4

on non-

cotton leaf 80 74

on non-

cotton square 63 52

Fall armyworm

cotton leaf 61 76

cotton square 33 25

on non-

cotton leaf 76 92

on non-

cotton square 45 42

Source: Modified and reprinted with permission from Jenkins et al. 1997.

obacco budworms and bollworms are not the

only insect pests that attack cotton. Unfortu-

nately, the Cry1Ac protein has essentially no

effect on many of them. Pests that Bt cotton does

not directly affect include boll weevils, cotton

aphids, cotton fleahoppers, cutworms, spider

mites, stink bugs, tarnished plant bugs, thrips,

and whiteflies. In some caterpillar species, Bt

cotton may provide only 10 to 50 percent control.

This partial suppression may be cause for concern

in the later years of an insect resistance manage-

ment program because it does not provide a high-

dose strategy (see Glossary) for insects such as

beet armyworms, fall armyworms (table 1),

southern armyworms, soybean loopers, and

yellowstriped armyworms. Bt cotton at this time

provides good to excellent control of cabbage

loopers, cotton leafperforators, European corn

borers, salt marsh caterpillars, and cotton square

borers. Future varieties of Bt cotton may produce

different Cry-proteins or other new toxins that

will control a wider variety of pests.

The effect of Bt cotton on some insect pests may

be indirect. For example, Bt cotton does not

directly affect the cotton aphid, but reductions in

insecticide use against tobacco budworms and

bollworms allow more of the aphid’s natural

enemies to survive, and they in turn reduce aphid

numbers.

On the other hand, reducing the amount of foliar

insecticide may also allow other pests normally

controlled by the insecticides to become more

abundant. Boll weevils, stink bugs, and plant

bugs, for example, have by chance been con-

trolled by foliar sprays applied against tobacco

budworms or bollworms. So, reduction in the use

of insecticides on Bt cotton has allowed these

pests to increase in some areas. Offsetting this

disadvantage, however, is Bt cotton’s usefulness

where the boll weevil has been eradiated.

Tar nished plant bug

nymph

Southern armyworms.

Courtesy of R. Smith.

Controlling Other Insects

any studies have shown that Cry1Ac in

Bt cotton is highly selective because it

kills only certain caterpillar species. Bt cotton

has minimal or no effect on beneficial insects,

including honey bees, lady beetles, spiders, big-

eyed bugs, pirate bugs, and parasitic wasps.

However, laboratory research has shown that

Cry1Ab protein can indirectly affect green

lacewing larvae that eat Bt-killed caterpillars. It

is not known if Cry1Ac in Bt cotton has a

similar effect on lacewing larvae. Theoretically, Bt

cotton may indirectly lower the general abundance

of some beneficial insects, since it causes caterpillar

populations to decline, resulting in less food for the

predators, parasites, and the pathogens that attack

them. Offsetting this effect is the positive influence

gained from reducing conventional broad spectrum

insecticide use in Bt cotton. The overall balance of

these contrasting influences is currently unknown

and is difficult to predict.

Does Bt Cotton Affect

Beneficial Insects?

Big-eyed bug

Lacewing larva

Table 2. Yield comparisons between

and non-

cotton

Yield (lb lint/acre)

cotton non-

cotton

Year (unsprayed) (sprayed)

1994 1369 1392

1995 1465 1425

Source: Reprinted with permission from Jenkins et al. 1997.

s is the case with most new technology, Bt

cotton offers value to the cotton farmer in

specific circumstances (table 2). Information on

economic benefits is limited due to the short time

the technology has been available and the many

new cotton varieties introduced each year. Re-

cently, the technology fee and the seed cost for Bt

cotton have decreased, which has affected the

economics of growing it. Yield data are available

from federal and university entomologists and

agronomists, as well as seed companies, in nearly

every cotton-growing state.

Comparisons of the Bt and non-Bt cotton varieties

generally show that Bt cotton offers an economic

advantage in instances where effective insecticid-

al control of certain caterpillar pests is difficult to

achieve or is very costly.

Examples of such situations

include

• insecticide-resistant to-

bacco budworms or boll-

worms

• high populations of sus-

ceptible tobacco budworms

or bollworms, such as in

outbreak seasons or during

the initial phases of boll

weevil eradication

• situations where a properly timed and applied

insecticide management program cannot be

achieved, such as in fields that do not allow

the proper operation of air or ground sprayers,

in remote fields, or in cases where cotton

acreage exceeds the amount of equipment or

personnel dedicated to insecticidal control

• situations where insecticidal control is exces-

sively costly (for example, more than $40/

acre), as may be the case when high infesta-

tions are coupled with newer, expensive

insecticide products—even though the insecti-

cide program may protect the crop

• situations where eliminating early tobacco

budworm sprays allows survival of beneficial

insects that reduce the risk of pest infestations

The Value of Bt Cotton to the

Cotton Farmer

associated with higher insecticide use (as with

beet armyworms or aphids).

These situations suggest that income from re-

duced insecticide input, along with higher yields

as a result of less insect damage, offset the tech-

nology fee and favor Bt cotton. However, if

infestations of tobacco budworms or bollworms

are low, or the yield of the Bt variety used is low,

the technology fee may exceed the value of the Bt

toxin. Also, a conventional insecticide spray

program may allow the farmer to grow certain

high-yielding cotton varieties that perform better

than available Bt varieties. Some economic com-

parisons show little or no economic advantage to

using Bt cotton (table 3), whereas the economic

returns in other circumstances have been positive

(table 4). Even in the areas that economically

favor Bt cotton, however, there are often situa-

tions where high-yielding, non-Bt varieties

grown under conventional spray programs

provide equal or greater economic returns than

Bt varieties.

Regardless of whether or not a particular variety

contains the Bt gene, yield potential continues to

be the primary consideration when selecting a

cotton variety. The Cry1Ac gene in Bt cotton is

only one of thousands of genes that affect the

yield and other characteristics of different cotton

varieties. Growers who are selecting cotton

varieties should carefully consider the traits of

each, including yield performance, relative

maturity, fiber quality, ability to withstand ad-

verse weather, and harvestability.

Bt cotton may offer value to the cotton farmer in

ways that are hard to measure by short-term

economic comparisons. Using insecticides in-

volves complying with certain laws, such as

worker protection and pesticide label restrictions.

Compliance often makes the grower’s job more

difficult and increases the risk of consequences

arising from noncompliance. Insecticide use in

sensitive areas—next to schools, fish ponds,

dwellings, medical facilities, roads—can be a

concern to the grower and his or her neighbors. If

legal and social risks are a concern, Bt cotton may

have value to the grower by reducing these risks.

Adopting Bt cotton may result in a more efficient

enterprise, while maintaining a high level of

insect pest control. Insect management with

insecticides can be time-consuming and involve a

significant amount of labor and equipment. If Bt

cotton reduces the insect control burden and

decreases the need for labor and equipment,

these resources may be diverted to other farm

obligations.

Bt cotton may reduce potential resistance to foliar

insecticides in tobacco budworms and

bollworms. Since resistance genes selected as a

result of one insecticide may be eliminated by a

different and unrelated insecticide, the rotation of

toxins, including Bt Cry-protein, may be able to

slow the selection of genes for resistance to any

single toxin. Farmers will be obliged to include a

non-Bt refuge to accompany any Bt cotton

planted (see refuge section, p. 23). Use of a

sprayed refuge provides an excellent opportunity

to use newer insecticide classes, as well as

effective older chemistries, as aids in reducing

development of insecticide resistance in tobacco

budworms and bollworms and in adopting

resistance management plans for other

chemistries. Maintaining effective refuge areas

that are close to Bt cotton also helps slow

resistance to the current Cry1Ac toxin cotton

varieties and may decrease resistance to other Bt

toxins as they are introduced.

Table 3. Cost of

cotton vs. non-

cotton in North Carolina, 1999

Expense

cotton Non-

cotton

Technology fee* $19.14 $ 0.00

Control costs 5.78 @ 0.76 applications/season 19.88 @ 2.65 applications/season

Damage† 0.00 @ 4.41% damage 8.50 @ 5.5% damage

Extra scouting 3.00 0.00

Total $27.92 $28.38

* Projected average cost; varies by seed rate and row spacing.

† Difference in late-season bollworm damage under grower conditions (N=614 fields, 1996–1998).

Source: J. Bacheler, unpublished data.

Table 4. Cost of

cotton vs. non-

cotton in Mississippi, 1995–97

Expense

cotton Non-

cotton

Number of sprays* 6.7 11.7

Control costs $61.48 $68.15

lb lint/acre 876 789

Economic advantage $63.22 —

* Average of 1995–1997

Source: Reprinted with permission from Stewart et al. 1998.

oth Bt cotton and boll weevil eradication

have great value for the cotton industry in

the United States. Available Bt cotton varieties are

highly effective against tobacco budworms and

provide significant suppression of bollworms and

certain other caterpillar species. Consequently,

the foliar insecticide treatments required to

control these pests in Bt varieties are substantially

lower than in non-Bt varieties (table 5). Many of

the treatments used to control tobacco budworms

and bollworms also are active against other pests,

such as boll weevils and tarnished plant bugs,

and lower insecticide use in Bt cotton reduces

coincidental control of such pests. As a result, Bt

cotton varieties grown in boll-weevil-infested

areas typically require more foliar insecticide

treatments (table 5). Eradication of the boll weevil

is, therefore, necessary to realize the maximum

potential benefit from growing Bt cotton.

Eradication of the boll weevil reduces the number

of foliar insecticide treatments necessary to

control other pests in non-Bt cotton. For example,

in Georgia, before the boll weevil was eradicated,

the amount of insecticide required for control of

other pests was notably higher than the amount

required following eradication. Data from Geor-

gia show an increase in the need for treatment for

other pests during the early years of boll weevil

eradication, because the insecticides used de-

stroyed beneficial insects. In the absence of

beneficial insects, populations of pests such as

tobacco budworms, bollworms, beet armyworms,

cotton aphids, and whiteflies often increase.

Bt Cotton and Boll Weevil Eradication:

Boll weevil

Can They Work Together?

Bt cotton has proven itself to be a useful tool in

minimizing the risk from certain caterpillar pest

outbreaks in the early years of a boll weevil

eradication program. In recent eradication pro-

grams in Alabama, Mississippi, and Louisiana,

producers chose to plant more than 80 percent of

their acreage to Bt varieties, primarily to mini-

mize risks of tobacco budworm outbreaks. How-

ever, there has been a negative aspect to this high

use of Bt cotton. When most acreage is planted to

Bt varieties, growers apply fewer sprays that

provide coincidental control of boll weevil. This

makes boll weevil eradication somewhat more

difficult and more costly. Greatly overshadowing

this negative influence are the positive effects of

Bt cotton in reducing the risks of secondary pest

problems during the initial years of an eradica-

tion effort.

Where boll weevils are eradicated, the overall

value of Bt cotton increases. In areas free of boll

weevils, insecticide sprays can be cut back and

beneficial organisms more successfully relied on

to reduce pest insects. The example from Georgia

shows a distinct decline in the need to treat for

other pests once the boll weevil was eradicated

(fig. 2). An additional reduction in foliar treat-

ments was observed after Bt cotton became

available in 1996. This example shows that boll

weevil eradication and Bt cotton are complemen-

tary in reducing total insecticide use and lower-

ing insect control costs. Reducing insecticide use

and relying more heavily on biological control

also benefits efforts to manage insecticide resis-

tance.

Table 5. Average number of annual insecticide sprays for tobacco

budworms, bollworms, and boll weevils in Mississippi, 1996–98

Insect 1996 1997 1998

Tobacco budworms and

bollworms

cotton 0.3 0.9 1.2

on non-

cotton 3.1 3.1 5.2

Boll weevils

cotton — 2.6 3.3

on non-

cotton — 1.9 1.9

Source: Modified with permission from Layton et al. 1999.

Figure 2. Non-boll weevil treatments in Georgia. Reprinted with permission from Layton et al. 1999.

86 87 88 89 90 91 92 93 94 95 96 97 98

Boll weevil eradication began began

Average no. treatments

Bt cotton introduced

t is commonly known that more than 500

species of insects and mites have developed at

least some degree of resistance to insecticides

(Georghiou and Saito 1983), knowledge clearly

showing that many arthropods have the genetic

potential for rapid adaptation to chemicals in

their environment. Most scientists agree that the

tobacco budworm and the bollworm will eventu-

ally become resistant to the Cry1Ac protein used

in current Bt cotton varieties. The tobacco bud-

worm has a well-known reputation for develop-

ing resistance to chemical insecticides. Currently

it is resistant to most conventional insecticides

used on cotton. However, for the time being, it is

extremely susceptible to the Cry1Ac protein in Bt

cotton. The bollworm is inherently more tolerant

to this toxin, and it is likely to develop resistance

faster than the tobacco budworm.

Field and laboratory studies document the devel-

oped resistance of several insects to spray formu-

lations of B.t. toxins. The best-known example is

the diamondback moth, a caterpillar pest that

attacks cabbage and related plants. It has shown

high levels of resistance to Bt sprays in Florida,

Hawaii, North Carolina, Asia, and other locations

(Tabashnik et al. 1990). It has also shown resis-

tance to Bt transgenic canola plants. Researchers

have already developed laboratory colonies of

Colorado potato beetles, European corn borers,

tobacco budworms, and bollworms that are

resistant to Cry-proteins (fig. 3). The resistant

laboratory colonies of tobacco budworms and

bollworms demonstrate these insects have the

genetic potential to become resistant.

Crop protection with Bt cotton is a form of host

plant resistance, like resistance of soybean variet-

ies to the soybean cyst nematode. Farmers are

familiar with resistant crops losing their protec-

tion from pests, like nematodes overcoming

soybean resistance and mildew adapting to

resistant wheat varieties. While the same fate is

predicted for Bt cotton, the time necessary to

reach economic resistance can be greatly influ-

enced by the way growers and consultants utilize

this crop.

Resistance to Bt Cotton in Tobacco

Budworms and Bollworms

Figure 3. Development of resistance to

cotton in tobacco budworm in laboratory experiments.

Reprinted with permission from Gould et al. 1992.

Cry1Ac Concentration (␮g/ml)

Mortality (percent)

Control strain

Control female X selected male

Selected female X control male

Selected strain

100

0.01

0.1

10 100

variety of factors may influence the rate at

which tobacco budworms and bollworms

become resistant to Cry-toxin in Bt cotton.

These factors include:

• The number of generations of tobacco bud-

worms and bollworms exposed each year to

plants containing the same or similar toxins

• The percentage of each generation exposed

plants containing the same or similar Cry-

toxins

• The mortality level that Cry-toxin causes

among tobacco budworms or bollworms

carrying one copy of a resistance allele and

one copy of a susceptible allele. The mortality

level is determined by the Cry-toxin concentra-

tion in the plant, which in turn may determine

the functional dominance of the allele affecting

resistance

• The frequency with which Cry-resistance

alleles are expressed in the tobacco budworm

or bollworm population before exposure to

Cry-toxins and the dominant or recessive

nature of the resistance alleles

• The migration patterns of tobacco budworm

and bollworm moths

• The survival advantage or disadvantage that

resistance allele(s) offer tobacco budworms or

bollworms both in the presence and absence

of Cry-toxins

• The number of susceptible moths available for

mating with moths carrying resistance gene(s).

Before exposure to Cry-toxins—by spraying

insecticides containing Bt or through planting Bt

crops—very few tobacco budworms and boll-

worms (perhaps 1 in 100,000 to 1 in 1 million)

carry two copies of a resistance allele (RR),

meaning they are fully resistant to Bt cotton.

Some tobacco budworms or bollworms have a

single copy of a resistance allele and a susceptible

allele (RS); these are called heterozygotes. The

overwhelming majority have two copies of a

susceptible allele (SS).

Most of the susceptible insects (SS) are killed after

feeding on Bt cotton, depending on the dose of

Cry-toxin in the plant. The heterozygous insects

(RS) usually are more difficult to kill than the

susceptible insects. Still, heterozygous insects are

not considered Bt resistant in most instances,

because most will die if the toxin dose in the

plant is high enough. Resistant insects (RR) are

not killed by Bt toxin. The difference in survival

rates among these three types represents a selec-

tive advantage for resistant (RR) tobacco bud-

worms and bollworms feeding on Bt cotton,

because they will survive while susceptible

individuals will die.

As the use of Bt cotton increases, a higher per-

centage of tobacco budworm and bollworm

populations will be exposed to Cry-toxins. Rela-

tively more caterpillars carrying resistance alleles

will survive to adulthood, while fewer suscep-

How Resistance Develops

tible caterpillars will survive. As a result, more

resistant insects will pass on alleles for resistance

to new generations.

Because Cry-toxins are expressed in transgenic

plants for the entire growing season—compared

to insecticides that remain active for short peri-

ods—Bt cotton further prompts tobacco bud-

worms and bollworms to select for resistance.

This exposure can greatly enhance selection for

resistant alleles and subsequently accelerate the

pace that these insects develop resistance, espe-

cially in areas where few alternate hosts are

available (fig. 4).

Due to the high, season-long selection for Bt-

resistant insects, scientists advise that develop-

ment of field-level resistance could take a rela-

tively short time. However, if Bt crops are not

used over a high proportion of the total acreage,

the rate that insects develop resistance should be

slower and Bt crops should remain effective for

many years (Gould et al. 1992). The rate that

resistance develops increases proportionately as

the acreage of Bt crops expands within a county,

state, or region. The presence or absence of

alternate non-Bt food plants in a particular area

may also influence the development of resistance,

which will probably occur first in a locale where

the use of Bt cotton is high and the availability of

non-Bt hosts, including cotton and other plants, is

low.

Figure 4. Increase of resistant larvae on

cotton. Reprinted with permission of

D. Sumerford.

everal types of Bt corn use different Bt

transformation events and express Cry1Ab,

Cry1Ac, Cry1F (not commercially available), or

Cry9c toxins. Some brands of Cry1Ab corn

express Bt toxin in the ears, where bollworm

larvae (called corn earworms on corn) may be

feeding, but other brands do not have toxin in the

ears. Cry9c toxin does not kill bollworms and

should not affect development of Bt resistance.

Currently, only the Yieldgard corn hybrids ex-

press Bt protein in ears and may have an effect on

Bt resistance in bollworms (fig. 5). Tobacco bud-

worms should not be affected by corn since they

very seldom infest this crop.

Growing corn in cotton production areas could

have a major influence on bollworms’ develop-

ment of resistance to Bt cotton. Bollworm cater-

Bt Corn: Does It Hasten Resistance

to Bt Cotton?

Figure 5. Computer simulation showing the influence of

corn and

cotton on the rate

that bollworms develop resistance to

. Reprinted with permission from ILSI Health and

Environmental Sciences Institute 1998.

100%

% of all cotton planted to

cotton

Years to resistance

* Spatially explicit model with 120 patches of corn, 160 patches of cotton,

40 patches of wild hosts, and 80 soybean patches. Five replicates and

noncross resistance.

20% of all corn planted in

cotton

40% of all corn planted to

corn

60% of all corn planted to

corn

20%

40%

60%

rowers know that production costs can

increase as insects develop resistance to

insecticides. How much of an increase

depends on the availability and cost-effectiveness

of alternative strategies. If such strategies do not

exist or are expensive, production costs and crop

losses may be very high. The resistance of tobacco

budworms and bollworms to Bt cotton is accom-

panied by costs similar to those encountered with

resistance to conventional insecticides.

There are also additional costs to resistance. Cry-

toxins in Bt plants and Cry-toxin-based insecti-

cides are not easily replaced when insects develop

resistance. In the past, growers relied on the

availability of new insecticides. There is no guar-

antee this process will continue. Developing new

insecticides and transgenic insecticidal crops is

time intensive, difficult, and expensive. Research-

ers may not be able to develop new insecticides at

reasonable costs that conform to environmental

and performance requirements as quickly as

Implications of Resistance

pillars infest the whorl and ear stages of corn.

Moths are strongly attracted to silking corn to lay

eggs, and a major portion of the second-genera-

tion population may develop in corn ears. Non-Bt

corn will not select for Bt-resistant bollworms but

will act as a refuge and delay resistance. The

more non-Bt corn—other non-Bt hosts—grown in

a corn and cotton production area, the slower Bt

resistance develops.

However, growing ear-expressing Bt corn can

hasten Bt resistance in two ways: (1) if non-Bt

corn is replaced with ear-expressing Bt corn, the

refuge effect that non-Bt corn provides dimin-

ishes and bollworm populations are exposed to

Bt toxin—both from the increase in Bt crops and

the decrease in non-Bt crops; and (2) as men-

tioned, greater exposure to the Bt toxin gives

bollworms that are carrying a resistance allele(s) a

survival advantage and hastens the pace to

resistance. If robust non-Bt refuges are main-

tained for both cotton and corn, the rate that

resistance develops is reduced and should allow

a prolonged life for both Bt products.

growers need them. A case in point is the high

cost of recently released tobacco budworm

insecticides for use on cotton.

The length of time that an insecticide or Bt cotton

remains effective may depend upon how well

growers and pest managers follow resistance

management guidelines. Improper usage dra-

matically decreases the effective life of a product.

If Bt products are carefully used, their effective-

ness may be extended for many years. But if the

technology is abused, budworms and bollworms

will quickly become resistant. Preserving the

effectiveness of Bt cotton is one way to keep pest

management costs at the lowest level.

Other less obvious risks could also occur. In the

past the appearance of resistant tobacco bud-

worm or bollworm infestations increased the

frequency of scouting and complicated decision-

making by growers and pest managers. If new,

selective insecticides are needed to combat Cry-

toxin-resistant pests, secondary insects may

become more numerous. Also, caterpillars ex-

posed to one Cry-protein (for example, Cry1Ac)

may develop resistance to other similar Cry-

proteins, even though they have not been ex-

posed to them; this process is known as cross-

resistance. When growing Bt crops, growers

should always use the refuge sizes specified in

the licensing agreement in order to slow the

development of resistance to the Cry-proteins

contained in sprayable Bt insecticides and Bt

crops.

logical, science-based, and proactive

resistance management strategy is neces-

sary to prevent tobacco budworms or bollworms

from developing resistance to Bt cotton in less

than 10 years. All members of the cotton industry

should practice this strategy to slow development

of resistance. EPA has registered products from

companies that sell Bt cotton seed, and these

companies are required to recommend and

support insect resistance management (IRM)

strategies for Bt cotton. IRM is a key element of a

good overall integrated pest management (IPM)

program.

EPA has accepted a resistance management

concept for Bt cotton known as the “high dose/

refuge strategy.” This approach has two comple-

mentary principles: (1) Bt plants must produce a

high dose of Cry-toxin throughout the season,

and (2) effective IRM refuges must be maintained.

An IRM refuge consists of a non-Bt host crop, and

it is intended to produce susceptible tobacco

budworms or bollworms or both.

In theory, if Bt plants express a high dose of toxin,

then all susceptible (SS) pests eating the plant will

die, almost all of the heterozygous (RS) insects

will die, and resistant (RR) insects will survive.

When resistant survivors from the Bt crop mate

with susceptible insects from a non-Bt refuge, the

offspring receive one allele—either an S or an R—

from each parent. Offspring from the cross-

mating will be heterozygotes (RS). If Bt plants

express a high dose of toxin, almost all heterozy-

gotes will be killed. Eliminating most heterozy-

gotes also eliminates most resistant alleles from

the surviving populations and greatly slows the

development of resistance.

Without a source for producing susceptible

insects—an IRM refuge—the development of

resistance is proportional to the dose; that is, the

higher the dose, the more rapidly resistance

develops. Therefore, the high dose/refuge strat-

egy is a high-risk strategy, depending upon the

availability of properly functioning IRM refuges.

The high dose, or in other words high effective-

ness, is very good for pest control, but it can

cause the rapid development of resistance in the

absence of effective IRM refuges.

As mentioned, the IRM refuge is acreage planted

with a non-Bt crop that serves as a host for to-

Can Growers Slow Resistance?

Future IRM Refuge Options

bacco budworms or bollworms or both. And the

refuge must be close enough to the Bt crop to

ensure that susceptible moths have an opportu-

nity to mate with resistant ones. This means that

moths from the Bt cotton and the refuge must

emerge at about the same time and be relatively

close to each other.

Commercial Bt cotton varieties (Bollgard) cur-

rently express enough Cry1Ac toxin throughout

most of the season to kill all susceptible (SS) and

almost all heterozygous (RS) tobacco budworms.

Only resistant (RR) tobacco budworms are ex-

pected to easily survive. Thus, commercially

efuge regulations originally mandated in

1995 for Bollgard varieties will remain in

effect through the 2000 growing season. When the

registration for Bollgard cotton expires after the

2000 growing season, new refuge requirements

may be forthcoming. At the time of publication,

no final refuge requirements had been deter-

mined for the 2001 growing season and beyond.

The issue will be debated before the final decision

is made; recommendations will range from

complete removal of Bt technology from the

marketplace, to a minimum of a 50-percent non-

Bt cotton refuge, to no change from the current 4-

percent-unsprayed or 20-percent-sprayed refuge

scenarios.

Computer simulation models, along with limited

evidence from field and laboratory studies,

suggest that the 4-percent-unsprayed refuge or

available Bt cotton varieties (Bollgard) should

qualify for the “high dose” definition for tobacco

budworms.

As mentioned, research shows that bollworms are

less sensitive to Cry1Ac toxin than tobacco

budworms. Researchers estimate that from 5 to 25

percent of susceptible (SS) bollworm larvae

survive on Bt cotton varieties now in use

(Bollgard), and estimates for heterozygote (RS)

survival are significantly higher. So the Bt cotton

grown now cannot be considered “high dose”

against bollworms.

the 20-percent-sprayed refuge required on the

original Bt cotton label (Bollgard cotton) may not

adequately delay resistance in bollworms and

tobacco budworms. Studies using computer

models (Gould et al. 1992, ILSI 1998) also suggest

that bollworm resistance to Bt cotton can occur

quickly if Bt cotton is extensively planted and

only small IRM refuges are used. Research indi-

cates that bollworms, and to a lesser extent

budworms, have the genetic ability to adapt

quickly to Bt toxin (Gould et al. 1992, Sumerford

et al. 2000, Burd et al. 2000).

In 1999, EPA and the U.S. Department of Agricul-

ture, proposed for discussion the following two

structured refuge options to mitigate the resis-

tance of tobacco budworms and bollworms to Bt

toxins expressed in Bollgard Bt cotton:

1 An external refuge of at least 30% non

cotton should be implemented. The

placement of the structured refuge

should be planted within 0.5 miles of

the farthest

cotton in a field to pro-

vide

-susceptible moths. The external

refuges of non

-Bt

cotton can be treated

with any other registered non

-Bt

insecti-

cide or other insect control measures.

2 In-field refuges of at least 10% non

-Bt

cotton refuge should be implemented.

In-field refuges should be planted

entirely within the field as blocks,

minimum size to be determined based

on planter size, to provide

Bt-

suscep-

tible moths. Cotton fields may be

treated with any registered non

-Bt

insecticide or other control measures,

as long as the entire field is treated in

the same manner. This means that

cotton rows cannot be treated indepen-

dently from non

-Bt

cotton rows with

insecticides or other insect control

measures.

In both options, (1) and (2), agronomic

practices used for farming the non

-Bt

cotton must ensure adequate produc-

tion of susceptible [tobacco budworm

and cotton bollworm] adults to mate

with resistant adults emerging from

cotton. In particular, termination of

growth of non

-Bt

cotton should not be

done until termination of growth of

cotton has begun. In general, agro-

nomic practices for non

-Bt

cotton

should be similar, as practical, to those

of the

cotton grown in the same

management unit, especially regarding

crop nutrition, irrigation, and termination

(U.S. Environmental Protection Agency

and U.S. Department of Agriculture

1999).

EPA and USDA co-authored this position paper to

provide stakeholders with a focal point for dis-

cussing future recommendations. The entire

document may be reviewed at http://

www.epa.gov/oppbppdl/biopesticides/

otherdocs/bt_position_paper_618.html. Updated

versions of refuge guidelines will be available at

this website.

Entomologists in the public sector are concerned

with cotton insects’ almost 50-year history of

developing resistance to sprayable insecticides

from almost every chemical class. Sound biologi-

cal reasons indicate that Bt insecticide within

plants will be even more vulnerable to the devel-

opment of insect resistance. Given the reduced

pace of developing new replacement insect-

control technology, the U.S. cotton industry may

face a greater risk of insecticide resistance than

ever before.

The refuge plans currently in use are designed for

Bollgard cotton varieties that perform like the

varieties first marketed (for example, NuCotn 33

and NuCotn 35). These refuges may be inappro-

priate for new Bt genes or stacked gene Bt prod-

ucts that may be marketed in the near future. One

hopes new Bt gene products will deliver a high

dose of toxin to the bollworm and secondary

lepidopterans and qualify for the high-dose refuge

strategy. If these products do qualify, scientific

theory should support less restrictive IRM plans

for such future products. The development of

highly effective transgenic insecticidal cotton

plants with multiple toxic genes should be

encouraged.

The Economics of Cotton Refuges

Can the Development of Resistance

Be Successfully Monitored?

he expenses and yield reduction associated

with refuges must be viewed as costs of

using Bt cotton technology. Balancing these are

the economic and other benefits realized from Bt

cotton. In non-Bt cotton when insecticides are

used for insect management, the chemical costs,

application costs, and risks of handling pesticides

and possible complaints about them, among

other costs, are balanced against the benefit of

insect control and higher yields. If, for example, a

90/10 in-field refuge plan is used, the 10-percent

refuge may be damaged by insects, but the yield

and insect control cost over the total acreage may

be more profitable than that achieved using other

insect management plans.

Preserving Bt cotton technology from resistance

helps ensure that growers will have effective

insect management options in the future. Experi-

ence shows that greater crop loss and higher cost

insect management typically follow the develop-

ment of resistance. So, clearly, an economic

benefit is often difficult to estimate. There is no

guarantee that new and cost-effective biotechnol-

ogy or insecticidal products will replace existing

technology in a timely fashion.

onitoring for the development of insect

resistance to Cry-proteins is a difficult

and imprecise task. It may include surveying the

annual use of Bt cotton in each county or parish,

annual testing of tobacco budworm and boll-

worm populations for Bt sensitivity, and checking

Bt crops for any changes in the survival rate of

tobacco budworms and bollworms. Monitoring

tobacco budworm and bollworm populations on

Bt cotton is important in providing the earliest

warning that they are developing resistance; it is

also required by EPA. And it improves resistance

management efforts.

Companies that register the proteins in Bt cotton

with EPA are required to keep annual sales

records on a county-by-county basis and submit

summaries for each state. Surveys of grower use

of Bt cotton, conducted by Cooperative Extension

Service personnel, also may be available and

could be used as a guide for monitoring areas

where circumstances favor the development of Bt

resistance by tobacco budworms and bollworms.

Researchers predict that resistance is more likely

in areas where the use of Bt cotton has been high

for several years. Other characteristics of a par-

ticular region—such as the amount of non-Bt

cotton acreage and alternative hosts of tobacco

budworms or bollworms—may be surveyed to

help determine the risk of resistance. Assigning

resistance risk categories to unique cotton-grow-

ing environments across the Cotton Belt can be

helpful in monitoring the efforts at specific sites.

A major difficulty in monitoring is that resistance

can develop to an advanced stage before it is

easily detected in the field. For example, if 1 of

every 100,000 tobacco budworms was resistant

when Bt cotton was first marketed, resistance

levels would advance many-fold—perhaps to 1

per 100 individuals—before field failures could

be detected.

Detection of resistance in the field by scouts and

growers would likely occur only in outbreak

years, unless the resistance level was more than 1

insect per 100. The development of resistance is

largely undetectable by measuring field perfor-

mance of Bt cotton. This invisible phase is due, in

part, to monitoring techniques that are not sensi-

tive enough to detect early shifts in tobacco bud-

worm and bollworm resistance.

A first step in establishing an effective monitoring

program is to document the initial susceptibility

level of tobacco budworms and bollworms to the

Bt toxin. With this information as a baseline, it

may be possible to spot small, early changes in

susceptibility before field control failure occurs.

Laboratory studies are uncovering more informa-

tion about the frequency with which resistant

genes occur and the dominance of these genes.

Detecting changes in susceptibility to the Cry-

toxins requires precise techniques that provide

information from a large number of insects and

many locations. The U.S. Department of Agricul-

ture and state universities are cooperating to

establish Cry-toxin susceptibility baselines and

measure resistance in tobacco budworm and

bollworm larvae collected from all across the

Cotton Belt, especially from the mid-South and

the Southeast (Summerford et al. 2000). Technol-

ogy companies are also conducting similar tests.

If these monitoring studies are sufficiently wide-

spread, changes in the susceptibility of tobacco

budworms and bollworms to Bt may be success-

fully detected prior to field control failures. EPA

is reevaluating annual monitoring requirements

for development of remedial action plans.

Counting the number of surviving caterpillars in

fields of Bt cotton may be helpful for detecting

the development of resistance. Susceptible popu-

lations of tobacco budworms are decimated in Bt

cotton, so scouting for their survival may uncover

resistance before economic field failures occur.

This method is not highly sensitive, however, and

insect resistance to Cry1Ac toxin must reach a

relatively high level—for example, one resistant

insect in 500 susceptible insects—before detection

is possible.

The tolerance of bollworms to Cry1Ac toxin is

already too high to allow close measurement of

changes in resistance using scouting and larval

survival. What’s more, less than adequate Bt

expression in the cotton plants may produce a

false positive for resistance. Field scouting will

detect only large shifts in the survival rate of

bollworms.

Fitting Bt Cotton Into an Insect

Management Program

s with all conventional insecticides, Bt

cotton must be managed wisely and used in

combination with other insect pest and crop

management practices. For more than 20 years,

IPM has focused on using insecticides on an as-

needed basis only. The decision to use insecti-

cides has generally been based on crop scouting

to determine insect pest densities and the use of

spray thresholds. Of course, Bt crops cannot be

used as-needed because the Bt toxin is in the

plant. For this reason, tobacco budworms and

bollworms have greater opportunities for expo-

sure to the toxin, and development of resistance

is more likely with Bt plants.

Consequently, resistance management must be

part of the total pest management package. On

the positive side, Bt plants, in the absence of

insecticide sprays, will help preserve beneficial

insects, allowing growers to take advantage of

them as a free resource, especially in areas where

the boll weevil has been eradicated.

When developing crop production plans, growers

should consider all insect management strategies

and tactics, with emphasis on the following areas:

Cultural practices. Early crop maturity (which

may be optimized with computerized manage-

ment programs such as COTMAN) should be

part of every pest management plan. Most grow-

ers realize that early fruit set and early fruit

maturity help reduce insect pest infestations.

Combined use of optimal planting times and

early-maturing cotton varieties reduce crop

damage, minimize insecticide use, and diminish

the chances of pests’ becoming resistant to Bt

toxin and insecticides. Early fruit set should be

protected so maturity is not delayed.

Rotating Bt crops with other non-Bt crops (tem-

poral refuge) that also are hosts of tobacco bud-

worms and bollworms is an example of resistance

management and offers overall pest management

benefits. Soybeans, peanuts, and tobacco can

harbor tobacco budworms, and these same crops,

plus corn and grain sorghum, are all hosts of

bollworms. Yieldgard Bt corn would not be

considered an adequate alternate host, but other

available Bt corn brands and non-Bt corn could

serve as alternate hosts. Certain forage crops,

such as crimson clover and alfalfa, and many

weeds may also serve as hosts to tobacco bud-

worms and bollworms. These alternate hosts can

provide an important refuge benefit and help

slow the development of resistance. In fact,

according to computer models, the rate that

bollworms develop Bt resistance is directly

related to the proportion of non-Bt corn (not

Yieldgard) in a cotton and corn cropping system.

Beneficial insects. Conventional insecticides

often have a significant adverse effect on benefi-

cial insects in cotton. Populations of cotton

aphids, beet armyworms, tobacco budworms,

bollworms, fall armyworms, and other pests

frequently increase following insecticide use due

to the destruction of beneficial arthropods. Bt

toxins do not have this effect. The loss of benefi-

cial insects can also be reduced by using fewer

applications of insecticide or by using an insecti-

cide that is less harmful.

Scouting and thresholds. Bt cotton is not

effective on pests such as boll weevils, plant bugs,

and stink bugs. When bollworms, beet army-

worms, fall armyworms, and looper moths lay

large numbers of eggs, damaging infestations

may occur because the level of Bt toxin produced

is not adequate to fully control them. As a result,

Bt cotton must be scouted in a fashion similar to

that used with conventional cotton but using

altered techniques and modified treatment

thresholds. Entomologists in each cotton-produc-

ing state publish variations for Bt cotton scouting

and treatment thresholds.

Bt cotton kills almost all newly hatched tobacco

budworm larvae and most bollworm larvae, so

using eggs as a criterion for insecticide applica-

tions may lead to unneeded foliar treatments.

When large numbers of bollworm eggs are laid,

survival of the larvae may lead to damaging

infestations. As a consequence, in a few states

where the tobacco budworm and bollworm

populations consist mostly of bollworms, exten-

sion entomologists have adopted a high egg

threshold for Bt cotton (table 6). In other states

(Arkansas, Mississippi, and Texas, for example)

egg thresholds are not recommended as a treat-

ment criterion. Use of the egg threshold is not

possible in states where the percentage of tobacco

Fi 14 C t i l ti h i th i fl f

tt th t th t b ll d l

The beneficial wasp,

Microplitis croceipes

, feeding on bollworm/budworm larva.

Courtesy of J. Powell.

budworms is high, since separating tobacco

budworms from bollworms without a microscope

is difficult and time-consuming in every stage but

the adult.

A potential solution to the problem of identifying

eggs and small larvae is the development of kits

containing species-specific monoclonal antibod-

ies. These kits would allow growers and consult-

ants to make better pest control decisions if

surviving larvae longer than 1/8 to 1/4 inch are

found in Bt cotton. This technology is still being

developed.

Scouting Bt cotton for tobacco budworms and

bollworms must employ somewhat different

techniques than scouting non-Bt cotton. Newly

hatched larvae do not die immediately after

feeding on Bt cotton. In fact, most states recom-

mend counting caterpillars only of a certain size,

such as 1/8 to 1/4 inch long. Caterpillars that

reach a minimum size have a greater ability to

survive on Bt cotton. As the caterpillars become

larger, they become more difficult to kill with the

Bt toxin and can damage the crop unless elimi-

nated quickly. The larvae that survive are most

likely to be bollworms. Caterpillars, usually

bollworms, often are found in flowers or “bloom

tags,” so greater emphasis is being placed on

examining the blooms and small bolls when

scouting.

Tobacco budworm and bollworm damage in Bt

cotton can also be a target for scouting. Caterpil-

lars that grow beyond a minimum size and feed

on terminals, squares, or bolls cause recognizable

Table 6. Bollworm and tobacco budworm thresholds on cotton in South Carolina

Spray threshold

Stage % Eggs % Square damage % Larvae

Before bloom

non-

cotton — 20 15

cotton — — —

After bloom

non-

cotton 20 3 5

cotton 75 5 (bolls) 30

Source: Roof 1999

Cotton boll damage from budworm/bollworm larvae

damage. The superficial damage of very small

caterpillars should not be mistaken for penetrat-

ing injury that may indicate a damaging insect

population. In some situations, fall armyworms

or beet armyworms can cause this damage, so

scouts should be careful to properly identify the

pest species. Some states have developed damage

thresholds for Bt cotton.

Insecticide use in

cotton. In Bt cotton, all

insect pests not affected by the Bt toxin should be

managed as they would be in non-Bt cotton. It

may be necessary to apply insecticide to control

thrips, aphids, boll weevils, tarnished plant bugs,

stink bugs, and a few tolerant caterpillars such as

beet armyworms and fall armyworms. The

decision to apply insecticides to Bt and non-Bt

cotton should be based on scouting and treatment

thresholds.

Insecticide use depends on several variables,

including the abundance of the pest insect and

the presence of beneficial insects. Using insecti-

cides against early-season pests, such as boll

weevils and tarnished plant bugs, also reduces

the number of beneficial insects. As mentioned

previously, this reduction may result in higher

populations of insect pests and fewer opportuni-

ties for reduced insecticide use in Bt cotton. In

areas where boll weevils have been eradicated

and tarnished plant bugs are infrequent, the

presence of Bt toxin and beneficial insects greatly

reduces the amount of insecticide needed on Bt

cotton.

Key Steps—Implementing a Resistance

Management Plan

ransgenic crops are one of the most revolu-

tionary developments in agricultural pro-

duction. As with most new technology, there are

exciting possibilities for the economic value of Bt

cotton and apprehensions about its wise use. In

order to preserve Bt cotton well into the 21st

century, producers, seed companies, scientists,

and regulators need to foster strong collaboration

to ensure longevity of the technology.

Companies are striving to develop new genes for

insertion into cotton plant DNA to provide other

he following summary, based on the

principles outlined in this publication,

assumes that a voluntary, proactive approach

by growers will provide product stewardship

for long-term yield benefits and profitability.

1 Use

cotton in fields where the risk of

severe tobacco budworm and bollworm

infestations warrants the price premium for

seed.

2 Plant

cotton and non-

refuge in the

required proportions and patterns.

Bt Cotton in the Future

possibilities for improving agronomic traits and

pest control characteristics. Genes for new insecti-

cidal toxins will be important for managing a

wider spectrum of insects and for slowing the

pace of resistance. If future varieties express a

high dose of toxin and the toxins do not have the

same physiological target site, the rate that insects

develop resistance could be greatly reduced.

3 Record where

and non

-Bt

cotton are

planted so

cotton performance can be

monitored and non

-Bt

cotton can be scouted

and treated as needed.

4 Use all cultural alternatives available for

avoiding high late-season tobacco budworm

and bollworm populations. These alternatives

include early planting, short-season varieties,

and protection of early fruit.

5 Continue using an IPM approach for all

pests. When designing scouting priorities,

keep in mind that other insects—such as

beet armyworms, cabbage loopers, cotton

square borers, European corn borers, fall

armyworms, southern armyworms, soybean

loopers, and yellowstriped armyworms—are

suppressed at various levels by

cotton.

Still other pests are not affected by

cotton,

including boll weevils, cotton aphids, cotton

fleahoppers, cutworms, spider mites, stink

bugs, tarnished plant bugs, thrips, and

whiteflies.

6 Monitor

cotton to verify tobacco budworm

and bollworm control throughout the season.

Since bollworms are not as readily controlled

cotton, carefully watch for a heavy egg-

lay and scout for surviving larvae lower in the

cotton plants, especially in bloom tags. If

feeding damage occurs in

cotton, investi-

gate the cause immediately. If needed, get

help in identifying feeding caterpillars. If

tobacco budworm or bollworm larvae or

excessive damage are discovered, resis-

tance or tolerance to

cotton is a possibility,

and the situation should be investigated.

Verify from field records that Bt cotton was

planted where excessive damage or larvae are

observed. Consult your grower’s guide for the

seed company’s procedure for investigating

suspected cases of resistance or tolerance. Imme-

diately notify seed company representatives or

extension agents or both if evidence indicates a

performance problem.

References

Burd, A.D., J.R. Bradley, J.W. Van Duyn, and F.

Gould. 2000. Resistance of bollworm, Helicoverpa

zea, to CryIAc toxin. In P. Dugger and D. Richter,

eds., Proceedings of the Beltwide Cotton Produc-

tion Research Conference, pp. 923–926. National

Cotton Council, Memphis, Tennessee.

Georghiou, G.P., and T. Saito, eds. 1983. Pest

resistance to pesticides. Plenum Press, New York.

Gould, F., A. Martinez-Ramirez, and A. Ander-

son. 1992. Broad-spectrum resistance to Bacillus

thuringiensis toxins in Heliothis virescens. Proceed-

ings of the National Academy of Sciences USA

89:7986–7990.

ILSI Health and Environmental Sciences Institute.

1998. Evaluation of insect resistance management

in Bt field corn; a science-based framework for

risk assessment and risk management. Report of

an expert panel. ILSI Press, Washington, DC.

Jenkins, J.N., J.C. McCarty, Jr., and R.E. Buehler.

1997. Resistance of cotton with endotoxin genes

from Bacillus thuringiensis var. kurstaki on se-

lected lepidopteran insects. Agronomy Journal

89:768–780.

Layton, M.B., S.D. Stewart, M.R. Williams, and

J.L. Long. 1999. Performance of Bt cotton in

Mississippi, 1998. In P. Dugger and D. Richter,

eds., Proceedings of the Beltwide Cotton Produc-

tion Research Conference, pp. 942–945. National

Cotton Council, Memphis, Tennessee.

Ostlie, K.R., W.D. Hutchison, and R.L. Hellmich,

eds. 1997. Bt corn and European corn borer. NCR

Publication 602. University of Minnesota, St.

Paul.

Roof, Mitchell E. 1999. Cotton insect manage-

ment. Clemson University Extension Information

Card 97, Clemson, South Carolina.

Stewart, S., J. Reed, R. Luttrell, and F.A. Harris.

1998. Cotton insect control strategy project:

Comparing Bt and conventional cotton manage-

ment and plant bug control strategies at five

locations in Mississippi (1995–1997). In P. Dugger

and D. Richter, eds., Proceedings of the Beltwide

Cotton Production Research Conference, pp.

1199–1203. National Cotton Council, Memphis,

Tennessee.

Sumerford, D.V., D.D. Hardee, L.C. Adams, and

W.L. Solomon. 2001. Tolerance to CryIAc in

populations of Helicoverpa zea and Heliothis

virescens (Lepidoptera: Noctuidae): Three-year

summary. Journal of Economic Entomology. In

press.

Tabashnik, B.E., N.L. Cushing, N. Finson, and

M.W. Johnson. 1990. Field development of resis-

tance to Bacillus thuringiensis in diamondback

moth (Lepidoptera: Plutellidae). Journal of

Economic Entomology 83:1671–1676.

U.S. Environmental Protection Agency and U.S.

Department of Agriculture. EPA and USDA

position paper on insect resistance management

in Bt crops. 1999. Posted on the World Wide Web

May 27, 1999, minor revisions July 12, 1999.

http://www.epa.gov/oppbppd1/biopesticides/

otherdocs/bt_position_paper_618.html

Glossary

Allele. An alternative form of a gene. For ex-

ample, a gene determining the reaction of an

insect to Bt toxin may occur in the form of an

allele for resistance or an allele for susceptibility.

Alternate host. Non-Bt plant types other than

cotton that support successful reproduction of

tobacco budworms or bollworms or both; soy-

bean and corn are examples.

Bacillus thuringiensis (Bt). A naturally occurring

soil bacterium that occurs worldwide and pro-

duces a toxin specific to certain insects (for

example, moths, beetles, blackflies, or mosqui-

toes).

Beneficial arthropods. Usually refers to insect

predators or parasites of pest insects in crops.

When abundant, these insects can assist in de-

creasing certain pests, although they are often

seriously reduced by insecticide.

Biotechnology. The science and art of genetically

modifying an organism’s DNA, such that the

transformed individual can express new traits

that enhance the quality of a product (for ex-

ample, seed oils or fiber quality) or can express

resistance to pests.

Boll weevil eradication program. A joint USDA,

producer, and state program designed to elimi-

nate the boll weevil as an economic pest from the

Cotton Belt.

Bt cotton. Commercial varieties of cotton that

contain a gene from Bacillus thuringiensis within

its DNA. This gene allows the plant to produce

Cry-protein within most or all plant tissues. The

toxin makes the plant tissue—terminals, squares,

or bolls—toxic to some important caterpillar

pests.

Bt insecticides. Formulations of Bt Cry-proteins

manufactured and sold for spraying a wide

variety of plants in order to control certain cater-

pillar or beetle pests. Some formulations are also

used for mosquito and fly control.

Cross resistance. Resistance to one or more toxins

from exposure to one or more other toxins, such

as bollworms becoming resistant to Cry1Ac in

cotton by being exposed to CryIAb in corn, or

vice versa.

Cry-proteins. Any of several crystalline proteins

found in Bt spores that are activated by enzymes

in the insect’s midgut. These proteins attack the

cells lining the gut, cause gut paralysis, and

subsequently kill the insect.

Cry1Ac protein (toxin). One of many Bt crystal-

line protein toxins. The Cry-protein used in the

first varieties of Bt cotton sold commercially to

growers.

DNA. Deoxyribonucleic acid, a double-stranded

molecule, consisting of paired nucleotide units

grouped into genes and associated regulatory

sequences. These genes serve as blueprints for

protein construction from amino acid building

blocks.

Dominance (of an allele). The ability of one allele

to determine a characteristic in a heterozygous

individual. For example, an allele for resistance to

Bt toxin may be dominant. As a consequence, an

insect heterozygous for resistance, that is, con-

taining an allele for resistance and an allele for

susceptibility, is resistant to Bt toxin.

Dose of toxin. The amount of toxin eaten per

insect. For example, the dose of toxin that a

caterpillar receives by eating part of a Bt plant

can be stated as micrograms of Bt toxin eaten per

caterpillar or per milligram of the caterpillar’s

total body weight.

Expression. Production of the desired trait (pro-

tein concentration, for example) in a transgenic

plant. Expression varies with the gene, its pro-

moter, and its insertion point in the host DNA.

Gene. The basic unit of inheritance; a section of

DNA that codes for a specific product (for ex-

ample, protein) or trait.

Genetic marker. See Marker.

Heterozygote. A diploid organism carrying two

different alleles (for example, susceptible and

resistant) of a gene.

High dose. A dose of toxin high enough to kill all

susceptible target pests and nearly all heterozy-

gotes. Such a dose can be delivered by plants

with sufficiently high concentrations of Bt toxin.

High-dose refuge strategy. A resistance manage-

ment approach that uses plants to minimize the

rapid selection for resistance to transgenic plants.

This strategy relies upon plants that produce Cry-

proteins at a concentration sufficient to kill all but

the most resistant insects and is used in combina-

tion with a non-Bt refuge that allows susceptible

insects to survive and mate with resistant indi-

viduals.

Host plant resistance. Ability of a plant to avoid

insect damage, to kill attacking insects, or to

tolerate their damage.

Insect resistance management (IRM). A proac-

tive approach to offset and slow insects’ resis-

tance to Bt crops or insecticides by reducing

selection or by counteracting the effects of selec-

tion for resistance genes.

Integrated pest management (IPM). A manage-

ment approach that integrates multiple, comple-

mentary control tactics to manage pests in a

profitable, environmentally sound manner.

Examples of the control tactics include conserva-

tion of natural enemies, crop rotation, host plant

resistance, and insecticides.

IPM. See Integrated pest management.

IRM. See Insect resistance management.

Larva. Immature stage of certain insect species;

examples include caterpillars and grubs.

Lepidoptera. The order of insects that includes

moths and butterflies. The plant-eating immature

stages are often called worms, caterpillars, or

larvae.

Marker. A genetic flag or trait used to verify

successful gene transformation and to indirectly

measure expression of the inserted genes.

Mode of action. Mechanism by which a toxin

kills an insect. For example, the mode of action of

Bt is ingestion and disruption of cells lining the

midgut.

Promoter. A DNA sequence that regulates where,

when, and to what degree an associated gene is

expressed.

Refuge or insect resistance management (IRM)

refuge. A wild host area or an area planted with

nontransgenic plants (for example, non-Bt cotton

or alternative hosts for tobacco budworms or

bollworms) where susceptible pests can survive

and produce a local population capable of mating

with resistant survivors from Bt cotton. This

mating, by decreasing the likelihood that Bt-

resistant insects will mate with one another,

dilutes resistance in the insect population.

Registration. Legal approval of pesticides and

transgenic crops for use in the United States by

the U.S. Environmental Protection Agency.

Registration is granted after extensive review of

toxicology (to mammals, birds, fish, and other

nontarget organisms), environmental fate, health

and safety issues, and precautions.

Resistance (by pests). The evolved capacity of an

organism to survive in response to selection from

exposure to a pesticide. The evolution of resis-

tance occurs through a process of genetic accu-

mulation, whereby a population becomes less

sensitive to the pesticide following repeated

exposure.

Selection. A natural or artificial process that

results in survival and better reproductive suc-

cess of some individuals over others. Selection

results in genetic shifts if survivors are more, or

less, likely to have particular inherited traits.

Stacked. Describes transgenic plants with more

than one introduced gene in a single crop plant

variety, such as a “stacked” cotton variety con-

taining a Bt gene and a gene for herbicide toler-

ance.

Transgenic. An organism genetically altered by

addition of foreign genetic material (DNA) from

another organism into its own DNA.