United States

Department of
Agriculture

Bt Cotton
&
Agricultural
Research
Service

ARS–154

January 2001

Management of the
Tobacco Budworm-Bollworm
Complex
United States
Department of
Agriculture

Agricultural

Bt Cotton
&
Research
Service

ARS–154

January 2001

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. Mention of trade names, commercial products, or
Bagwell. 2000. Bt Cotton & Management of the companies in this publication is solely for the purpose
Tobacco Budworm-Bollworm Complex. U.S. Depart- of providing specific information and does not imply
ment of Agriculture, Agricultural Research Service, recommendation or endorsement by the U.S. Depart-
ARS–154. 40 pp. ment of Agriculture over others not mentioned.

Preservation of Bt technology is critical for cotton This publication reports research involving pesticides.
producers across the U.S. Cotton Belt because of It does not contain recommendations for their use nor
increasing insecticide resistance and production costs. does it imply that uses discussed here have been
Frequent introduction of new transgenic cotton registered. All uses of pesticides must be registered by
varieties creates a need to continuously evaluate their appropriate state or Federal agencies or both before
cost-effectiveness and develop efficient plans for their they can be recommended.
deployment. This publication explains how Bt cotton
is developed, how it controls insect pests, and how it While supplies last, copies of this publication may be
can most effectively be used in insect pest manage- obtained at no cost from D.D. Hardee, USDA–ARS,
ment. Restrictions and limitations to the use of Bt P.O. Box 346, Stoneville, MS 38776, e-mail:
cotton are discussed, such as insects’ development of dhardee@ars.usda.gov. Copies also available from F.L.
resistance to it and approaches to preserving the Carter, National Cotton Council, P.O. Box 820285,
technology for long-term profits. Memphis, TN 38182.

Audiences for the publication consist of research and Copies of this publication may be purchased from the
extension entomologists in the public and private National Technical Information Service, 5285 Port
sectors, consultants, and cotton producers. Royal Road, Springfield, VA 22161; telephone (703)
605–6000.
Keywords: bollworm, Bt cotton, budworm, cotton,
Heliothis virescens, Helicoverpa zea, refuge, resistance
monitoring, transgenic cotton

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.

Issued January 2001

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.
Bt Cotton &
Management of the
Tobacco Budworm-Bollworm
Complex

Tobacco budworm,
Heliothis virescens
(F.), larva

Bt cotton is one of the first crop

protection products from biotechnology.

All Bt cotton plants contain one or more foreign budworms and bollworms. This new technology
genes derived from the soil-dwelling bacterium, for managing insect pests was approved for
Bacillus thuringiensis; thus, they are transgenic commercialization in the United States by the
plants. U.S. Environmental Protection Agency (EPA) in
October 1995 and is now available from several
The insertion of the genes from B. thuringiensis seed companies in this country, as well as in
causes cotton plant cells to produce crystal many other cotton-growing countries around the
insecticidal proteins, often referred to as Cry- world.
proteins. These insecticidal proteins are effective
in killing some of the most injurious caterpillar Cotton varieties containing the Cry1Ac Bt protein
pests of cotton, such as the larvae of tobacco provide protection against three major U.S.

1
 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
Tobacco budworm,
controls insect pests, and
Heliothis virescens how it can be most effec-
(F.), moth tively used in insect pest
management. Restrictions
on and limitations to the use
of Bt cotton are discussed,
cotton pests—tobacco bud- less than that provided by such as insects’ develop-
worms, bollworms, and pink registered insecticides. ment of resistance to it and
bollworms. Bt cotton also approaches to preserving
reduces survival of other The preservation of the Bt the technology for long-
caterpillar pests such as beet technology is critical because (1) term profits.
armyworms, cabbage loopers, bollworms, tobacco budworms,
cotton leafperforators, fall and pink bollworms continue to Tobacco budworm, Heliothis
armyworms, southern army- develop resistance to foliar- virescens (F.), and bollworm,
worms, and soybean loopers. applied insecticides, (2) use of Helicoverpa zea (Boddie),
The protection it provides insecticides is tied to ecological cause more damage to
against tobacco budworms, concerns, and (3) Bt genes are cotton than any other insect
pink bollworms, and European valuable resources. This publi- pest in the U.S. Cotton Belt.
corn borers is greater than the cation focuses on how Bt cotton The combined cost of
protection provided by the affects the tobacco budworm controlling these pests and
most effective foliar insecti- and bollworm. It is intended to the losses they inflict on
cides. Unfortunately the provide information that will cotton production exceeds
protection it affords cotton guide producers, research and $300 million a year. Insecti-
against bollworms is generally extension entomologists, con- cides used against tobacco
sultants, and industry in the budworms and bollworms

2
often create other problems, such as higher cides vary across the Cotton Belt, both between
populations of beet armyworms and cotton and within the states. Because of this variation
aphids and an increased pesticide load in the and the price of the technology, not all areas of
environment. Frequent exposure of insect pests to the Cotton Belt are able to economically justify
insecticides results in the development of insecti- the use of Bt cotton. However, where insect
cide resistance, which reduces the overall effec- infestations are severe, Bt cotton offers a new
tiveness of available insecticides, increases crop management tool for producers, helps ensure
losses, and leads to higher pest control costs and against yield loss in the presence of heavy infesta-
lower farm profits. tions of insecticide-resistant tobacco budworms,
and aids in reducing bollworm damage.
The severity of tobacco budworm and bollworm
infestations and resistance to synthetic insecti-

Bt—What Is It?

T 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-

Bollworm larva
proteins, and there are hundreds of known
feeding in boll
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-

3
sects, such as the larvae of certain kinds of flies, used for more than 30 years in various liquid and
beetles, and moths. For example, Colorado potato granular formulations of natural Bt insecticides,
beetle larvae are affected by Cry3A proteins; mainly to control caterpillars on a variety of
Cry1Ac is used against tobacco budworms; and crops. The Bt cotton varieties presently used
European corn borers can be killed with Cry1Ab, against tobacco budworms, bollworms, and
Cry1F, Cry1Ac, and Cry9c proteins. Other Cry- certain other caterpillars produce the Cry1Ac
proteins are active against mosquito larvae, flies, protein.
or even nematodes. Some Cry-proteins have been

The Why’s and How’s of Creating

Bt Cotton
overcomes most of the aforementioned limita-
B
ioinsecticides like Bt that are sprayed on
crops may perform as well as synthetic tions. The plant-produced Bt proteins are pro-
insecticides in very limited situations, but the tected from rapid environmental degradation
performance of Bt insecticides has been inconsis- since they are not directly exposed to the environ-
tent in many instances. ment. Incomplete coverage is not usually a
problem because the plants produce the proteins
The erratic performance in cotton is attributed to in all tissues where larvae feed, thus ensuring
four reasons: that the larvae will eat the Cry-protein. The
protein is always present whenever newly
• The toxin is rapidly degraded by ultraviolet hatched larvae feed, eliminating the timing
light, heat, high leaf pH, or desiccation. problem associated with foliar application. The
result is that Bt cotton has a built-in system that
• Caterpillars must eat enough treated plant efficiently and consistently delivers Cry-toxins to
tissue to get a lethal dose of the toxin, since the target pests from the time a newly hatched
the toxin has no contact effect. larva takes its first bite (fig. 1).

• The sites where tobacco budworms and Bt cotton offers a vastly improved method for
bollworms feed are difficult to cover with the delivering Cry-insecticides to target insects,
foliar-applied sprays. compared to traditional Bt sprays. Bt cotton may
also be considered a form of host plant resistance,
• Bt Cry-proteins are less toxic to older larvae. in that the Cry-protein trait is carried in the
plant’s genes, as is traditional plant resistance to
A cotton plant modified to produce Cry-protein insects.
within the plant tissues that caterpillars eat

4
Figure 1. Mode of action for Bt toxin after eaten by a tobacco budworm larva.
Modified with permission from Ostlie et al. 1997.

5
Biotechnologists created Bt cotton by inserting Genetic marker. A genetic marker allows re-
selected exotic DNA, from a Bt bacterium, into searchers to identify successful insertion of a
the cotton plant’s own DNA. DNA is the genetic gene into the plant’s DNA. It also assists plant
breeders in identifying and developing new cotton
material that controls expression of a plant’s or an
lines with the Bt gene. A common marker is an
animal’s traits. Following the insertion of modi- herbicide tolerance gene linked to the Bt gene.
fied Bt DNA into the cotton plant’s DNA, seed Following a transformation attempt to place the Bt
companies moved the Cry-protein trait into high- and marker gene into the plant’s DNA, plants are
performance cotton varieties by traditional plant treated with herbicide. Plants that were success-
breeding methods. Agronomic qualities for yield, fully transformed have the Bt gene and the
harvestability, fiber quality, and other important herbicide resistance gene and will survive herbi-
cide treatment; plants without the marker gene,
characteristics were preserved at the same time
and hence without the linked Bt gene, will be
the Cry-protein gene was added to commercial
killed by the herbicide.
varieties.

This genetic package—a Bt gene plus a promoter

The three primary components of the genetic
and marker—can be inserted into cotton plant
package inserted into cotton DNA include:
DNA through a variety of plant transformation
techniques. Transformed plants may be affected
Protein gene. The Bt gene, modified for im-
by the genetic package, as well as the location of
proved expression in cotton, enables the cotton
plant to produce Cry-protein. The first varieties of the new genes in the plant DNA. The insertion
Bt cotton produced in the United States contained site may affect Bt protein production and other
one Cry-protein gene—Cry1Ac. Other varieties plant functions as well. So biotechnology compa-
contain a “stacked” gene complex, for example— nies carefully scrutinize each transformation to
one gene for insect control (Cry1Ac) and one ensure adequate production of Bt protein and to
gene to protect the cotton from application of the
limit possible negative effects on agronomic
herbicide glyphosate. Future cotton varieties may
traits.
include these genes, other genes that allow the
plant to produce different Cry-proteins, or insecti-
cidal proteins from sources other than Bt. There Following a successful transformation, plants are
are many possible combinations for crop im- entered into a traditional backcross breeding
provement traits. program with the variety chosen to receive the
foreign Bt gene package. The final product, a Bt
Promoter. A promoter is a DNA segment that
cotton variety, is developed after four or five
controls the amount of Cry-protein produced and
backcross generations. Even though the new
the plant parts where it is produced. Some pro-
moters limit protein production to specific parts of transgenic Bt cotton variety may be named after
the plant, such as leaves, green tissue, or pollen. the parent variety, agronomic qualities can be
Others, including those used in Bt cotton and considerably different.
certain Bt 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 Bt cotton
produce some Bt protein throughout the growing
season.

6
The Safety of Bt and Bt Cotton

B efore registering Bt cotton, EPA reviewed

data on Bt insecticides that had been accu-
mulated for decades. Bt Cry-proteins were found
from the requirement of tolerance in food because
of their history of safety and because they de-
grade rapidly in the environment. These Cry-
to be toxic only to certain insect groups and to proteins are considered among the safest and
have no known negative effects to humans, most environmentally friendly insecticides
domestic animals, fish, wildlife, or other organ- known.
isms. EPA exempted Bt-produced Cry-proteins

Controlling Tobacco Budworms and

Bollworms: Does Bt Cotton Do the Job?

W 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
Bollworm/budworm
eggs on a leaf. Courtesy than 98 percent control of tobacco
of S. Stewart. 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.

7
Bt cotton is less effective against bollworms. Still, hatch, the larvae may move into open blooms
it can eliminate as many as 60 to 90⫹ percent of where they feed on flower parts, including
the bollworms infesting a cotton field (table 1). pollen, that are known to have a lower level of
When there are high numbers of bollworms on Bt Cry-protein than other plant parts. This feeding
cotton during the bloom stage, growers may need on less toxic parts may result in lower mortality.
to apply one or more supplemental insecticide The bollworm in addition is naturally more
treatments to prevent economic damage. This has tolerant of Cry1Ac Bt protein than the tobacco
been well documented through field research. budworm. These two factors help explain why
Fortunately, the bollworm can still be managed more bollworms than budworms survive on Bt
more effectively and inexpensively with currently cotton. Moreover, Cry-protein expression in Bt
available insecticides—except perhaps in South cotton decreases about 80 days postplanting,
Carolina where resistance to insecticides has been which may allow higher survival of bollworms.
detected. This late decline may also reduce Cry-protein
effectiveness against other insect pests that occur
Bollworm moths lay their eggs on cotton plant later in the growing season and feed on mature
terminals, leaves, buds, and flowers. As the eggs leaves.

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

Bt and non-Bt cotton genotypes

Percent survival

Insect 1994 1995

Tobacco budworm
on Bt cotton leaf 1 0
on Bt cotton square 2 0

on non-Bt cotton leaf 86 84

on non-Bt cotton square 69 67

Bollworm
on Bt cotton leaf 7 23
on Bt cotton square 5 4

on non-Bt cotton leaf 80 74

on non-Bt cotton square 63 52

Fall armyworm
on Bt cotton leaf 61 76
on Bt cotton square 33 25

on non-Bt cotton leaf 76 92

on non-Bt cotton square 45 42

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

8
Controlling Other Insects

Southern armyworms.
Tarnished plant bug Courtesy of R. Smith.
nymph

T obacco budworms and bollworms are not the

only insect pests that attack cotton. Unfortu-
nately, the Cry1Ac protein has essentially no
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
effect on many of them. Pests that Bt cotton does insecticide use against tobacco budworms and
not directly affect include boll weevils, cotton bollworms allow more of the aphid’s natural
aphids, cotton fleahoppers, cutworms, spider enemies to survive, and they in turn reduce aphid
mites, stink bugs, tarnished plant bugs, thrips, numbers.
and whiteflies. In some caterpillar species, Bt
cotton may provide only 10 to 50 percent control. On the other hand, reducing the amount of foliar
This partial suppression may be cause for concern insecticide may also allow other pests normally
in the later years of an insect resistance manage- controlled by the insecticides to become more
ment program because it does not provide a high- abundant. Boll weevils, stink bugs, and plant
dose strategy (see Glossary) for insects such as bugs, for example, have by chance been con-
beet armyworms, fall armyworms (table 1), trolled by foliar sprays applied against tobacco
southern armyworms, soybean loopers, and budworms or bollworms. So, reduction in the use
yellowstriped armyworms. Bt cotton at this time of insecticides on Bt cotton has allowed these
provides good to excellent control of cabbage pests to increase in some areas. Offsetting this
loopers, cotton leafperforators, European corn disadvantage, however, is Bt cotton’s usefulness
borers, salt marsh caterpillars, and cotton square where the boll weevil has been eradiated.
borers. Future varieties of Bt cotton may produce
different Cry-proteins or other new toxins that
will control a wider variety of pests.

9
Does Bt Cotton Affect
Beneficial Insects?

Big-eyed bug

Lacewing larva

any studies have shown that Cry1Ac in similar effect on lacewing larvae. Theoretically, Bt
M Bt cotton is highly selective because it
kills only certain caterpillar species. Bt cotton
cotton may indirectly lower the general abundance
of some beneficial insects, since it causes caterpillar
has minimal or no effect on beneficial insects, populations to decline, resulting in less food for the
including honey bees, lady beetles, spiders, big- predators, parasites, and the pathogens that attack
eyed bugs, pirate bugs, and parasitic wasps. them. Offsetting this effect is the positive influence
However, laboratory research has shown that gained from reducing conventional broad spectrum
Cry1Ab protein can indirectly affect green insecticide use in Bt cotton. The overall balance of
lacewing larvae that eat Bt-killed caterpillars. It these contrasting influences is currently unknown
is not known if Cry1Ac in Bt cotton has a and is difficult to predict.

10
The Value of Bt Cotton to the
Cotton Farmer
s is the case with most new technology, Bt • situations where a properly timed and applied
A cotton offers value to the cotton farmer in
specific circumstances (table 2). Information on
insecticide management program cannot be
achieved, such as in fields that do not allow
economic benefits is limited due to the short time the proper operation of air or ground sprayers,
the technology has been available and the many in remote fields, or in cases where cotton
new cotton varieties introduced each year. Re- acreage exceeds the amount of equipment or
cently, the technology fee and the seed cost for Bt personnel dedicated to insecticidal control
cotton have decreased, which has affected the
economics of growing it. Yield data are available • situations where insecticidal control is exces-
from federal and university entomologists and sively costly (for example, more than $40/
agronomists, as well as seed companies, in nearly acre), as may be the case when high infesta-
every cotton-growing state. tions are coupled with newer, expensive
insecticide products—even though the insecti-
Comparisons of the Bt and non-Bt cotton varieties cide program may protect the crop
generally show that Bt cotton offers an economic
advantage in instances where effective insecticid- • situations where eliminating early tobacco
al control of certain caterpillar pests is difficult to budworm sprays allows survival of beneficial
achieve or is very costly. insects that reduce the risk of pest infestations

Examples of such situations

include
Table 2. Yield comparisons between Bt and non-Bt cotton
• insecticide-resistant to-
bacco budworms or boll- Yield (lb lint/acre)
worms

Bt cotton non-Bt cotton

• high populations of sus-
Year (unsprayed) (sprayed)
ceptible tobacco budworms
or bollworms, such as in
outbreak seasons or during 1994 1369 1392
the initial phases of boll 1995 1465 1425
weevil eradication

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

11
 associated with higher insecticide use (as with worker protection and pesticide label restrictions.
beet armyworms or aphids). Compliance often makes the grower’s job more
difficult and increases the risk of consequences
These situations suggest that income from re- arising from noncompliance. Insecticide use in
duced insecticide input, along with higher yields sensitive areas—next to schools, fish ponds,
as a result of less insect damage, offset the tech- dwellings, medical facilities, roads—can be a
nology fee and favor Bt cotton. However, if concern to the grower and his or her neighbors. If
infestations of tobacco budworms or bollworms legal and social risks are a concern, Bt cotton may
are low, or the yield of the Bt variety used is low, have value to the grower by reducing these risks.
the technology fee may exceed the value of the Bt
toxin. Also, a conventional insecticide spray Adopting Bt cotton may result in a more efficient
program may allow the farmer to grow certain enterprise, while maintaining a high level of
high-yielding cotton varieties that perform better insect pest control. Insect management with
than available Bt varieties. Some economic com- insecticides can be time-consuming and involve a
parisons show little or no economic advantage to significant amount of labor and equipment. If Bt
using Bt cotton (table 3), whereas the economic cotton reduces the insect control burden and
returns in other circumstances have been positive decreases the need for labor and equipment,
(table 4). Even in the areas that economically these resources may be diverted to other farm
favor Bt cotton, however, there are often situa- obligations.
tions where high-yielding, non-Bt varieties
grown under conventional spray programs Bt cotton may reduce potential resistance to foliar
provide equal or greater economic returns than insecticides in tobacco budworms and
Bt varieties. bollworms. Since resistance genes selected as a
result of one insecticide may be eliminated by a
Regardless of whether or not a particular variety different and unrelated insecticide, the rotation of
contains the Bt gene, yield potential continues to toxins, including Bt Cry-protein, may be able to
be the primary consideration when selecting a slow the selection of genes for resistance to any
cotton variety. The Cry1Ac gene in Bt cotton is single toxin. Farmers will be obliged to include a
only one of thousands of genes that affect the non-Bt refuge to accompany any Bt cotton
yield and other characteristics of different cotton planted (see refuge section, p. 23). Use of a
varieties. Growers who are selecting cotton sprayed refuge provides an excellent opportunity
varieties should carefully consider the traits of to use newer insecticide classes, as well as
each, including yield performance, relative effective older chemistries, as aids in reducing
maturity, fiber quality, ability to withstand ad- development of insecticide resistance in tobacco
verse weather, and harvestability. budworms and bollworms and in adopting
resistance management plans for other
Bt cotton may offer value to the cotton farmer in chemistries. Maintaining effective refuge areas
ways that are hard to measure by short-term that are close to Bt cotton also helps slow
economic comparisons. Using insecticides in- resistance to the current Cry1Ac toxin cotton
volves complying with certain laws, such as varieties and may decrease resistance to other Bt
toxins as they are introduced.

12
Table 3. Cost of Bt cotton vs. non-Bt cotton in North Carolina, 1999

Expense Bt cotton Non-Bt 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 Bt cotton vs. non-Bt cotton in Mississippi, 1995–97

Expense Bt cotton Non-Bt 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.

13
Bt Cotton and Boll Weevil Eradication:
Can They Work Together?

Boll weevil

B oth Bt cotton and boll weevil eradication

have great value for the cotton industry in
the United States. Available Bt cotton varieties are
is, therefore, necessary to realize the maximum
potential benefit from growing Bt cotton.

highly effective against tobacco budworms and Eradication of the boll weevil reduces the number
provide significant suppression of bollworms and of foliar insecticide treatments necessary to
certain other caterpillar species. Consequently, control other pests in non-Bt cotton. For example,
the foliar insecticide treatments required to in Georgia, before the boll weevil was eradicated,
control these pests in Bt varieties are substantially the amount of insecticide required for control of
lower than in non-Bt varieties (table 5). Many of other pests was notably higher than the amount
the treatments used to control tobacco budworms required following eradication. Data from Geor-
and bollworms also are active against other pests, gia show an increase in the need for treatment for
such as boll weevils and tarnished plant bugs, other pests during the early years of boll weevil
and lower insecticide use in Bt cotton reduces eradication, because the insecticides used de-
coincidental control of such pests. As a result, Bt stroyed beneficial insects. In the absence of
cotton varieties grown in boll-weevil-infested beneficial insects, populations of pests such as
areas typically require more foliar insecticide tobacco budworms, bollworms, beet armyworms,
treatments (table 5). Eradication of the boll weevil cotton aphids, and whiteflies often increase.

14
Bt cotton has proven itself to be a useful tool in Where boll weevils are eradicated, the overall
minimizing the risk from certain caterpillar pest value of Bt cotton increases. In areas free of boll
outbreaks in the early years of a boll weevil weevils, insecticide sprays can be cut back and
eradication program. In recent eradication pro- beneficial organisms more successfully relied on
grams in Alabama, Mississippi, and Louisiana, to reduce pest insects. The example from Georgia
producers chose to plant more than 80 percent of shows a distinct decline in the need to treat for
their acreage to Bt varieties, primarily to mini- other pests once the boll weevil was eradicated
mize risks of tobacco budworm outbreaks. How- (fig. 2). An additional reduction in foliar treat-
ever, there has been a negative aspect to this high ments was observed after Bt cotton became
use of Bt cotton. When most acreage is planted to available in 1996. This example shows that boll
Bt varieties, growers apply fewer sprays that weevil eradication and Bt cotton are complemen-
provide coincidental control of boll weevil. This tary in reducing total insecticide use and lower-
makes boll weevil eradication somewhat more ing insect control costs. Reducing insecticide use
difficult and more costly. Greatly overshadowing and relying more heavily on biological control
this negative influence are the positive effects of also benefits efforts to manage insecticide resis-
Bt cotton in reducing the risks of secondary pest tance.
problems during the initial years of an eradica-
tion effort.

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
on Bt cotton 0.3 0.9 1.2
on non-Bt cotton 3.1 3.1 5.2

Boll weevils
on Bt cotton — 2.6 3.3
on non-Bt cotton — 1.9 1.9

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

15
 16
Boll weevil eradication began began
14
Average no. treatments

12
10
8
Bt cotton introduced
6
4
2
0
86 87 88 89 90 91 92 93 94 95 96 97 98

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

16
Resistance to Bt Cotton in Tobacco
Budworms and Bollworms

t is commonly known that more than 500 high levels of resistance to Bt sprays in Florida,
I species of insects and mites have developed at
least some degree of resistance to insecticides
Hawaii, North Carolina, Asia, and other locations
(Tabashnik et al. 1990). It has also shown resis-
(Georghiou and Saito 1983), knowledge clearly tance to Bt transgenic canola plants. Researchers
showing that many arthropods have the genetic have already developed laboratory colonies of
potential for rapid adaptation to chemicals in Colorado potato beetles, European corn borers,
their environment. Most scientists agree that the tobacco budworms, and bollworms that are
tobacco budworm and the bollworm will eventu- resistant to Cry-proteins (fig. 3). The resistant
ally become resistant to the Cry1Ac protein used laboratory colonies of tobacco budworms and
in current Bt cotton varieties. The tobacco bud- bollworms demonstrate these insects have the
worm has a well-known reputation for develop- genetic potential to become resistant.
ing resistance to chemical insecticides. Currently
it is resistant to most conventional insecticides Crop protection with Bt cotton is a form of host
used on cotton. However, for the time being, it is plant resistance, like resistance of soybean variet-
extremely susceptible to the Cry1Ac protein in Bt ies to the soybean cyst nematode. Farmers are
cotton. The bollworm is inherently more tolerant familiar with resistant crops losing their protec-
to this toxin, and it is likely to develop resistance tion from pests, like nematodes overcoming
faster than the tobacco budworm. soybean resistance and mildew adapting to
resistant wheat varieties. While the same fate is
Field and laboratory studies document the devel- predicted for Bt cotton, the time necessary to
oped resistance of several insects to spray formu- reach economic resistance can be greatly influ-
lations of B.t. toxins. The best-known example is enced by the way growers and consultants utilize
the diamondback moth, a caterpillar pest that this crop.
attacks cabbage and related plants. It has shown

17
 100

80
Mortality (percent)

60 Control strain
Control female X selected male
Selected female X control male
Selected strain
40

20

0
0.01 0.1 1 10 100

Cry1Ac Concentration (␮g/ml)

Figure 3. Development of resistance to Bt cotton in tobacco budworm in laboratory experiments.

Reprinted with permission from Gould et al. 1992.

18
How Resistance Develops
variety of factors may influence the rate at • The number of susceptible moths available for
A which tobacco budworms and bollworms
become resistant to Cry-toxin in Bt cotton.
mating with moths carrying resistance gene(s).

Before exposure to Cry-toxins—by spraying

These factors include: insecticides containing Bt or through planting Bt
crops—very few tobacco budworms and boll-
• The number of generations of tobacco bud- worms (perhaps 1 in 100,000 to 1 in 1 million)
worms and bollworms exposed each year to carry two copies of a resistance allele (RR),
Bt plants containing the same or similar toxins meaning they are fully resistant to Bt cotton.
Some tobacco budworms or bollworms have a
• The percentage of each generation exposed single copy of a resistance allele and a susceptible
to Bt plants containing the same or similar Cry- allele (RS); these are called heterozygotes. The
toxins overwhelming majority have two copies of a
susceptible allele (SS).
• The mortality level that Cry-toxin causes
among tobacco budworms or bollworms Most of the susceptible insects (SS) are killed after
carrying one copy of a resistance allele and feeding on Bt cotton, depending on the dose of
one copy of a susceptible allele. The mortality Cry-toxin in the plant. The heterozygous insects
level is determined by the Cry-toxin concentra- (RS) usually are more difficult to kill than the
tion in the plant, which in turn may determine susceptible insects. Still, heterozygous insects are
the functional dominance of the allele affecting not considered Bt resistant in most instances,
resistance because most will die if the toxin dose in the
plant is high enough. Resistant insects (RR) are
• The frequency with which Cry-resistance not killed by Bt toxin. The difference in survival
alleles are expressed in the tobacco budworm rates among these three types represents a selec-
or bollworm population before exposure to tive advantage for resistant (RR) tobacco bud-
Cry-toxins and the dominant or recessive worms and bollworms feeding on Bt cotton,
nature of the resistance alleles because they will survive while susceptible
individuals will die.
• The migration patterns of tobacco budworm
and bollworm moths As the use of Bt cotton increases, a higher per-
centage of tobacco budworm and bollworm
• The survival advantage or disadvantage that populations will be exposed to Cry-toxins. Rela-
resistance allele(s) offer tobacco budworms or tively more caterpillars carrying resistance alleles
bollworms both in the presence and absence will survive to adulthood, while fewer suscep-
of Cry-toxins

19
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
Bt cotton. Reprinted with permission of
D. Sumerford.

20
Bt Corn: Does It Hasten Resistance
to Bt Cotton?

S everal types of Bt corn use different Bt

transformation events and express Cry1Ab,
Cry1Ac, Cry1F (not commercially available), or
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-
Cry9c toxins. Some brands of Cry1Ab corn worms should not be affected by corn since they
express Bt toxin in the ears, where bollworm very seldom infest this crop.
larvae (called corn earworms on corn) may be
feeding, but other brands do not have toxin in the Growing corn in cotton production areas could
ears. Cry9c toxin does not kill bollworms and have a major influence on bollworms’ develop-
should not affect development of Bt resistance. ment of resistance to Bt cotton. Bollworm cater-

60

50
Years to resistance

20% of all corn planted in Bt cotton

40 40% of all corn planted to Bt corn
60% of all corn planted to Bt corn
30

20

10

0
20% 40% 60% 100%
% of all cotton planted to Bt cotton

* 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.

Figure 5. Computer simulation showing the influence of Bt corn and Bt cotton on the rate
that bollworms develop resistance to Bt. Reprinted with permission from ILSI Health and
Environmental Sciences Institute 1998.

21
pillars infest the whorl and ear stages of corn. corn is replaced with ear-expressing Bt corn, the
Moths are strongly attracted to silking corn to lay refuge effect that non-Bt corn provides dimin-
eggs, and a major portion of the second-genera- ishes and bollworm populations are exposed to
tion population may develop in corn ears. Non-Bt Bt toxin—both from the increase in Bt crops and
corn will not select for Bt-resistant bollworms but the decrease in non-Bt crops; and (2) as men-
will act as a refuge and delay resistance. The tioned, greater exposure to the Bt toxin gives
more non-Bt corn—other non-Bt hosts—grown in bollworms that are carrying a resistance allele(s) a
a corn and cotton production area, the slower Bt survival advantage and hastens the pace to
resistance develops. resistance. If robust non-Bt refuges are main-
tained for both cotton and corn, the rate that
However, growing ear-expressing Bt corn can resistance develops is reduced and should allow
hasten Bt resistance in two ways: (1) if non-Bt a prolonged life for both Bt products.

Implications of Resistance
rowers know that production costs can growers need them. A case in point is the high
G increase as insects develop resistance to
insecticides. How much of an increase
cost of recently released tobacco budworm
insecticides for use on cotton.
depends on the availability and cost-effectiveness
of alternative strategies. If such strategies do not The length of time that an insecticide or Bt cotton
exist or are expensive, production costs and crop remains effective may depend upon how well
losses may be very high. The resistance of tobacco growers and pest managers follow resistance
budworms and bollworms to Bt cotton is accom- management guidelines. Improper usage dra-
panied by costs similar to those encountered with matically decreases the effective life of a product.
resistance to conventional insecticides. If Bt products are carefully used, their effective-
ness may be extended for many years. But if the
There are also additional costs to resistance. Cry- technology is abused, budworms and bollworms
toxins in Bt plants and Cry-toxin-based insecti- will quickly become resistant. Preserving the
cides are not easily replaced when insects develop effectiveness of Bt cotton is one way to keep pest
resistance. In the past, growers relied on the management costs at the lowest level.
availability of new insecticides. There is no guar-
antee this process will continue. Developing new Other less obvious risks could also occur. In the
insecticides and transgenic insecticidal crops is past the appearance of resistant tobacco bud-
time intensive, difficult, and expensive. Research- worm or bollworm infestations increased the
ers may not be able to develop new insecticides at frequency of scouting and complicated decision-
reasonable costs that conform to environmental making by growers and pest managers. If new,
and performance requirements as quickly as selective insecticides are needed to combat Cry-

22
toxin-resistant pests, secondary insects may should always use the refuge sizes specified in
become more numerous. Also, caterpillars ex- the licensing agreement in order to slow the
posed to one Cry-protein (for example, Cry1Ac) development of resistance to the Cry-proteins
may develop resistance to other similar Cry- contained in sprayable Bt insecticides and Bt
proteins, even though they have not been ex- crops.
posed to them; this process is known as cross-
resistance. When growing Bt crops, growers

Can Growers Slow Resistance?

logical, science-based, and proactive will die, and resistant (RR) insects will survive.
A resistance management strategy is neces-
sary to prevent tobacco budworms or bollworms
When resistant survivors from the Bt crop mate
with susceptible insects from a non-Bt refuge, the
from developing resistance to Bt cotton in less offspring receive one allele—either an S or an R—
than 10 years. All members of the cotton industry from each parent. Offspring from the cross-
should practice this strategy to slow development mating will be heterozygotes (RS). If Bt plants
of resistance. EPA has registered products from express a high dose of toxin, almost all heterozy-
companies that sell Bt cotton seed, and these gotes will be killed. Eliminating most heterozy-
companies are required to recommend and gotes also eliminates most resistant alleles from
support insect resistance management (IRM) the surviving populations and greatly slows the
strategies for Bt cotton. IRM is a key element of a development of resistance.
good overall integrated pest management (IPM)
program. Without a source for producing susceptible
insects—an IRM refuge—the development of
EPA has accepted a resistance management resistance is proportional to the dose; that is, the
concept for Bt cotton known as the “high dose/ higher the dose, the more rapidly resistance
refuge strategy.” This approach has two comple- develops. Therefore, the high dose/refuge strat-
mentary principles: (1) Bt plants must produce a egy is a high-risk strategy, depending upon the
high dose of Cry-toxin throughout the season, availability of properly functioning IRM refuges.
and (2) effective IRM refuges must be maintained. The high dose, or in other words high effective-
An IRM refuge consists of a non-Bt host crop, and ness, is very good for pest control, but it can
it is intended to produce susceptible tobacco cause the rapid development of resistance in the
budworms or bollworms or both. absence of effective IRM refuges.

In theory, if Bt plants express a high dose of toxin, As mentioned, the IRM refuge is acreage planted
then all susceptible (SS) pests eating the plant will with a non-Bt crop that serves as a host for to-
die, almost all of the heterozygous (RS) insects

23
bacco budworms or bollworms or both. And the available Bt cotton varieties (Bollgard) should
refuge must be close enough to the Bt crop to qualify for the “high dose” definition for tobacco
ensure that susceptible moths have an opportu- budworms.
nity to mate with resistant ones. This means that
moths from the Bt cotton and the refuge must As mentioned, research shows that bollworms are
emerge at about the same time and be relatively less sensitive to Cry1Ac toxin than tobacco
close to each other. budworms. Researchers estimate that from 5 to 25
percent of susceptible (SS) bollworm larvae
Commercial Bt cotton varieties (Bollgard) cur- survive on Bt cotton varieties now in use
rently express enough Cry1Ac toxin throughout (Bollgard), and estimates for heterozygote (RS)
most of the season to kill all susceptible (SS) and survival are significantly higher. So the Bt cotton
almost all heterozygous (RS) tobacco budworms. grown now cannot be considered “high dose”
Only resistant (RR) tobacco budworms are ex- against bollworms.
pected to easily survive. Thus, commercially

Future IRM Refuge Options

efuge regulations originally mandated in the 20-percent-sprayed refuge required on the
R 1995 for Bollgard varieties will remain in
effect through the 2000 growing season. When the
original Bt cotton label (Bollgard cotton) may not
adequately delay resistance in bollworms and
registration for Bollgard cotton expires after the tobacco budworms. Studies using computer
2000 growing season, new refuge requirements models (Gould et al. 1992, ILSI 1998) also suggest
may be forthcoming. At the time of publication, that bollworm resistance to Bt cotton can occur
no final refuge requirements had been deter- quickly if Bt cotton is extensively planted and
mined for the 2001 growing season and beyond. only small IRM refuges are used. Research indi-
The issue will be debated before the final decision cates that bollworms, and to a lesser extent
is made; recommendations will range from budworms, have the genetic ability to adapt
complete removal of Bt technology from the quickly to Bt toxin (Gould et al. 1992, Sumerford
marketplace, to a minimum of a 50-percent non- et al. 2000, Burd et al. 2000).
Bt cotton refuge, to no change from the current 4-
percent-unsprayed or 20-percent-sprayed refuge In 1999, EPA and the U.S. Department of Agricul-
scenarios. ture, proposed for discussion the following two
structured refuge options to mitigate the resis-
Computer simulation models, along with limited tance of tobacco budworms and bollworms to Bt
evidence from field and laboratory studies, toxins expressed in Bollgard Bt cotton:
suggest that the 4-percent-unsprayed refuge or

24
 1 An external refuge of at least 30% non- (U.S. Environmental Protection Agency
Bt cotton should be implemented. The and U.S. Department of Agriculture
placement of the structured refuge 1999).
should be planted within 0.5 miles of
the farthest Bt cotton in a field to pro- EPA and USDA co-authored this position paper to
vide Bt-susceptible moths. The external provide stakeholders with a focal point for dis-
refuges of non-Bt cotton can be treated cussing future recommendations. The entire
with any other registered non-Bt insecti- document may be reviewed at http://
cide or other insect control measures. www.epa.gov/oppbppdl/biopesticides/
2 In-field refuges of at least 10% non-Bt otherdocs/bt_position_paper_618.html. Updated
cotton refuge should be implemented. versions of refuge guidelines will be available at
In-field refuges should be planted this website.
entirely within the field as blocks,
minimum size to be determined based Entomologists in the public sector are concerned
on planter size, to provide Bt-suscep- with cotton insects’ almost 50-year history of
tible moths. Cotton fields may be developing resistance to sprayable insecticides
treated with any registered non-Bt from almost every chemical class. Sound biologi-
insecticide or other control measures, cal reasons indicate that Bt insecticide within
as long as the entire field is treated in plants will be even more vulnerable to the devel-
the same manner. This means that Bt opment of insect resistance. Given the reduced
cotton rows cannot be treated indepen- pace of developing new replacement insect-
dently from non-Bt cotton rows with control technology, the U.S. cotton industry may
insecticides or other insect control face a greater risk of insecticide resistance than
measures. ever before.

In both options, (1) and (2), agronomic The refuge plans currently in use are designed for
practices used for farming the non-Bt Bollgard cotton varieties that perform like the
cotton must ensure adequate produc- varieties first marketed (for example, NuCotn 33
tion of susceptible [tobacco budworm and NuCotn 35). These refuges may be inappro-
and cotton bollworm] adults to mate priate for new Bt genes or stacked gene Bt prod-
with resistant adults emerging from Bt ucts that may be marketed in the near future. One
cotton. In particular, termination of hopes new Bt gene products will deliver a high
growth of non-Bt cotton should not be dose of toxin to the bollworm and secondary
done until termination of growth of Bt lepidopterans and qualify for the high-dose refuge
cotton has begun. In general, agro- strategy. If these products do qualify, scientific
nomic practices for non-Bt cotton theory should support less restrictive IRM plans
should be similar, as practical, to those for such future products. The development of
of the Bt cotton grown in the same highly effective transgenic insecticidal cotton
management unit, especially regarding plants with multiple toxic genes should be
crop nutrition, irrigation, and termination encouraged.

25
The Economics of Cotton Refuges
be more profitable than that achieved using other
T he expenses and yield reduction associated
with refuges must be viewed as costs of
using Bt cotton technology. Balancing these are
insect management plans.

the economic and other benefits realized from Bt Preserving Bt cotton technology from resistance
cotton. In non-Bt cotton when insecticides are helps ensure that growers will have effective
used for insect management, the chemical costs, insect management options in the future. Experi-
application costs, and risks of handling pesticides ence shows that greater crop loss and higher cost
and possible complaints about them, among insect management typically follow the develop-
other costs, are balanced against the benefit of ment of resistance. So, clearly, an economic
insect control and higher yields. If, for example, a benefit is often difficult to estimate. There is no
90/10 in-field refuge plan is used, the 10-percent guarantee that new and cost-effective biotechnol-
refuge may be damaged by insects, but the yield ogy or insecticidal products will replace existing
and insect control cost over the total acreage may technology in a timely fashion.

Can the Development of Resistance

Be Successfully Monitored?
onitoring for the development of insect Companies that register the proteins in Bt cotton
M resistance to Cry-proteins is a difficult
and imprecise task. It may include surveying the
with EPA are required to keep annual sales
records on a county-by-county basis and submit
annual use of Bt cotton in each county or parish, summaries for each state. Surveys of grower use
annual testing of tobacco budworm and boll- of Bt cotton, conducted by Cooperative Extension
worm populations for Bt sensitivity, and checking Service personnel, also may be available and
Bt crops for any changes in the survival rate of could be used as a guide for monitoring areas
tobacco budworms and bollworms. Monitoring where circumstances favor the development of Bt
tobacco budworm and bollworm populations on resistance by tobacco budworms and bollworms.
Bt cotton is important in providing the earliest Researchers predict that resistance is more likely
warning that they are developing resistance; it is in areas where the use of Bt cotton has been high
also required by EPA. And it improves resistance for several years. Other characteristics of a par-
management efforts.

26
ticular region—such as the amount of non-Bt toxins requires precise techniques that provide
cotton acreage and alternative hosts of tobacco information from a large number of insects and
budworms or bollworms—may be surveyed to many locations. The U.S. Department of Agricul-
help determine the risk of resistance. Assigning ture and state universities are cooperating to
resistance risk categories to unique cotton-grow- establish Cry-toxin susceptibility baselines and
ing environments across the Cotton Belt can be measure resistance in tobacco budworm and
helpful in monitoring the efforts at specific sites. bollworm larvae collected from all across the
Cotton Belt, especially from the mid-South and
A major difficulty in monitoring is that resistance the Southeast (Summerford et al. 2000). Technol-
can develop to an advanced stage before it is ogy companies are also conducting similar tests.
easily detected in the field. For example, if 1 of If these monitoring studies are sufficiently wide-
every 100,000 tobacco budworms was resistant spread, changes in the susceptibility of tobacco
when Bt cotton was first marketed, resistance budworms and bollworms to Bt may be success-
levels would advance many-fold—perhaps to 1 fully detected prior to field control failures. EPA
per 100 individuals—before field failures could is reevaluating annual monitoring requirements
be detected. for development of remedial action plans.

Detection of resistance in the field by scouts and Counting the number of surviving caterpillars in
growers would likely occur only in outbreak fields of Bt cotton may be helpful for detecting
years, unless the resistance level was more than 1 the development of resistance. Susceptible popu-
insect per 100. The development of resistance is lations of tobacco budworms are decimated in Bt
largely undetectable by measuring field perfor- cotton, so scouting for their survival may uncover
mance of Bt cotton. This invisible phase is due, in resistance before economic field failures occur.
part, to monitoring techniques that are not sensi- This method is not highly sensitive, however, and
tive enough to detect early shifts in tobacco bud- insect resistance to Cry1Ac toxin must reach a
worm and bollworm resistance. relatively high level—for example, one resistant
insect in 500 susceptible insects—before detection
A first step in establishing an effective monitoring is possible.
program is to document the initial susceptibility
level of tobacco budworms and bollworms to the The tolerance of bollworms to Cry1Ac toxin is
Bt toxin. With this information as a baseline, it already too high to allow close measurement of
may be possible to spot small, early changes in changes in resistance using scouting and larval
susceptibility before field control failure occurs. survival. What’s more, less than adequate Bt
Laboratory studies are uncovering more informa- expression in the cotton plants may produce a
tion about the frequency with which resistant false positive for resistance. Field scouting will
genes occur and the dominance of these genes. detect only large shifts in the survival rate of
Detecting changes in susceptibility to the Cry- bollworms.

27
Fitting Bt Cotton Into an Insect
Management Program

A s with all conventional insecticides, Bt Combined use of optimal planting times and
cotton must be managed wisely and used in early-maturing cotton varieties reduce crop
combination with other insect pest and crop damage, minimize insecticide use, and diminish
management practices. For more than 20 years, the chances of pests’ becoming resistant to Bt
IPM has focused on using insecticides on an as- toxin and insecticides. Early fruit set should be
needed basis only. The decision to use insecti- protected so maturity is not delayed.
cides has generally been based on crop scouting
to determine insect pest densities and the use of Rotating Bt crops with other non-Bt crops (tem-
spray thresholds. Of course, Bt crops cannot be poral refuge) that also are hosts of tobacco bud-
used as-needed because the Bt toxin is in the worms and bollworms is an example of resistance
plant. For this reason, tobacco budworms and management and offers overall pest management
bollworms have greater opportunities for expo- benefits. Soybeans, peanuts, and tobacco can
sure to the toxin, and development of resistance harbor tobacco budworms, and these same crops,
is more likely with Bt plants. plus corn and grain sorghum, are all hosts of
bollworms. Yieldgard Bt corn would not be
Consequently, resistance management must be considered an adequate alternate host, but other
part of the total pest management package. On available Bt corn brands and non-Bt corn could
the positive side, Bt plants, in the absence of serve as alternate hosts. Certain forage crops,
insecticide sprays, will help preserve beneficial such as crimson clover and alfalfa, and many
insects, allowing growers to take advantage of weeds may also serve as hosts to tobacco bud-
them as a free resource, especially in areas where worms and bollworms. These alternate hosts can
the boll weevil has been eradicated. provide an important refuge benefit and help
slow the development of resistance. In fact,
When developing crop production plans, growers according to computer models, the rate that
should consider all insect management strategies bollworms develop Bt resistance is directly
and tactics, with emphasis on the following areas: related to the proportion of non-Bt corn (not
Yieldgard) in a cotton and corn cropping system.
Cultural practices. Early crop maturity (which
may be optimized with computerized manage- Beneficial insects. Conventional insecticides
ment programs such as COTMAN) should be often have a significant adverse effect on benefi-
part of every pest management plan. Most grow- cial insects in cotton. Populations of cotton
ers realize that early fruit set and early fruit aphids, beet armyworms, tobacco budworms,
maturity help reduce insect pest infestations. bollworms, fall armyworms, and other pests

28
frequently increase following insecticide use due ing state publish variations for Bt cotton scouting
to the destruction of beneficial arthropods. Bt and treatment thresholds.
toxins do not have this effect. The loss of benefi-
cial insects can also be reduced by using fewer Bt cotton kills almost all newly hatched tobacco
applications of insecticide or by using an insecti- budworm larvae and most bollworm larvae, so
cide that is less harmful. using eggs as a criterion for insecticide applica-
tions may lead to unneeded foliar treatments.
Scouting and thresholds. Bt cotton is not When large numbers of bollworm eggs are laid,
effective on pests such as boll weevils, plant bugs, survival of the larvae may lead to damaging
and stink bugs. When bollworms, beet army- infestations. As a consequence, in a few states
worms, fall armyworms, and looper moths lay where the tobacco budworm and bollworm
large numbers of eggs, damaging infestations populations consist mostly of bollworms, exten-
may occur because the level of Bt toxin produced sion entomologists have adopted a high egg
is not adequate to fully control them. As a result, threshold for Bt cotton (table 6). In other states
Bt cotton must be scouted in a fashion similar to (Arkansas, Mississippi, and Texas, for example)
that used with conventional cotton but using egg thresholds are not recommended as a treat-
altered techniques and modified treatment ment criterion. Use of the egg threshold is not
thresholds. Entomologists in each cotton-produc- possible in states where the percentage of tobacco

The beneficial wasp, Microplitis croceipes, feeding on bollworm/budworm larva.

Courtesy of J. Powell.

29
budworms is high, since separating tobacco feeding on Bt cotton. In fact, most states recom-
budworms from bollworms without a microscope mend counting caterpillars only of a certain size,
is difficult and time-consuming in every stage but such as 1/8 to 1/4 inch long. Caterpillars that
the adult. reach a minimum size have a greater ability to
survive on Bt cotton. As the caterpillars become
A potential solution to the problem of identifying larger, they become more difficult to kill with the
eggs and small larvae is the development of kits Bt toxin and can damage the crop unless elimi-
containing species-specific monoclonal antibod- nated quickly. The larvae that survive are most
ies. These kits would allow growers and consult- likely to be bollworms. Caterpillars, usually
ants to make better pest control decisions if bollworms, often are found in flowers or “bloom
surviving larvae longer than 1/8 to 1/4 inch are tags,” so greater emphasis is being placed on
found in Bt cotton. This technology is still being examining the blooms and small bolls when
developed. scouting.

Scouting Bt cotton for tobacco budworms and Tobacco budworm and bollworm damage in Bt
bollworms must employ somewhat different cotton can also be a target for scouting. Caterpil-
techniques than scouting non-Bt cotton. Newly lars that grow beyond a minimum size and feed
hatched larvae do not die immediately after 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-Bt cotton — 20 15
Bt cotton — — —

After bloom
non-Bt cotton 20 3 5
Bt cotton 75 5 (bolls) 30

Source: Roof 1999

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 Cotton boll damage from budworm/bollworm larvae

damage. The superficial damage of very small cotton should be based on scouting and treatment
caterpillars should not be mistaken for penetrat- thresholds.
ing injury that may indicate a damaging insect
population. In some situations, fall armyworms Insecticide use depends on several variables,
or beet armyworms can cause this damage, so including the abundance of the pest insect and
scouts should be careful to properly identify the the presence of beneficial insects. Using insecti-
pest species. Some states have developed damage cides against early-season pests, such as boll
thresholds for Bt cotton. weevils and tarnished plant bugs, also reduces
the number of beneficial insects. As mentioned
Insecticide use in Bt cotton. In Bt cotton, all previously, this reduction may result in higher
insect pests not affected by the Bt toxin should be populations of insect pests and fewer opportuni-
managed as they would be in non-Bt cotton. It ties for reduced insecticide use in Bt cotton. In
may be necessary to apply insecticide to control areas where boll weevils have been eradicated
thrips, aphids, boll weevils, tarnished plant bugs, and tarnished plant bugs are infrequent, the
stink bugs, and a few tolerant caterpillars such as presence of Bt toxin and beneficial insects greatly
beet armyworms and fall armyworms. The reduces the amount of insecticide needed on Bt
decision to apply insecticides to Bt and non-Bt cotton.

31
Bt Cotton in the Future
ransgenic crops are one of the most revolu- possibilities for improving agronomic traits and
T tionary developments in agricultural pro-
duction. As with most new technology, there are
pest control characteristics. Genes for new insecti-
cidal toxins will be important for managing a
exciting possibilities for the economic value of Bt wider spectrum of insects and for slowing the
cotton and apprehensions about its wise use. In pace of resistance. If future varieties express a
order to preserve Bt cotton well into the 21st high dose of toxin and the toxins do not have the
century, producers, seed companies, scientists, same physiological target site, the rate that insects
and regulators need to foster strong collaboration develop resistance could be greatly reduced.
to ensure longevity of the technology.

Companies are striving to develop new genes for

insertion into cotton plant DNA to provide other

Key Steps—Implementing a Resistance

Management Plan
he following summary, based on the 3 Record where Bt and non-Bt cotton are
T principles outlined in this publication,
assumes that a voluntary, proactive approach
planted so Bt cotton performance can be
monitored and non-Bt cotton can be scouted
by growers will provide product stewardship and treated as needed.
for long-term yield benefits and profitability.
4 Use all cultural alternatives available for
1 Use Bt cotton in fields where the risk of avoiding high late-season tobacco budworm
severe tobacco budworm and bollworm and bollworm populations. These alternatives
infestations warrants the price premium for include early planting, short-season varieties,
seed. and protection of early fruit.

2 Plant Bt cotton and non-Bt refuge in the 5 Continue using an IPM approach for all
required proportions and patterns. pests. When designing scouting priorities,
keep in mind that other insects—such as

32
32
 beet armyworms, cabbage loopers, cotton feeding damage occurs in Bt cotton, investi-
square borers, European corn borers, fall gate the cause immediately. If needed, get
armyworms, southern armyworms, soybean help in identifying feeding caterpillars. If
loopers, and yellowstriped armyworms—are tobacco budworm or bollworm larvae or
suppressed at various levels by Bt cotton. excessive damage are discovered, resis-
Still other pests are not affected by Bt cotton, tance or tolerance to Bt cotton is a possibility,
including boll weevils, cotton aphids, cotton and the situation should be investigated.
fleahoppers, cutworms, spider mites, stink
bugs, tarnished plant bugs, thrips, and Verify from field records that Bt cotton was
whiteflies. planted where excessive damage or larvae are
observed. Consult your grower’s guide for the
6 Monitor Bt cotton to verify tobacco budworm seed company’s procedure for investigating
and bollworm control throughout the season. suspected cases of resistance or tolerance. Imme-
Since bollworms are not as readily controlled diately notify seed company representatives or
by Bt cotton, carefully watch for a heavy egg- extension agents or both if evidence indicates a
lay and scout for surviving larvae lower in the performance problem.
cotton plants, especially in bloom tags. If

33
References

Burd, A.D., J.R. Bradley, J.W. Van Duyn, and F. Publication 602. University of Minnesota, St.
Gould. 2000. Resistance of bollworm, Helicoverpa Paul.
zea, to CryIAc toxin. In P. Dugger and D. Richter,
eds., Proceedings of the Beltwide Cotton Produc- Roof, Mitchell E. 1999. Cotton insect manage-
tion Research Conference, pp. 923–926. National ment. Clemson University Extension Information
Cotton Council, Memphis, Tennessee. Card 97, Clemson, South Carolina.

Georghiou, G.P., and T. Saito, eds. 1983. Pest Stewart, S., J. Reed, R. Luttrell, and F.A. Harris.
resistance to pesticides. Plenum Press, New York. 1998. Cotton insect control strategy project:
Comparing Bt and conventional cotton manage-
Gould, F., A. Martinez-Ramirez, and A. Ander- ment and plant bug control strategies at five
son. 1992. Broad-spectrum resistance to Bacillus locations in Mississippi (1995–1997). In P. Dugger
thuringiensis toxins in Heliothis virescens. Proceed- and D. Richter, eds., Proceedings of the Beltwide
ings of the National Academy of Sciences USA Cotton Production Research Conference, pp.
89:7986–7990. 1199–1203. National Cotton Council, Memphis,
Tennessee.
ILSI Health and Environmental Sciences Institute.
1998. Evaluation of insect resistance management Sumerford, D.V., D.D. Hardee, L.C. Adams, and
in Bt field corn; a science-based framework for W.L. Solomon. 2001. Tolerance to CryIAc in
risk assessment and risk management. Report of populations of Helicoverpa zea and Heliothis
an expert panel. ILSI Press, Washington, DC. virescens (Lepidoptera: Noctuidae): Three-year
summary. Journal of Economic Entomology. In
Jenkins, J.N., J.C. McCarty, Jr., and R.E. Buehler. press.
1997. Resistance of cotton with endotoxin genes
from Bacillus thuringiensis var. kurstaki on se- Tabashnik, B.E., N.L. Cushing, N. Finson, and
lected lepidopteran insects. Agronomy Journal M.W. Johnson. 1990. Field development of resis-
89:768–780. tance to Bacillus thuringiensis in diamondback
moth (Lepidoptera: Plutellidae). Journal of
Layton, M.B., S.D. Stewart, M.R. Williams, and Economic Entomology 83:1671–1676.
J.L. Long. 1999. Performance of Bt cotton in
Mississippi, 1998. In P. Dugger and D. Richter, U.S. Environmental Protection Agency and U.S.
eds., Proceedings of the Beltwide Cotton Produc- Department of Agriculture. EPA and USDA
tion Research Conference, pp. 942–945. National position paper on insect resistance management
Cotton Council, Memphis, Tennessee. in Bt crops. 1999. Posted on the World Wide Web
May 27, 1999, minor revisions July 12, 1999.
Ostlie, K.R., W.D. Hutchison, and R.L. Hellmich, http://www.epa.gov/oppbppd1/biopesticides/
eds. 1997. Bt corn and European corn borer. NCR otherdocs/bt_position_paper_618.html

34
Glossary

Allele. An alternative form of a gene. For ex- toxin makes the plant tissue—terminals, squares,
ample, a gene determining the reaction of an or bolls—toxic to some important caterpillar
insect to Bt toxin may occur in the form of an pests.
allele for resistance or an allele for susceptibility.
Bt insecticides. Formulations of Bt Cry-proteins
Alternate host. Non-Bt plant types other than manufactured and sold for spraying a wide
cotton that support successful reproduction of variety of plants in order to control certain cater-
tobacco budworms or bollworms or both; soy- pillar or beetle pests. Some formulations are also
bean and corn are examples. used for mosquito and fly control.

Bacillus thuringiensis (Bt). A naturally occurring Cross resistance. Resistance to one or more toxins
soil bacterium that occurs worldwide and pro- from exposure to one or more other toxins, such
duces a toxin specific to certain insects (for as bollworms becoming resistant to Cry1Ac in
example, moths, beetles, blackflies, or mosqui- cotton by being exposed to CryIAb in corn, or
toes). vice versa.

Beneficial arthropods. Usually refers to insect Cry-proteins. Any of several crystalline proteins
predators or parasites of pest insects in crops. found in Bt spores that are activated by enzymes
When abundant, these insects can assist in de- in the insect’s midgut. These proteins attack the
creasing certain pests, although they are often cells lining the gut, cause gut paralysis, and
seriously reduced by insecticide. subsequently kill the insect.

Biotechnology. The science and art of genetically Cry1Ac protein (toxin). One of many Bt crystal-
modifying an organism’s DNA, such that the line protein toxins. The Cry-protein used in the
transformed individual can express new traits first varieties of Bt cotton sold commercially to
that enhance the quality of a product (for ex- growers.
ample, seed oils or fiber quality) or can express
resistance to pests. DNA. Deoxyribonucleic acid, a double-stranded
molecule, consisting of paired nucleotide units
Boll weevil eradication program. A joint USDA, grouped into genes and associated regulatory
producer, and state program designed to elimi- sequences. These genes serve as blueprints for
nate the boll weevil as an economic pest from the protein construction from amino acid building
Cotton Belt. blocks.

Bt cotton. Commercial varieties of cotton that Dominance (of an allele). The ability of one allele
contain a gene from Bacillus thuringiensis within to determine a characteristic in a heterozygous
its DNA. This gene allows the plant to produce individual. For example, an allele for resistance to
Cry-protein within most or all plant tissues. The Bt toxin may be dominant. As a consequence, an

35
insect heterozygous for resistance, that is, con- tion with a non-Bt refuge that allows susceptible
taining an allele for resistance and an allele for insects to survive and mate with resistant indi-
susceptibility, is resistant to Bt toxin. viduals.

Dose of toxin. The amount of toxin eaten per Host plant resistance. Ability of a plant to avoid
insect. For example, the dose of toxin that a insect damage, to kill attacking insects, or to
caterpillar receives by eating part of a Bt plant tolerate their damage.
can be stated as micrograms of Bt toxin eaten per
caterpillar or per milligram of the caterpillar’s Insect resistance management (IRM). A proac-
total body weight. tive approach to offset and slow insects’ resis-
tance to Bt crops or insecticides by reducing
Expression. Production of the desired trait (pro- selection or by counteracting the effects of selec-
tein concentration, for example) in a transgenic tion for resistance genes.
plant. Expression varies with the gene, its pro-
moter, and its insertion point in the host DNA. Integrated pest management (IPM). A manage-
ment approach that integrates multiple, comple-
Gene. The basic unit of inheritance; a section of mentary control tactics to manage pests in a
DNA that codes for a specific product (for ex- profitable, environmentally sound manner.
ample, protein) or trait. Examples of the control tactics include conserva-
tion of natural enemies, crop rotation, host plant
Genetic marker. See Marker. resistance, and insecticides.

Heterozygote. A diploid organism carrying two IPM. See Integrated pest management.
different alleles (for example, susceptible and
resistant) of a gene. IRM. See Insect resistance management.

High dose. A dose of toxin high enough to kill all Larva. Immature stage of certain insect species;
susceptible target pests and nearly all heterozy- examples include caterpillars and grubs.
gotes. Such a dose can be delivered by plants
with sufficiently high concentrations of Bt toxin. Lepidoptera. The order of insects that includes
moths and butterflies. The plant-eating immature
High-dose refuge strategy. A resistance manage- stages are often called worms, caterpillars, or
ment approach that uses plants to minimize the larvae.
rapid selection for resistance to transgenic plants.
This strategy relies upon plants that produce Cry- Marker. A genetic flag or trait used to verify
proteins at a concentration sufficient to kill all but successful gene transformation and to indirectly
the most resistant insects and is used in combina- measure expression of the inserted genes.

36
Mode of action. Mechanism by which a toxin Resistance (by pests). The evolved capacity of an
kills an insect. For example, the mode of action of organism to survive in response to selection from
Bt is ingestion and disruption of cells lining the exposure to a pesticide. The evolution of resis-
midgut. tance occurs through a process of genetic accu-
mulation, whereby a population becomes less
Promoter. A DNA sequence that regulates where, sensitive to the pesticide following repeated
when, and to what degree an associated gene is exposure.
expressed.
Selection. A natural or artificial process that
Refuge or insect resistance management (IRM) results in survival and better reproductive suc-
refuge. A wild host area or an area planted with cess of some individuals over others. Selection
nontransgenic plants (for example, non-Bt cotton results in genetic shifts if survivors are more, or
or alternative hosts for tobacco budworms or less, likely to have particular inherited traits.
bollworms) where susceptible pests can survive
and produce a local population capable of mating Stacked. Describes transgenic plants with more
with resistant survivors from Bt cotton. This than one introduced gene in a single crop plant
mating, by decreasing the likelihood that Bt- variety, such as a “stacked” cotton variety con-
resistant insects will mate with one another, taining a Bt gene and a gene for herbicide toler-
dilutes resistance in the insect population. ance.

Registration. Legal approval of pesticides and Transgenic. An organism genetically altered by

transgenic crops for use in the United States by addition of foreign genetic material (DNA) from
the U.S. Environmental Protection Agency. another organism into its own DNA.
Registration is granted after extensive review of
toxicology (to mammals, birds, fish, and other
nontarget organisms), environmental fate, health
and safety issues, and precautions.

37