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Protecting Farmers' Investment in Bt Cotton
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In studies to track the development of
Bt resistance, this undersized, 12-day-old
larva was fed a diet containing Bt proteins.
(K9242-1)
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In the past, farmers have used toxic
chemicals such as calcium arsenate, Paris green,
dichlorodiphenyltrichloroethane (DDT), and toxaphene to rid their cotton crops
of insects. Today, however, farmers are buying transgenic seeds that have
built-in pest control. ARS researchers
are pitching in to ensure that American farmers have made a sound investment
and to ensure global competitiveness.
Cotton pests, like pests of many crops, have developed some resistance to many
of the insecticides used to control them. Insects persist, almost mockingly, in
spite of farmers' fierce attempts to snuff them out.
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A normal, 12-day-old cotton bollworm
larva raised on a control diet.
(K9241-1)
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Tobacco budworm (Heliothis
virescens) and bollworm (Helicoverpa zea) are two of the most
destructive pests in cotton and other crops, with costs of control, production,
and lost yield of up to $300 million per year in the United States alone. To
add to our arsenal of control agents for these and other lepidopteran pests, in
the late 1980s industry began to develop crops with built-in pest control from
Bacillus thuringiensis (Bt) genes, which produce proteins toxic
to several insects, including tobacco budworm and bollworm.
Cotton was one of the first crops to benefit from biotechnology-supplied pest
protection, and Bt cotton is now one of the most widely used transgenic
crops. It is currently grown throughout the United States, China, India, and
Australia. More than 2 million acres of Bt cotton are grown in the
United States alone. Other crops, including corn, potatoes, and soybeans, also
now contain Bt genes.
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Entomologist D.D. Hardee
examines a cotton bollworm
larva on a cotton boll.
(K9240-1)
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But Bt resistance
is cropping up, too. "In addition to synthetic insecticide-resistance
development worldwide, several pests have developed resistance to foliar
Bt, including Indianmeal moths, diamondback moths, and at least nine
other insects," says entomologist D.D. Hardee, who heads ARS' Southern
Insect Management Research Unit in Stoneville, Mississippi. Because toxins are
present in cotton for the entire life of the plant and in all parts of the
plant, scientists are concerned that tobacco budworm and bollworm may rapidly
develop resistance to Bt cotton. Since Bt cotton's commercial
release in 1996, Hardee's unit has been tracking bollworm and tobacco budworm
tolerance to Bt proteins as part of a Bt-resistance monitoring
program.
"Tobacco budworm is the key pest for which Bt cotton is used,
because it has rapidly developed resistance to previous insecticides. Even
though bollworm is more Bt-tolerant, it's easily managed with less
expensive insecticides and hasn't shown much propensity for developing
resistance to insecticides. Nevertheless, we think it's essential that both
insects be monitored simultaneously," says Hardee.
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To assess resistance to Bt
proteins, entomologist Douglas
Sumerford weighs a cotton
bollworm larva that was exposed
to the Bt-insecticidal protein
Cry1Ac for 12 days.
(K9243-1)
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"We solicit voluntary
participation in our program from all cotton-growing states. Entomologists at
universities, other ARS units, and private industry, as well as growers and
consultants have responded," says Hardee. "But we need even more
people to send us samples because our challenge is always getting enough
insects from all areas."
When insect samples arrive in Stoneville, they're sent to ARS entomologist
Larry Adams, who raises them through one complete generation on a diet without
toxic proteins. The next larval generation from field colonies is then placed
on a Bt diet and compared with the Stoneville laboratory colony. ARS
entomologist Douglas V. Sumerford receives the same colonies for detailed
resistance monitoring using a rapid and sensitive assay he developed to help
identify pests with tolerance.
"We evaluate the second generation with an artificial diet containing
Bt protein levels so we can distinguish among larvae in their ability to
tolerate Bt proteins," says Sumerford. "The more tolerant to
Bt protein the pest is, the faster it'll grow on the artificial diet and
the more likely it'll survive higher doses of Bt in the artificial diet.
We check the weights of individual larvae, and if a worm is growing fast on the
diet, we look for cues indicating a genetic basis for tolerance."
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Close-up of cotton bollworm larva
on the scale.
(K9244-1)
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Sumerford is checking for genetic
differences between individuals and families. He's developing a genetic map to
pinpoint insects that could become resistant to Bt. One adult female can
lay 400 to 500 eggs, and a moth can fly several miles. Sumerford says, "We
want to know if the resistance trait can be spread to other regions. This kind
of information combined with the genetic map will help us better monitor any
changes in field populations."
"Resistance monitoring is so critical," he adds. "In past years,
some cotton growers considered quitting farming, because of the expense
involved in controlling tobacco budworm once it developed resistance to
pyrethroid pesticides. Our information will help develop remedial action plans
for farmers and help preserve the Bt technology."
Another part of the monitoring program, carried out by entomologist John J.
Adamczyk, Jr., is assessing Bt cotton varieties to see how well they
control insects. In 1996, only one Bt cotton variety was commercially
available; today, more than 25 are.
"Many varieties are marketed on their insecticidal qualities, but we want
to do further analysis to help growers make a sound investment," says
Adamczyk.
He says that by using assays, he and his colleagures have found that some
varieties express Bt proteins better than others, and certain parts of
plants express the proteins better than others. They now want to learn if
differences in insect control could also be due to incidents like a farmer's
accidentally mixing Bt and non-Bt seeds or to environmental
factors like irrigation or soil salinity. They hope to take high-yielding
varieties with the best Bt protein levels and incorporate them into a
variety-development program.
"We're investigating the next generation of Bt cotton, an
experimental variety that has two Bt genes. It should be commercially
available in 2003," says Adamczyk. "Under an experimental use permit
from the U.S. Environmental Protection Agency, we are profiling the season-long
activity of this double-Bt-gene product against various caterpillar
pests. Even if there is no cross-resistance between the two Bt genes, we
still need to make sure that both Bt proteins are giving year-round
control."
Since the resistance monitoring program started, the Stoneville group hasn't
detected any change in the tolerance of tobacco budworm to Bt cotton.
They must gather data from more areas of the Cotton Belt before they can be
certain about bollworm.
"In the meantime, this technology significantly reduces insecticide
use," Hardee notes, "and since Bt is present in the entire
plant for the life of the plant and only affects target pests feeding on the
cotton, beneficial insects are left unharmed."
To attempt to delay the development of Bt resistance, federal
regulations allow growers to choose one of the following three
Bt-growing plans:
- Plant a maximum of 80 percent Bt cotton along with 20 percent
non-Bt, with the option of spraying the latter as needed with anything
but foliar Bt insecticides.
- Plant a maximum of 95 percent Bt cotton along with 5 percent
non-Bt, but the latter cannot be sprayed with any insecticides.
- Plant a maximum of 95 percent Bt cotton along with 5 percent
non-Bt, with the latter planted in the same field with the Bt and
sprayed only if the 95 percent is sprayed with non-Bt foliar
insecticides.
"The regulations are an attempt to prolong the effectiveness of Bt
cotton for decades," says Hardee, "and they are subject to change
after the 2001 growing season."
A new publication, "Bt Cotton & Management of the Tobacco
Budworm-Bollworm Complex," is available from Hardee's group.—By
Tara
Weaver-Missick, Agricultural Research Service Information Staff.
This research is part of Plant, Microbial, and Insect Genetic Resources,
Genomics and Genetic Improvement, an ARS National Program (#301) described on
the World Wide Web at http://www.nps.ars.usda.gov.
D.D. Hardee,
Douglas V. Sumerford,
John J. Adamczyk, Jr., and
Larry C. Adams are in the USDA-ARS
Southern
Insect Management Research Unit, P.O. Box 346, Stoneville, MS 38776; phone
(662) 686-5231, fax (662) 686-5421. |
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"Protecting Farmers' Investment in Bt
Cotton" was published in the
February 2001
issue of Agricultural Research magazine.
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