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Resistance to Bt complicating cotton insect control

Update on current Bt landscape and recent introduction of three-gene Bt cotton.

Cotton producers have increasingly relied on the first and now the second generations of Bt technology to help them control lepidopterous pests such as cotton bollworms, tobacco budworms and, in the Far West, pink bollworms.

Dawson Kerns, a Ph.D. candidate at the University of Tennessee, traced the history of Bt cotton and how resistance has developed to the Cry 1Ac and Cry 2Ab proteins in the 25 years since the first Bacillus thuringiensis traits became commercially available.

Kerns and Scott Stewart, professor of entomology and now the director of the University of Tennessee’s West Tennessee AgResearch and Education Center in Jackson, Tenn., gave growers an update on the current Bt landscape and the recent introduction of three-gene Bt cotton in this presentation from the first-ever virtual Milan No-Till Field Day.

“The take-home point here is that these traits have high efficacy against most pests,” said Kerns. “However, with bollworm they don’t work quite as well as they do on other pests, such as tobacco budworms and pink bollworms.”

Bollworms, whose scientific name is Helicoverpa zea, typically become most threatening to cotton after first bloom. In recent years, field resistance has been detected in bollworms to the Cry1Ac and Cry2Ab proteins, which has sometimes required growers to make insecticide applications for bollworms.

“Unfortunately, the traits that are expressed in Bt cotton are very similar to the traits expressed in Bt corn,” said Kerns. “When bollworms, also known as corn earworms, are feeding on corn they can also select for cross-resistance to the traits that are expressed in Bt cotton.

“There has been evidence of this in Tennessee. Bioassays carried out by David Kerns at Texas A&M University have shown a high-resistance ratio in some of our local populations,” he said. “What we can see is we have high resistance ratios to both Cry 1Ac and Cry 2 Ab.

In the next segment, Stewart will discuss new developments in the fight against bollworms and other cotton pests.

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