By Susan McGinley
Bt cotton was first grown commercially in the United States in 1996 and has accounted for more than half of the cotton acreage in Arizona since 1997.
For several years, a team of scientists at the University of Arizona and the Arizona Cotton Research and Protection Council has tracked resistance to Bt cotton in pink bollworm caterpillars, which attack bolls of cotton in the southwestern U. S.
They discovered that in 1997 the frequency of a resistance gene in pink bollworm was higher than expected.
The estimated frequency of a gene conferring resistance to the toxin in Bt cotton was about 1 in 10 for pink bollworm caterpillars from 10 Arizona cotton fields in 1997. This is roughly 100 times higher than estimates for other pests of Bt crops.
Based on this relatively high estimate and projections from computer models, rapid increases in resistance were expected in subsequent years. Surprisingly, the estimated frequency of resistance did not increase from 1997 to 1999 and Bt cotton remained effective against pink bollworm. "Bt cotton is working extremely well in Arizona," said Bruce Tabashnik, UA entomology professor and lead author of the study.
"Resistance has not evolved as quickly as expected," said Tabashnik.
To help delay resistance, the EPA requires that farmers who grow Bt cotton must also plant refuges of cotton without Bt toxin to allow survival of pests that are not resistant.
To understand why resistance has not evolved as predicted, the scientists are developing new models that incorporate the latest data and are expanding field monitoring and research on resistance genetics and ecology.
Recent results suggest that on ordinary cotton, resistant pests suffer a competitive disadvantage relative to non-resistant pests. Nonetheless, the scientists continue to prepare for resistance problems in the future.
"Bt cotton has helped to reduce insecticide use in Arizona cotton to the lowest levels in the past 20 years," said coauthor Larry Antilla of the grower-supported Arizona Cotton Research and Protection Council. "This benefits the public, farm workers, and the environment."
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