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2-Plants: Three years of Bt-cotton did not increase pest resistance

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-------------------------------- GENET-news --------------------------------

TITLE:  Study shows no increase in pest resistance to genetically modified
SOURCE: University of Arizona press Release, by Susan McGinley
DATE:   November 20, 2000

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Study shows no increase in pest resistance to genetically modified cotton

Results of a new study published in the Nov. 21 issue of the Proceedings of 
the National Academy of Sciences may diminish fears about one of the 
potential pitfalls of genetically modified crops. Bt cotton has a gene 
transferred from the bacterium Bt (Bacillus thuringiensis) that lets plants 
produce a natural insecticide, thus reducing reliance on sprays of chemical 
insecticides. A major concern is that pests could quickly evolve resistance 
to the Bt toxin in genetically modified cotton. This has not happened yet, 
according to the new study.

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."


Prepared by Susan McGinley
tel: (520) 621-7182
fax: (520) 626-8688

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