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Transgenic cotton controls pest

TUCSON, Feb. 3 (UPI) -- Genetically engineered cotton in Arizona, grown using a strategy mandated by the Environmental Protection Agency, is effectively controlling a common crop pest without causing increased pesticide resistance, a study released Monday concluded.

The findings suggest transgenic crops could help the environment through reduced insecticide use.

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"Transgenic crops have potential to improve agriculture, but we must be careful when we use them," researcher Yves Carriere, an evolutionary ecologist at the University of Arizona in Tucson, told United Press International.

Repeated experience with pesticides reveals insects frequently and rapidly evolve resistance against such compounds. While the insecticides may kill off nearly all members of a given pest species, a few lucky insects might have a natural insecticide resistance they spread to all descendants.

Carriere said pesticides have come a long way in environmental friendliness since DDT but it is best to limit their use in case of any unintended damage.

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"Also, when you use too much insecticide, you kill off the natural enemies of any pests," Carriere explained. "More than 40 species of herbivores feed on cotton, but only three or four of them are significant, with the rest controlled by other things like natural enemies. You apply too much insecticide, and the pests that were not a problem become a problem."

One group of genetically engineered crops designed to reduce pesticide use are so-called Bt crops, which produce a toxin made by a gene taken from the natural soil microbe Bacillus thuringiensis. Transgenic crops now cover more than 150 million acres worldwide. However, little is known about their long-term consequences and the possibility such crops quickly would be rendered useless once insects evolved resistance against them.

"In the laboratory, we have easily grown pink bollworms resistant to Bt cotton in the laboratory," Carriere said. "So their relatives in the field have a potential to evolve resistance that's quite high."

To minimize the risk of evolved resistance against Bt crops, the EPA in 1995 required all growers of Bt cotton to plant crops that do not generate toxins alongside Bt crops. Insects vulnerable to such toxins are kept alive in such clean fields to mix their insecticide-susceptible genes with resistance traits to dilute and therefore delay the evolution of resistance among their descendants.

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"The EPA was right in doing so," Carriere explained. "This refuge strategy is absolutely needed to delay the evolution of resistance."

Carriere and colleagues looked at the population density of pink bollworm, a key cotton pest, across 300,000 acres of cotton in Arizona over a 10-year span -- five years before Bt cotton was deployed and five years after. More than 1,000 traps containing sex pheromones were deployed to capture insects for study.

Findings reported Monday online by the Proceedings of the National Academy of Sciences show that investigators found up to a four-fold decrease in pesticide applications following the introduction of Bt cotton. This led to an up to six-fold decrease in pink bollworm population density.

This steady decline suggests the pink bollworm soon could be eliminated as a key pest, Carriere said.

"A key pest has never been transformed into a non-important pest with insecticides," he explained. His team will continue to track pink bollworm numbers.

Jane Rissler, senior staff scientist for the Union of Concerned Scientists in Washington, was pleased to see these results.

"The Arizona scientists deserve a lot of credit for their systematic study of the use of Bt cotton," she said. "Frankly, this is what we'd like to see in many other places in the country."

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Rissler and Carriere both feel these results do not necessarily extrapolate for Bt crops in other parts of the country.

"The pink bollworm is specialized on cotton, while other key pests of Bt cotton in other parts of the country are generalists that eat other crops," Rissler said.

Carriere added: "Here in Arizona it is clear Bt cotton has caused a dramatic reduction in the use of synthetic insecticides. But I'm talking about Arizona. This is not my general position for every transgenic."

(Reported by Charles Choi, UPI Science News, in New York.)

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