8 February 2008

GM Cotton Gets First Resistant Pest

by Kate Melville

Bollworm are the first pests to evolve resistance in the field to cotton plants modified to produce an insecticide called Bt, according to a new research report in Nature Biotechnology. Bt-resistant populations of bollworm, Helicoverpa zea, were found in more than a dozen crop fields in Mississippi and Arkansas between 2003 and 2006. "What we're seeing is evolution in action," said lead researcher Bruce Tabashnik. "This is the first documented case of field-evolved resistance to a Bt crop."

Bt crops are so named because they have been genetically altered to produce Bt toxins, which kill some insects. The data documenting bollworm resistance were first collected seven years after Bt cotton was introduced in 1996.

The researchers write in their report that Bt cotton and Bt corn have been grown on more than 400 million acres worldwide since 1996, "generating one of the largest selections for insect resistance ever known." Even so, the researchers found that most caterpillar pests of cotton and corn remained susceptible to Bt crops. "The resistance occurred in one particular pest in one part of the U.S.," Tabashnik said. "The other major pests attacking Bt crops have not evolved resistance. And even most bollworm populations have not evolved resistance."

The field outcomes refute some experts' worst-case scenarios that predicted pests would become resistant to Bt crops in as few as three years, he said. "The only other case of field-evolved resistance to Bt toxins involves resistance to Bt sprays," Tabashnik said. He added that such sprays have been used for decades, but now represent a small proportion of the Bt used against crop pests.

To delay resistance, non-Bt crops are planted near Bt crops to provide "refuges" for susceptible pests. Because resistant insects are rare, the only mates they are likely to encounter would be susceptible insects from the refuges. The hybrid offspring of such a mating generally would be susceptible to the toxin. In most pests, offspring are resistant to Bt toxins only if both parents are resistant.

In bollworm, however, hybrid offspring produced by matings between susceptible and resistant moths are resistant. Such a dominant inheritance of resistance was predicted to make resistance evolve faster. The UA researchers found that bollworm resistance evolved fastest in the states with the lowest abundance of refuges. The field outcomes documented by the global monitoring data fit the predictions of the theory underlying the refuge strategy, Tabashnik said.

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Source: University of Arizona