Five of 13 major crop pests evolve resistance to Bt corn and cotton, crops engineered to be toxic to certain insects, as of 2010, compared with only one in 2005, study says

Nevin Barich

WASHINGTON , June 11, 2013 (Xinhua News Agency) – Five of 13 major crop pests have evolved resistance to Bt corn and cotton, crops engineered to be toxic to certain insects, as of 2010, compared with only one in 2005, a study said Monday.

Researchers at the University of Arizona reported in the journal Nature Biotechnology that while the increase in resistance was expected, the main question was how fast the insects evolve resistance and why.

They analyzed data from 77 studies of 13 pest species in eight countries on five continents. Three of the five cases of resistance are in the United States, where about half of the world 's Bt crop acreage is planted.

The new study found that in the worst cases, resistance evolved in 2 to 3 years; but in the best cases, effectiveness of Bt crops has been sustained more than 15 years.

What's behind the difference is whether farmers plant "refuges" on adjacent lands using non-Bt crops. Researchers explained that the inheritance of resistance is recessive, meaning that insects survive on Bt plants only if have two copies of a resistance gene, one from each parent.

Planting refuges near Bt crops reduces the chances that two resistant insects will mate with each other, making it more likely they will breed with a susceptible mate, yielding offspring that are killed by the Bt crop, they said.

"Perhaps the most compelling evidence that refuges work comes from the pink bollworm, which evolved resistance rapidly to Bt cotton in India, but not in the U.S.," Bruce Tabashnik, a University of Arizona entomologist and the paper's lead author, said in a statement. "Same pest, same crop, same Bt protein, but very different outcomes."

He explained that in the southwestern U.S., scientists worked with growers to craft and implement an effective refuge strategy. In India, on the other hand, the refuge requirement was similar, but without the collaborative infrastructure, compliance was low.

Consequently, the researchers concluded that setting aside a relatively small area of land for refuges can delay resistance substantially and failure to meet this signifies a higher risk of resistance.

"Either take more stringent measures to delay resistance such as requiring larger refuges, or this pest will probably evolve resistance quickly to this Bt crop," Tabashnik said.

(c) 2013 Xinhua News Agency

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