"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.," Tabashnik said. "Same pest, same crop, same Bt protein, but very different outcomes."

He explained that in the Southwestern U.S., scientists from the EPA, academia, industry and the USDA 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.

One of the paper's main conclusions is that evaluating two factors can help to gauge the risk of resistance before Bt crops are commercialized. "If the data indicate that the pest's resistance is likely to be recessive and resistance is rare initially, the risk of rapid resistance evolution is low," Tabashnik said. In such cases, setting aside a relatively small area of land for refuges can delay resistance substantially. Conversely, failure to meet one or both of these criteria signifies a higher risk of resistance.

When higher risk is indicated, Tabashnik describes a fork in the road, with two paths: "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."

Two leading experts on Bt crops welcomed publication of the study.

Kongming Wu, director of the Institute for Plant Protection at the Chinese Academy of Agricultural Sciences in Beijing, said, "This review paper will be very helpful for understanding insect resistance in agricultural systems and improving strategies to sustain the effectiveness of Bt crops."

Fred Gould, professor of entomology at North Carolina State University, said, "It's great to have an up-to-date, comprehensive review of what we know about resistance to transgenic insecticidal crops."

Although the new report is the most comprehensive evaluation of pest resistance to Bt crops so far, Tabashnik emphasized that it represents only the beginning of using systematic data analyses to enhance understanding and management of resistance. 

"These plants have been remarkably useful and in most cases, resistance has evolved slower than expected," Tabashnik said. "I see these crops as an increasingly important part of the future of agriculture. The progress made provides motivation to collect more data and to incorporate it in planning future crop deployments. We've also started exchanging ideas and information with scientists facing related challenges, such as herbicide resistance in weeds and resistance to drugs in bacteria, HIV and cancer."

But will farmers ever be able to prevent resistance altogether? Tabashnik said he doesn't think so. 

"You're always expecting the pest to adapt. It's almost a given that preventing the evolution of resistance is not possible."

 

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