It’s well established that an evolutionary tug-of-war exists between transgenic crops and the pests they’re meant to repel—as one changes, the other often adapts accordingly.

In some cases, pests have become quickly resistant to the transgenic crop-produced Bacillus thuringiensis (Bt) meant to thwart them. In others, they have not developed any such resistance at all.

But just how much has the evolution of resistance in pests reduced the effectiveness of insecticidal proteins produced by transgenic crops? Looking back on the first billion acres of transgenic crops, a group of scientists at the University of Arizona aimed to find out.

Analyzing field and lab data from 77 studies of 13 pest species in eight countries across five continents, entomologist Bruce Tabashnik, Ph.D., and his colleagues found well-documented cases of field-evolved resistance to Bt crops in five major pests as of 2010, compared with only one such case in 2005. They also found that three of the five cases occurred in the U.S. (which accounts for about half of the world’s Bt crop acreage), and that in the worst cases, resistance to Bt evolved within just two to three years. In the best cases, though, some Bt crops have retained their effectiveness for more than 15 years.

“With a billion acres of these crops planted over the past 16 years, and with the data accumulated over that period, we have a better scientific understanding of how fast the insects evolve resistance and why,” Dr. Tabashnik said in a statement.

Added co-author Yves Carrière, Ph.D.: “The factors we found to favor sustained efficacy of Bt crops are in line with what we would expect based on evolutionary theory.”

Indeed, the researchers report in their paper that factors that appear to delay Bt resistance include recessive inheritance of resistance, low initial frequency of resistance alleles, and abundant refuges of non-Bt host plants.

“Computer models showed that refuges should be especially good for delaying resistance when inheritance of resistance in the pest is recessive,” Dr. Carrière added.

The scientists also provide practical advice for farmers. Explained Dr. Tabashnik: “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.” When resistance risk is higher, though, farmers ought to “take more stringent measures to delay resistance such as requiring larger refuges, or this pest will probably evolve resistance quickly to this Bt crop,” he added.

At any rate, the researchers added, preventing resistance altogether is an unlikely outcome. “You’re always expecting the pest to adapt,” Dr. Tabashnik said. “It’s almost a given that preventing the evolution of resistance is not possible.”

“Insect resistance to Bt crops: lessons from the first billion acres” appeared online in Nature Biotechnology June 10.

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