Pollen wafting from bioengineered corn to traditional varieties may be undermining the fight to keep pests from evolving resistance to pesticides, according to a new study.
Farmers who plant Bt corn, which is genetically engineered to make an insecticide produced by the bacterium Bacillus thuringiensis, by law must plant non-Bt corn nearby, explains Bruce Tabashnik of the University of Arizona in Tucson. By harboring susceptible pests, those non-Bt rows of corn, called a refuge, are supposed to slow down a pest population’s tendency to develop resistance to the Bt pesticide.
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Tests show, however, that pollen from the Bt corn drifts over into the refuge and creates Bt-laced kernels in the ears of otherwise non-Bt plants, say Tabashnik and Charles Chilcutt of Texas A&M University in Corpus Christi. The rules for refuges may need revising, the researchers say in the May 18 Proceedings of the National Academy of Sciences.
The impact of the finding comes from the context, Tabashnik says. Botanists probably could have predicted the results, but he says refuge designers, including himself, hadn’t factored in the migration of engineered genes. “It’s not in any of the models I’ve ever seen, and I’ve been doing this for 20 years,” says Tabashnik.
Another specialist in insect resistance, David Onstad of the University of Illinois at Urbana-Champaign, calls the new report “an important paper with practical consequences.”
Tabashnik says that farmers might have to plant wider refuges or use varieties of corn in the refuges that bloom at slightly different times than do the nearby Bt varieties.
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Previous studies have examined how the drift of Bt pollen affects monarch butterflies or wild plants related to the bioengineered crops (SN: 9/15/01, p.164: Available to subscribers at Bt Corn Risk to Monarchs Is ‘Negligible’).
Both fans and critics of Bt crops have fretted about how the technology increases the odds that insects will eventually evolve ways to defeat the toxin. Lab and field studies have shown that insects can develop such resistance, although researchers have found resistance on farms developing only from Bt sprays. To delay resistance, U.S. farmers plant at least 20 percent of their cornfields with a non-Bt variety.
Chilcutt first got an inkling that the refuges might not be Bt-free when he noticed that white Bt-free corn growing near a plot of yellow Bt corn looked as if it had a mixture of kernel colors.
To study the drift of Bt-genes into refuges, Chilcutt and Tabashnik planted pairs of Bt and non-Bt corn varieties in various configurations. The prevailing wind in the test plots blew from the Bt to the non-Bt plants.
In corn ears harvested from the supposed Bt-free zone, researchers found that the amount of Bt toxin decreased as the distance from the Bt-corn increased.
Overall, Bt concentrations in corn kernels from the refuges were “low to moderate,” the researchers report.
Allison Snow of Ohio State University in Columbus says, “I think what scientists will start arguing about, once they have time to read this paper, is ‘Is this a big deal or a small effect?'”