Organic Refuge Limits Superpests

Environmentalists and others have voiced worries that genetically modified crops might stimulate a boom in superpests. Now, in the first field test of its kind, researchers directly pitted a crop modified to produce an insecticidal toxin against a moth that carries a gene for resistance to that toxin. The results suggest that carefully managed planting can slow the proliferation of pests with new powers.

Last year, an estimated 24% of corn and 5% of cotton grown worldwide carried genes for a toxin derived from the bacterium Bacillus thuringiensis (Bt). These so-called Bt crops are designed to repel insects that try to eat them. But no one knows how insect populations will respond to such novel plant defenses. One breed of insect, the diamondback moth, has already evolved genes for resistance to the Bt toxin. When insects from such a species carry two copies of a resistance gene, they can survive high doses of Bt. To avoid that, the U.S. Environmental Protection Agency (EPA) mandated that farmers plant part of their acreage with nonmodified crops. Insect populations there, the thinking goes, would remain susceptible to Bt and keep that susceptibility trait active in the gene pool.

No one had tested the EPA strategy in the field, however. To compare various refuge strategies, a team led by entomologist Anthony Shelton of Cornell University's agricultural experiment station in Geneva, New York, planted experimental plots of broccoli. In one group, two rows of conventional broccoli plants (20% of the total) bordered one side of the GM broccoli field. The other plot mixed the two broccoli types at random. Then the team let loose a special strain of diamondback moth pupa, some of which carried at least one copy of a resistance gene to the Bt toxin. The team collected and counted the larvae that grew on the crops and tracked whether the insects had inherited one or two copies of the resistance genes. The mixed planting fared far worse at keeping resistance genes from spreading through the population, the researchers report in the March issue of Nature Biotechnology.

A second experiment examined the effects of spraying these refuge plots with insecticides, as the EPA guidelines allow. The active ingredient in the insecticide was a second toxin also derived from the same Bt bacteria. Although the sprayed fields had fewer larvae overall, those survivors were more likely to carry resistance genes, presumably because the double-whammy of Bt crops in neighboring rows and the added Bt spray increased the selection pressure on the insects.

The research supports the idea that a refuge from GM plants will foster a population of insects that aren't resistant to the modified crops, says entomologist Fred Gould at North Carolina State University, Raleigh. But he warns that, as the second experiment showed, "if you spray the refuge, you dramatically decrease the utility of it."