American farmers have planted 20 million acres of genetically modified corn, engineered to repel predatory insects without using noxious chemicals. If farmers are not careful, a new report warns, the scheme could backfire and breed even more troublesome strains of pests.
The modified crops pump out Bt toxin, a natural insecticide created by bacteria. Some insects, in turn, carry genes that protect them from the toxin. To check the spread of those genes, the U.S. Environmental Protection Agency requires that farmers intersperse fields of genetically modified crops with islands of non-modified plants. The idea is that insects in the refuges would remain vulnerable to Bt toxin, and the bugs would interbreed to keep the whole population susceptible.
For the first time, scientists have proof the tactic works. Entomologist Anthony Shelton of Cornell University and his colleagues sowed fields of altered broccoli and then unleashed broccoli-eating diamondback moths. In fields containing refuges of non-modified plants, resistance genes were less likely to be spread through the moth population. But there is a big caveat. Spraying the refuges with pesticides—a common practice among farmers, and one permitted by EPA regulations—sped the appearance of toxin-resistant moths. "Any time you spray insects on a plant, you are never going to get 100 percent mortality," Shelton says. "You end up putting a mosaic of doses on the plants—some high, and some low. Where the doses are low, you won't kill all the insects, and those that survive will be more likely to develop resistance."