Corn Conundrum

How do you keep destructive insects from developing resistance to the toxins in genetically modified plants – resistance that turns insects into efficient and effective crop-killing machines?

In the case of corn plants and the western corn rootworm beetle, you need to more than double the amount of non-toxic corn that is planted around the genetically modified corn, according to a new study.

Western corn rootworm beetles are some of the most destructive and economically important crop pests in the United States. Studies have shown that the western corn rootworm beetle has already worked its resistant wiles against so-called Bt corn – corn infused with insect-killing power from the bacterium Bacillus thuringiensis.

The idea behind resistance prevention is elegant in its simplicity. Imagine what happens when you plant a refuge of insecticide-free “normal” corn close to Bt corn: Bugs will munch from both sets of corn and then mate, creating children that can’t quite stomach the toxic corn, although they may be able to handle it a bit better than their parents or grandparents.

Without normal corn planted nearby, insects would rather quickly develop immunity to the Bt toxins and pass this immunity down to their children.

Hello immune crop-killers, goodbye corn.

But how much normal corn is needed to effectively keep resistance at bay?

A recent study by NC State entomologist Fred Gould and Bruce Tabashnik, an entomologist from the University of Arizona, suggests a lot more normal corn is needed. Right now, the Environmental Protection Agency mandates that farmers plant 20 percent normal corn around a given acreage of Bt corn that contains one bug toxin and 5 percent normal corn around a given acreage of Bt corn that contains two bug toxins. The researchers recommend more than doubling the amount of normal corn – planting 50 percent normal corn around a given acreage of Bt corn that contains one bug toxin and 20 percent normal corn around a given acreage of Bt corn that contains two bug toxins.

How can farmers deal with planting corn that they know will serve as a sacrificial lamb of sorts? Tabashnik and Gould recommend that farmers move away from silver-bullet solutions and toward integrated pest management practices that combine a number of ecologically appropriate resistance-prevention methods, including some use of Bt corn, crop rotation and judicious use of insecticide sprays.