The new study tested these results in a broad sample of western corn rootworms from Illinois, Iowa, Nebraska and Missouri.

“We indeed found that the rotation-resistant rootworms could eat more foliage than the wild type,” Seufferheld says. “They are also able to survive a little longer on the soybean than the wild-type rootworms.”

When insects feed on their leaves, soybeans ramp up production of proteinase inhibitors to combat the insects’ ability to digest proteins in their leaves. The researchers hypothesized that the rotation-resistant rootworms had evolved the ability to compete a little longer in this chemical warfare with the soybeans.

Tests confirmed that rotation-resistant rootworms had higher levels of a special class of proteinases than wild-type rootworms to begin with, and that they increased production of one of these proteinases, Cathepsin-L, in response to soybean defenses. The wild-type rootworms increased levels of another proteinase, Cathepsin-B, when feeding on soybeans, the researchers found. But this enzyme appears to be ineffective against the plant’s defenses.

This difference allows the rotation-resistant beetles to survive on soybeans for two or three days – just long enough, the researchers say, for some of them to lay their eggs in bean fields. In spring, when the same fields are planted in corn, the rootworm larvae emerge to feed on corn roots.

Illinois insect behaviorist and co-author Joseph Spencer of the Illinois Natural History Survey, part of the U of I Prairie Research Institute, says that before this study, researchers studying rotation resistance were looking at the insects and insect behavior in isolation, thus missing their interaction with plants as a potential clue to the problem.

“You have to include the soybean in the equation,” Spencer says. “It is not a passive player. The beetle has changed its behavior but what facilitates this change in behavior is this change in expression of these digestive proteases. That allows them to stay in the soybeans longer. We had ignored this aspect of the biology.”

The study team also included Matias Curzi, of the U of I, who earned his master’s of science in Seufferheld’s laboratory and now is working at Pioneer Argentina. Funding for this work was provided by the USDA National Institute of Food and Agriculture.