A cutworm you won't find on any Bt corn's label may be cutting into your corn. The western bean cutworm, historically only a pest in the western Corn Belt, has been slowly moving out of its home territory and becoming more troublesome.

Mike Catangui, extension entomologist at South Dakota State University, hypothesizes that the prevalent use of Bt hybrids may be causing a species shift in the insects that are attacking corn.

“Most Bt hybrids screen out the corn borer and let western bean cutworm pass through,” says Catangui. “Obviously, western bean cutworm will be favored if we continue planting this kind of corn.”

While he isn't against using Bt corn, Catangui reminds farmers to be aware of what insects their Bt hybrids control.

“I think we're selecting for the western bean cutworm because it's not controlled by the most widely used transgenic Bt hybrids,” he says. “Farmers shouldn't be lulled into thinking that because they grow Bt corn it will take care of everything. This insect is successful in attacking most Bt corn.”

Ron Seymour, extension educator at the University of Nebraska, conducted research on Bt hybrids and western bean cutworm last year and is continuing his work this summer.

“From preliminary studies we're finding there is suppression of western bean cutworm larvae in Herculex hybrids, or those carrying the Cry1F gene,” Seymour says. “With more data we may be able to say it controls it. But with just one year's data I would be reluctant to say it's fully controlling it.”

Based on data from multiple research and strip trial locations in Nebraska and South Dakota in 2002, Dow AgroSciences and Pioneer Hi-Bred, International have submitted an application for an addition to the Herculex I label to the EPA for control of western bean cutworm. It's under consideration for approval.

“We have submitted it as an addition to the label because the data collected from 2002 shows that Herculex I provides very good protection against western bean cutworm,” says Wally Thingelstad, communications manager, Dow AgroSciences.

While Seymour isn't ready to agree with Catangui that there's a full-fledged species shift occurring, he does admit that it's something to watch.

“I'm not sure we've got enough data to say that a species shift is indeed happening,” he says. “But based on the prevalence of planting Bt corn, in theory, it's possible.”

Seymour is more inclined to believe that western bean cutworm is doing better because of droughty conditions over the past several years. The larval stage of the western bean cutworm's life cycle is susceptible to a disease, nosema, that the researchers assume is more abundant in areas of higher moisture, causing it to thrive in the West.

Seymour says the insect is a strong flier and can blow several hundred miles. Then, if conditions are favorable, it can become established in a new area.

“There is a possibility that this pest is expanding its range to some degree,” says Seymour. “I don't think it's a rapid expansion, but those who are on the edges of its traditional range — Iowa, Kansas, Minnesota — need to be aware that they could experience some feeding damage by this particular pest that they haven't had to deal with previously.”