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Corn+Soybean Digest

Bt-Resistant Borer Search Comes Up Empty

They combed fields for what they hoped wasn't there and found a bit of it, but nothing they could use.

University of Illinois entomologists, closely monitoring for signs of European corn borer resistance to Bt corn, found two surviving larvae on Bt plants last summer. But, because of the circumstances, they can't yet draw conclusions.

"We believe an important step in resistance monitoring at this stage in Bt corn development is to search for the single corn borer that might survive a full dose of Bt," says entomologist Rick Weinzierl. "We consider it essential to continue our surveys in 1998 and beyond."

Weinzierl and his colleagues, Kevin Steffey and Christopher Pierce, are conducting the project. It's funded by the Illinois Council for Food &Agricultural Research.The three scientists launched their project in an effort to locate the "needle-in-a-haystack" corn borer that could signal the first signs of resistance to the Bt toxin.

Crews surveyed 370 acres of Bt corn across 22 counties looking for surviving borer larvae.

"Those 370 acres would have been home to 5.18 million corn borers," says Weinzierl. (That's based on 28,000 plants per acre x 370 acres x 0.5 borer per plant.)

"We found about 200 corn borers surviving in Bt fields," he reports. "However, all but two were on plants that were not producing Bt toxin. Based on seed company information and our own observations, we expected at least a small portion of the plants in a Bt field would not be producing Bt toxin."

The researchers used a special gene-check test that indicates whether a plant is Bt-positive or Bt-negative.

Of the two larvae found on Bt-positive plants, one was parasitized by an organism not related to the Bt toxin. That ended any further chance for study. The other was collected and placed on a lab diet but did not survive.

"Our intent was to rear these needle-in-a-haystack larvae from Bt plants to the adult stage, mate them with each other or with lab-colony moths, and then continue to rear subsequent generations for crosses and evaluation," explains Weinzierl.

"These steps would allow us to investigate the nature of their resistance to Bt, if, in fact, their survival resulted from resistance. These steps will now wait until more borer larvae are collected from Bt plants."

In the meantime, entomologists urge farmers to plant a portion of Bt cornfields to non-Bt hybrids. That allows susceptible corn borers to survive and potentially mate with any resistant borers.

University of Illinois 1997 testing showed that some non-Bt hybrids tolerate borer infestations and still produce acceptable yields. Those kinds of hybrids, say the entomologists, would be excellent choices for use in refuges in a resistance-management program.

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