Despite some recent media reports to the contrary, corn rootworms have yet to show any resistance to Bt corn rootworm hybrids in the field, says Kevin Steffey, University of Illinois (U of I) Extension entomologist.

“We don’t have any empirical evidence that western corn rootworms are becoming tolerant or resistant to Bt,” emphasizes Steffey. “However, there is some scientific evidence that it takes more Bt protein to kill the variant form of the western corn rootworm in the eastern areas of Illinois around Urbana than it does to kill the ‘normal’ western corn rootworm that populates areas of western Illinois.”

Bt corn rootworm-resistant hybrids and most soil insecticides are still the best choices for protecting roots from corn rootworm feeding, says Steffey. However, he adds that farmers who do decide to use a Bt corn rootworm-resistant hybrid must also plant a refuge area to a non-Bt corn rootworm-resistant hybrid to deter corn rootworm resistance to Bt from developing.

“The great thing about Bt corn hybrids is that they provide host plant resistance,” says Steffey. “Host plant resistance is a primary recommendation for insect control—that’s the way we have defeated the Hessian fly and any number of insects in the past. The only difference is that the host plant resistance in this case is transgenic.”

Farmers who see lodged corn in their fields should check stalks and roots to determine the cause. “It still may not be too late to examine lodged corn to determine if lodging was a result of stalk rot, root rot or rootworm feeding,” says Steffey. “Rootworm damage is much more ragged in appearance than root rot and you can sometimes still see the scars from feeding.”

For more information on managing the western corn rootworm variant, click on this pdf Web link from the U of I:

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