Rootworms in Illinois and Iowa have become resistant to an insecticide produced by Monsanto Co. corn, and four more states (South Dakota, Wisconsin, Minnesota and Nebraska) likely have resistant worms, according to the U.S. Environmental Protection Agency

The resistance finding is based on a test developed at Iowa State University rather than the regulatory definition, which is “flawed” and should be changed, said the EPA in a report examining Monsanto’s product. At least 23 counties have damaged fields that need to be tested for resistance.


Kelly J. Clauss, a Monsanto spokeswoman, said the company is working with growers to reduce the number of fields with unexpected damage, a sign of suspected resistance. “Corn rootworm resistance is suspected, but not confirmed, according to the regulatory definition of resistance,” said Clauss. “We are very much in line with the EPA in terms of what needs to be done.” The EPA recommended Monsanto work with the agency to revise the definition of confirmed resistance by March 31. Meanwhile, Monsanto has agreed to transition growers from Yieldgard to SmartStax corn, which has a second mode of action for controlling rootworms.