Gray said that resistance has not been proven in Illinois fields, but the rootworms found in Illinois have been bred in a lab at Iowa State to determine if resistance is developing, or being passed from one generation to the next.

“That's the suspicion. We're careful not to use the resistance word here in Illinois,” he said. “But it matches what Gassmann has seen so far.”

Gray said he got a call from a seed dealer who told him he had spotted the damage in a Cass County field. Last year, farmers reported damage in LaSalle, Whiteside and Henry counties.

Monsanto said it was working with farmers to help them monitor any potential damage, and was dispatching specialists to Illinois, Iowa, Nebraska, Minnesota, South Dakota and Kansas.

“We're working really closely with our dealers as well as growers to understand the circumstances and the history to provide some solutions,” said Danielle Stuart, a Monsanto spokesperson. “We're recommending that farmers keep a close eye on their fields.”

The company, earlier this week, posted information on its website, alerting growers to the problem.