Monsanto officials have denied reports that corn pests in the U.S. are becoming resistant to its genetically modified corn seed.

Researchers at Iowa State University say they have discovered corn rootworms in Iowa farms that may have developed a tolerance to the company's corn seed, which Monsanto genetically modified to kill the pests. At a recent UBS investor conference, Brett Begemann, Monsanto's executive vice president, said the rootworm crop damage occurred in areas where the corn pest overwhelms the plant's genetically engineered defense mechanisms. "We have been watching it," said Begemann. "It's not spreading. It's not getting bigger."

About 100 farmers a year have reported damage from the corn pests, according to Begemann. He said the company is working with farmers to suggest ways to limit pest problems, such as by rotating corn and soybean crop growth cycles or using pesticides.