Researchers in the key corn-growing state of Illinois are finding significant damage from rootworms in farm fields planted in a rotation with a genetically modified corn that is supposed to protect the crop from the pests, according to a new report.

Evidence gathered from fields in two Illinois counties suggests that pest problems are mounting as the rootworms grow ever more resistant to efforts to fight them, including crop rotation combined with use of the biotech corn, according to the report issued by Michael Gray, a professor of crop sciences at the University of Illinois.

Farmers across "a wide swath of Illinois" could face formidable challenges in protecting their corn crops from the hungry insects, Gray said in the Aug. 27 report.

The crop damage was found in fields where the specialized biotech corn had been planted in a rotation following soybeans, a practice which typically helps beat back the rootworm problems as western corn rootworm adults typically lay eggs in cornfields and not in soybean fields.

But a large number of adult western corn rootworms were collected in both the damaged corn fields and from adjacent soybean fields, Gray said.

Western corn rootworm is one of the most devastating corn rootworm species in North America, especially in the Midwestern corn-growing areas. The rootworm larva feed on the roots of corn plants, reducing a plant's ability to grow normally, and can dramatically cut production.

Monsanto introduced genetically modified corn designed to protect from the rootworms in 2003. The corn, which contains a protein referred to as Cry3Bb1, has been popular with farmers in key growing areas around the country, and is supposed to reduce the need to put insecticides into the soil.

But last year, a group of academic corn experts warned the Environmental Protection Agency that they were worried about long-term corn production prospects because of growing resistance of the rootworm to the genetic modifications in corn.

Rodney Williamson, director of research and development with the Iowa Corn Growers Association, said there have been reports of resistance problems in Iowa but not to the level of concern seen in Illinois.

He stresses that farmers do not want to stop using the biotech corn because it helps reduce pesticide use, and said there was an effort underway to pursue various field and crop management practices to try to mitigate crop losses tied to rootworms.

"People are taking efforts to address it," he said. "Rotating modes of action; that will be one of the best things we can do."

A Monsanto official had no immediate response to a request for comment, but the company has said in the past that it is working with growers to address resistance concerns.