Aug 28 (Reuters) - 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, a combination of measures that are supposed to protect the crop from the pests, according to a new report.

“It’s very alarming,” said Joe Spencer, an insect behaviorist with the Illinois Natural History Survey who is researching the issue.

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, in conjunction with Spencer.

Farmers across “a wide swath of Illinois” could face formidable challenges protecting 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 that 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. And they appear to also be resistant to the biotech corn, a double whammy for farmers.

“It looked like continuous corn and use of the same trait year after year is what produced resistant beetles,” said Spencer. “Growers thought their get-out-of-jail-free card was just to rotate to soybeans. But what we’re seeing in northeast and east-central Illinois is beetles that are also resistant to crop rotation.”

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 Co 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 they were worried about long-term corn production prospects because of growing resistance of the rootworm to the genetic modifications in corn.

Monsanto spokesman Jeffrey Neu acknowledged “pockets of heavy corn rootworm pressure in isolated areas of Illinois,” and said the company was working with farmers to address the problems. Crop rotation is key, as well as using different corn seed products, he added.

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 added that 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.”