Widely grown corn plants that Monsanto Co. genetically modified to thwart a voracious bug are falling prey to that very pest in a few Iowa fields, the first time a major Midwest scourge has developed resistance to a genetically modified crop.

The discovery raises concerns that the way some farmers are using biotech crops could spawn superbugs.

Iowa State University entomologist Aaron Gassmann's discovery that western corn rootworms in four northeast Iowa fields have evolved to resist the natural pesticide made by Monsanto's corn plant could encourage some farmers to switch to insect-proof seeds sold by competitors of the St. Louis crop biotechnology giant, and to return to spraying harsher synthetic insecticides on their fields.

"These are isolated cases, and it isn't clear how widespread the problem will become," said Dr. Gassmann in an interview. "But it is an early warning that management practices need to change."

The finding adds fuel to the race among crop biotechnology rivals to locate the next generation of genes that can protect plants from insects. Scientists at Monsanto and Syngenta AG of Basel, Switzerland, are already researching how to use a medical breakthrough called RNA interference to, among other things, make crops deadly for insects to eat. If this works, a bug munching on such a plant could ingest genetic code that turns off one of its essential genes.