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.

Monsanto said its rootworm-resistant corn seed lines are working as it expected "on more than 99% of the acres planted with this technology" and that it is too early to know what the Iowa State University study means for farmers.

The discovery comes amid a debate about whether the genetically modified crops that now saturate the Farm Belt are changing how some farmers operate in undesirable ways.

These insect-proof and herbicide-resistant crops make farming so much easier that many growers rely heavily on the technology, violating a basic tenet of pest management, which warns that using one method year after year gives more opportunity for pests to adapt.

Monsanto is already at the center of this issue because of its success since the 1990s marketing seeds that grow into crops that can survive exposure to its Roundup herbicide, a glyphosate-based chemical known for its ability to kill almost anything green.

These seeds made it so convenient for farmers to spray Roundup that many farmers stopped using other weedkillers. As a result, say many scientists, superweeds immune to Roundup have spread to millions of acres in more than 20 states in the South and Midwest.

Monsanto became the first company to sell rootworm-resistant biotech corn to farmers in 2003. The seed contains a gene from a common soil microorganism called Bacillus thuringiensis, or Bt, from which crop biotechnology has been used to mine several genes for making insecticidal proteins.

One of the genes Monsanto developed makes a crystalline protein called Cry3Bb1. It rips apart the gut of the rootworm but is harmless to mammals, birds and most beneficial insects. Competitors, which use other Bt genes to attack the rootworm, estimate that roughly one-third of the corn grown in the U.S. carries Monsanto's Cry3Bb1 gene.

Monsanto said it generated world-wide sales of $4.26 billion from corn seed and biotechnology traits, about 40% of its overall sales, in its last full year.

Until insecticide-producing corn plants arrived, Midwest farmers typically tried to keep pests like the corn borer and the rootworm in check by changing what they grew in a field each year, often rotating between corn and soybeans. That way, the offspring of corn-loving insects would starve the next year.

Some farmers began to plant corn in the same field year after year. The financial incentive to grow corn has increased in recent years in part because the ethanol-fuel industry's exploding appetite for corn has helped to lift prices to very profitable levels for growers.

According to Dr. Gassmann, the Iowa fields in which he found rootworms resistant to the Cry3Bb1 toxin had been producing Monsanto's Bt-expressing corn continuously for at least three years. Dr. Gassmann collected rootworm beetles from four Iowa cornfields with plant damage in 2009. Their larvae were then fed corn containing Monsanto's Cry3Bb1 toxin. They had a survival rate three times that of control larvae that ate the same corn.

Dr. Gassmann found that Monsanto's Bt toxin still had some lethal impact on the larvae from the problem Iowa fields, and that the bugs were still highly susceptible to a rootworm-resistant corn plant from a competitor that uses a different Bt toxin, called Cry34/35Ab1.

Scientists in other Farm Belt states are also looking for signs that Monsanto's Bt corn might be losing its effectiveness. Mike Gray, a University of Illinois entomologist, said he is studying rootworm beetles he collected in northwest Illinois earlier this month from fields where Monsanto's Bt-expressing corn had suffered extensive rootworm damage.

The government requires that farmers who plant the genetically modified corn take certain steps aimed at preventing insects from developing resistance. Farmers are told to create a refuge for the bugs by planting non-modified corn in part of their fields. The refuge, which can be as much as 20% of a farmer's field, is supposed to reduce the chances that two toxin-resistant bugs mate and pass along that trait to their offspring.

Dr. Gray said the confirmation of toxin-resistant rootworms in Iowa could force the U.S. Environmental Protection Agency to revisit its policy of allowing the size of these insect refuges to shrink to as little as 5% of a cornfield as crop biotechnology companies begin to sell seed for corn plants that can make two different rootworm-killing toxins.

Part of what has attracted some farmers to Monsanto's new SmartStax corn line is that it allows them to plant a smaller refuge. But one of the two anti-rootworm toxins in that variety is the Cry3Bb1 protein at the center of Dr. Gassmann's study.

The EPA said it is too early to comment on any implications arising from Dr. Gassmann's paper.