Western rootworm beetles climb on a corn stalk. Photo courtesy John Obermeyer, Purdue University

Seed companies and federal regulators are studying reports that a major corn pest has apparently outsmarted a line of genetically modified corn.

The plant is designed to kill a bug called the corn rootworm, but in several Midwest states, including Minnesota, it looks like the corn is losing its effectiveness.

Many farmers consider the worm-like larvae of the corn rootworm beetle the No. 1 enemy of their cornfields. In a state like Minnesota, with a roughly $7 billion corn crop, the rootworm commands attention.

But one factor that may have contributed to the resurgence of rootworm is a drop in farmer compliance with provisions calling for them to plant a percentage of their land with non-genetically modified corn.

Farmers in a broad stretch of the corn belt are telling seed companies and others about unusual amounts of corn rootworm damage they're finding this summer. One of the people they're calling is University of Minnesota entomologist Ken Ostlie.

Ostlie spent a recent afternoon digging up corn plants in a southern Minnesota field. He doesn't like what he saw.

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"We found that there is significant amount of corn rootworm feeding in the field, indicating that [rootworm] populations have built up," Ostlie said.

Western rootworm beetles forage on corn silk. Photo courtesy Jim Boersma, Pioneer

The bug's larvae loves corn roots. If enough roots are destroyed, plants can't stand up and they tip over, something Ostlie saw in the field.

The affected type of corn has been sold since 2003 by Monsanto, the nation's leading seller of genetically modified corn seed. Genetic modification produces a protein which until recently was deadly to the pest.

But in half a dozen states, stretching from Illinois to South Dakota, farmers may be facing a bug that can survive after eating the normally fatal protein.

"Somewhere in here, the rootworm has adapted to that trait," Ostlie said.

Exactly how the pest adapted is unknown, but it's not surprising. The corn rootworm has amazed scientists with its adaptability. It's evolved to become immune to certain insecticides and crop rotation practices that kept it at bay. Now it's apparently conquering genetically modified corn.

"I'm not pleased to see that we have the resistance evolving," said Greg Jaffe, biotechnology director for the Center for Science in the Public Interest. "That shows that we might begin losing the benefits of this technology."

Reports of rootworm resistance have grown in number for several years. But the first scientific confirmation of the problem came a month ago, in an Iowa State University study.

So far, only Monsanto's corn is implicated.

The findings did not surprise Jaffe, who supports genetically modified corn is because it reduces the amount of insecticides farmers spray on their fields. In research he published a couple years ago, he warned of coming problems.

His report showed a declining number of farmers complying with contractual obligations with Monsanto and other seed companies -- measures that are designed to forestall rootworm resistance to the protein.

The problem Jaffe found was that fewer farmers were planting what are called 'refuges.' These are fields of non-genetically modified corn. Corn prices more than doubled over Jaffe's study period, and, at least on paper, farmers had an incentive to skip the refuge fields and devote more land to the higher-yielding genetically modified corn.

To obtain the seed, farmers had to agree to plan a fifth of their acreage with regular corn. The goal is to dilute the population of resistant bugs and make it harder for them to pass on their resistance to the next generation of rootworms. Jaffe found only 75 percent of farmers were following the refuge requirement by 2008 -- compared to about 90 percent in 2003.

Jaffe said it's not known whether the decline in refuge promoted the growth of resistant varieties. But he said the federal Environmental Protection Agency will have to make a decision about just how serious a threat the resistant rootworms are.

"If in fact this is widespread, then EPA should use all those tools that it has available to it -- including restricting sales in locations that may have high levels of resistance to prevent that from spreading," Jaffe said.

Monsanto is sure to fight any sales restrictions. Company officials have said little other than calling rootworm issues a fairly limited problem. They also say the company has new lines of corn which will kill any insects found to be resistant.

Monsanto may address the subject more thoroughly at an investor conference this week. Meanwhile, EPA officials will only say that the agency is studying the issue.

When farmers start harvesting corn this month, they'll be on the lookout for unusual rootworm damage. Their observations should help clarify the extent of rootworm resistance to genetically modified corn.