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An insect that can fatally weaken the root system of corn crops is overcoming the main line of defense that farmers have against the pest, posing a threat to Minnesota's nearly $5 billion corn crop.

The corn rootworm is developing resistance to genetically modified corn, which contains a protein that normally is deadly to the pest. Researchers at Iowa State University have documented what they say is the first known example of rootworms able to survive the toxin.

By killing the insect, genetically modified corn saves farmers the extra step and cost of spraying insecticide on their fields. Farmers like it so much that an estimated three quarters of the nation's corn crop is genetically modified.

But if the corn rootworm can eat the corn and survive, it will cost farmers money. As the name implies, the pest loves to feed on the roots of the plant, said Iowa State University entomologist Aaron Gassmann, the lead researcher on the school's research.

"The rootworm makes it very difficult for the corn to remain standing straight in the face of winds that are associated with storms, for example," Gassmann said.

Farmers in northeast Iowa first alerted university researchers in 2009 that they were seeing rootworm damage in genetically modified corn fields where the pest should have been killed. The farmers suspected that the rootworms had developed immunity to the protein implanted in the corn.

Laboratory tests of worms collected from the problem fields showed that the farmers were right, Gassmann said.

"These corn rootworm populations had developed resistance to the toxin that had been used to control them for several years," he said.

Gassmann saw the damage the resistant rootworms caused -- corn plants that had fallen over or were tilted at extreme angles.  

Iowa is not the only state seeing the problem. About 15 southern Minnesota farmers have reported rootworm damage in fields of genetically modified corn, University of Minnesota Extension entomologist Ken Ostlie said.

"The data at the moment suggests that we are probably dealing with resistance," he said.

Ostlie said most of the reports have been from southeast Minnesota, but problems have surfaced in other areas of the state. He said it's not known why the rootworms are able to survive.  

"Larvae are in some way dealing with or bypassing the protein," Ostlie said. "Either they're feeding in different parts of the root system where the concentration may be lower. Or in fact, part of the population may be inherently resistant."

Most of the resistance problems have been found in corn seed developed by Monsanto. Company officials did not respond to an MPR interview request.

But according to its website, Monsanto is collaborating with the Iowa State University research team to figure out what implications the research has for farmers.  

It's too early to say if the Iowa State research is inaccurate or whether rootworms are in fact developing resistance, say representatives of National Corn Growers Association. 

But if the research holds up, farmers should be able to limit the problem, said Nathan Fields, the association's director of biotechnology and economic analysis. He said famers will largely have to adjust what sort of seed they use.

For example, since a specific Monsanto corn seed is most linked to the resistance issue, farmers might try another Monsanto variety or a seed from a different company.  

"If you change that mode of action, you should quell any kind of resistance pockets that are coming up," Fields said.

Another step farmers can take is to change what crops they're planting. Rootworms feed only on corn. If they hatch in a soybean field, they'll quickly die.

So one of the best courses of actions farmers can take is to avoid planting corn in the same field two years in a row.

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Mark Steil

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An insect that can fatally weaken the root system of corn crops is overcoming farmers' main line of defense against the pest, and could threaten Minnesota's nearly $5 billion corn crop.

Rootworm could be resistant to genetically modified crops

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