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DAVID GREENE, HOST:

Across the Midwestern Corn Belt, there is a battle going on, hidden in the soil. On one side, tiny white insect larvae. They're trying to eat the roots of corn plants. On the other side, all the weapons that farmers can assemble to stop them. And this year, farmers are rolling out more weapons than ever before, including more chemical pesticides.

NPR's Dan Charles reports.

DAN CHARLES, BYLINE: When farmers in northeastern Nebraska want advice about how much fertilizer to use or how to kill insects, some of them go to Dan Steiner. He's an independent crop consultant, and he's no friend of pesticides.

DAN STEINER: We used to get sick, because we always dig to see how the corn's coming. We didn't wear the gloves and everything. And you'd kind of puke in the middle of the day or something. I think we were low-dosing poison on ourselves.

CHARLES: If you notice, Steiner said we used to get sick. It's changed, he says, because of biotechnology. Fifteen years ago, new types of corn came on the market. They had new genes inserted in the laboratory. The genes came from bacteria that are naturally poisonous to certain insects. With these so-called Bt genes, corn plants became poisonous to the European corn borer and the corn rootworm. So farmers didn't need so many chemicals.

STEINER: And ever since then, you know, I'm like, hey, we feel good every spring.

(LAUGHTER)

CHARLES: But maybe this was a little too good to last. Those inserted genes worked great for a while. And everyone agrees, they're still a rock-solid defense against the European corn borer. But in parts of Illinois, Iowa, Minnesota and Nebraska, farmers say the corn rootworm is back.

STEINER: You never really know for sure, until that big rain with the strong wind, and then the next morning the phone starts ringing and says, you know, what's going on out there?

CHARLES: You'd see a hillside of corn blown flat. Many of the roots were eaten away. But why? Have these genes really lost their power? It's a question for science.

At the University of Nebraska, graduate student David Wangila, who came here from Kenya, is turning colonies of rootworms loose on potted corn plants. The corn stalks contain different versions of the anti-rootworm gene.

Just yesterday, he took some of these pots and dumped the soil from each one into a steel bucket. If there are insect larvae in the soil, bright, hot, lights will gradually drive them out into little glass jars filled with alcohol.

DAVID WANGILA: They try, like, to escape from the heat.

CHARLES: Now, if the rootworm poisoning genes in the corn are working well, there shouldn't be any larvae. But there are. Wangila points to one of the little glass jars.

WANGILA: I don't know if you can see them.

CHARLES: Those little worms, right?

WANGILA: Yes. Yes. Those are the corn rootworm larvae.

CHARLES: Corn rootworm larvae. Already, three nice plump ones. This is not good. Those insects came from a cornfield in Nebraska and here in the laboratory, they were feeding on corn that contained the first anti-rootworm gene - one that farmers started using in 2003.

It's evidence - not proof yet, but still, a sign - that these rootworms have adapted, so the gene is not killing them anymore.

Similar experiments are going on at half a dozen universities with rootworms collected all over the Corn Belt. Researchers in Iowa have already found corn rootworms resistant to that same gene.

Nobody knows how widely those insects have spread, but farmers are not waiting to find out. Some are switching to other versions of biotech corn with anti-rootworm genes that do still work. Many, though, are going back to pesticides.

Companies that sell soil insecticides for use in cornfields are reporting huge increases in sales; 50, even 100 percent over the past two years.

Dan Steiner, the Nebraska crop consultant, is not happy about this. He tells farmers there's a better way to fight the rootworm. Starve it. Just switch that field to another crop.

STEINER: One rotation can take care of a lot of problems. Go to beans, wheat, oats. Number one right thing to do.

CHARLES: Insect experts say it's also likely to work better in the long run.

Lance Meinke, an entomologist at the University of Nebraska, tells farmers, if you just plant corn, year after year, the rootworm will probably overwhelm any weapon you throw at it someday. The problem, Meinke says, is farmers are thinking about the money they can make today.

LANCE MEINKE: I think economics are driving everything. Corn prices have been so high the last three years, every farmer is trying to protect every kernel. People are just really going for it right now, to try to be as profitable as they can.

CHARLES: So they may just keep growing corn, fighting rootworms with insecticides until those chemicals stop working, too.

Dan Charles, NPR News.

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