A corn field in Iowa shows rootworm damage in 2012.
A corn field in Iowa shows rootworm damage in 2012. - 
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In Sac Country, Iowa, corn stands a foot high at Darwin Bettin’s farm on a breezy summer day. But one recent morning after a storm, he found a big surprise.

"I could see corn laying down in my field and none of my neighbors' fields," Bettin says. "I told my wife if I didn't know better, that looks like rootworm damage."

The rootworm he’s talking about was the scourge of the cornfield for years. The insect’s larvae feed on the roots of the corn, knocking the plants over row by row. Then ten years ago, Monsanto introduced what’s called Bt corn, which is genetically engineered to emit a toxin that kills the larvae.

“And I didn't need insecticide," Bettin says. "I could just use that. And so we used that and got along good.”

Until they didn’t. In time, the rootworms the toxin didn’t kill emerged in force, slashing Bettin’s yield in half. This year the farmer applied a trusted soil insecticide. And that’s what concerns the Environmental Protection Agency. The EPA signed off on Bt corn in the first place.

“One of the points in our decision to register the Bt toxins was that would eliminate the need to use soil-applied corn rootworm insecticides," says Steven Bradbury, who directs the office of pesticide programs at the EPA.

Surveys show that, with their profits at stake, as many as half of Midwestern farmers are still using Bt corn. But they’re applying insecticide, too. And that adds up.

Aaron Locker, a marketing director for the farm products manufacturer FMC, says sales and profits are on the rise, "due in part to resistance to corn rootworm."

And the gains are industry-wide. Luke Samuel, Monsanto’s corn insect traits manager, says rootworms have been a challenge in the past and they’ll be a challenge in the future.

“Resistance is not a new concept in agriculture," he says.

The EPA says Monsanto is moving in the right direction. The company introduced seed with a new toxin to confound the rootworm larvae, and they’re encouraging farmers to plant corn every other year to control the infestations. With corn prices high, that’s a hard sell.