sealPurdue News

October 1996

Plant smart to avoid Bt-resistant corn borer, experts advise

WEST LAFAYETTE, Ind. -- Corn farmers could learn a lesson from cotton producers: Planting crops genetically engineered to be insect-resistant works in the short run, but also could increase the number of insects resistant to pesticides in the future, Purdue University experts advise.

The genetically engineered crops in question use a naturally occurring insecticide produced by the bacterium Bacillus thuringiensis , which is commonly known as "Bt." This past summer, cotton farmers in the South who used Bt-enriched cotton saw their crops attacked by insecticide-resistant bollworms.

Scientists have moved the Bt gene into certain crops, allowing the plants themselves to fight off the insects. In corn, the Bt gene has proven to be effective against the corn borer. Overall, the long-term prognosis for Bt-enriched crops is still up in the air, experts say.

Marshall Martin, professor of agricultural economics and director of Purdue's Center for Agricultural Policy and Technology Assessment, says Bt-enriched crops offer distinct advantages. "Bt is from a soil bacterium, and it's often found in organic gardening products," he says. "It is much safer for people than insecticides. It has been approved by the EPA, the FDA and the USDA, which found no negative human health aspect."

The natural insecticide from the bacterium not only eliminates the need for synthetic chemical insecticides, but it also doesn't affect animals or beneficial insects, and it degrades in sunlight. Also, because the gene works by producing a protein that is easily digested by people and animals -- but not by some insects -- there is no concern about it being placed in food.

According to the experts, problems occur when farmers overuse the technology, just as over-prescribing antibiotics to people can result in treatment-resistant bacteria. If farmers use too much of the Bt crops, it will kill nearly all of the pest insects. A few that are immune to the insecticide will survive, however, and these will multiply and eventually come storming back.

This problem arose this past summer when a small number of cotton farmers in Texas, Louisiana, Mississippi, Alabama and Georgia reported that they had to supplement the protection of the Bt-enriched cotton with an insecticide in order to protect their crops from the cotton bollworm.

"It's our anticipation that problems with resistant pests are likely to occur from Bt crops," says Purdue entomologist Larry Bledsoe. "The fact that they documented problems in cotton fields this year wasn't too surprising to entomologists.

"As long as just a few corn borer moths survive, or bollworms in the case of cotton, you'll create a resistant strain of insects. If just 1 percent of the insects are resistant, a farmer probably won't see any insects for a few years. But when they return, 99 percent of them will be insecticide-resistant, and they will be very difficult to control."

Martin says that although everyone knew that Bt-resistant bollworms might someday appear, the speed at which nature adapted took the experts by surprise: "They knew there would be problems at some point, perhaps years down the road. And they might have considered that it could occur within just a few years. But I don't think anyone expected there to be these types of problems in the first year or two.

"The last thing a seed company wants is for seed not to work after they've put all of the R-and-D time and effort into it. The farmer doesn't want the seed to fail because that would deny him another tool, and it's one that he needs. The environmentalists don't want the seed to fail because then farmers will go back to spraying insecticide.

"All parties agree that we don't want these types of problems to happen again."

Bledsoe says any problems with Bt corn -- which are completely hypothetical at this point -- will take a long time to appear, because Bt corn hasn't been embraced by Midwestern farmers as quickly as Bt cotton was by Southern producers. The reason is twofold: For the past few years the corn borer hasn't been as common in the Midwest, and currently the genetically engineered seed costs $20 more per bag.

"Farmers don't typically pay a lot of attention to corn borer," Bledsoe says. "Before the Bt corn there wasn't much you could do about it, so they just didn't think about it. But this year we did have an strong outbreak in northeastern Indiana, southeastern Michigan and northwestern Ohio, so farmers may be in the mood to do something about it.

"In terms of the total seed planted this summer, Bt corn was a minuscule amount. I'd estimate that only about 200,000 bags were planted this year, which was the first year the seed has been available commercially." With roughly 25 million bags of seed corn planted, Bt corn probably made up less than 1 percent of the total.

According to Bledsoe, although many are concerned about resistant insects, many individuals would rather have someone else to sacrifice for the greater good.

"We have suggested methods of preventing the problem to the companies, such as mixing 80 percent resistant seed and 20 percent susceptible seed in a bag," he says. "But the seed producers don't want to be the first one out there who has seed that's 20 percent susceptible. Likewise, we've told the farmers to plant 80 percent resistant corn and 20 percent susceptible corn. But the farmer doesn't want to do it, he wants his neighbor to do it."

Martin predicts farmers soon will realize that controlling the corn borer within limits makes economic sense. "A farmer can withstand some damage," he says. "There's an economic threshold. You can live with a yield loss of three to four bushels per acre, but not 10 to 15 bushels per acre. So we don't need to kill every insect out there, just suppress them."

Martin says the reason farmers will adopt the proper management technique for Bt crops is that for some insects, Bt crops are potentially a more effective way to treat an infestation. "It's a corn borer ," Martin says. "It bores inside the plant, and once it's there you can't do anything about it." With Bt crops, the insecticidal trait is in the plant tissue and is able to stop the insects.

The greatest damage from corn borers occurs late in the growing season. As the borer chews holes through stalks, corn plants weaken and can be knocked over and made unharvestable by wind or storms. But often the infestation isn't discovered until the crop is harvested.

"If you ride in the cab of a combine, as I have done, and you see these stalks hit the corn head and fall over where the grain can't be harvested, it just hits the farmer right in the pocketbook," Martin says. "That's money that's falling on the ground."

Farmers whose fields are infested can lose 10 percent to 15 percent or more of their crop. "At corn prices that we're seeing right now, there's good incentive to try to control it," he says.

Sources: Larry Bledsoe, (765) 494-8324; e-mail,
Marshall Martin, (765) 494-4268; e-mail,
Writer: Steve Tally, (765) 494-9809; e-mail,
Purdue News Service: (765) 494-2096; e-mail,

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