Key Issues Perspective

Key Issues Perspective: Corn earworm a major problem in soybeans

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How far north will corn earworms be a problem for soybean growers in 2012? The degree of the problem and where the problem will occur north of the Mason Dixon line are nearly impossible to predict, but Mid-South entomologists know how intense the corn earworm problem can be in any given year.

Moth flights can be continuous for weeks on end in the Mid-South, and it is from this region that moths spread north. They can migrate a long distance. The caterpillar pest infests the main three crops grown in the Mid-South—corn, cotton and soybeans. And it is given a different common name depending on the crop in which it infests—corn earworm, bollworm and soybean pod worm—but it is mainly referred to as bollworm in the South and corn earworm in the North.

“We are having really high bollworm numbers in soybeans. The moth flights have been sustained, sometimes up to 12 weeks,” said Angus Catchot, Mississippi State University entomologist.

“We are using pyrethroid insecticides in all of those crops. Growers want to use them, but we have to utilize other compounds now,” he said. “We are beginning to see more tolerance or lack of control from pyrethroids.”

Multiple applications of a pyrethroid can be necessary each year in any of the main crops for larvae control because the residual is usually less than a week. As noted by Catchot, when one pyrethroid or another fails, then resistance is to the entire class of chemistry because there are only subtle differences between one or another.     

John Smith, Bayer CropScience technical service representative, said controlling corn earworms when the beans are most vulnerable at pod set is important. Even though Smith is based in Arkansas, he has received questions about corn earworm control in soybeans from as far north as Kansas in the past.

“A mild winter may have let more corn earworms overwinter farther north. Typically with most insects and corn earworms, there always is a northern limit as to where they can overwinter. With the winter being as mild as it was, I would really suspect a lot more corn earworms may have overwintered farther north than they would have in a typical year when we have more cold weather,” Smith said. “It definitely is hard to predict insect populations as you go into a summer.”

Applying an insecticide that has much longer residual than a pyrethroid is an answer for worm control in the Midwest and especially the Mid-South these days. Catchot said, “Farmers will start with these compounds in the future rather than take a chance on the cheaper priced pyrethroids that are not providing adequate control of corn earworms.”

The new longer residual insecticide most used in 2011 was Belt from Bayer CropScience. Smith said, “I tell growers and applicators that if you stay within the labeled use rate, two to three ounces per acre, expect at least two weeks control of corn earworms. You might get a lot more. We’ve had university trials showing up to 30 days when the application was made in ideal conditions, and that is really more than you can expect out of most other insecticides.”

He also noted that some other caterpillar pests, such as loopers, are even more sensitive to Belt than corn earworm.

Smith said typically a Belt insecticide application for corn earworms would occur when soybeans are in the R3 to R4 stage when pods are developing, and this could also be the only application necessary to get through the earworm larvae attack.

For more information, you can contact Smith by e-mail at johnfsmith@bayer.com.


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