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Corn+Soybean Digest

Innocent Insects

You see leaf damage, round up the usual suspects and prepare to defend your fields. But be careful, you might just be getting one of the ‘good guys’ by mistake.

Experts agree farmers are being more vigilant about making sure they're spraying for the right insect at the right time. But with tight profit margins and an application cost of $10-15/acre, producers need to take even greater steps and ensure it makes sense both economically and ecologically to spray, says John Obermeyer, Purdue University entomologist.

“One of the most common ‘good guys’ out in the field, especially early in the season, are ground beetles,” says Obermeyer. “They're eating on a lot of the worms that may be actual pests to the crop.”

Ground beetles move quickly and are brown, shiny, and fairly good-sized. Not only does the ground beetle stand out in a crowd, there are no crop pests that look similar enough to be confusing.

Unfortunately, that's not the case with every insect.

The crane fly larvae is one that often deals with a case of mistaken identity. It's a worm that feeds on dead and decaying matter, but is often confused with black cutworm (see photos above). While black cutworm can cause severe leaf damage by feeding on seedling corn, crane fly larvae is actually good for the soil, breaking down dead and decaying organic matter.

Black cutworm is the first suspect when seedling corn damage occurs. But it can be tricky to convict, according to Obermeyer.

“I visited a field where both cutworms and wireworms were being considered as the source of the problem. But it turned out the field was full of millipedes that were not damaging the crop,” he says. “A small amount of black cutworms were in there, but not enough to reach the economic threshold.”

Wayne Bailey, University of Missouri entomologist, has seen nearly identical damage caused by stinkbugs and billbugs in corn. Both insects create feeding holes that appear in even patterns in the whorl of a corn seedling. The only damage difference is that holes caused by stinkbug feeding tend to have yellow halos, while billbug's feeding holes tend to stay green.

“There are different thresholds. In fact, the timing for control is different,” Bailey says. “My concern is that if stinkbugs are present in low numbers, it may be non-economic. Yet they're seeing holes in plants that may have been caused by billbugs, which happened very early season, and they may assume they're stinkbug-caused. It can be very confusing.”

Both entomologists say that farmers generally do a good job of diagnosing pest problems.

But they also suggest talking with local extension agents and chemical dealers.

Bailey says the best way to get a positive identification on the problem insect is to collect a sample. However, if you can't catch the crook, a photo lineup is the next best thing.

“We use a lot of digital cameras now and that has really speeded things up,” Bailey says. “They can send it in to us on e-mail and we can look at it right away.”

For more information on insect identification, visit: The following Missouri extension documents are available: M166, Corn Insect Pests: A Diagnostic Guide; PS6, Common Soybean Insects; or PS13, Beneficial Insects in Field Crops. Or call 800-292-0969.

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