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Post-planting weather could decide insect threat, specialist says

WEST LAFAYETTE, Ind. - Farmers should look to the skies for an idea of how active below-ground insects could be in cornfields later this spring, a Purdue Extension entomologist says.

Weather conditions following planting are a good barometer for infestations of seedcorn maggots, wireworms, grubs and slugs, Christian Krupke said. Those insects, and others like them, are beginning to move toward the surface of the soil after overwintering beneath the surface. They feed on plant roots and green material and are a greater threat if the corn plant is not well established.

"All of those pests get a foothold only when we get a cool, wet post-planting period," Krupke said. "If you don't have those weather conditions the corn plant generally pops out of the ground and does just fine in terms of insect pests. So in the early stages of the crop season it really is weather-dependent."

Corn planting is off to a slow start in Indiana. As of Monday (April 15) farmers had planted less than 1 percent of the 6.1 million acres they told the U.S. Department of Agriculture they intend to plant in a March survey. That compares with 21 percent planted by this time one year ago and the mid-April five-year average of 6 percent.

A weeklong forecast of cooler and wetter weather in much of Indiana could slow planting progress but won't necessarily contribute to post-planting insect problems, Krupke said. Aside from timely control of winter annual weeds that can attract egg-laying moths like black cutworm, there's little farmers can do ahead of planting to reduce insect damage risk and only certain steps they can take after seeds are in the ground.

"Sometimes you'll get insect protection from insecticidal seed treatments at this time of year. Seedcorn maggot is one example," Krupke said. "For others such as rootworms, white grubs and wireworms, you also can get some protection from in-furrow insecticides - for those still using granular and liquid products. The Bt corn hybrids have no affect on wireworms, white grubs, seedcorn maggots or slugs. There's not much that can be done in terms of managing below-ground pests once damage has occurred, however."

If the weather cooperates and corn plants are able to outgrow those pests, other insects - notably, armyworm and black cutworm - could present problems, Krupke said.

"Right now these moths are in the Gulf states where they can develop year-round," he said. "In the spring they move north on prevailing winds and with storm fronts - some years many, some years few - and look for places to lay eggs. They prefer to lay eggs in green material such as weedy fields and fields with cover crops, and when those cover crops or weeds are killed the larvae will then move over to the corn that is out of the ground."

Purdue entomologist John Obermeyer coordinates a network of volunteers who monitor the moths and others using pheromone traps placed throughout the state. They report their findings in the weekly Purdue Pest & Crop Newsletter.

The good news for farmers is that last year's drought hurt some insects as much as it did crops. Krupke believes overwintering insect populations generally are lower and, with the exception of spider mites, which thrive in hot, dry conditions, could portend fewer insect problems this year.

But farmers should not be overconfident.

"If conditions are just right and these insects are in contact with germinating seed for a long time, that's when you have a recipe for bad things to happen," Krupke said. "Insects in general, and pests in particular, can increase their populations extremely rapidly when conditions are right."

Source: Purdue university

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