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College of Food, Agricultural, and Environmental Sciences


Assess Corn for Rootworm Injury

July 12, 2007

WOOSTER, Ohio -- Like other insects and plant pests, Western corn rootworm is making an early appearance in Ohio due to hot, dry conditions -- prompting field scouting for feeding injury.

Ron Hammond, an Ohio State University Extension entomologist, said that now is the time to assess root injury from larvae of Western corn rootworm and the variant, as well as take notice of adults that might be eyeing corn silks in uneven fields.

"This is the time when it's imperative we determine how heavy the populations are this year, how well control tactics have worked and how common the first-year corn rootworm variant has been in Ohio," said Hammond, with the Ohio Agricultural Research and Development Center.

The rootworm larvae cause the most significant damage by feeding on corn roots. In severe cases, corn lodging can occur and reduce yields. Growers can determine how extensive larval root feeding is by grading the roots on a 0-to-3 scale. Zero indicates no visible damage to roots, "1" indicates one node of roots destroyed, "2" indicates two nodes of roots destroyed, and "3" indicates three nodes of roots destroyed. The higher the number, the greater the potential for economic loss.

Hammond said that despite the damage caused by larvae, adult corn rootworm beetles are also a concern. They not only cause cosmetic damage to plants through leaf feeding, but they can also cause significant injury to plants through silk clipping.

"Silk clipping from adults can be severe in cases where we have uneven cornfields, which is what we are experiencing now with the lack of significant rainfall across the state," said Hammond. "Because of the potential for uneven silking with the variation in corn growth within fields, there may be only a small portion of the field that has fresh silks at any one time. In these cases, the beetles will congregate on those emerged silks and populations per plant may be larger than if the whole field had silked."

Rescue treatment for rootworm beetle silk clipping is warranted if five or more beetles are found per silk mass when 75 percent of the plants have silked and silk clipping to one-quarter of an inch or less is observed.

Growers can take steps to help control the corn rootworm and the damage it causes.

• In areas not impacted by the Western corn rootworm variant, crop rotation is the best management practice. Continuous cornfields are most at risk for corn rootworm problems since adults lay eggs in cornfields for larvae emergence in the following year's corn crop.

• In fields impacted by the variant (adults that lay eggs in soybean fields for larvae emergence in the following year's corn crop), planting Bt rootworm corn is one of the best management techniques. "With Bt corn, you still have to leave a 20 percent refuge (non-Bt corn) so the insect doesn't develop resistance," said Hammond. According to the U.S. Department of Agriculture, 20 percent of this year's corn crop in Ohio contains stacked varieties that include herbicide and insect resistance.

• Other management practices include soil insecticides and seed treatments. "The biggest problem with seed treatments is that sometimes they don't work as well against heavy insect populations," said Hammond.

For the latest developments regarding the Western corn rootworm, log on to the OSU Extension Agronomic Crops Team Web site at Information on corn rootworm management can be found at

Candace Pollock
Ron Hammond