The emergence of corn rootworm adults and the first reports of inadequate performance of soil insecticides often coincide. I have received one report (with more than one field involved) that a soil insecticide applied at planting time has failed to provide acceptable root protection. The individual who reported the problem in fields in a county in east-central Illinois had noticed lodged corn near the field edges. Upon further investigation, he found as many as eight to nine rootworm larvae per root mass and noticeable pruning damage. It is not necessary at this time to divulge the product involved, the individual who reported the problem, or the county in which it occurred. If additional information becomes available, I will provide more details.|
Healthy roots (left) and roots damaged by corn rootworm larvae (right).
Goose-necked corn as a result of feeding damage by corn rootworm larvae.
Many people will soon begin assessing the effectiveness of insecticides applied at planting to protect corn roots from corn rootworm larvae, especially when they begin to observe the adults in the fields. But remember what we have said before: The presence of rootworm adults in cornfields does not mean that the soil insecticide failed to prevent root damage. Also, lodged or goose-necked corn can occur for several reasons, including, but not limited to, strong winds, shallow root systems, and herbicide injury. The only way to fairly assess the effectiveness of a soil insecticide for rootworm control is to dig up corn plants, clean off the dirt, and examine the roots for feeding injury. And remember, you rarely will find corn roots without any evidence of feeding injury by rootworm larvae. Some feeding scars occur in almost all fields. Only severe root pruning should be cause for concern.
As we have urged in the past, make certain you have verified rootworm larvae as the culprit before you decide that the soil insecticide did not perform adequately. During most years, most soil insecticides protect corn roots quite well. In years when "failures" occur, we want to focus on the possible reasons and not be sidetracked by reports of rootworm damage that turn out to be attributable to other causes.--Kevin Steffey