European corn borer showing up in conventional corn

Editor’s note: The following was written by Adam Varenhorst, associate professor and South Dakota State University Extension field crop entomologist, Philip Rozeboom and Patrick Wagner for the university’s website Aug. 8.

European corn borer caterpillars have been observed in conventional corn at several locations in South Dakota.

Indicators for the presence of European corn borer caterpillars in corn are a line of straight holes in the upper leaves that is often referred to as shot-hole injury. These holes are an indicator that a caterpillar fed on the leaf while it was still curled.

In South Dakota, there are populations of European corn borer that have one generation per year in the northern areas of the state, and populations that have two generations per year in the southern areas of the state. The fields that are observing caterpillar defoliation and caterpillars in the upper whorl are in areas where it has two generations per year.

Scouting for caterpillars is time intensive. For each field, 20 plants from five areas need to be examined (100 plants total for each field) for signs of defoliation.

If defoliation is present, the whorl and upper stem need to be dissected to determine the number of European corn borer caterpillars that are present and their location within the plant. The University of Illinois has a worksheet at bit.ly/4oAWXOgthat can be used to determine if an insecticide application would be profitable.

European corn borers don’t receive as much attention now as they did prior to the release of Bt corn hybrids that have traits for managing the caterpillars. However, in areas where conventional corn is planted, the corn borer can still be an issue.

Without management from either Bt hybrids or insecticides, the European corn borer caterpillars can cause 2-6% yield losses per plant. Some plants will have more than one European corn borer caterpillar feeding within the stem, which can result in yield losses as high as 12% per plant.

Insecticides must be applied when the caterpillars are still in the whorl and haven’t tunneled into the stalk. Once they begin tunneling, the insecticides won’t be able to reach them.