Look for small larvae down inside the whorls or feeding holes in the new leaves. Larvae that are feeding in the whorl, but haven't tunneled into the plant, can be killed with an insecticide. The smaller the corn, the more likely it can be killed by stalk borers. Stalk borers don't migrate very far from grass, so only the first four rows of corn next to grass would need to be sprayed. If the number of infested plants exceeds the percent given for the plant stage and market value selected, then an insecticide application can be economically justified.
Young plants have a lower threshold because they are more easily killed than older plants.
These thresholds were developed by Larry Pedigo , entomologist at Iowa State University. Always read and follow label directions. This information subject to a usage policy .