Corn earworms in whorl-stage corn

The corn earworm has surprised a number of people this summer because populations of the insect are fairly abundant in whorl-stage cornfields in central and western Iowa. Reports of 30-50 percent of the plants being infested are common from those field scouts that have recognized the insect and its injury. Many light green larvae are being found, resulting in some misidentification. The corn earworm typically has a freckled orange head (although sometimes it is green), alternating stripes of darker and lighter colors, and dark setae (hairs), with four pairs of prolegs in the middle of the body. The light green larvae might be confused with the green cloverworm that has three pairs of prolegs and occurs in soybeans; the dark larvae might be confused with armyworm or fall armyworm. None of these other insects have been reported in Iowa this year, although it might be possible to find armyworm. The species of larvae most likely to be found in corn the first week of July are European corn borer, stalk borer, and corn earworm. The photographs should help with identification.

Corn earworms are striped with an orange-freckled head.
Dark color phase of the corn earworm.
Heads of armyworm, fall armyworm, and corn earworm.

Here is a brief description of corn earworm injury from the soon-to-be-published Handbook of Corn Insects: "Small larvae produce pinhole feeding injury on the leaves and eat tender tassels and silks. Larger larvae may feed on leaves, causing severe defoliation; severely damage the tassel, which can result in poor pollination; and consume the silks and developing kernels."

In the southeastern states, where corn earworm is often an annual problem, control of larvae has been suggested if 50 percent of the whorls are infested. There is not a more exact economic threshold that takes into account the value of the corn crop and the cost of control. Many of the insecticides that are labeled for European corn borer also are labeled for corn earworm. Check labels for specific rates and read and follow all label directions.

This article originally appeared on page 136 of the IC-482(18) -- July 12, 1999 issue.

Updated 07/11/1999 - 1:00pm