Armyworms have been reported by Jim Jensen, extension field specialist-crops, defoliating a field of young corn in Lee County. These insects rarely causes losses in conventional-tilled fields, but they should be scouted for in minimum- or no-tilled fields with any kind of living ground cover. The moths migrate from the southern United States, similar to black cutworms, and are attracted to fields that were planted into a cover crop such as rye, or where grassy weeds exist. Females lay their eggs on the rye or grass and the young larvae feed on the leaves. After consuming the grass (or if the grass is killed with a herbicide), the larvae move onto the corn. They usually confine their feeding to the leaf margins except when populations are very large; then they consume all the leaves except for the tougher midrib. Feeding starts on the lower leaves and as these leaves are eaten, the armyworms move to the upper leaves. Larvae do not tunnel into the stalk and they do not feed on the growing point. A field can be completely defoliated in a couple of days when armyworm populations are large. Cornfields that have grassy weeds sprayed with a herbicide should be closely scouted as the weeds begin to die. Armyworms, if present, will move immediately to the corn.

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Corn plants stripped by armyworms.

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Armyworm injury to a cornfield.

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Armyworm injury to whorl-stage corn.

Young corn plants have a remarkable ability to compensate for early-season defoliation. Experiments at Iowa State University have shown that corn in the 7-8- and 9-10-leaf growth stages that sustained 50 percent defoliation in 1 day exhibited only a 2-3 percent and 4-6 percent yield loss, respectively. For corn that is in the 7-8-leaf stage, treatment (see table) of armyworms should be considered when larvae are less than 3/4 inch in length, the population exceeds 8 larvae per plant, and 25 percent of the leaf area has been removed. If armyworms are less than 3/4 inch in length they still have another week or so to feed. If larvae are mostly 11/2 inches in length, then they are nearly done feeding and very little additional leaf injury will occur so the field should not be sprayed; it is too late for the insecticide to be of any economic benefit.

Table 1. Insecticides labeled for armyworms.

Read and follow all label directions. *Restricted use pesticide.