Integrated Crop Management

Armyworms damage corn in southern Iowa

One of the first reports of significant armyworm damage has come from Tracy Cameron, agronomist at Crestland Co-op. He reports a cornfield in Adams County with V1-V2 stage plants that had several acres eaten to the ground. The armyworms were seen crawling on the soil surface during the day and moving to other plants to feed.

Armyworms rarely cause losses in conventional-tilled fields, but they should be scouted for in minimum- or no-tilled fields with any kind of living ground cover, especially grassy weeds. The adult moths migrate from the southern United States 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 rye or 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, at least on larger plants. 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.

Armyworm injury to a cornfield.

Armyworms typically have black stripes on the four paris of larger prolegs.

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 of armyworms should be considered when larvae are less than 3/4 inch in length, the population exceeds eight 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 1 1/2 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. Unfortunately, I do not have data on economic thresholds for armyworms in plants smaller than V7 so treatment decisions would be a judgement call.

Table 1 lists commonly available insecticides labeled for armyworm control in corn. The label for Warrior (although registered for use in corn) states that it should be used only against first and second instars. Armyworms this size have recently hatched and are unlikely to be found by most field scouts. Therefore, by the time a problem is detected, most armyworms will be much larger and beyond the control of Warrior.

Table 1. Insecticides labeled for armyworms.

Insecticide Rate per Acre<br>(Low and High Rates)
Ambush 2E 6.4-12.8 ounces
Asana XL 5.8-9.6 ounces
Lorsban 4E 1-2 pints
Penncap-M 2-3 pints
Pounce 3.2EC 4-8 ounces
Sevin XLR+ 2-4 pints
Warrior 1E or T 2.56-3.84 ounces (for control of 1st and 2nd instars only)

Read and follow all label directions.

This article originally appeared on page 105 of the IC-486(13) -- June 11, 2001 issue.

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