Armyworms reported in Illinois

Armyworms have been reported in central Illinois as far north as Peoria by Kevin Black, Growmark agronomist. There have been no reports from Iowa, but farmers and field scouts in southeastern Iowa should be aware of potential problems over the next couple of weeks. These insects rarely cause losses in conventional-tilled fields, but they should be scouted in minimum- or no-tilled fields with any kind of living ground cover.

Armyworm injury to a cornfield.

The adult 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. Armyworms can be recognized by the black stripes on the four pairs of prolegs near the middle section of the body. 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. Another option would be to scout for armyworms before the herbicicde is applied; if they are found, then an insecticide could be tank mixed.

Armyworm injury to whorl-stage corn, eating all of corn except for leaf midrib and stalk.

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, 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.

Insecticides labeled for armyworms in corn include

  • 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
  • and Sevin XLR+, 2-4 pints

Warrior also is labeled for armyworms in corn, but the label states that it should be used against first and second instars only. It is unlikely that most field scouts will find armyworms this small.

This article originally appeared on pages 81-82 of the IC-486(10) -- May 21, 2001 issue.

Updated 05/20/2001 - 1:00pm