True Armyworm And Fall Armyworm

True Armyworm and Fall Armyworm

True armyworms do most of their feeding damage to cereals as nearly-mature larvae during July.

Larvae feed on the leaves, stripping the leaf margins and move up the plants to feed on the panicles and flowers, stripping off the awns and kernels.

Fall armyworms may feed on dense, lodged cereal crops but are most common in corn. They may feed on the leaves and developing ears but prefer to feed on the succulent leaves in the whorl first. Feeding is usually confined to leaf margins. If the growing bud is injured, corn can suffer a significant loss in yield.

Host Crops

Armyworms feed on oats, wheat, fall rye, corn, barley, and forage grasses. Broad-leafed plants such as alfalfa, cabbage and turnips may also be attacked but the damage is minor. Fall and true armyworms may also feed on field and sweet corn, especially late-planted corn. They feed on both the leaves and ears.


The moths are active at night, feeding on nectar, mating, and searching for egg laying sites. Lodged plants provide a favourable habitat for moth concealment and egg laying. During June, adult female armyworms lay their tiny, white eggs at night, in clusters of up to 100, at the base of host plants. The eggs hatch in one to two weeks.

The larvae feed at night on the leaves and crowns of their hosts. During the day, they rest at the base of the host plants. There are six larval instars. It takes three to four weeks for the larvae to reach maturity.

When mature, the larvae construct pupal chambers a few centimetres below the surface of the soil. The pupae are about 20 mm (3/4 in.) long and reddish-brown in colour. Pupation may last about two weeks. There are usually two generations per summer in Canada with the first generation doing the greatest damage in late June and early July.

May June July August September
Moths blown in Moths blown in and egg laying begins; larval feeding starts First generation larval feeding continues New moths emerge, lay eggs Second generation larval feeding

Scouting Techniques

Influxes of moths from the south, during May and June, can be monitored using light traps or pheromone traps. Infestations of armyworm larvae are not easily detected by casual observation because the caterpillars hide during the day. However, black birds (grackles and red-winged black birds) commonly search for armyworms in small grain. Any field that has significant bird activity should be scouted.

In cereal and grass crops, check at least five areas of a field in late June. Pay special attention to patches of lodged plants and areas of grassy weeds. During the day, look for notched leaves on the plants. Late at night, with the aid of a flashlight, shake plants over a one-square metre area and count the dislodged larvae.

If sampling for inactive larvae during the day, look under plant debris and lumps of soil, in soil cracks, and deep in the leaves. Daytime checking for armyworms is difficult.

Economic Thresholds

Localized infestations may require action. In cereals, control is usually necessary when larval densities exceed 10 larvae per square metre (square yard). Only infested areas of the field should be treated. Spraying should be done in the evening when armyworms are feeding on the plants.

There is little benefit in applying an insecticide once the armyworm is nearly full grown, pupae are present, parasitism is extensive, or the crop is nearing maturity. By that time most of the damage will have been done.

Control Tips

Destroying grassy weeds, one to two weeks before seeding, will minimize the risk of attracting egg-laying moths and subsequent infestations.