Northern Plains Integrated Pest Management GuideIPM

Authors: Buyung Hadi, Robert Wright, Phillip Glogoza, Mark Boetel, R. Jeff Whitworth, Holly Davis, and J.P. Michaud

Identification

True armyworm and fall armyworm are minor lepidopteran pests of corn in the Northern Great Plains.

Full grown true armyworm larvae (Mythimna unipuncta formerly Pseudaletia unipuncta) are 1.5-1.6 inches (3.7-4.1 cm) long, green to brown in color with black and white flecks. A net-pattern is visible on the head capsule of the larva.

A newly-hatched fall armyworm larva (Spodoptera frugiperda) is light-colored with a black head. The body color darkens as it ages. Fully-grown fall armyworms are 1.5 inches (3.81 cm) long, with colors varying from light tan to almost black. Fall armyworm caterpillars can be identified by a prominent white inverted Y-shape on the head. Additionally, black spots with spines are scattered along the body with four dark spots on top side of the caterpillar's hind end forming a conspicuous square pattern.

A true armyworm, note the body color and net-pattern visible on the head capsule

White inverted Y-shaped marking on fall armyworm's head

A fall armyworm on peanut leaf, note the square pattern formed by four large black spots on the eighth abdominal segment of the fall armyworm

Life Cycle and Seasonal History

Both true and fall armyworm moths survive the winter in the southern states and the adults migrate northward throughout the spring and summer. Armyworms are usually found in the Northern Great Plains beginning in June or July. Adult moths mate and lay egg masses at night, usually on the underside of the leaves. True armyworm females prefer to lay their eggs on grasses including small grains and weeds within field and field margins. Upon hatching, the larvae feed on the nearby hosts' leaves, and may move to nearby corn fields. Larvae of armyworms can feed on the leaves, developing tassels or ears. The larvae of true armyworm feed at nights and hide in the soil during the day. Fall armyworm larvae feed during day and night. In the Northern Great Plains, armyworms produce 1-2 generations per year.

Plant Injury and Damage

Serious feeding injury from armyworms may be more apparent in late-planted corn. The armyworms can cut ragged-edged holes in the leaves of whorl-stage plants, consume whole leaves except for the midribs in severe cases, and leave excrement/frass on the leaves. The effect of armyworm leaf feeding injuries on yield loss is correlated with the length of feeding period. Corn plants are usually able to compensate for foliar injuries incurred over a short period of time. Later generation of the fall armyworms may enter the developing corn ear, cutting an entry hole through the husk, and feed on the kernels.

Fall armyworm with some associated frass on a corn leaf

Fall armyworm feeding deep inside the corn whorl

Fall armyworm and entry hole on a corn ear

True armyworm related injuries where whole leaf tissues were consumed, leaving the midribs bare

Management Approaches

Scouting and Threshold

Control at the whorl stage is usually not economical and should not be attempted unless 75 percent of the plants are infested and the larvae are less than 1-1/4 inches (3.1 cm) long. To estimate the field infestation, choose 20 plants randomly in one spot of the field and thoroughly examine the plants for fall armyworm presence or injury. Repeat this in four other spots scattered across the field and calculate the percentage of plants with fall armyworm infestation out of the total samples.

Treatment to control armyworms already tunneling into the ears is not effective. If larvae are less than 1-1/4 inches (3.1 cm) long and are detected during corn silking, control may be warranted before the larvae tunnel into the ears.

It is possible to have hot spots of armyworm infestation in the field, in which case spot treatment may be warranted.