We have gotten many phone calls from farmers whose crops are being seriously damaged by armyworms. These calls have come in from Minnesota, Iowa, and Wisconsin. The crops that we have taken calls on are corn, barley, and wheat.
Armyworms have earned their name for their tendency, at high populations, to ‘march’ across a field, consuming whatever vegetation is in their path. Two species of armyworm, true armyworms and fall armyworms, may affect crops in the Upper Midwest, predominantly in the grass family (corn, wheat, other small grains) and in fall grassy cover crops.
Early summer crop damage is caused by the true armyworm. True armyworm larva can cause damage in the early summer to corn, wheat, barley, oats, and occasionally to soybeans and sunflowers. The adults begin arriving in the upper Midwest in April or early May, with peak moth flights later in May or early June. These dates vary from year to year, based on weather conditions.
Neither true armyworms (Mythimna unipuncta, formerly Pseudaletia unipuncta) nor fall armyworms (Spodoptera frugiperda) overwinter in the Upper Midwest. Moths of both species fly north each year to lay eggs and complete one or more lifecycles before migrating south in the fall. (A third species, wheat head armyworm, has been identified in our area, but is rarely a pest.)
You can view armyworm reports from university researchers who trap and monitor armyworm worm arrivals online:
Trapping results don’t necessarily translate into armyworm infestations. Temperature and rain can alter establishment of larval populations, so base treatment methods on later field scouting.
Armyworms prefer to lay eggs in dense, grassy vegetation, such as weedy fields, small grains, small grain cover crops, and grassy field edges. Moths lay eggs for 7 to 10 days in rows or clusters on the lower leaves of grasses or at the base of plants. The eggs hatch in 7 to 10 days and the larva progress through six growth stages, called instars. The early instar stages take from 3 to 5 days each, and the consumption of vegetation nearly triples with each stage. Heavy feeding during the fifth and sixth instars can result in yield damage. Larvae will spent about 7-10 days (depending on temperature) in the sixth instar and consume 80 percent of all the foliage eaten during larval growth before they finish feeding and pupate.
When larva feed on leaf tissue in small grains, they may separate leaves from stems and hurt yields occurs with heavy defoliation, especially if the flag leaf is eaten. The larvae may then move up to the seed heads.
Once fine-leaved grasses are consumed, true armyworms may migrate to corn, where they begin feeding on lower leaves and progress up the plant. They feed on leaf margins: armyworms don’t tunnel into the stalk or feed on ears. Defoliation can be severe, leaving only the corn leaf midribs and stalk
Armyworm larvae coloration ranges from tan to olive to nearly black. They have a tan or orangish head capsule with a vein-like stripe. Larvae are striped down their sides with a dark band bordered by white stripes and a pink to orangish stripe on each side.
Scouting, thresholds, and treatment
Armyworms are active at night and on cooler, cloudy days. During sunny days, larva tend to hide under leaf litter and at the base of plants, so scout either near dawn or dusk. Scout areas first that are most prone to infestations; low, wet and/or weedy areas of a field, grassy field margins, and in lodged small grains.
Small grains. In small grains look in at least five field locations for leaves with notched leaf margins and partially eaten leaf fragment and small round pellets of frass (excrement) on the ground. Shake plants within a square foot to release larva, then count armyworm larvae.
Corn. In corn examine 20 plants in each of at least five field locations. Calculate the percent of corn plants affected by feeding. Count the number of larvae, measure their length, and calculate their average length. As corn reaches V5 or V6, armyworms will hide in the whorl during the day to avoid the sun rather than retreating to the soil surface.
Population thresholds for treating
- Before heading: 4 or more larvae/square foot
- At heading: 2 or more larvae/square foot
- VE-V2: 10% or more of plants are injured and lava are less than ¾-inch long
- V7-V8: Larva are ¾ inch long, there are more than 8 larva per plant and 25% or leaf area has been removed
- Post-pollen shed: Treat if plants are being defoliated above the ear
Once larvae are 1” or longer, insecticide treatments are less effective. In addition, larval of this size will soon finish feeding and pupate.
* There is some discrepancy among the universities about corn threshold levels.
Organic Treatment Options
Organic farmers have four OMRI-approved insect treatments labeled for armyworms.
|Mode of ‘entry’
|Length of residual
|contact and ingestion
|contact and ingestion
|Dipel ES or WP
Additional Armyworm Information for Upper Midwest Farmers
Photo credit: Frank Peairs, Colorado State University