Keith Waldron, NYS IPM
True Armyworm moths are long-range migrants that arrive on spring storms from their southern overwintering locations. Migration is sporadic, cyclic from year to year, and difficult to predict. True armyworms are primarily a pest of plants in the grass family: forage, pasture (and lawns), corn, and wheat and other small grains. Note: Under hunger stress, these armyworms will also attack some legumes and other plants.
Armyworm moths lay their eggs on weeds or grasses along field margins, corn leaves, or small grains. Larvae hatch about a week later and develop for about three weeks. They feed mostly at night.
Larvae range from 1/8 to 1 ½ inch long. They look pale green too brownish while still small and have smooth, cylindrical bodies. Mature larvae are smooth and marked with two orange, white-bordered strips on each side.
True Armyworm Lifecycle
True Armyworm Larvae
Keep a lookout for armyworms early while larvae are still small, since large larvae do most of the feeding and quickly destroy whole stands of corn, grasses and small grains. Fields at most risk for armyworms feeding are:
- grass or mostly grass hayfields, pastures
- wheat and other small grain fields and cut hay fields
- cornfields that:
- were planted into a small grain cover crop such as rye grass
- have grassy weeds, quackgrass, crabgrass and bluegrass and other perennials
- were planted into burned down sods, have grass weed issues, no-till or reduced tillage fields, fields with crop residue
- fields near severely infested small grain and cut hay fields, and in no-tillage corn established in grain stubble or on grassy land.
True Armyworm Damage
Armyworms can move from field to field very quickly. Start scouting for the armyworms in May and repeat scouting every 3 days to 5 days. Monitor fields for armyworm larvae when they are less than 1/2 inch long.
Because armyworms feed at night, look for chewed leaves, cut stems, lodged plants, pellet-like frass on the ground, and larvae hidden under plant canopy and surface residue.
Armyworms have two (sometimes three) generations in New York. Each takes about five weeks to complete. In a normal year, late-generation impacts are usually minimal or isolated. But if you see armyworm larvae of varied sizes (1/2 inch and greater), it suggests several flights on storm fronts—meaning we may see an extended period of armyworm activity.
Most years, natural enemies—various fungal and viral diseases as well as parasites such as tachinid flies—help suppress armyworms.
Recommended economic thresholds for corn:
- seedling: 10 percent or more plants show damage and larvae are still present.
- whorl-stage: apply an insecticide when there are three or more larvae per plant.
Tall corn seldom needs treatment unless the leaves above the ear are also damaged.
Note: control can be challenging if caterpillars are greater than one-inch long.
Is treatment necessary? Check that the insecticide is labeled for true armyworm and the crop, be it corn, wheat, grass hayfields, or similar crops. Where possible, treat only the infested portion of the field and a 20- to 40-foot border around it. Treating the border prevents armyworms from invading from an adjacent infested field. Because the larvae are active at night, apply treatments late in the day.
Several transgenic corn hybrids with certain Bt. genes are resistant. Learn more at Michigan State University: Handy Bt Trait Table for U.S. corn production.