Armyworms are occasionally a problem in corn, especially in weedy fields, in fields near severely infested small grain, and in no-tillage corn established in grain stubble or on grassy land. Armyworm moths are long-range migrants which arrive on the spring storms from their southern overwintering locations. While there are more than one generation per season of armyworms once they arrive, it is the first generation which causes economic losses in NY. Check fields regularly for ragged holes chewed from the leaf margins and pellet-like droppings (frass) in the whorls and scattered on the ground. The larvae will be found in the leaf whorls or at the surface of the soil.
For whorl-stage corn, apply an insecticide (see table below) only if most plants show damage and about three larvae per plant are found. Tall corn will seldom need to be treated unless the leaves above the ear are also damaged. Only the infested portion of the field and a 20- to 40-foot border around it need be treated. A border 20 to 40 feet wide treated with insecticide will prevent armyworms from invading from an adjacent infested field. Because the larvae are active at night, apply treatments late in the day.