Description & Life Cycle Of Armyworm - The larvae are nearly hairless, smooth, striped caterpillar, varying in color from green to brown. The stripes, one along each side and a broad one down the back, are dark, often nearly black. The stripe along the back usually has a fine light-colored broken line running down its center. The head is pale brown with a green tinge and mottled with dark brown. Larvae will range in size from 1/2 to 2 inches in length. They typically feed at night and during the day, theyâll drop to the soil surface and burrow under the debris. In Vermont, true armyworms overwinter as partially grown caterpillars and complete development in May. Two to three weeks later, adults emerge and lay eggs two to three weeks later. There are usually three generations in a season. Often, but not always, it is the last generation that does the most damage. Natural predators and disease normally keep armyworm populations in a controllable state. However, cool, wet weather during the spring favors development of armyworms and retards development of the disease or the parasites. Although May was warm and dry this year, the intermittent cool, wet conditions in late May/early June, as well as the delay in first cut, may have favored the armyworm over its natural predators.

Patterns Of Damage - True armyworm larvae prefer grasses including corn, small grains and grass hay and pasture crops. They first strip the leaves and then consume midribs, seedheads and finally stems. A large population can strip a field in a couple of days. When the field is eaten, they "march" to adjacent fields, hence the name "armyworm." In corn, armyworms usually confine their feeding to the leaf margins except when populations are very large, then they consume all the leaves except for the tougher midrib.

Feeding starts on the lower leaves and as these leaves are eaten, the armyworms move to the upper leaves. Look for ragged holes and pellet-like droppings in the whorls and scattered on the ground. Larvae do not tunnel into the stalk and they do not feed on the growing point, at least on larger plants.

Cornfields that are most susceptible include fields that are either minimum or no-tilled into grass sod or fields infested with grass weeds. Cornfields that have grassy weeds sprayed postemergence with a herbicide should be closely scouted as the weeds begin to die. Armyworms, if present, will move immediately to the corn. This happened to one field in Franklin County.