Description: Armyworms are striped larvae, which feed on the foliage of corn and small grains in the late spring and early summer. Mature armyworm are about 1 1/2 inches in length. Corn planted no-till into a grassy cover crop such as rye or into an alfalfa stand mixed with orchardgrass is very likely to become heavily infested with armyworm.
Damage Symptoms: Armyworms feed on the leaves of both seedlings and mature plants and can cause severe defoliation. They consume the softer leaf tissue, leaving the tougher midribs and stalk. Larvae typically start feeding on the lower leaves before moving to the upper leaves.
Scouting: June or when grassy weeds are killed with herbicides. Early season problems occur in no-tilled fields that were in pasture or sod previously or that have heavy grassy weed populations. Late season problems occur when small grain crops mature and the armyworms migrate into adjacent corn.
Management: Eliminating the use of rye or other grassy cover will reduce the potential of armyworm outbreaks. At-plant insecticides can be used for a rescue treatment.
The Ohio State University
Iowa State University Extension, Corn Field Guide: A reference for identifying diseases, insect pests and disorders of corn. 2009: 52.