Armyworms are the larvae of moths. The caterpillars are light green or tan in their early growth stage and dark green or brown in later stages. Full-grown larvae are 1 ½ to 2 inches long and almost hairless. They can be identified by a series of green, yellow, or brown stripes down the length of their bodies. They eat grass blades, and large infestations can completely defoliate a lawn in a few days.