On Crops: Corn and other grain crops
Most of eastern North America
Armyworms are dusky brown mottled caterpillars that eat the foliage of corn and other grasses. As they mature to about 1.5 inches (3cm) long, armyworms darken and show lengthwise stripes. The larvae of a night-flying moth, armyworms can suddenly become numerous if a resident population overwinters nearby, but the adult moths are also quite mobile. Sometimes the moths overwinter in the mid-Atlantic region and move northward in large numbers in early summer.
The primary feeding point are the new leaves, which will have raggedly-chewed edges and dark pebbles of caterpillar excrement nearby. During the day, the caterpillars hide in the shelter of the corn plant's whorl of new leaves, or beneath rocks or other shelter near the base of small plants.
Clean up the previous year's grain crops from year to year, and watch your corn closely if you live in an area where low-till corn is grown commercially. Armyworms have many natural enemies, including birds and several beneficial wasps, so encourage these into your garden.
Late in the day, treat the plants with a Bt or spinosad-based organic pesticide by applying it to the leaves most likely to be tasted during the night's feeding. Damage should stop within a few days.