Armyworms are numerous in field crops throughout Illinois. Armyworms feed primarily on wheat and other small grains, as well as pasture grasses. If their food source is depleted by their feeding, maturation of the crop, or herbicide application, the caterpillars can move en masse, like an army, into nearby lawns, golf courses, and other turfgrass areas. When numerous, they can eat off every blade of grass in several thousand square feet of turf per night. In the evening, you have nice, green turf, and in the morning, all you have are crowns and thatch.
True armyworm caterpillars grow to about 1-1/2 inch long. Mature larvae are brown to black, with five orange stripes, one down the back and two on each side. Small caterpillars are dark in color, with less noticeable stripes. Caterpillars hide in the thatch during the day and come out to feed at night.
Because only the blades are eaten, irrigation helps the grass crowns grow new grass blades quickly. Control can be accomplished with a treatment of bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban), diazinon, halofenozide (Mach 2 or Grubex), spinosad (Conserve), trichlorfon (Dylox or Proxol), or other labeled insecticides. Insecticidal nematodes, either Steinernema carpocapsae (BioSafe) or Heterorhabditis bacteriophora(Cruiser), should also be effective.