Sod Webworm is a pest especially of bluegrass lawns. The adult is gray-tan, small moth (½ to ¾ inches in length) that flies in an irregular zig-zag pattern over the turf, usually late in the day near dusk. Female moths can deposit as many as 200 eggs. The larvae themselves are ¼ to ¾ inch long gray or light brown caterpillars with black spots. They hide during the day in silky white tubes in the soil, and feed on the turf grass at night.
Sod Webworm larvae or caterpillars (the life stage that damages the lawn) feed on blades of grass from late spring until fall. They particularly like thatchy lawns. You'll first notice brown, circular patches the size of small plates in the driest areas of your lawn, where the grass will be clipped very close to the ground, then the same area will turn brown. Nestled in the soil will be the telltale white tubes, with the webworm caterpillars inside. You can use a flashlight to look for night-feeding caterpillars. The buff-colored moths are also visible at night, especially swarming near lights. Or soak a small area of lawn with soapy water (2 tablespoons dish soap in 2 gallons of water). This will bring the pests to the surface.
Sod Webworms are found along the West Coast, but are especially prevalent in the eastern half of the United States, east of the Rockies.