There have been scattered reports of sod webworm causing lawn damage in northern and central Illinois. This insect is most likely to occur in areas that have not received as much rainfall as most of the state or in well-drained locations such as slopes and berms. This is because sod webworm larvae are killed by microsporidia, which are microorganisms that are present in damp conditions.
Sod webworm damage will first appear as indistinct areas of brownish turf. Close examination will show that many of the grass blades have been eaten off at the base. You may also see tiny balls of green caterpillar feces—about twice the size of a pinhead. Large numbers of starlings, cowbirds, red-winged blackbirds, or other blackbirds on the turf may also indicate the presence of sod webworms.
Flush the caterpillars to the surface with a tablespoon of 2% pyrethrin insecticide or with a mixture of dishwashing detergent and water (one tablespoon detergent per gallon of water). Apply the treatment over a square foot of turf and look for brown-spotted, slender greenish or tannish caterpillars coming to the surface. Two to three larvae per square foot are enough to cause turf damage. Many insecticides are available and will provide effective control.