Significant sod webworm damage has been found in McHenry County, south of Crystal Lake in northeastern Illinois. Sod webworm overwinters as partially grown larvae in the thatch. In the spring, the larvae resume feeding by clipping grass blades off at the base and eating them. Damage appears as indistinct brownish areas. Close examination reveals firmly rooted grass with few grass blades. The brownish areas are the thatch showing. Commonly, one can find small, green fecal pellets from the caterpillars. One-half-inch diameter holes may also be numerous where insectivorous birds such as starlings, robins, grackles, and cowbirds have been feeding on the larvae.
The larvae themselves are up to 1 inch long, with dark brown spots. The background color varies and may be white, gray, tan, or greenish. The caterpillars emerge at night to feed, spending the day in silk-lined tunnels in the thatch. One can find the caterpillars at the soil--thatch interface, but it is easier to flush them out. A disclosing solution can be made by mixing a teaspoon of 5% pyrethrum insecticide or a tablespoon of dishwashing detergent in a gallon of water. Spread this evenly over a square foot of turf; a watering can works well for doing this. This solution irritates the larvae, causing them to come out onto the turf surface within a minute or so. Two or more larvae per square foot are enough to cause damage.
Sod webworm larvae are attacked by a naturally occurring microsporidium that typically kills most of the overwintering generation in Illinois. This disease is more prevalent during cool, moist weather. With the weather of recent weeks through much of Illinois, sod webworm damage is not likely to be widespread. Look for it to show up first in well-drained turf areas such as south-facing slopes and the tops of berms.
Later in the season, be watchful for large numbers of tan moths that fly up from the turf, fly just a few feet above the ground in a jerky motion, and drop back into the turf within 30 feet or so. Adult moths are 3/4 to 1 inch long, with elongated "snouts" and wings that fit tight against the body, making them look tubelike. Sod webworms have two generations per year in northern Illinois, three in southern Illinois. Generations overlap, making it likely to see them at any time during the growing season.
Bifenthrin (Talstar), carbaryl (Sevin), spinosad (Conserve), trichlorfon (Dylox), and many other insecticides--as well as insecticidal nematodes&--are effective against sod webworm larvae. Apply control materials as spot treatments in infested areas and 2 weeks after a heavy moth flight under warm, dry conditions. Turf damaged by light to moderate sod webworm injury will recover with irrigation, as the grass crowns will grow new grass blades to replace those that were eaten.