Large numbers of sod webworm moths were flying in central Illinois about 2 weeks ago and probably also in other portions. These tan moths 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, looking tubelike. Sod webworms have two generations per year in northern Illinois and three in southern Illinois. Generations overlap, making it likely to see them at any time during the growing season.
Sod webworm larvae are attacked by a naturally occurring microsporidium that typically kills most of the overwintering generation and subsequent generations in Illinois. This disease is more prevalent during cool, moist weather. With the current dry weather, sod webworm damage may show up yet this fall if rainfall is not timely. Look for damage to show up first in well-drained turf areas, such as south-facing slopes and the tops of berms.
Damage appears as indistinct brownish areas. Close examination reveals firmly rooted grass with few 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 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.
To make a disclosing solution, mix 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. 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.