During dry periods, keep an eye out for sod webworm attack in turf. Sod webworm larvae are normally controlled naturally by microsporidia in Illinois. These microbes are most effective under moist conditions. With our rather consistent rainfall in Illinois, sod webworms are not a problem that we expect every year. However, during dry spells when unwatered grass goes brownish and dormant, sod webworms can become numerous without the damage being noticed. This happened in McHenry, Lake, and parts of Kane counties in northeastern Illinois in late July.

Larvae are slender and greenish to grayish in color with brown spots. They emerge at night to clip the grass blades close against the crown, causing brownish areas of turf because the thatch shows through. Close observation reveals that there are not as many grass blades present as there should be. And, if you look closely, you can usually see numerous pinhead-sized, green balls of feces left by the larvae. During the day, larvae are in their silk-lined tunnels in the thatch.

You can flush the caterpillars out of their tunnels by applying an irritant disclosing solution. This solution can be a teaspoon of 5 percent pyrethrin or 2 tablespoons of dishwashing detergent per gallon of water. Use a watering can or another method to distribute this mixture evenly over a square foot of turf. Within 30 seconds or so, sod webworms, black cutworms, ground beetles, rove beetles, earthworms near the surface, and various other creatures will come out onto the turf surface. The sod webworms may be over 1 inch long if they are fully grown. All of these creatures will go back into the thatch in a few seconds, so keep an eye on the area for 3 or 4 minutes. If there are at least two or three sod webworms per square foot, their numbers are high enough to cause damage.

You can also scout sod webworms by watching for the adult moths. Large numbers of 1-inch-long tan moths that fly low to the ground in a zigzagging and dipping motion are probably sod webworms. When at rest, these moths hold their wings tight against their bodies in a tubelike fashion. If you have large numbers of moths in turf and the weather stays dry, applying insecticide two to three weeks later will kill the young caterpillars that result. Sod webworms have several generations per year, so you may find these insects at any time during the growing season. Several insecticides provide effective control.