All turfgrass species grown in the Midwest may be damaged by sod webworms.
Damage will appear as scattered brownish areas of turf, with these areas combining into large brown areas later. Close examination reveals firmly rooted turf that appears brown because many of the grass blades are missing and the thatch is exposed. Close inspection of the crowns of the turfgrass plants reveals very short stubs of green grass blades remaining from their being eaten off by the sod webworm larvae. Green, pinhead-sized, fecal pellets will be easily seen at the base of the plants. Damage may occur from spring into fall but is most common in midsummer.
Sod webworms are usually a serious pest problem in Illinois during seasons of drought on non-irrigated turf. The larvae are very susceptible to diseases, particularly under cool, damp conditions. These conditions occur often enough in Illinois that outbreak numbers usually do not occur. There are several species of sod webworms that occur in Illinois. The most common is the larger sod webworm, the species primarily discussed here. Other Illinois species of turfgrass-feeding webworms appear similar but may be slightly different in size and may prefer some turfgrass species to others.
Sod webworms overwinter as larvae in the thatch. Sod webworm larvae are slender caterpillars that are grayish to tan and covered with brown spots. They construct a silk-lined burrow in the thatch where they hide during the day. At night they leave their burrow to feed on grass blades. They cut individual grass blades within 1/8 inch of the plant's crown and take many of the blades back to their burrow to eat. Fully grown, 1-inch long caterpillars pupate in their burrow, emerging later as adult moths.
The adult moths are tan, about 3/4-inch long, with a wingspan of about 1 inch. The palps on the front of the head are elongated, giving the moth the appearance of having a long snout. When at rest, the wings are held tightly against the body so that the body appears tube-shaped; these moths are commonly called tube moths or closed-wing moths. They sit in the turf during the day. When disturbed, the moths rarely fly higher than 5 or 6 feet above the turf and fly in a jerky up-and-down manner before settling back into the turf within 30 feet of where they were disturbed. If a moth is approached slowly and quietly, it can be observed at rest, typically sitting crosswise on a grass blade. The above characteristics can be observed and the moth identified.
Probably the most common method of scouting is to observe when sod webworm moths are present in large numbers in turf areas. Through turf maintenance such as mowing or fertilization, the moths fly up and are easily recognized by their jerky, short flights. Note heavy numbers on the calendar and plan to scout for larvae or treat with insecticide about 2 weeks after large numbers of moths are seen.
Insect-feeding birds, particularly starlings, feed heavily on sod webworm larvae in turf. The presence of starlings and other birds in large numbers on turfgrass areas for several days in midsummer is a clue that they may be feeding on sod webworm larvae. Close examination of the turf will reveal holes (poked into the turf by beaks) that will be 1 to 2 inches deep and almost 1/2 inch in diameter. Further turf examination should reveal sod webworm feeding damage or frass.
Although sod webworm larvae are difficult to locate in the thatch, they can be brought to the surface where they can be easily seen and counted. Irritating drenches of 1 teaspoon of 6 percent pyrethrin (or 1 ounce of liquid dishwashing detergent per gallon of water) applied to 1 square foot of turf will drive out the sod webworms. Two or more sod webworm caterpillars per square foot are sufficient to cause obvious damage.
Control sod webworms by applying a labeled insecticide to the turf, usually as a liquid, and allow it to dry on the grass blades to poison the sod webworm larvae when they feed on the grass blades at night. Granular insecticides are effective, but they must be watered lightly to activate them.
Nonchemical control in Illinois occurs naturally almost every year by microsporidian diseases of the sod webworm larvae, particularly during the winter. Endophyte-containing turfgrasses, such as some fescues and ryegrasses, effectively control sod webworm larvae as well. However, because sod webworm is a sporadic problem in Illinois and because endophyte grasses are not effective against white grubs, the major pests in Illinois turfgrasses, paying extra to purchase them may not be warranted.