With rare exceptions, sod webworms feed primarily on plants of the grass family. Turfgrasses most commonly recorded as hosts include Kentucky bluegrass, perennial ryegrass, fine fescue, and bentgrass. Records of damage to warm-season grasses are relatively few; however, some species will damage bermudagrass and zoysiagrass.
Young larvae feed only on the surface layers of leaves and stems. The first evidence of damage to a normally growing lawn may be small patches of leaves that are yellow to brown during the summer. Larger larvae cut off grass blades just above the thatch line, pull them into their tunnels, and eat them. The injury appears as small brown patches of closely cropped grass. If many larvae are present, the patches run together to form large, irregular brown patches. The presence of webworms under a drought condition constitutes the most serious situation for potentially serious turfgrass damage. Not only can the dormancy of the grass restrict the manifestation of early feeding symptoms, but all too often the dead turf does not become evident until fall rains revitalize the normal turf.
Sod webworms overwinter as larvae in the thatch or soil. Some species do so as mature or nearly mature larvae, while others overwinter as small larvae. In the spring, larvae resume feeding or pupate. Moths emerge during late spring or early summer, depending on species. Adult activity occurs mostly during the early hours of the night. After mating, the females scatter their eggs indiscriminately as they fly over turfgrasses. The eggs hatch in 7 to 10 days and young larvae immediately begin to feed and construct their silken tunnels. During hot weather, most feeding is at night or on cloudy days. Large larvae may cut off entire grass leaves and pull them into their silk-lined tunnels. Some species have only one generation per year but many have two or three per year, with approximately 6 weeks elapsing between egg deposition and adult emergence.
Sod webworm moths can be distinguished from most other moths by their appearance. When they are at rest, their long labial palpi extend, snoutlike, in front of their heads. Moths also fold their wings partially around their bodies so that they appear very slender. Most species are brownish or dull ash gray in color with white or grayish hind wings. Many of the Crambus species have a white streak from the base to the outer margin on the front wings. Most species are about 1/2 inch in length and have a wingspread of about 3/4 inch. The eggs are very small, oblong, and white to pale yellow. Larvae vary from pinkish white to yellowish to light brown with yellowish brown, brown, or black heads. They have thick bodies, coarse hairs, and paired dorsal and lateral dark spots on each segment. Most species are about 1 inch long when mature.
Treat when 15 or more larvae are found per square yard. Please contact your local county extension office for current information.