Sod webworm moths can be distinguished from most other moths by their appearance. When at rest, their mouthparts extend snout-like in front of their heads. The moths also fold their wings partially around their bodies so that they appear very slender. Most species are brownish or dull ash gray with white or grayish hindwings. Most species measure about 1/2 inch in length and have a wingspan of about 3/4 inch. 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.
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. Larvae resume feeding or pupate in spring. Moths emerge during late spring or early summer, depending on species. Adult activity occurs most often during the early hours of night. After mating, females scatter their eggs indiscriminately as they fly over turfgrass. Eggs hatch in seven to ten days and young larvae immediately begin to feed and construct silken tunnels in the soil. During hot weather, most feeding occurs at night or on cloudy days. Large larvae may cut off entire grass leaves and pull them into their silken tunnels. Some species have only one generation per year, but many have two or three per year with approximately six weeks elapsing between egg deposition and adult emergence.
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. The buffalograss webworm, Surattha indentella Kearfott, feeds almost exclusively on buffalograss.
Young larvae feed only on the surface layers of leaves and stems. The first evidence of damage to non-stressed turfgrass may be small patches of leaves that are yellow to brown during summer. Larger larvae cut off grass blades just above the thatch line and pull them into their tunnels to eat them. The injury appears as small brown patches of closely cropped grass. If many larvae are present, these patches run together to form large, irregular brown patches.
The presence of webworms under drought conditions constitutes the most serious situation for potentially heavy turfgrass damage. Not only can the dormancy of the grass conceal development of early feeding symptoms, but all too often the dead plants do not become evident until fall rains revitalize the turf.
Inspection & Control
Adult moths are often noted flying in erratic, zigzag patterns close to the turf surface at dusk; however, the presence of a large number of small moths flying over the lawn does not necessarily mean that a heavy larval infestation will occur. Nor does the presence of flocks of birds feeding on the lawn during the day signify a heavy infestation. These phenomena, however, should cause the presence of webworm larvae to be suspected and should not be ignored.
Since the presence of adults is not a positive indication of potentially damaging larval populations, larval abundance should be determined before a decision is made to apply an insecticide. Infestations can be detected by applying irritants such as dish detergent. Add one-quarter cup of dry or one ounce of liquid detergent to one gallon of water in a sprinkling can and apply the solution to one square yard of turfgrass where an infestation is suspected. Also effective is mixing one tablespoon of a commercial garden insecticide containing 1 percent to 2 percent pyrethrin in one gallon of water. If the thatch is dry, irrigation before the test is advisable. Caterpillars will surface within 10 minutes of soaking and can be found by separating the blades of grass, particularly at the interface between living and dead areas of turf. Treat when 10 to 30 or more larvae are found per square yard of turf. Use the lower threshold for more highly managed turf. Rougher turf areas can tolerate 30 or more per square yard. Preventative insecticides (e.g., imidacloprid, clothianidin) are suggested for newly planted lawns.