Sod Webworms

Adult sod webworms, called lawn moths, are typical snout moths: they have sensory appendages called labial palps that extend in front of the head. The moth holds its wings close to and over its body at rest, giving it a slender appearance. When disturbed, the moth makes a short flight close to the grass. At night, these moths drop their eggs indiscriminately onto turf. The creamy larvae have a distinctive double row of brown or black spots down their backs, located at the base of long bristles. The Lucerne moth larva is somewhat larger than the other sod webworm larvae. During the day larvae reside in silk-lined burrows, writhing when disturbed. At night they emerge to feed.

Bluegrasses and bentgrasses often suffer the most damage, while perennial ryegrasses and turf-type tall fescues infected with endophytes (symbiotic fungi) and warm-season turfgrasses are more resistant.

First instar sod webworm larvae are leaf skeletonizers. Later instars notch or cut off leaf blades and pull them into the burrow. Heavily infested turf (more than 100/sq. yd.) quickly appears moth eaten, with irregular patches of brown grass or bare areas. Significant damage can occur on drought-affected bluegrass and on bentgrass green and tee areas. Lucerne moths are primarily a problem where clover and dichondra are mixed with turfgrass.

When sod webworms are present, dethatching the turfgrass may help. Monitor to determine if treatment is needed. Treatment choices include parasitic nematodes and Bacillus thuringiensis (Bt).

Natural enemies in California include a parasitic tachinid fly and two parasitic braconid wasps, along with earwig, rove beetle, robber fly, paper wasp, ant, and vertebrate predators. The extensive soil or thatch contact of sod webworms makes Steinernema carpocapsae nematodes a valuable control measure. Bacillus thuringiensis ssp. kurstaki (Bt), a microbial insecticide, can be used but it breaks down rapidly in sunlight, washes readily off leaves, and is ineffective against late instar larvae.

Thatch removal can assist in removing sod webworm habitat, although larvae do not require a thatch layer to be present in very high numbers. Control of clover and dichondra may help minimize damage. Damage is usually not noticeable in turf mowed at heights above 2.5 inches.

Monitor for these pests from June to early October. Consider treating only when a drench test (see section on MONITORING AND TREATING INSECTS AND MITES) indicates there are more than 5 larvae per square yard on stressed greens or 15 larvae per square yard in other situations. If Bt is used, apply it when there are predominantly early instar larvae. Other materials are also most effective on small larvae but will kill larger ones more effectively.

Mow and irrigate the site before applying insecticide and do not mow or irrigate the turfgrass for at least 24 hours after treatment unless nematodes were applied, in which case apply a post-treatment irrigation. When Bt is applied, do not irrigate for 2 days after treatment.