Figure 1. Adult vagabond sod webworm (Agriphila vulgivagellus). Photo by Patrick Wagner.
Sod webworm moths are emerging throughout South Dakota. Although these pests are common during the fall, the number of moth sightings and population densities in the Western half of the state are higher than normal. The particular species being found is the vagabond sod webworm (Figure 1). Unlike several other webworm species found in the United States, vagabond sod webworms rarely cause much damage and the adult moths are no more than just a short-term nuisance.
Depending on the instar, sod webworm larvae (caterpillars) can reach up to one inch in length. Their bodies are brown to green in color with dark spots and covered with coarse hairs. Their heads are solid brown. Adult sod webworms are ¾ to 1 inch long. The adult moths have long snouts which is characteristic of the family Pyralidae. Their wings are dull white or grey with a pattern of dark stripes and a delicate fringe along the edges of the hindwings (Figure 1). While at rest, adult webworms hold their wings in a tube-like position over their bodies. The vagabond sod webworms have bronze or gold colored scales near the fringe.
Overwintering larvae become active in the spring and start feeding on grass blades. Larvae molt several times throughout the summer and then pupate in late summer to early fall. Webworm adults emerge 7-10 days later. The adults will be active for approximately 2 weeks. During this time, the adults mate and females lay their eggs on grass. A week after the eggs are laid, new larvae hatch out and begin feeding until colder weather sets in and they go dormant for the winter. The larvae form web-like chambers in the thatch layer of lawns where they overwinter until the following spring. Most sod webworms have two generations per year, but the vagabond sod webworms typically have only one generation.
Lawn Damage & Management
Sod webworm damage is characterized by lawns looking thin, drought stressed, or by having dead patches of grass in more severe cases. However, the vagabond sod webworms that are being found in South Dakota usually do not cause any significant damage. Typically, lawns are able to tolerate the larval feeding as it occurs during peak growth periods of the grass.
Little additional management is needed for vagabond sod webworm. Several natural enemies such as parasitic wasps and flies can act as biological control agents to reduce webworm abundance. Birds also play a large role in managing webworm populations as they will feed on both the developing larvae in the grass as well as the adult moths flying in the air. With proper care, most lawns are able to withstand any injury. Be sure to water and fertilize lawns so that the grass can easily recover and outgrow webworm feeding. Chemical control should be a last resort and is not necessary unless there have been multiple back-to-back years of high emergences.