Small, white moths flitting across your lawn, especially at dusk, often signal an impending problem. These moths are the adult stage of the sod webworm, a type of snout moth, but they do not cause turf damage. The actual destruction is caused by the caterpillars (larvae) that hatch from the eggs dropped while the moths fly. This damage can appear quickly, transforming healthy turf into unsightly brown patches, making accurate identification and targeted treatment essential.
Identifying the True Culprit
The adult sod webworm moth is typically small, about three-quarters of an inch long, appearing dull white, tan, or gray. They often have snout-like projections extending from the head. When disturbed, these moths fly low and erratically in a short, zigzag pattern before quickly settling back into the grass. This flight activity is a warning sign, as the females are dropping hundreds of eggs onto the turf.
The larvae are the voracious feeders that cause the visible damage. Sod webworm caterpillars are slender, ranging from light green or grayish to brown, and grow up to one inch long. They are easily identified by four parallel rows of distinct dark spots along their bodies. These caterpillars are nocturnal, spending their days hidden in silk-lined tunnels. They emerge at night to chew grass blades down to the crown, causing damage that first appears as small, irregular brown patches resembling drought stress.
Cultural and Biological Control Methods
Controlling the larvae can be accomplished using biological agents, which target the feeding stage with minimal impact on beneficial insects and the environment. One effective method involves using Bacillus thuringiensis (Bt) var. kurstaki, a naturally occurring bacterium. Bt is a microbial insecticide that produces a protein toxin. When ingested by the caterpillar, this toxin paralyzes its gut, causing it to stop feeding immediately.
Bt must be applied when the larvae are small, as it is less effective on mature caterpillars. It should be applied late in the day since sunlight rapidly breaks down the active ingredient. Do not irrigate the lawn for approximately 48 hours after application to ensure the toxin remains on the grass blades where the larvae feed. Another biological option is the application of entomopathogenic nematodes, specifically Steinernema carpocapsae.
These beneficial roundworms are living organisms that actively seek out and infect sod webworm larvae, often killing them within 48 hours. S. carpocapsae is an “ambush” forager, waiting near the soil surface for the host to pass by. The nematodes should be applied during the coolest time of day. They require post-application irrigation to wash them into the thatch layer where the larvae reside.
Targeted Insecticide Application
For severe infestations, chemical control must be directed at the larval stage, as treating the adult moth will not stop current turf damage. The most effective strategy is to apply the insecticide when the nocturnal caterpillars are actively feeding on the surface, typically in the late afternoon or early evening. This timing ensures the product is fresh and available when the pests emerge from their silken tunnels.
Effective active ingredients for sod webworm control include pyrethroids like bifenthrin or cyfluthrin, and the carbamate insecticide carbaryl (often sold as Sevin). A newer class of insecticide, the anthranilic diamide chlorantraniliprole, offers excellent control and extended residual activity, sometimes providing protection for up to 90 days. These products work by contact or ingestion as the larvae feed on the treated grass.
Liquid formulations are preferred over granular products because they provide better coverage of the grass blades where the webworms feed. After applying most liquid contact insecticides, avoid watering or mowing the lawn for at least 24 to 48 hours. This practice keeps the chemical on the grass blades at the surface, maximizing contact with the feeding caterpillars.
Preventing Future Infestations
Long-term management focuses on making the lawn environment less hospitable to the sod webworm life cycle. A healthy, vigorous lawn is naturally more tolerant of pest feeding damage. Proper mowing height is one of the simplest preventative measures, as turf mowed at three to four inches is less susceptible to noticeable damage than shorter grass.
Managing thatch is another important element, as excess thatch provides shelter for the eggs and larvae. Periodically dethatching the lawn when the layer exceeds half an inch can help reduce the webworm habitat. Using slow-release fertilizers and avoiding excessive nitrogen applications can also prevent the rapid, soft growth that sod webworms prefer and reduce thatch buildup.
Watering Practices
Proper watering practices can also discourage infestation. Sod webworms thrive in hot, dry soil conditions, but avoiding overwatering, especially in the evening, helps reduce favorable conditions for the pest.
For a more robust defense, consider planting turfgrass varieties enhanced with naturally occurring, symbiotic fungi called endophytes. These fungi produce alkaloids that deter sod webworm feeding, offering built-in resistance to the pest. By combining these cultural practices, the lawn can better withstand and recover from future webworm activity.