Small, light-colored moths fluttering erratically across your lawn at dusk signal the presence of sod webworms. While the adult moths are harmless, their offspring—the larval stage—are highly destructive to grass blades. The caterpillar feeds near the soil line, causing damage often mistaken for drought stress or disease. This guide provides actionable steps to eliminate an active infestation and implement cultural practices to prevent their return.
Identifying the Culprit and the Damage
Sod webworm damage often begins subtly, presenting as small, ragged patches of brown or thinning grass that quickly expand. Upon close inspection, the grass blades appear chewed or notched, indicating the caterpillar is feeding on the leaf tissue. The larvae also produce fine, silky tunnels in the thatch layer and leave behind tiny, green, pellet-like droppings called frass near the soil surface.
To confirm an infestation, perform a “soap flush test” on a suspected area. Mix two tablespoons of liquid dish soap into two gallons of water and pour the solution slowly over a one-square-yard section. The soapy water irritates the larvae, forcing the small, grayish-green caterpillars to crawl to the surface within five to fifteen minutes. If you observe more than ten to fifteen larvae in that area, treatment is recommended.
Immediate Treatment Options
Successfully treating an active infestation requires targeting the webworms during their nocturnal feeding hours. Both biological and chemical controls are effective, but timing is crucial for the product to reach the larvae hidden in the thatch layer. All treatments should be applied in the late afternoon or evening, just before the webworms emerge to feed.
Biological controls offer a targeted approach, with Bacillus thuringiensis subspecies kurstaki (Btk) being a common choice. Btk is a naturally occurring soil bacterium that must be ingested by the caterpillar to be effective, causing it to stop feeding almost immediately. The biological insecticide Spinosad is also highly effective against webworms and provides residual control.
Beneficial nematodes, specifically Steinernema carpocapsae, are microscopic roundworms that actively seek out and parasitize the larvae in the soil. For biological agents, the lawn should be lightly watered before application to move the product into the thatch layer. When using Btk, avoid heavy irrigation for at least twenty-four to forty-eight hours after application, as sunlight and washing can quickly degrade its effectiveness.
When rapid control is necessary for a severe outbreak, chemical options such as synthetic pyrethroids (e.g., bifenthrin, permethrin) or carbaryl can be used. These broad-spectrum insecticides provide quick knockdown of the pest population. A light watering after application helps move the active ingredient into the thatch where the larvae reside. Always follow the manufacturer’s label instructions for application rates and safety precautions.
Preventing Future Infestations
Long-term management relies on cultural practices that make the lawn less hospitable to the webworm life cycle. These practices include:
- Adjusting the mowing height to above 2.5 inches to discourage moths from laying eggs and allow the grass to withstand minor feeding damage.
- Deep, infrequent watering to encourage deeper root growth, improving turf resilience and preventing the dry soil conditions webworms prefer.
- Thatch management, through aeration or dethatching, to reduce the dense, protective layer where larvae hide during the day.
- Avoiding excessive nitrogen fertilizer, which prevents the flush of tender, new growth that webworms find most attractive.