Sod webworms are the destructive larval stage of the common lawn moth and pose a significant threat to turfgrass health. These small caterpillars are typically tan, gray, or greenish with distinct dark spots. They feed voraciously on grass blades, often causing serious damage before they are noticed. While the adult moths are harmless, their egg-laying signals a looming infestation of larvae that reside and feed near the thatch layer. Understanding the signs of their activity and implementing targeted control measures is the most effective way to eliminate these pests.
Identifying the Signs of Infestation
The initial evidence of a sod webworm problem often appears as small, irregular patches of brown or yellowed grass that mimic drought stress. Upon closer inspection, the grass blades in these damaged areas look ragged or chewed, as the larvae clip them off near the soil line. You may also notice small, silky tunnels or webbing near the base of the grass, particularly in the thatch layer, which the caterpillars use for daytime protection.
A definitive diagnosis requires performing a simple drench test, which forces the nocturnal larvae to the surface for counting. Mix one to two tablespoons of liquid dish soap with one to three gallons of water and pour the solution slowly over a two- to three-square-foot section of lawn. The soapy water irritates the caterpillars, causing them to emerge from the thatch within five to ten minutes. If you find more than 5 to 15 larvae per square yard, a control treatment is necessary to prevent further destruction.
Non-Chemical and Biological Control Strategies
Homeowners seeking to avoid synthetic chemicals have several effective non-chemical strategies for managing webworm populations. Simple cultural practices can make the lawn less hospitable, such as maintaining a higher mowing height, typically above 2.5 inches, which helps the turf tolerate minor feeding damage. Reducing excessive thatch buildup, which serves as a protective daytime habitat for the larvae, also exposes them to natural predators and makes control treatments more effective.
Targeted biological controls offer a potent means of elimination that specifically attacks the caterpillars without harming beneficial insects or pets. The bacterium Bacillus thuringiensis subspecies kurstaki (Bt) is a microbial insecticide that must be ingested by the larvae. Once consumed, the toxin paralyzes the caterpillar’s gut, causing it to stop feeding almost immediately. Since sunlight quickly breaks down Bt, application should occur in the late afternoon or evening when webworms are actively feeding, and the area should not be irrigated for at least two days afterward.
Another highly effective biological option involves the use of beneficial nematodes, specifically the species Steinernema carpocapsae. These microscopic roundworms actively seek out and penetrate the webworm larvae, releasing symbiotic bacteria that cause a fatal infection. Since nematodes are vulnerable to desiccation and ultraviolet light, they must be applied to warm, moist soil during the coolest time of the day, preferably in the evening. The treated area requires immediate, light irrigation to wash the nematodes into the thatch layer where the webworms hide.
Selecting and Applying Insecticide Treatments
For moderate to severe infestations, chemical controls are often the fastest and most reliable solution for sod webworm eradication. Insecticides are broadly categorized into curative treatments, which quickly eliminate an existing problem, and preventative products, which offer longer residual control. Common active ingredients in curative products include synthetic pyrethroids like bifenthrin, permethrin, cyfluthrin, and lambda-cyhalothrin.
The timing of application is crucial because the larvae feed at night, so insecticides should be applied in the late afternoon or early evening just before the caterpillars emerge. For maximum effectiveness, treatments should target the younger, more vulnerable larval stages. These stages typically appear after a peak in adult moth flight activity in the late spring and early summer. Some products, such as granules containing bifenthrin, may offer both quick knockdown and a residual effect lasting up to 90 days.
Proper application technique ensures the product reaches the webworms in their sheltered location near the soil surface. After applying the insecticide, water the turf lightly to wash the active ingredient down through the grass canopy and into the thatch and soil. This ensures contact with the pest while reducing the risk of product breakdown from sunlight exposure. Always read and strictly adhere to the instructions on the product label to ensure both safety and efficacy.
Long-Term Lawn Management for Prevention
Preventing future sod webworm problems relies heavily on maintaining a dense, vigorous lawn that can naturally withstand minor pest feeding. Avoid over-fertilizing with high-nitrogen products, as this promotes the lush growth attractive to egg-laying female moths. Instead, focus on a balanced fertilization program that promotes overall turf health and strong root development.
Monitoring for the presence of adult moths is an important early warning sign, as their erratic, low-level flight at dusk indicates an impending generation of larvae. Choosing endophyte-enhanced turfgrass varieties can also provide a built-in defense, as the fungi living inside the grass tissue produce natural deterrents to webworm feeding. Regular lawn aeration and dethatching contribute to prevention by reducing the density of the thatch layer, making it a less hospitable environment for the pests.