Sod webworms, the destructive larval stage of lawn moths, are common turfgrass pests that rapidly turn a healthy lawn into an unsightly patchwork of brown, chewed-up grass. These caterpillars feed on grass blades near the soil surface, causing damage that often resembles drought stress or disease. Accurate identification and applying treatments at the optimal time are necessary for successful elimination.
Confirming the Pest and Optimal Treatment Timing
Before applying any treatment, confirm that sod webworms are the cause of the lawn damage, which appears as ragged, thinning grass blades and irregular brown patches. The most reliable confirmation method is the “flush test,” which irritates the nocturnal larvae and forces them to the surface. To perform this, mix a few tablespoons of mild liquid dish soap into a gallon of water and pour the solution over a one-square-foot area of affected turf.
If sod webworms are present, the caterpillars typically emerge from the thatch within five to ten minutes due to the soap solution’s irritating effect. Confirm the pests visually by looking for small, grayish-brown larvae less than an inch long, often with dark spots. Telltale signs include silken tunnels and small, green, pellet-like droppings, known as frass, near the soil line. Treatment is most effective against the small, young larvae, which are the most actively feeding stage.
The sod webworm life cycle often involves multiple generations per year, but the most damaging generation typically occurs in late summer or early fall when the turf is under heat stress. Targeting small larvae, usually about two weeks after a peak in adult moth flight activity is observed, maximizes control effectiveness. Adult moths and pupae are not affected by most treatments, so precise timing against the feeding caterpillars is crucial.
Biological and Environmentally Friendly Treatments
Homeowners seeking low-toxicity controls have effective biological options that specifically target sod webworms. One powerful method involves applying beneficial entomopathogenic nematodes, particularly Steinernema carpocapsae. These microscopic, non-chemical organisms are infective juveniles that actively seek out and enter the sod webworm larvae through natural openings like the mouth or anus.
Once inside the caterpillar, the nematodes release symbiotic bacteria, which multiply rapidly and cause septicemia, typically killing the host within 48 hours. Because S. carpocapsae are sensitive to ultraviolet light and require moisture for movement, they should be applied in the late afternoon or evening and immediately watered into the turf. This ensures the nematodes reach the larvae hiding in the thatch and soil and are protected from the sun.
Another effective biological control is the bacterium Bacillus thuringiensis (Bt), often sold as the subspecies kurstaki (Btk). Bt is a stomach poison, effective only when ingested by the feeding webworm larvae. Once the larvae consume the treated grass blades, the bacteria produce a protein crystal toxin that paralyzes the caterpillar’s gut, causing it to stop feeding immediately before death.
Since Bt rapidly degrades when exposed to sunlight, it must be applied in the evening to ensure it remains active when nocturnal sod webworms emerge to feed. It is most effective against very young larvae, and multiple applications may be necessary due to its short residual life. Reducing excessive thatch buildup through dethatching can also help, as it eliminates one of the primary hiding places for the larvae.
Targeted Chemical Solutions
When a sod webworm infestation is severe or rapid results are required, synthetic chemical insecticides provide quick and effective control. These products are generally fast-acting and are categorized by the active ingredient, often belonging to the pyrethroid class. Common examples of active ingredients include bifenthrin, cyfluthrin, and lambda-cyhalothrin.
Pyrethroids are contact and stomach poisons that disrupt the insect’s nervous system, providing a swift knockdown effect on the feeding larvae. These insecticides are available in both liquid and granular formulations. Liquid applications are often preferred for better coverage of the grass blades and thatch where the pests reside. To maximize contact with the nocturnal larvae, chemical treatments should be applied in the late afternoon or early evening.
Proper application technique is crucial for chemical effectiveness and safety. It is recommended to lightly irrigate the lawn a few days before treatment to encourage the larvae to move closer to the surface where the insecticide will be concentrated. If using a granular product, a light watering after application is necessary to wash the chemical off the grass blades and into the thatch layer. Avoid heavy watering for at least 24 hours after applying liquid sprays, as this can wash the product away from the larval feeding zone.