Sod webworms are a common lawn pest found across the United States. They are the destructive caterpillar stage of the sod webworm moth. These small, greenish-tan larvae feed primarily on grass blades, causing noticeable damage to turfgrass. The resulting ragged, brown patches are often mistaken for drought stress or disease, leading to delayed or incorrect treatment. Controlling these pests requires confirming their presence and applying the correct solutions at the most effective time in their lifecycle.
Confirming the Pest and Timing Treatment
Identifying a sod webworm infestation involves more than simply observing brown patches in the lawn. The damage typically appears as ragged, chewed grass blades, expanding rapidly, especially during dry periods. You can confirm the pest by looking for small, silk-lined tunnels within the thatch layer, which the larvae use for shelter during the day. The presence of tiny, green, sawdust-like droppings, known as frass, near the grass crown is a strong indicator.
To confirm an infestation, a simple soap flush test is highly effective. Applying a mixture of water and mild dish soap over a small, affected area irritates the larvae, causing them to wriggle to the surface within minutes. This test helps determine the population density and confirms that treatment is necessary.
Timing the application of control measures is important because insecticides are most effective against young larvae. Sod webworms often have multiple generations per year. The most noticeable damage usually occurs in late summer or early fall when the new generation is actively feeding. Targeting the first and second larval stages, which appear shortly after high adult moth activity, provides the best control results. Treating large, mature larvae or the pupae stage is significantly less effective.
Chemical Control Options
Effective chemical control relies on selecting the right active ingredient based on whether a quick knockdown or long-lasting residual effect is desired. The most common and reliable insecticides belong to two main groups: pyrethroids and carbamates/diamides. These treatments must penetrate the thatch layer to reach the feeding caterpillars.
Pyrethroids, such as bifenthrin, permethrin, and cyfluthrin, are contact killers that provide immediate results. These active ingredients disrupt the insect’s nervous system upon direct contact or ingestion of treated grass blades. They are generally applied as liquid sprays to ensure thorough coverage and quick action, but they offer a shorter residual period.
For longer-lasting control, ingredients like carbaryl (a carbamate) or chlorantraniliprole (an anthranilic diamide) are often used. Carbaryl acts as a broad-spectrum nerve toxin, providing both contact and residual activity. Chlorantraniliprole is a newer chemistry that acts on the insect’s muscle function, offering a longer residual effect and lower toxicity to many beneficial insects. It often comes in granular formulations that require watering to activate and move the chemical into the thatch layer.
Biological and Cultural Control Methods
Several biological and cultural methods offer effective alternatives for managing sod webworms. Biological controls use naturally occurring organisms to target the pest. One popular biological insecticide is Bacillus thuringiensis (Bt), specifically the kurstaki variety.
Bt is a soil bacterium that produces a protein crystal toxic only to certain caterpillars, including sod webworms. When the larvae ingest the treated grass, the toxin paralyzes their gut, causing them to stop feeding almost immediately and leading to death within a few days. Because sunlight can break down this product quickly, application timing is crucial for its success.
Another effective biological control option is the use of beneficial nematodes, particularly Steinernema carpocapsae. These microscopic roundworms are natural parasites that actively seek out and enter the sod webworm larvae. Once inside, the nematodes release symbiotic bacteria, which multiply and ultimately kill the host, often within 48 hours.
Cultural control methods focus on creating a turf environment that is less favorable to the pest and more resilient to damage. Sod webworms thrive in thick thatch, so managing this layer prevents them from building extensive tunnel systems. Ensuring appropriate watering and avoiding over-fertilization, especially with nitrogen, helps the grass remain healthy and better tolerate minor feeding damage.
Effective Application Techniques
The success of any sod webworm treatment depends heavily on the application technique. A light watering of the turf before the application is beneficial, as it encourages the webworms to move closer to the surface to escape the moisture. This pre-watering step maximizes the chance of contact with the insecticide.
It is best to apply the product in the late afternoon or early evening because this is when the sod webworm larvae emerge from their silken tunnels to feed. Applying during their active period ensures they ingest or contact the product immediately. Following the application, a light irrigation is necessary to wash the treatment off the grass blades and into the thatch layer where the caterpillars reside.
For liquid sprays, avoid heavy watering afterward, as this can dilute the product or wash it away from the target zone. If using granular products, post-application watering is necessary to dissolve the granules and activate the active ingredient. Always wear protective gear like gloves and follow the manufacturer’s label directions precisely for safe and effective use.