Webworms, specifically the sod webworm, are common pests that can quickly turn a healthy lawn into a patchy, brown landscape. These insects are the larval stage of a small moth, and their feeding habits cause significant turf damage, especially during the warm summer months. Homeowners often mistake the initial signs of an infestation for drought stress or disease, leading to delayed treatment. This guide focuses on immediate eradication and long-term prevention strategies.
Identifying Webworms and Damage
The destructive stage is the larva, a small caterpillar that is typically grayish-green to tan and grows up to three-quarters of an inch long. Larvae have small dark spots and a dark head capsule, often curling into a “C” shape when disturbed. The adult is a small, pale brown or tan snout moth, named for its protruding mouthparts.
These moths are harmless, but their zig-zag flight pattern low over the grass at dusk signals egg-laying. The feeding larvae hide in the thatch layer during the day, emerging at night to chew on grass blades near the soil line. This nocturnal feeding causes damage that first appears as small, irregular brown patches that look “scalped.”
A telltale sign of webworm presence is the silken web or tunnel constructed in the thatch, often mixed with small, green fecal pellets known as frass. The damage is distinct because the grass blades are visibly chewed off or notched, not simply dead or yellowed. A simple diagnostic tool is the “soap flush” test, which confirms the infestation. To perform the test, mix two tablespoons of liquid dish soap into one to two gallons of water and pour the solution slowly over a square yard of damaged turf. The soap irritates the webworms, forcing them to surface within five to fifteen minutes, allowing you to count them and assess the population density.
Immediate Eradication Methods
Once an infestation is confirmed, immediate action is necessary to halt the feeding damage. The most effective methods target the larvae deep within the thatch layer near the soil surface. Treatment should always be applied in the late afternoon or early evening because the larvae are nocturnal and become active at dusk.
Biological Controls
Biological options offer a less toxic approach that is highly effective against the larval stage. Bacillus thuringiensis subspecies kurstaki (Bt) is a naturally occurring soil bacterium that, when ingested, paralyzes the digestive system of the webworm caterpillar. Bt is most effective against young, early-instar larvae and will not harm beneficial insects, pets, or people.
Entomopathogenic nematodes, specifically species like Steinernema carpocapsae, are microscopic worms that seek out and kill the larvae in the thatch layer. When applying nematodes, the soil must be moist, and application should occur during the coolest part of the day to ensure their survival. Light irrigation immediately after application is required to wash the nematodes into the thatch where the webworms reside.
Chemical Controls
For severe or rapidly spreading infestations, chemical insecticides provide a quick-acting solution. Common active ingredients effective against sod webworms include synthetic pyrethroids, such as bifenthrin or cyfluthrin, and carbaryl. Liquid formulations often provide better coverage, penetrating the thatch layer more effectively than granular products.
It is imperative to read and follow all label instructions and safety warnings when using chemical controls. After applying the insecticide, a light watering is often recommended to move the product down to the base of the grass blades and into the thatch where the larvae are hiding. Avoid heavy irrigation, which can wash the product away, and do not mow the lawn for at least 24 hours after treatment.
Mechanical Suppression
While not a standalone solution, certain mechanical practices can supplement eradication efforts. For small, localized areas, the soap flush test itself can act as a localized control, as the soapy water can kill a small number of caterpillars. Mowing the grass at a precise height for your turf type can also expose the larvae, making them more vulnerable to chemical treatments and natural predators.
Long-Term Prevention Through Cultural Practices
Long-term webworm control relies on maintaining a healthy, resilient lawn environment that is less attractive to pests and better able to recover from damage. A dense, vigorous turf will naturally suppress webworm populations. Proper watering techniques are a significant deterrent to pest stress. Watering deeply but infrequently encourages the grass to develop deep, robust root systems, which increases the turf’s overall health and ability to withstand feeding damage. Overly stressed or drought-afflicted grass is more susceptible to webworm attack.
Thatch management is also important because excessive thatch provides a protective layer and ideal hiding place for the webworm larvae during the day. Dethatching or aerating the lawn when necessary reduces this shelter, exposing the larvae to predators and environmental factors. This practice also helps insecticides penetrate more effectively during a treatment.
A balanced fertilization program promotes healthy growth without creating the tender, succulent new turf that webworms prefer. Avoid over-fertilizing with nitrogen, which can lead to excessive, soft growth and increased thatch accumulation. Regular monitoring of the lawn, especially during peak webworm season in late spring through summer, allows for early detection and targeted intervention before a major outbreak occurs.