Sod webworms are turfgrass pests that cause damage to lawns across North America. These pests are the larval, or caterpillar, stage of a small, non-damaging moth, often called a “lawn moth” or “snout moth.” The destruction to turf is caused by the feeding habits of the growing larvae, which can quickly turn a healthy lawn into a patchy landscape. Understanding the appearance of both the larval and adult stages is the first step in identifying and addressing a potential infestation.
Identifying the Webworm Larva
The sod webworm larva is the most destructive stage. A fully grown caterpillar reaches approximately three-quarters of an inch to one inch, presenting a plump, soft-bodied appearance. Their coloring ranges from grayish-green to dull tan or brownish shades, helping them camouflage within the grass thatch layer.
A distinguishing feature is the presence of small, dark spots arranged in parallel rows down the body. These spots often have short, stiff hairs, or setae, protruding from them. When disturbed, the larvae instinctively curl into a tight, C-shaped coil. They are primarily active at night, retreating to silk-lined tunnels constructed in the thatch during the day.
Identifying the Adult Moth
The adult sod webworm is a slender-bodied moth that does not feed on grass. These moths are small, typically having a wingspan between one-half and three-quarters of an inch. Their wings are often a dull color, ranging from white to buff, gray, or light brown, sometimes featuring subtle dark markings.
A characteristic trait is their resting posture, where the wings are folded tightly back and wrapped around the body, giving the insect a narrow, tubular appearance. The moth’s head features two small, forward-projecting mouthparts that resemble a snout, leading to the nickname “snout moths.” They are frequently seen flying low and erratically over the lawn at dusk as they deposit eggs onto the turf.
Recognizing Signs of Lawn Damage
The clearest indication of a sod webworm problem is the appearance of irregular, brown patches in the lawn. These damaged areas often look “scalped” because the larvae chew the grass blades off near the crown. Unlike damage from grubs, which feed on roots and cause the turf to lift easily, webworm damage is concentrated on the grass blades themselves.
A closer inspection of the damaged area will often reveal the presence of silken tunnels or fine, silky webbing near the soil line and within the thatch layer. These small, webbed shelters are where the caterpillars hide and rest during daylight hours. Another telltale sign is the presence of frass, which are tiny, dark green or brownish pellet-like droppings left by the feeding larvae.
To confirm the presence of these pests, a simple soap flush test can be performed on the suspect area. This involves mixing liquid dish soap with water and pouring the solution over a one-square-foot section of the turf. The soapy water irritates the webworms, forcing them to emerge from their tunnels to the surface within minutes, allowing for a visual count.