Sod webworms are the larval stage of small, dull-colored moths, often called “snout moths.” The adult moths are harmless, but the caterpillars, which grow up to three-quarters of an inch long, are voracious feeders on turfgrass. Their feeding activity is often mistaken for drought stress or a lawn disease because initial visual symptoms are similar. Accurately diagnosing the cause requires examining the affected areas beyond simple browning. This guide provides specific visual characteristics to distinguish sod webworm damage from other problems.
Identifying the Initial Signs of Damage
The first signs of webworm activity are subtle, concentrating on individual grass blades near the soil line. Young larvae initially “skeletonize” the blades by consuming soft tissue, giving the grass a translucent or shredded appearance. Mature caterpillars chew off entire sections of the blade, resulting in a notched or jagged edge.
This direct feeding causes the grass to lose its vibrant color, starting as a dull, off-green or yellowish hue. Damage progresses quickly from these subtle injuries to small, distinct patches of brown, dead turf. These patches, often the size of a quarter to a baseball, look like the grass has been closely “scalped.”
A distinctive feature is that the grass blades are missing or clipped, unlike simple drought stress where blades are wilted or dead from the tip down. Webworm damage starts as scattered, irregular spots, while drought stress affects large, uniform sections.
Distinguishing Damage Patterns and Location
The macro-level pattern of webworm damage is characterized by irregularity and expansion. Small, dead spots eventually merge into larger, straw-colored areas with uneven borders, creating a patchwork look that can resemble a fungal infection or heat stress.
The location of the most severe damage often indicates a webworm presence. Larvae prefer sunny, warm, and dry areas, such as along sidewalks, driveways, or slopes prone to drying out.
Damage becomes most noticeable during late summer (July through September), when the second generation of larvae is most active and the grass is stressed by heat. Although larvae feed nocturnally, the damage becomes visible during the day as clipped grass blades dry out. Heavily infested areas can appear “moth-eaten” with patches of bare soil.
Confirming the Culprit: Physical Evidence
Definitive confirmation relies on finding the physical evidence left behind by the larvae, as visual inspection alone can be misleading. Sod webworms construct silk-lined tunnels or tubes near the soil surface within the thatch layer, where they hide during the day. These silken structures, sometimes mixed with soil and plant debris, are a direct sign of their presence.
Another key identifier is frass, the larvae’s excrement. This frass appears as tiny, pellet-like droppings, usually moist green or brown, scattered around the web-lined burrows. The discovery of these small pellets among the thatch provides certain confirmation that caterpillars caused the injury.
The larvae themselves can also be found. The caterpillars are grayish-green or brownish, often covered with small, dark spots. Because they are nocturnal, they are usually curled up in a C-shape within their silken tunnels during the day. Applying a soapy water solution to the turf can irritate the larvae and force them to the surface, allowing for positive identification.