Some insects are not harmful to a lawn, but some can damage the lawn grass. The sod webworm is one of the most common grass pests in the Rocky Mountain region. Adult webworms are small moths which flit about the lawn during mowing, but do not feed on the lawn.
Young webworms are caterpillars, measuring about one-quarter to one inch in length.
They are brown or gray and with rows of dark spots on their backs. During the day, young webworms live in silk-lined tunnels in thatch and the surface of the soil. At night, they feed on grass leaves. A heavily infested lawn may not grow very much, the grass may become thin and some dead patches may develop. Most lawns are healthy enough to withstand substantial webworm populations without dead spots or thinning grass.