Tropical sod webworms (Herpetogramma phaeopteralis Guenée) are caterpillars that chew on blades of grass, resulting in a lawn that looks ragged, thin and brown.
Despite their name, they are not limited to the tropics and have been found throughout the Southeast. The larvae of a tiny brown moth (Lepidoptera Crambidae), sod webworms live in the thatch that accumulates just above the soil in turf grasses.
According to the University of Florida’s Institute of Food and Agricultural Sciences, the peak season of infestation in North Florida is fall. A number of organic products will kill sod webworms, including products containing Bt (Bacillus thuringiensis) and spinosad, which won’t harm birds, bees or mammals.
It’s important not to use a broad-spectrum insecticide, because it would also kill other insects that are beneficial and that attack webworms, including ladybugs and Trichogramma wasps. The best option for keeping your lawn sod webworm free, however, is prevention. The main cause of caterpillar infestations is overuse of fertilizer. Keep your lawn mowed to the appropriate height, and use fertilizer sparingly.
If you must water your lawn, water it deeply once a week. Shallow watering leads to thatch, which provides a home for the pest.