Though several species of sod webworms are common, they are all similar in appearance, behavior, and life history. Sod webworm adults are pale brown moths, 1⁄2 to 3⁄4 inch from the tips of their feathery snouts to the ends of their folded wings. In early summer, females fly over turf, dropping eggs onto the tips of the grass below. Green, brown, or flesh-colored caterpillars grow to just under an inch, completing one or two more life cycles before temperatures drop. Caterpillars emerge from burrows in the thatch at night to feed, cutting new grass shoots off at the crown. One larva can “deforest” a baseball-sized patch, but symptoms may go unnoticed until growth slows.
The dry times of July and August make insect damage more visible in your lawn, simply because grass cannot regrow munched blades as quickly. And given the coincidence of webworm activity with damage from drought, grubs, and other turf pests, diagnosis is more involved than a visual once-over. Mix 2 gallons of water with 2 tablespoons of dish soap and pour it over the afflicted area. The soap will irritate critters living in the thatch, forcing them to surface. Once you’ve scoped out your turf and made a positive identification, control becomes much easier.
Prevention is the best medicine, and long turf—3 inches or taller—tolerates pests better, exhibiting less damage than shorter turf under the same pest pressure. A light layer of compost applied in early autumn will break down the excessive thatch in which webworms, chinch bugs, and billbugs reside, while providing nitrogen to regrow bare spots. You can also encourage webworms’ natural enemies (such as the common black ground beetle) by growing a garden of flowering plants alongside the lawn.
Illustration by Jack Unruh
Originally published in Organic Gardening magazine, June/July 2014