What Sod Webworms Are
Sod webworms are also called lawn moths, named for the silk-lined tunnels the larvae build in the thatch, where they hide during the day. The adults are small white, tan, or brown moths with snout-like projections on the head and wings that wrap around the body, as the University of Nebraska-Lincoln Department of Entomology describes them. They do not fly far; walk across an infested lawn at dusk and they flit up in a zigzag and land a few feet away.
Entomologists count nearly 20 species across the country, and in Florida’s climate they produce two or three generations a season. The adults do not feed on grass. The females scatter up to 200 eggs over the lawn as they fly, the caterpillars hatch in about a week, and those caterpillars chew grass leaves down to the crown at night. They feed on St. Augustine, zoysia, Bermuda, and the northern grasses too. A severe outbreak can kill large areas of a lawn that looked healthy a week earlier.
Signs of an Infestation
- Brown patches the size of a saucer that grow and merge, looking like drought damage, where the grass has been chewed to the soil surface. Look closely and the blades are cut, not wilted, and the damage is worst in the hottest, sunniest part of the yard.
- Small moths lifting out of the lawn in the late afternoon and at dusk.
- Green pellets (frass) in the thatch, and birds working the lawn in the morning.
Confirm it with a soap flush. Mix one to two tablespoons of liquid dish soap in two gallons of water and pour it over a square yard at the edge of a damaged patch. Within a few minutes the caterpillars, about three-quarters of an inch long, green to tan with dark spots, come to the surface. More than a handful per square yard means treatment. The same flush brings up chinch bugs, which cause similar patches in St. Augustine, so you learn which pest you have.
Getting Rid of Sod Webworms
Several lawn insecticides labeled for caterpillars control sod webworms, in liquid or granular form; granules go on with a fertilizer spreader if you prefer not to spray. Biological products with Bacillus thuringiensis subspecies kurstaki (Bt), a bacterium that produces a natural toxin the caterpillars cannot survive, work as well but more slowly, and are safe around people and pets.
Timing matters more than product. Water the lawn a day or two before treating, apply late in the day, before the caterpillars come up to feed, and water lightly afterward if the label calls for it, to move the product off the blades and into the thatch where the larvae are. Retreat if a second flush a week later still brings caterpillars up. Damaged St. Augustine usually fills back in from runners once the feeding stops; where the crowns are gone, the patch is re-sodded.
Preventing the Next One
Webworms, like most pests, go for a weak lawn. Keep the grass healthy and cut it right: St. Augustine at the tall end of its range, 3 to 4 inches, and zoysia at 1 to 2 inches, never removing more than a third of the blade in one mowing. Scalped, thin turf dries out faster and gives the caterpillars an open canopy. Water deeply and less often once the lawn is established, and feed it according to a soil test rather than on a heavy schedule; lush, overfed growth is what the moths look for.
Bayside Sod’s Turf Care plans cover insect monitoring and treatment across Manatee and Sarasota counties, including webworms and chinch bugs, and include the soil testing that keeps the lawn from inviting them. If a patch is past saving, we grow the sod to replace it; request a quote or call (941) 702-1746.