If your lawn suddenly looks ragged and chewed-down by late summer, thinning out in patches even though you have watered it fine, you may have tropical sod webworms. They are small caterpillars that feed at night, hide during the day, and can chew a healthy-looking yard threadbare in a week or two once a generation gets going. We see them every year on lawns around Monticello, and St. Augustine gets hit the hardest.
What they are and when they show up
Tropical sod webworms are the larvae of a small, dull tan-to-gray moth. The caterpillars are green to grayish, usually under an inch long, with dark spots down the body, and they curl up in the thatch during daylight where you will never spot them casually. They come out after dark and chew on the grass blades.
The timing is the key to recognizing them. In North Florida the pressure builds through the summer and peaks in the warm, wet stretch from roughly July through October. Generations overlap, so you can knock them down and see fresh feeding a couple of weeks later as the next batch matures. They love St. Augustine but will feed on bermuda, zoysia, and centipede too, so no warm-season lawn around here is completely off the menu.
What the damage looks like
Sod webworms chew the blade, not the root, so the damage looks different from grubs or chinch bugs. Get down and look at individual blades and you will see the story.
- Ragged, notched, chewed blade edges. Early on they scrape a thin window in the blade. As they grow they eat notches out of the edges and chew blades off short.
- Thin, close-cropped patches that look almost like somebody set the mower too low in spots, with the turf getting shorter and sparser than the grass around it.
- Green frass pellets down in the thatch. Part the grass and look for the little green-to-tan droppings, which are a dead giveaway.
- Moths flushing up at dusk. Walk the yard in the evening and small tan moths flutter up ahead of your feet in a low, zigzag flight, then dive back into the grass. Lots of moths means eggs are being laid.
- Birds working the lawn in the morning, picking caterpillars out of the turf.
Confirm it with a soap flush
Before you treat, prove they are there. The soap flush is quick, cheap, and it drives the caterpillars up out of the thatch so you can count them. Mix a couple of tablespoons of dish soap into a gallon or two of water in a watering can and pour it slowly over a two-foot-square section right at the edge of a thinning patch, where the fresh feeding is. Within a few minutes the soap irritates them and they wriggle up to the surface. Do it in two or three spots. If you flush up several caterpillars per square yard, you have your answer and it is time to treat.
This same flush will bring up armyworms too, which brings up an important distinction below.
Treating tropical sod webworms
Timing and technique matter as much as the product. A few rules make the difference:
- Treat in the evening. The caterpillars feed at night and hide by day, so a late-afternoon or dusk application puts the product on the grass right when they come up to eat it.
- Mow and water first, then treat. Cut the grass and water it a day ahead so the plants are not stressed and the canopy is even. But do not mow right after you treat, or you will haul the product off in the clippings and lose your coverage.
- Match the product to the pressure. For light infestations a product based on Bacillus thuringiensis (Bt), a naturally occurring bacterium that targets caterpillars specifically, works well and is easy on beneficial insects. For heavy, fast damage, a pyrethroid such as a bifenthrin-based granular knocks them down harder and faster.
- Expect to repeat. Because generations overlap through the season, one application often is not the end of it. Scout again a couple of weeks later and re-treat if the flush test says they are back. Rotate chemistry if you spray repeatedly.
As always, the label is the instruction sheet, not a suggestion. It tells you the rate, whether to water in or let it dry, and how soon you can re-apply. Read it and follow it.
Do not confuse them with armyworms
Sod webworms are not the only caterpillar chewing lawns in late summer. Fall armyworms do very similar damage, and the soap flush brings both to the surface. Armyworms are generally larger, often move across a lawn in a visible marching front, and can strip an area shockingly fast, sometimes overnight. The treatments overlap, but the scale and speed are different, so it helps to know which one you have. Our armyworm guide covers how to tell them apart. And if the damage turns out to be spreading straw-brown patches in the hottest, sunniest part of the yard rather than chewed blades, you are likely looking at chinch bugs instead, which is a different fix entirely. When in doubt, run through the full diagnosis walk-through.
Help the lawn shrug it off
A vigorous, well-managed lawn recovers from webworm feeding far faster than a stressed one, and since the caterpillars only eat the blades, a plant with a healthy crown and root system will grow right back out of the damage once the pest is handled. Do not panic-fertilize in the middle of an outbreak, though, because a flush of soft new growth is exactly what the next generation wants to eat. Keep your mowing height where it belongs for your grass type, water deep and infrequent, and let the turf rebuild. Our guide to mowing height by grass type lays out where to set the deck.
Sod webworms come in waves, and a lawn that looked fine on Friday can look chewed to the ground by the next weekend. If your yard is going ragged in late summer and you would rather not be out there after dark pouring soapy water on it, we will scout it, confirm what is feeding, and time the treatment right. Get in touch and we will get ahead of the next generation.