Sod Webworms in St. Charles County: Night-Feeding Caterpillars and How to Stop Them

If your St. Charles County lawn has brown patches that keep spreading in August, and you’ve noticed small tan moths fluttering over the grass at dusk, sod webworms are a strong possibility. These caterpillars feed at night in the thatch, and they can chew a lot of grass before you ever see them. Here’s how to confirm them and what actually works.

What Are Sod Webworms?

Sod webworms are the caterpillar stage of small, dingy-colored moths. Around here you might hear the adults called close-winged moths or snout moths, because they hold their wings tight against their bodies and have two little snout-like parts on their heads.

The caterpillars are the problem. They range from about a quarter inch up to an inch long, grayish-brown to greenish to dirty white, with four rows of dark spots running down the back. They live in the thatch layer of the lawn, not in the soil. Each one builds a small burrow lined with silk and covered with dirt and bits of grass. During the day they hide in those burrows. At night they come out, cut grass blades, and drag them back in.

If you’ve read our guide to armyworms, you know that pest feeds from the top down. Sod webworms are different. They work from the thatch, and they’re far less visible.

The Telltale Damage

Webworm damage is easy to mistake for drought, especially in August. The patches look similar at first: irregular, brown, often starting near sidewalks, driveways, or hot dry spots where the soil is compacted.

The difference shows up when you look closely. Drought-stressed grass is still rooted. The blades are just brown. With webworms, the blades are actually missing. The caterpillars cut them off at the thatch line and drag them underground.

Two other clues:

• Green fecal pellets in the thatch. They look like tiny specks of sawdust.
• Silk-lined tunnels. Part the dead grass near the soil line and you can see them.

Missouri Botanical Garden’s pest guide notes that webworm damage is usually worst on unirrigated lawns during drought. That fits St. Charles County in August. If you’ve held off on watering and the lawn is stressed, it’s a better target.

Sod Webworms vs Everything Else

The tricky part is that lots of things cause brown patches in late summer. Here’s how to sort them out:

Sign	Sod Webworms	Armyworms	Chinch Bugs	Grubs	Drought / Fungus
When damage shows	Late summer, builds slowly	Overnight, strips blades	July heat, fast	Late summer, turf peels	Gradual, or spreads in patterns
Where the pest is	In thatch, silk tunnels	On top, moving in groups	At the soil surface	In the soil, C-shaped	No insect present
What’s damaged	Blades cut off at thatch	Blades chewed to stubs	Grass turns yellow then brown	Roots eaten, turf lifts like carpet	Grass just turns brown
After watering	Doesn’t green back up	Doesn’t green back up	Doesn’t green back up	Doesn’t green back up	Greens up, if not dormant

If you can pull dead turf up like a carpet and find white C-shaped grubs underneath, that’s grubs, not webworms. If the blades are chewed down to stems and caterpillars are on top, that’s armyworms. Webworms stay down in the thatch and cut blades at the base.

The Soap Flush Test

Here’s the fastest way to confirm sod webworms. It takes about ten minutes and costs nothing.

1. Mix about two tablespoons of liquid dish soap into a gallon of water.
2. Pour it over a square yard of turf at the edge of a brown patch, where the grass is still half alive.
3. Wait five to ten minutes and watch.

The soap irritates the caterpillars and they crawl to the surface. If you count five or more per square yard, treatment is worth it. If you find none, keep looking before you spend money. A few here and there is normal.

You can also loosen the thatch with a trowel and look for the pellets and silk tunnels. Webworms live in that dead grass layer, not the soil, so that’s where you check.

Why August Is Prime Time

Understanding the timing helps you catch them early. The moths start showing up in late spring and early summer. Females fly low over the lawn at dusk, dipping down to drop as many as 200 eggs at a time. The eggs are tiny and cream-colored, and they need moisture to survive.

That moisture requirement matters. In a dry summer, more eggs die, but the lawn also gets stressed, and the larvae that do hatch find easy feeding on weak turf. Then the generations stack up. In Missouri you can get two to three generations in a season, and by late July through September the caterpillars from the summer generations are at their biggest. A mature larva can eat twice its own body weight in a single night. With enough of them, that adds up fast.

What Actually Kills Them

The good news is sod webworms are treatable. The bad news is timing and placement matter.

For young caterpillars: Bacillus thuringiensis, usually sold as Bt, is the gentlest option that works. It only affects caterpillars, and they have to eat it, so you want to hit the young ones right after a heavy moth flight. Missouri Botanical Garden recommends applying Bt about two weeks after you see a large number of moths, especially during hot, dry weather, to catch the new hatchlings.

For most home infestations: Products with spinosad, bifenthrin, lambda-cyhalothrin, permethrin, or cyfluthrin are labeled for sod webworms. Spinosad is a good middle ground if you want something a little softer. The pyrethroid products come as granules you can spread with a regular spreader, or as liquids.

Three application notes:

• Water it in. Webworms live in the thatch, so the insecticide has to get down to where they are.
• Treat in the late afternoon or evening. They feed at night, and that’s when they’ll pick it up.
• Don’t expect your July grub product to help here. Grub products work in the soil, and webworms are in the thatch. Different pest, different layer, different response. Our white grubs guide explains the soil side of that story.

Preventing Next Year

The long-term fix is making the lawn less inviting.

• Seed with endophyte-enhanced fescue this fall. Endophytes are beneficial fungi that live inside certain grass varieties and make them less appetizing to caterpillars. Fall is the right time to overseed in Missouri, and the seed you pick matters. Our guide to choosing grass seed for overseeding covers what to look for.
• Manage the thatch. If the thatch layer is more than about half an inch to an inch thick, it’s prime webworm habitat. A dethatching pass helps, and so does core aeration, which also tackles the compacted clay soil a lot of us have around here.
• Water during drought. Webworms do their worst on unirrigated lawns. Keeping the grass from going bone dry doesn’t just help the turf. It keeps the habitat less favorable.
• Hold the heavy summer nitrogen. A lush, heavily fed lawn in August is a buffet. We say the same thing about armyworms.
• Let the birds work. Grackles, robins, and starlings love caterpillars. If birds are working a patch of lawn, something’s alive in there. Give them room.

FAQ

Are sod webworms harmful to people or pets? No. They feed on grass blades and stay in the thatch. They don’t bite, sting, or carry diseases. The damage is to the lawn only.

Can sod webworms kill my lawn? They can thin it badly and, in severe cases on already-stressed turf, kill patches outright. Most healthy St. Charles County fescue lawns recover once the caterpillars are controlled and the grass gets some fall care.

How long do sod webworms last? Each generation feeds for a few weeks before pupating. In Missouri you can get two to three generations per season, so damage can keep showing up from late July into September. The last generation of the year overwinters as larvae and finishes feeding in spring.

Will my grub treatment also kill sod webworms? Usually not. Grub products work in the soil, and webworms live in the thatch. They’re different pests in different layers. That’s why confirming what you have matters before you buy anything.

Should I treat if I only find a few webworms? No. The treatment threshold is roughly five or more caterpillars per square yard. A handful of webworms in a healthy lawn isn’t worth spraying — birds and other predators usually handle them.

When to Call a Professional

Call someone if you can’t confirm what you’re looking at, if the damaged area is large, or if you treated and the caterpillars are still feeding. A local provider can identify the pest, apply the right product at the right rate, and set up a fall overseed if the lawn needs it.

A good provider should be able to tell you what they found and what they’re using, and why. If the answer is vague, that’s a red flag.

That’s what Midwest Lawn Care is here for — tell us what your lawn is doing and we’ll point you to a dependable local provider who fits the job. Request lawn care help and we’ll help you compare your options.

Last updated: August 13, 2026