Fall Armyworms vs. Sod Webworms in Polk County Lawns — Reading the Damage Before You Pay for the Wrong Treatment

Fall armyworms chew a lawn from the edges of the blade inward and move across it like a wave — you’ll see a ragged, advancing front with green grass on one side and shredded stubble on the other. Sod webworms graze in scattered patches, leave notched blades that look like a bad mowing job, and drop tiny green frass pellets down in the thatch.

It’s mid-September, which in Polk County means both are running at the same time. Get the ID right and a single correctly timed application usually handles it. Get it wrong and you’ll spend $150 treating for something that isn’t there while the real culprit eats another 500 square feet.

Quick answer

Fall armyworms in a Polk County lawn move fast and defoliate in an advancing line, often overnight, and hit bahiagrass and pasture edges hardest. Sod webworms work slower, leaving grazed patches and green frass pellets, and favor St. Augustine. Both chew the blade. Chinch bugs don’t chew at all — they suck, and the grass turns straw yellow in full-sun patches that water won’t fix. A soap flush tells you which in under ten minutes.

Reading the damage: armyworm vs. sod webworm

Fall armyworm damage

The name is literal. They march. A fall armyworm outbreak (Spodoptera frugiperda) starts where a moth laid an egg mass — often on a fence post, a mailbox, a light fixture, the underside of a gutter — and the larvae fan out from there and feed together in a group.

What you’ll actually see:

• A distinct front. Healthy green on one side of a line, chewed-down brown stubble on the other. It creeps across the yard a few feet a day and can take out a big section in 48 hours.
• Ragged, torn blade edges. Big mouthparts. Early on you’ll see "windowpaning" where tiny larvae scrape one side of the blade and leave a translucent membrane; by the time they’re an inch long they’re eating whole blades down to the stem.
• Birds. A flock of cattle egrets, grackles, or ibis working one section of your yard hard is one of the most reliable early tells in Central Florida. The birds find them before you do.
• The larvae themselves. Up to about 1.5 inches, striped, with an upside-down pale Y on the head capsule and four dark dots arranged in a square on the second-to-last segment. That Y is the giveaway.
• Timing of feeding. Small larvae feed anytime; bigger ones feed early morning and evening and hide in the thatch during the heat of the day. Don’t go looking at 2 p.m. and conclude there’s nothing there.

Sod webworm damage

Tropical sod webworm (Herpetogramma phaeopteralis) is the more common one in irrigated Lakeland residential St. Augustine, and it’s sneakier.

• Notched blades. The classic early sign. Look at individual blades up close — you’ll see chunks taken out of the edges, like someone went at them with tiny pinking shears.
• Grazed patches, not a front. Instead of a marching line you get irregular scalped-looking areas an inch or two shorter than the surrounding grass. People routinely blame the mower or a dull blade.
• Green frass pellets. Part the grass and look at the thatch line. Sod webworm droppings are small, green, and pellet-shaped. They’re the single most useful confirming sign, and they’re green because the caterpillar is eating fresh leaf tissue right now — old frass darkens.
• Silk webbing in the thatch. They build silk-lined tunnels down at the soil line. Morning dew makes these visible.
• Little tan moths. Walk the yard at dusk and you’ll flush small dingy tan-to-gray moths that fly a few feet in a zigzag and drop back into the grass. Moths don’t cause damage, but they mean eggs.
• The larvae. Greenish to gray, up to about 3/4 inch, with dark spots, and they curl into a tight C when disturbed. Strictly night feeders — during the day they’re curled in the thatch.

The soap flush: run this before you call anybody

This is the single highest-value ten minutes you can spend, and it costs about fifty cents. It works on armyworms, sod webworms, and chinch bugs.

1. Mix 1 to 2 ounces of lemon-scented liquid dish soap into 1 to 2 gallons of water. Lemon-scented is what everybody recommends — the citrus oils irritate the insects and drive them up.
2. Pick a spot right at the edge of the damage, not in the dead center. Dead turf has already been abandoned. You want the boundary between damaged and healthy.
3. Pour the mix slowly over about 4 square feet (a 2-by-2-foot square). Soak it.
4. Wait and watch for 2 to 10 minutes. Don’t walk off. Get down on one knee and stare at the grass.
5. Count what comes up.

Rough action thresholds people use: roughly 2 to 3 armyworm larvae per square foot, or several sod webworm larvae per square foot alongside visible damage, is enough to justify treating. If you flush one or two total, you probably have a population your lawn can outgrow. If you flush fifteen, you’ve got a problem that’s going to keep spreading.

Do two or three flushes in different spots. One sample is an anecdote.

Two practical notes: do it in the early morning or evening when the larvae are nearest the surface, and rinse the area with plain water afterward so the soap doesn’t burn the turf in our afternoon heat.

Got a spreading brown patch and no idea why?

Run the flush, take a photo of what comes up, then call. We’re a 24/7 dispatch line for Lakeland and Polk County, and we’ll connect you with a vetted local pro who holds the right FDACS license category for turf work — which is not the same license as general household pest control. It’s free to call and free to get matched; the exterminator gives the quote. Start at lawn pest control in Lakeland, FL.

The misdiagnosis that costs the most money: chinch bugs

If you take one thing from this page, take this. Southern chinch bug damage looks nothing like caterpillar damage, and it’s the most common wrong guess on St. Augustine lawns in Polk County.

Chinch bugs (Blissus insularis) are sucking insects. They stick a straw into the plant and pull sap out, then inject a toxin that plugs the plant’s plumbing. The plant doesn’t get chewed — it gets strangled.

Here’s the side-by-side:

• Blade condition. Caterpillar damage = chewed, notched, ragged, missing tissue. Chinch bug damage = blades fully intact, just yellowed and then straw-brown.
• Color and pattern. Caterpillars leave green stubble or scalped patches. Chinch bugs leave irregular yellow-to-straw-colored patches that expand outward, often with a yellowing halo at the edge where the bugs are actively working.
• Location. Chinch bugs love heat and drought stress. They start in full-sun areas — along the driveway and sidewalk edges, the strip by the road, south- and west-facing exposures, next to the hot concrete of a pool deck. Caterpillar damage doesn’t care about sun exposure.
• The water test. This is the tiebreaker. Water a suspected drought-stressed patch thoroughly for a week. Real drought stress greens up. Chinch bug damage does not. That’s because the plant is dead, not thirsty.
• Grass type. Southern chinch bugs are essentially a St. Augustine problem. They don’t do meaningful damage to bahiagrass.

To confirm chinch bugs, part the grass at the edge of the yellowing area down at the soil line and look for tiny insects moving — adults are about 1/5 inch, black with white wings folded in an X pattern; nymphs are orange-red with a white band. A soap flush brings them up too. We went deeper on chinch bug season and how it stacks up against webworm pressure here: Lakeland lawn pest pressure — chinch bugs, sod webworms, and what Polk County lawns are up against.

Polk County timing windows

Right now is the overlap, which is exactly why this gets confusing in September.

• Fall armyworm: active from roughly June on, but the big flare-ups in Central Florida land August through October. The name refers to when the damage shows up, not the season the insect appears. Outbreaks often follow a stretch of heavy rain — and Polk County’s rainy season plus a wet tropical pattern does exactly that. Sod farms, pastures, and new bahia sod are hit first and hardest.
• Tropical sod webworm: builds all summer and peaks September through November. They can’t survive hard freezes, so every year’s population re-invades from the south and takes until late summer to build to damaging numbers. This is their month.
• Southern chinch bug: peaks in the heat, roughly April through September, with the worst of it in the dry, hot stretch before the rains. In our mild winters they never fully shut down, so a bad infestation can carry over.

What that means practically: in mid-September you can genuinely have chinch bug damage from August and fresh webworm damage from this week in the same yard. That’s why reading the blade matters more than reading the color from the porch.

Why bahiagrass and St. Augustine react differently

Polk County has a lot of both, and they don’t respond the same way.

Bahiagrass is the pasture-and-unirrigated-yard grass across a lot of the county — out toward Bartow, Fort Meade, Mulberry, and the grove-edge properties. It’s coarse, deep-rooted, and tough. Fall armyworms will absolutely strip it — a pasture can go from green to brown in two days — but because the growing points sit low and the root system runs deep, bahia usually recovers on its own in two to four weeks with rain and a little nitrogen, as long as the crowns weren’t destroyed and it isn’t already drought-stressed. On bahia, the real question is often whether treating is worth it at all versus just letting it grow back.

St. Augustine — mostly Floratam in Polk County subdivisions — is a different story. It spreads by stolons across the surface rather than by deep roots, and it has no meaningful seed bank to fall back on. Sod webworm grazing looks alarming but is usually cosmetic; with irrigation and light fertilizer it grows out in two to three weeks. Chinch bugs are the opposite: they kill the stolon, and dead St. Augustine does not come back. You’re buying sod. That’s why the chinch-bug-versus-caterpillar call has real money riding on it.

One more wrinkle that’s specific to us: our sandy, fast-draining soil means St. Augustine sits closer to drought stress than homeowners realize, and drought-stressed turf both attracts chinch bugs and recovers far slower from any caterpillar defoliation. Fix the irrigation problem and half the pest problem gets easier.

What this actually costs in Polk County

Dollar figures help you tell a fair quote from a bad one. These are what Polk County lawn companies typically charge — not our prices, and not a quote. The operator you’re connected with gives you the real number after they look at the lawn.

• Single curative lawn insect application, typical quarter-acre residential lot: roughly $75 to $150. Larger lots, half-acre and up: $150 to $300.
• Ongoing lawn pest and fertility program: commonly $50 to $90 per visit on a six-to-eight-visit annual schedule, which works out to about $350 to $700 a year for a standard yard.
• Chinch bug curative treatment with follow-up: roughly $100 to $250, often with a re-treat included because one pass rarely gets them all.
• St. Augustine sod replacement: roughly $0.55 to $1.00 per square foot installed in this market. A 1,000-square-foot dead area is therefore somewhere around $550 to $1,000. This is the number that makes correct diagnosis worth ten minutes with a bucket of soapy water.

Prices move with lot size, turf type, how far out you are, and whether you’re buying a one-time visit or a program. Treat these as a sanity check, not a quote.

Ready to get it handled while it’s still fixable?

Armyworms can cost you a lawn in a week, so this is a bad one to sit on. Tell us what your soap flush turned up and we’ll connect you with a vetted local pro carrying the right license for turf work, across Lakeland, Winter Haven, Auburndale, Bartow, Lake Alfred, Highland City, Lakeland Highlands, Polk City, and the rest of Polk County. Head to lawn pest control in Lakeland, or if you want to verify a company’s license category first, read how FDACS licensing works for Florida pest control.

Lawn pest call or general pest call?

This trips people up, and in Florida it’s a real distinction, not a marketing one. FDACS licenses pest control by category, and lawn and ornamental is a separate category from general household pest control. A company that’s great at roaches in your kitchen may not be licensed — or equipped — to treat turf.

It’s a lawn pest call when: the damage is in the grass, you’re dealing with armyworms, sod webworms, chinch bugs, mole crickets, grubs, or spittlebugs, and the work is broadcast turf application plus an agronomic conversation about mowing height, irrigation, and nitrogen.

It’s a general pest call when: the bugs are getting into the house — ants trailing inside, roaches, the perimeter band around the foundation. A heavy armyworm year does sometimes push caterpillars up against the house and into a garage, but that’s still a turf problem with an indoor symptom.

Plenty of Polk County companies hold both categories. Just ask which one they’re licensed for before they quote you, and confirm the license is current.

Frequently asked questions

How fast can fall armyworms destroy a lawn?

A heavy infestation can defoliate a large section in 24 to 72 hours, which is why people describe waking up to a brown lawn. Most of the total feeding happens in the last few days of the larval stage, when the caterpillars are biggest, so damage that looked minor over a week can explode suddenly. If you flush armyworms and see a defined advancing front, treat now rather than next weekend.

Will my lawn grow back after armyworms?

Usually, yes. Armyworms eat leaf tissue, not roots or crowns, so bahiagrass typically recovers in two to four weeks with water and a light nitrogen application. St. Augustine also recovers, though more slowly. The exception is turf that was already drought-stressed, freshly laid, or scalped short — that grass has no reserves and can die outright. Water it and feed it lightly rather than hitting it hard with fertilizer.

How do I know it’s not just drought stress or a bad sprinkler zone?

Run water on the area deeply for about a week and watch. Genuine drought stress greens back up; chinch bug and dead-turf damage does not. Also check the shape of the problem — irrigation issues make geometric patterns that match head coverage and spray arcs, while insect damage makes irregular blobs. And look at individual blades. Chewed edges mean caterpillars, and drought doesn’t chew.

Does the soap flush work for chinch bugs too?

Yes. Use the same lemon dish soap mix over a 4-square-foot area, but sample at the edge of the yellowing patch rather than the dead middle, because chinch bugs move outward ahead of the damage. They surface fast, often in under two minutes. You’re looking for tiny black adults with white X-marked wings and orange-red nymphs. A hand lens helps.

Can I treat armyworms or sod webworms myself?

Homeowner products exist and can work on a small, early, well-identified infestation, especially Bacillus thuringiensis products on small caterpillars. Where people waste money is applying the wrong product to the wrong pest, treating in the heat of the day, or mowing and watering right after application and washing it off. If the affected area is large, spreading daily, or you’re not confident in the ID, a licensed applicator with the right product and timing is cheaper than replacing sod.

Do armyworms come back after treatment?

They can. Moths keep flying and laying through fall, so a new generation can arrive a few weeks after a successful treatment, particularly in an outbreak year and particularly near pastures, sod fields, and grove edges. That’s not a failed treatment — it’s re-infestation. Keep scouting for the advancing front and fresh windowpaning every week or so through October, and check fence posts and structures for egg masses, which look like fuzzy gray-tan cotton patches.

One last note

Lakeland Exterminators is a dispatch service. We connect Polk County homeowners with licensed local pest control operators. We are not a pest control company and we don’t perform treatments ourselves.