Sod Webworms and Armyworms in Coastal Maine: How to Spot and Stop Late-Summer Lawn Caterpillars

If your Kennebunkport lawn was green and healthy in June but suddenly developed ragged brown patches in late July, grubs aren't the only suspect. Two surface-feeding caterpillars — sod webworms and armyworms — hit coastal Maine lawns hardest from mid-July through early September, and they can do serious damage before most homeowners realize what's happening.

At Wakem Lawn Care, we see these pests every summer on properties from Kennebunkport and Cape Porpoise up through Scarborough and Southern Maine. The good news: caught early, both are very treatable, and a healthy lawn recovers quickly. Here's how to tell them apart, what to do about them, and how to keep them from coming back.

What Are Sod Webworms?

Sod webworms are the larvae of small, buff-colored "lawn moths" you'll see fluttering in a zigzag pattern over the grass at dusk. The caterpillars live in silk-lined tunnels in the thatch layer and emerge at night to chew grass blades off just above the soil line, leaving scattered brown patches that slowly expand and merge.

Signs of sod webworm activity in a coastal Maine lawn include:

• Small moths flying low over the lawn at dusk in a jerky, zigzag flight pattern
• Irregular brown patches that start small (the size of a baseball) and gradually grow
• Grass blades chewed off cleanly near the crown, not yellowed or wilted
• Increased bird activity — starlings and robins pecking at the same spots repeatedly
• Green pellet-like droppings (frass) in the thatch when you part the grass

Webworm damage is easy to mistake for drought stress, especially during a dry Maine August. The key difference: drought-stressed grass wilts and turns straw-colored evenly, while webworm damage appears as distinct chewed-down patches with healthy grass around them. If you're unsure whether you're looking at dormancy, disease, or insect damage, our lawn maintenance team can diagnose it on site.

What Are Armyworms and Why Are They Different?

Armyworms are the more dramatic of the two. These striped caterpillars are the larvae of a migratory moth that flies north each summer — they can't survive Maine winters, so every infestation starts fresh with moths carried up the coast on summer storm fronts. When eggs hatch, the caterpillars feed together and move across a lawn in a literal advancing front, capable of stripping a wide band of turf almost overnight.

Where sod webworm damage builds gradually over weeks, armyworm damage explodes in days. A lawn can look fine on Friday and be half-browned by Monday. Coastal communities like Kennebunkport, Wells, and Old Orchard Beach often see armyworm arrivals after big southerly storm systems in late summer.

How Do I Check My Lawn for Webworms or Armyworms?

The soap flush test is the fastest way to confirm caterpillars in a Maine lawn. Mix two tablespoons of dish soap in two gallons of water and pour it over one square yard at the edge of a damaged patch. Within ten minutes, irritated caterpillars will wriggle to the surface where you can count and identify them.

What you find tells you what to do:

1. Fewer than 2–3 webworms per square foot — a healthy lawn can outgrow this. Water, mow high, and monitor.
2. More than 4–5 webworms, or any cluster of armyworms — treatment is warranted, and quickly in the case of armyworms.
3. Nothing surfaces — look elsewhere: grubs feed below the soil on roots (tug test: damaged turf lifts like carpet), while fungal diseases show lesions on the blades themselves.

How Do You Treat Lawn Caterpillars in Coastal Maine?

Treatment works best in late afternoon or early evening, right before the caterpillars come up to feed. Options range from biological to conventional:

• Bt (Bacillus thuringiensis) — an organic bacterial insecticide that targets caterpillars specifically and spares pollinators. Best on young larvae.
• Beneficial nematodes — applied with water to moist soil, effective on webworms in the thatch layer.
• Targeted conventional insecticides — fast knockdown for heavy armyworm infestations where a lawn is at real risk. Timing and label compliance matter, especially on coastal properties near marshes, ponds, and tidal water where runoff is a concern.

Mow the lawn and lightly water before applying any product so it reaches the thatch where the caterpillars hide. And skip the temptation to blanket-spray a healthy lawn "just in case" — unnecessary insecticide kills the beneficial insects that keep pest populations down naturally.

Recovery: Why Fall Is on Your Side

Here's the silver lining for Maine homeowners: unlike southern lawns, our cool-season grasses are heading into their strongest growing window just as caterpillar season ends. Damaged areas overseeded in late August through mid-September recover remarkably well.

A smart recovery plan for a caterpillar-damaged coastal Maine lawn looks like this:

1. Treat and confirm the infestation is done with a follow-up soap flush
2. Core aerate to relieve compaction and open the soil — if you're in the Scarborough area, see our Scarborough lawn care and aeration services
3. Overseed damaged patches with a salt-tolerant tall fescue or ryegrass blend suited to coastal conditions
4. Fertilize in early September to fuel recovery before the first frost
5. Reduce thatch — a thick thatch layer over half an inch is prime webworm habitat, so dethatching prevents next year's problem

Because webworms overwinter in thatch (unlike armyworms), a fall cleanup and healthy soil program is genuine prevention, not just cosmetics. It's the same reason we build aeration and overseeding into our fall visits — and why properties on a year-round program, from summer pest monitoring through winter services, rarely see the same problem twice.

When Should You Call a Professional?

Call a professional if brown patches are spreading week over week, if a soap flush turns up armyworms, or if you've treated once and damage continues. Armyworm infestations move too fast for trial and error, and misdiagnosis — treating caterpillars when the real problem is grubs or fungus — wastes both money and recovery time.

Wakem Lawn Care provides pest diagnosis, treatment, and full lawn recovery services throughout Kennebunkport, Kennebunk, Arundel, Wells, Biddeford, Scarborough, and the surrounding coastal Maine communities. Whether you need a one-time rescue or a season-long lawn maintenance program that catches problems before they spread — with landscaping support for beds and plantings, too — we're here to help.

Spotted moths at dusk or patches you can't explain? Contact us today for an inspection before a small problem marches across your whole lawn.