Armyworm and Sod Webworm Outbreak Season: How to Spot Damage Before You Lose the Yard Overnight

The Adult Moth

The most common species is the fall armyworm (Spodoptera frugiperda). The adult is a small gray-brown moth with a wingspan around one and a half inches. Females lay egg masses of 100 to 200 eggs each on fences, light poles, leaves, and structures near lawns. Eggs hatch in two to four days.

The Destructive Larvae

Young armyworms (first instar) are only a sixteenth of an inch long and barely feed. By the fifth and sixth instars — in the last three to five days before pupation — they consume 80 percent of their lifetime food intake. That is the window when entire lawns disappear overnight.

Where They Come From

Adult armyworms migrate north from Florida, Texas, and the Gulf Coast each summer, riding storm fronts and prevailing winds. Outbreak intensity depends on weather patterns in the Gulf states in May and June, plus the timing and severity of summer storms. Some years produce massive outbreaks that move across the central and eastern U.S. in waves.

The Adult Moth

The sod webworm adult is a small tan or buff-colored moth that flies low over the lawn at dusk in a distinctive zigzag pattern. If you have been seeing moths jumping ahead of you as you walk the lawn after dinner, you are seeing sod webworm adults.

How They Damage Your Lawn

Larvae build small silken tunnels at the soil line, hidden in the thatch, and feed on grass blades at night. Damage starts as small brown patches roughly the size of a softball that expand over one to two weeks. The timeline is more forgiving than armyworms — you usually have one to two weeks to act, versus 24 to 48 hours.

Which Grasses Are Affected

Sod webworms are most damaging on Kentucky bluegrass, perennial ryegrass, tall fescue, and bentgrass — more of a cool-season and transition zone problem. Armyworms hit warm-season turf hardest (bermuda, zoysia, St. Augustine) but will feed on almost any grass when populations are high enough.

Armyworm Damage Signature

Large, sharply defined brown areas, often expanding from a starting point. Grass blades are chewed off near the soil line, leaving short stubble. Damage spreads in a marching pattern. You may see frass (small dark caterpillar droppings) on the soil surface and on lower leaf surfaces.

Sod Webworm Damage Signature

Smaller, scattered brown patches roughly four to eight inches across, sometimes connected into larger irregular shapes. You may see silken webbing in the thatch when you part the grass with your fingers. Damage is typically heaviest in sunny areas and on south-facing slopes.

The Soap Flush Test

Pour a gallon of water mixed with two tablespoons of dish soap over a one-square-foot section at the edge of a damaged area. Wait three to five minutes. If you see five or more armyworms or 10 or more sod webworms per square foot, you have a treatable population. Above 15 to 20 of either, you have an outbreak.

Homeowner-Applied Products

Look for active ingredients like bifenthrin, lambda-cyhalothrin, spinosad, or chlorantraniliprole. Apply in late afternoon or early evening so the product is fresh when caterpillars come out to feed. Water in lightly (a quarter inch) after application to move the product into the thatch.

Professional Applications

Lawn Squad typically uses chlorantraniliprole-based products, which provide longer residual control and pose less risk to pollinators when applied correctly. The product moves into the plant and into the soil interface where the caterpillars feed.

Re-Scout After 72 Hours

A successful application kills 90 to 95 percent of the population. If you still find significant activity in your soap flush test, a second application is sometimes needed, particularly during heavy outbreak years where successive waves of moths lay eggs on top of treated areas.

Treated in Time

If you treat before the caterpillars eat the crowns, the lawn recovers. Armyworms typically eat the blades and stop. The crowns and roots remain intact, and the lawn pushes new growth within two to three weeks given adequate water. Damaged areas may benefit from a light fall overseed on cool-season turf.

Treated Too Late

If the caterpillars have eaten the crowns — particularly on bermuda and zoysia where caterpillars sometimes feed below the soil surface — you are looking at sprigging, sodding, or full renovation. The clock matters that much.

Do Not Fertilize Immediately

Resist the temptation to apply fertilizer right after a damaging event. The lawn is under stress. Pushing growth with nitrogen during recovery creates weak top growth on depleted roots. Wait two to three weeks until you see new green emerge, then apply a balanced feeding to support recovery.

Can armyworms actually destroy a lawn in one night?

Yes, when populations are high and the larvae are in their final instar. We have walked properties where 1,500 square feet went from green to brown stubble in less than 24 hours.

Will rain wash off the treatment?

Most modern armyworm products are rainfast within two to four hours after application. For the chemistries we use professionally, a thunderstorm an hour after treatment usually does not require a re-application. Check the label.

Are these caterpillars harmful to pets?

The caterpillars themselves are not toxic, but treatments should be allowed to dry and irrigation should move the product into the thatch before pets walk on the lawn. Standard reentry intervals apply.

Will birds handle the problem for me?

Birds eat armyworms and sod webworms enthusiastically, and a heavy concentration of grackles, starlings, robins, or crows poking the lawn is itself a signal of caterpillar activity. But birds rarely take down a population fast enough to prevent damage during a heavy outbreak.