“Grass worms” is a catch-all term for several lawn-destroying larvae that feed on turfgrass blades or roots. The most common culprits are armyworms, sod webworms, cutworms, and white grubs. Each one damages your lawn differently, shows up at different times of year, and requires a slightly different approach to control. Identifying which one you’re dealing with is the first step toward saving your turf.
Four Types of Grass Worms and How to Tell Them Apart
Armyworms are 1 to 2 inches long and range in color from gray to yellow-green to pink. The easiest way to identify fall armyworms specifically is the inverted Y-shaped marking on their head. They feed mostly at night, and the adult moths are brown with fuzzy abdomens that are easy to spot around outdoor lights after dark. A single egg mass contains 50 to 200 eggs and looks like a cottony, fuzzy patch, often laid on structures near your lawn.
Sod webworms are smaller, maturing to about 1 inch long, and are brown or green with dark spots. Their name comes from the silky webbing they spin near the soil surface, binding together grass clippings, feces, and debris as they prepare to pupate. On closely mowed turf (under 1 inch), this webbing is visible to the naked eye. The adult moth is a dingy gray-tan color with a distinctive double-snout projection on its head.
Cutworms come in several species, most about 2 inches long and gray or brown with some striping. They tend to curl into a C-shape when disturbed. Their adult moths are dull gray with brown or black markings. Cutworms typically clip grass blades at the base near the soil line, leaving small bare patches that can look like divots.
White grubs are the odd ones out because they live underground. These plump, C-shaped larvae are the immature stage of scarab beetles, including Japanese beetles and masked chafers. Rather than eating grass blades, they feed on roots just below the soil surface during spring, summer, and early fall.
What Grass Worm Damage Looks Like
The tricky thing about grass worm damage is that it often mimics drought stress or disease. Your lawn turns brown in patches, and your first instinct might be to water more. But the pattern of damage and a few telltale details can point you to the real cause.
Armyworm damage typically starts near the edge of a structure where eggs were laid, then spreads outward. Green turf gradually turns brown as the caterpillars grow into their later stages, roughly 2 to 3 weeks after hatching. The first three stages of the larvae are so small they’re nearly impossible to spot, which is why damage seems to appear suddenly. In severe infestations, armyworms can eat through entire grass stems, and re-sodding may be the only fix.
Sod webworm feeding creates a distinctive look: the grass in affected areas is noticeably shorter than surrounding turf, and individual blades have a ragged, notched appearance from chewing. This damage hits hardest in mid to late summer, when heat and drought stress are already weakening your lawn and making recovery slower.
White grub damage has its own signature. Because the larvae eat roots rather than blades, the grass loses its ability to absorb water and nutrients. Early signs include wilting turf that doesn’t bounce back after watering. In severe cases, the root system is so destroyed that you can grab a handful of grass and peel it up like loose carpet. A couple of grubs per square foot won’t cause visible harm, but a large population will.
How to Confirm an Infestation
For surface-feeding caterpillars like armyworms, a simple soap flush works well. Mix a few tablespoons of dish soap in a gallon of water and pour it over the brown area of your lawn. If caterpillars are present, they’ll wriggle to the surface within minutes. You can also look for early feeding signs before major damage appears: tiny white “windowpanes” on grass blades or a subtle white frosting at the blade tips. These marks come from recently hatched caterpillars skeletonizing the leaf surface.
For grubs, cut a one-square-foot section of turf about 3 inches deep in a damaged area and peel it back. Count the white, C-shaped larvae you find. One or two is normal and not a concern.
Treating Active Infestations
Timing matters more than product choice. Caterpillar larvae are far easier to kill when they’re small and newly hatched. Once armyworms reach their later stages at 1.5 to 2 inches long, they’re harder to control and have already done most of the damage. If you spot the early signs of feeding (frosted blade tips, tiny windowpane marks), that’s the ideal window to act.
For homeowners, insecticide products containing spinosad or chlorantraniliprole are effective against armyworms, sod webworms, and cutworms. Spinosad at lower concentrations handles small armyworms and sod webworms well, though cutworms require a higher rate. After applying any treatment to your lawn, avoid watering or mowing for 24 hours to let the product work.
For white grubs, the approach is different because the larvae are underground. Treatments need to be watered into the soil to reach the root zone where grubs feed. Preventive products applied in early summer, before grub populations build, tend to be more effective than trying to knock down an established infestation in late summer.
Biological Controls That Work
If you prefer to avoid synthetic chemicals, beneficial nematodes are a proven option for white grubs. Two types, from the genera Steinernema and Heterorhabditis, are parasites that actively hunt grub larvae in the soil. Commercially available products typically contain Heterorhabditis bacteriophora, a species that swims through soil moisture to find its host. As a bonus, it also kills flea larvae but leaves beneficial insects, pets, and people unharmed.
Nematodes need specific conditions to work. The soil temperature should be between 68 and 86°F. You’ll need to water the area thoroughly before applying, deliver the nematodes in water using a sprayer or watering can, then irrigate with at least a quarter inch of water afterward. Keep the soil moist for three days following application, and avoid applying during the heat of the day since UV exposure kills nematodes. Early morning or late evening is best. Order them when you’re ready to use them and apply immediately after they arrive, since they don’t store well and need a food source quickly.
One important caveat: nematodes are a curative treatment, not a preventive one. Grubs must already be present in the soil for the nematodes to survive and do their job. Spot-treating confirmed infestation areas gives better results than blanket applications.
For surface caterpillars, Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium available as a spray. It’s most effective against young, actively feeding caterpillars and breaks down quickly in sunlight, so evening application works best.
Preventing Grass Worms From Coming Back
A healthy, well-maintained lawn resists pest damage better and recovers faster when damage does occur. Several cultural practices make your turf less hospitable to grass worms.
Thatch management is one of the most important. Thatch, the layer of dead organic material between the grass blades and the soil, provides shelter for sod webworm larvae and egg-laying sites for moths. If your thatch layer exceeds half an inch, consider dethatching with a vertical mower or power rake. Core aeration also helps by improving moisture and air penetration into compacted soils, stimulating deeper root growth, and boosting the microbial activity that naturally decomposes thatch. Fertilize about a week before aerating to help the grass fill in the holes more quickly. Let the soil cores dry partially on the surface, then rake them back in or let them crumble naturally. This “top dresses” the lawn with soil microorganisms that continue breaking down thatch.
Irrigation habits matter too. Overwatering creates lush, soft growth that’s more attractive to egg-laying moths. Water deeply but infrequently to encourage deeper root systems that better withstand both drought and grub feeding. Fall fertilization in September promotes steady growth without the rapid, lush flush that spring feeding produces, and a soil test can tell you exactly how much nitrogen, phosphorus, and potassium your lawn actually needs.
One often overlooked detail: heavy pesticide use can actually make thatch problems worse by killing the earthworms and microbes responsible for breaking down organic matter. Use targeted treatments only when you’ve confirmed an infestation rather than applying preventive chemicals as a routine. This keeps your lawn’s natural decomposition cycle intact and reduces the sheltered habitat that grass worms thrive in.