Sod webworms are the larvae of small, dingy brown moths that feed on grass blades, leaving behind irregular brown patches that can quickly expand across a lawn. They’re one of the most common turf pests in the United States, active from spring through fall, with peak damage typically occurring in late summer and early autumn. If you’ve noticed small circular spots of closely clipped grass or moths zigzagging low over your yard at dusk, webworms are a likely culprit.
What Sod Webworms Look Like
The adults are easy to spot if you know what to look for. Sod webworm moths are brown to dull gray, about 3/4 inch long with a similar wingspan. Most North American species fold their wings partially when at rest, giving them a distinctive tube-like silhouette. Some species have prominent forward-pointing, snout-like mouthparts. You’ll typically notice them at dusk, flying in a jerky zigzag pattern just above the grass as they drop eggs onto leaf blades.
The larvae are the ones doing the damage. When they first hatch, they’re whitish and tiny, gradually shifting to light green and then yellowish-brown as they grow. They have dark brown heads and brown spots on each body segment that become more prominent as the caterpillars mature. The larvae go through five to six growth stages before pupating, and the pupae are reddish-brown, buried in the thatch layer just below the grass surface.
Life Cycle and Timing
Female moths lay creamy white, flat, oval egg masses on the upper surface of grass blades. These eggs darken to brownish-red as they develop and hatch in about 10 to 14 days. From egg to adult moth, the entire cycle takes 21 to 47 days depending on temperature, with warmer conditions speeding things up considerably. That fast turnaround means multiple generations can occur in a single growing season, and populations can build rapidly during warm stretches.
In southern states, peak activity runs from September through November. Populations decline when cold weather arrives, and larvae overwinter in the thatch layer. They resume feeding in spring as temperatures rise. In northern climates the window is shorter, but mid-to-late summer remains the period when damage is most visible, partly because heat-stressed grass recovers more slowly from feeding injury.
How to Spot the Damage
Sod webworm damage starts as small, quarter-sized circular areas where the grass looks closely cropped, almost scalped. These spots are brown thatch showing through because the grass blades are gone, not just discolored. That’s a key distinction from drought stress or fungal disease, where the blades are still present but wilted or browned. As the infestation grows, those small patches merge into large, irregular dead areas.
Young larvae notch the edges of grass blades, leaving a ragged, chewed appearance. Older, more mature caterpillars build silk-lined burrows in the thatch and pull entire grass blades down into these tunnels to eat. Two other telltale signs confirm a webworm problem: green fecal pellets (called frass) scattered in the thatch, and the silk tubes themselves, which you can spot by parting the grass near damaged areas and looking closely at the thatch layer.
The Soap Flush Test
Before treating, it’s worth confirming what you’re dealing with and how many larvae are present. The standard method is a soap flush: mix 1 to 2 tablespoons of lemon-scented liquid dish detergent into 2 gallons of water and pour it slowly over about one square yard of turf. Place the flush in healthy grass at the edge of a damaged area, not in the dead zone itself. Watch closely for 5 to 10 minutes. The soapy water irritates the larvae and drives them to the surface where you can count them.
Repeat in a few different spots to get a reliable picture of the population. The general treatment threshold is 10 to 15 larvae per square yard (roughly 4 to 6 per square foot), though this can vary with grass type and how stressed your lawn already is. A healthy, well-watered lawn tolerates more feeding than one already weakened by drought or poor soil.
Lawn Care Practices That Reduce Webworms
Thick thatch gives webworm larvae shelter, food, and a place to build their silk burrows. Keeping thatch under control through vertical mowing or power raking directly reduces the habitat these caterpillars depend on. Removing grass clippings after mowing can also help limit population buildup.
Over-fertilizing and over-irrigating both promote excessive thatch accumulation, which creates ideal webworm conditions. Moderate, balanced fertilization and deep but infrequent watering produce a lawn that’s healthy enough to tolerate some feeding while being less hospitable to pests. Mowing at the appropriate height for your grass type (rather than scalping it low) also helps turf recover faster from any damage that does occur.
Resistant Grass Varieties
If you’re establishing a new lawn or overseeding, grass selection matters. Several St. Augustinegrass cultivars, including Captiva and Amerishade, show moderate resistance to tropical sod webworms. Certain zoysiagrass varieties, particularly Cavalier, offer even higher resistance. Choosing a resistant cultivar won’t eliminate the problem entirely, but it reduces the severity of infestations and can mean the difference between cosmetic damage and a lawn that needs complete renovation.
Biological Control Options
For homeowners who prefer to avoid synthetic pesticides, several biological options work well against sod webworms, with one important caveat: timing is everything. These products are most effective against young, early-stage larvae. Once the caterpillars are large and well-established in their silk burrows, biological controls become far less reliable.
Bt (a naturally occurring soil bacterium sold under various brand names) is the most widely available biological option. It must be ingested by the larvae to work, and it’s essentially ineffective against late-stage caterpillars. Apply it when you’re seeing mostly small larvae. After application, avoid irrigating for two days so the product stays on the grass blades where larvae are feeding.
Beneficial nematodes (specifically the species Steinernema carpocapsae) are another biological approach. These microscopic organisms actively seek out and parasitize webworm larvae in the soil and thatch. Apply them during the coolest part of the day to prevent them from drying out, and irrigate immediately after application to wash them into the thatch where the larvae live. This is the opposite of the Bt rule, so don’t mix the two in the same application.
Neem-based products and spinosad (derived from a naturally occurring soil organism) round out the biological toolkit. Both are most effective on young larvae.
Chemical Treatments
When populations are high and damage is spreading fast, chemical insecticides provide faster knockdown. Chlorantraniliprole is a newer option with low toxicity to pollinators and other non-target organisms, making it a good choice for lawns where bees are active. Bifenthrin and carbaryl are broader-spectrum options that kill larger larvae more reliably but are also harder on beneficial insects.
Regardless of which product you use, the application protocol matters as much as the product itself. Mow and water your lawn before applying any insecticide. This shortens the grass so the product reaches the thatch where larvae hide and softens the thatch so the active ingredient penetrates. After treatment, do not mow or irrigate for at least 24 hours (or 48 hours for Bt products). The exception is nematode applications, which need immediate post-treatment watering.
Apply in the evening whenever possible. Webworm larvae feed primarily at night, so evening applications maximize contact. Many products also break down in direct sunlight, so late-day timing gives them more hours of effectiveness before UV exposure degrades them.
Recovery After Treatment
Lawns damaged by sod webworms generally recover well once the larvae are controlled, because webworms eat the grass blades but typically leave the crowns and roots intact. With proper watering and a light application of fertilizer, most warm-season grasses fill back in within a few weeks. Cool-season grasses may need overseeding if the damage is extensive, especially heading into fall when growth slows. The key is getting the population under control before the feeding strips the grass down to bare thatch, which takes much longer to recover from and invites weed colonization in the meantime.