Sod webworms are a common lawn pest, the larval stage of small moths often seen fluttering over turfgrass. These caterpillars feed aggressively on grass blades, leading to significant turf damage that often mimics drought stress or disease. Successful management relies much more on the precise timing of application than on the choice of treatment product. Understanding the pest’s life stages and peak activity periods is the most effective approach to protecting your lawn.
Recognizing Signs of Sod Webworm Damage
The first indication of a problem is typically irregular brown patches in the lawn that do not recover despite regular watering. Unlike grass killed by grubs, which can be rolled back like a carpet, webworm-damaged areas remain firmly rooted. Closer inspection reveals grass blades that are ragged, notched, or appear closely clipped near the soil line. In severe infestations, the grass may look shredded or “scalped.”
A definitive diagnosis requires finding physical evidence left by the larvae. These pests live in silk-lined tunnels within the thatch layer, where you may find small, bright green fecal pellets, known as frass, near the entrances. The most reliable way to confirm presence and population density is using the “soap flush” test.
To perform this test, mix about two tablespoons of liquid dish detergent into two gallons of water and pour the solution evenly over one square yard of the affected turf. The soapy water irritates the caterpillars, forcing them to the surface within five to fifteen minutes. If you count between 10 to 15 larvae in that one-square-yard area, a curative treatment is warranted.
Understanding the Life Cycle and Vulnerable Stages
Sod webworms generally complete two to three generations annually, depending on climate conditions. The cycle begins when adult moths scatter their eggs randomly across the lawn, often while flying in a zigzag pattern at dusk. The eggs hatch into larvae—the destructive stage—which immediately begin feeding on the grass blades.
The larvae spend several weeks feeding before pupating and emerging as the next generation of moths. They overwinter as partially grown larvae deep in the soil or thatch, resuming feeding when temperatures rise in the spring. Damage caused by these overwintering larvae is often the first noticeable sign of activity.
Targeting the pest during its most vulnerable phase is key to effective control. The most susceptible stage is the newly hatched, small larvae, known as the first and second instars. These young caterpillars feed close to the surface and have not yet constructed their protective, silk-lined tunnels. Once the larvae mature and burrow deeper, they become significantly harder to reach with surface-applied products.
Strategic Timing for Effective Treatment
The best time to apply curative treatment is 10 to 14 days after the peak flight of the adult moths. Monitoring for these tan moths, which fly low over the turf in the evening, should begin in late spring or early summer. A high number of adult moths signals that the next generation of eggs is being laid, forecasting the upcoming larval hatch.
For many regions, the first generation of peak moth flight occurs in late June or early July, meaning the optimal treatment window to target the young larvae is shortly thereafter. This initial application prevents the first generation from causing significant damage and producing a larger second generation. The second application window typically opens in late August or early September, timed to intercept the larvae from the second, and often more damaging, moth flight.
Curative products are most effective when ingested by the larvae, which feed primarily at night. Therefore, application should occur in the late afternoon or early evening to ensure the chemical residue is fresh and available when the caterpillars emerge. If using a contact insecticide, a light watering immediately after application can help move the product down to the feeding zone near the thatch layer.
If using a synthetic pyrethroid, do not water or mow the area for up to 48 hours following the application. This ensures the product remains concentrated on the grass blades where the pests feed. Proper timing ensures the treatment contacts the larvae when they are actively feeding, maximizing control.
Cultural Practices for Long-Term Control
Beyond immediate treatment, specific lawn care practices can help reduce the attractiveness of turf to webworms and minimize future outbreaks. Maintaining a taller mowing height helps deter the pests; damage is less noticeable in turf kept above 2.5 inches. Taller grass also shades the soil, making the environment less favorable for egg-laying moths, which prefer hot, sunny areas.
Adjusting irrigation practices is beneficial, as drought-stressed turf is far more susceptible to severe damage. Avoid excessive watering, however, which can contribute to thatch buildup that provides shelter for the larvae. A healthy, well-maintained lawn is more capable of recovering quickly from minor feeding damage.
Consider overseeding with endophyte-enhanced grass varieties for natural resistance. Certain cultivars of perennial ryegrasses and turf-type tall fescues contain a symbiotic fungus called an endophyte. This endophyte produces chemicals toxic to sod webworms, providing a built-in defense mechanism that discourages feeding.