Steinernema carpocapsae
Entomopathogenic nematode- Rate
- 250k–500k /m² (≈1 billion per acre at the low end)
- Temp
- 14–34 °C; best 19–31 °C
- Best for
- Late-day drench, moist soil
Pest
Sod webworms
Crambidae (Crambus, Parapediasia and relatives)
The short answer
Sod webworms are the caterpillars of small, pale 'lawn moths', snout moths in the genera Crambus, Parapediasia and relatives. The moths make short, darting flights over the turf at dusk. The larvae live in silk-lined tunnels at the soil surface, come out at night and chew the grass blades, and the first sign is small, ragged brown patches, about the size of a quarter at first, in late spring and again in mid to late summer. Low numbers are tolerable, so count before treating: UC IPM treats at 15 larvae per square yard, and Minnesota Extension tolerates up to 12 per square foot.
hero macro · spotted larva in a silk tunnel in thatch
A sod webworm larva in its silk-lined tunnel, with green frass pellets. Photo: pending.
Identification
Full-grown larvae are 3/4 to 1 inch (2–2.5 cm) long and slender, greyish, brownish or greenish (UC IPM calls them creamy), with dark spots along the body in a double row down the back. Minnesota describes a black head; UMass says the head of most species is light brown with dark markings. By day they rest in silk-lined tunnels in the thatch and writhe when disturbed. Cutworms and armyworms in lawns are larger, up to 2 inches (5 cm).
The adults are buff to whitish moths, 1/2 to 3/4 inch (13–19 mm) long. At rest they fold their wings tightly around the body, which makes them look narrow, and two projections at the front of the head look like a snout. They make short, erratic flights over the lawn at dusk, and their flights tell you when to look for larvae.
Easily confused with
- Cutworms and armywormsAlso a pestFat, dull grey, green or brownish larvae up to 2 inches long, where sod webworms are slender, greyish and ¾ to 1 inch at most.
Damage symptoms
The young larvae skeletonise the grass blades; older ones notch or cut them off and pull them into the burrow. The first sign is small, ragged brown spots in late spring and again in mid to late summer, most often in sunny areas. They grow and merge as feeding continues. UC IPM says turf with more than 100 larvae per square yard quickly looks moth-eaten.
- Small, ragged brown patches, about the size of a quarter, that grow and merge
- Grass blades notched or clipped off near the thatch, with brown scars
- Silk-lined tunnels in the thatch lined with green, pellet-like frass
- Buff moths darting over the lawn at twilight in summer
- Birds and other animals digging in the damaged patches for caterpillars
Lifecycle & degree days
The larvae spend the winter in silk shelters (hibernacula) up to a few inches below the lawn surface, resume feeding in spring, and pupate. Moths fly and lay in early summer, and the next generation of larvae feeds in mid to late summer, when the damage is usually worst. Minnesota has two generations. In New England some species have one and others two, and Purdue says several generations may occur in a season.
- Overwinters
- Large larvae in silk shelters below the surface
- Generations / yr
- 1–2 in New England and Minnesota; several per season (Purdue)
- Larvae (New England)
- Late April to early June, then again in summer
- Moths (New England)
- Late June to late August
- Scout
- June and again in early August (Minnesota)
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Host crops
Sod webworms feed on lawn grasses; UC IPM names bentgrass and bluegrass as especially favoured, along with clover. Damage is commonest in sunny spots. UC IPM rates endophyte-infected perennial ryegrass and turf-type tall fescue, and warm-season grasses, as more resistant, and UMass suggests endophytic cultivars when a lawn is due for renovation.
Monitoring & action thresholds
Action threshold
15 per yd² (UC IPM); up to 12 per ft² (Minnesota)
The two figures are far apart. UC IPM treats at more than 15 larvae per square yard in most turf (about 1.7 per ft²), or 5 per square yard on stressed greens. Minnesota Extension tolerates up to 12 larvae per square foot, about 108 per square yard, or seven times more. Neither source explains the gap, and UMass says no good action level is available for sod webworms. Count first and use your local extension figure.
Scout in June and again in early August, and whenever small brown spots appear. The soap flush is the standard test. UC IPM mixes 3–4 tablespoons of dishwashing liquid in 2 gallons of water, pours it over 1 square yard and counts for 10 minutes. UMass uses 1–2 tablespoons of lemon-scented detergent in 1–2 gallons over a 2 ft by 2 ft square; most caterpillars come up within 2–5 minutes. Moth flights at dusk tell you larvae will follow: UMass times insecticides two to three weeks after the peak flight.
Beneficial
Biological control
Sod webworms have many natural enemies. UC IPM lists a tachinid fly and two braconid wasps as parasites, and earwigs, rove beetles, robber flies, paper wasps, ants and vertebrates as predators. Two biological products fit. The nematode Steinernema carpocapsae ambushes insects moving through the thatch, and UC IPM rates it a valuable control. Bt kurstaki works on young larvae, but it breaks down quickly in sunlight, washes off the leaves and does not work on late-instar larvae.
Bacillus thuringiensis kurstaki
Bt (kurstaki)- Rate
- Label rate — ½–4 tsp per gallon on home concentrates; varies by product
- Best for
- Young larvae; reapply after rain
Cotesia congregata and other Braconidae
Braconid wasps- Kind
- Wild — attract it
- Best for
- Wild parasitoids — spare them
Staphylinidae
Rove beetles- Kind
- Wild — attract it
- Best for
- Background predation in thatch
Cultural controls
The cultural measures hide feeding damage and take away the thatch the larvae live in. Resistant grass is the longer-term fix.
- Mow high, above 2½ inches (6 cm), to hide feeding damage
- Remove excess thatch, where the larvae live (it helps, though they do not need it)
- Overseed or renovate with endophyte-infected perennial ryegrass or fescue where webworms recur
- Water and fertilise appropriately so the turf can regrow
- Water the lawn thoroughly a day or so before any insecticide (Minnesota)
Chemical, last resort
Broad-spectrum lawn insecticides kill the parasites and predators that keep webworms down.Check any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Management Guidelines: Turfgrass — Sod Webworms (Sutherland et al., 09/09)
UC IPM Pest Notes 7476, Lawn Insects (Sutherland, Dreistadt & Harivandi, 09/2014)
University of Minnesota Extension, Sod webworms (Krischik & Ascerno, reviewed 2018)
UMass Extension Turf Program, Sod Webworms (rev. 05/2011)
UMass Extension, Turf BMPs Section 11: Insect Management
Purdue Extension E-61-W, Turfgrass Insects (Gibb & Richmond)
DiPel DF label, EPA Reg. 73049-39 (2022)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Sod webworms by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.