Sod webworm
Click for larger image Sod webworm adult (Lepidoptera) on lawn grass

The sod webworm is the larval or caterpillar stage of a small whitish or dingy-colored moth. The caterpillars range in size from 1/4 to 1 inch long and may be grayish-brown to greenish to dirty white in color, with four parallel rows of dark brown spots on the abdomen. The adult moths are commonly known as close-winged moths (for their resting habit) and/or snout moths (for the two finger-like horns protruding from their head). Their primary hosts in the home landscape are Kentucky bluegrass, fine fescue and pernnial ryegrass. Usually they are only a problem in unirrigated lawns during periods of drought. At other times, naturally occuring diseaes, to which they are highly susceptible, keep them in check.

Symptoms and Diagnosis

The worms construct little burrows in the thatch layer of lawns. The burrows are covered with bits of dirt, lined with silk, and reinforced with excrement and pieces of grass. The worms can then cut off blades of grass, drag them into their lair, and feast safely in the comfort of their own home. All webworms feed at night. The first sign of damage can easily be overlooked, appearing only as small, irregular, brown patches in the grass. However, as the worm grows older, it begins to eat entire leaves and can consume twice its own body weight in one night. Large areas of grass can be severely damaged if worm populations are high. Because the brown patches caused by webworm can superficially resemble those caused by other pest insects or diseases, accurate diagnosis is important. Webworms live in the thatch layer of the lawn, not the soil. Check for them in the dead grass just above the soil line. You can loosen the thatch with a trowel. Look for grass blades that are missing, not just dead, as well as green fecal pellets, and the worm located in the silk tunnels.

Life Cycle

In late spring and early summer, the female sod webworm moths fly over the lawn at dusk, dipping down to lay as many as 200 eggs in the grass. The eggs resemble tiny cream-colored beads and are preferentially dropped in humid areas of the lawn. The eggs need moisture to develop; otherwise mortality rate would be high. When the larvae have completed their growth and are about 1 inch long, they leave their burrow and construct cocoons of silk and bits of earth in the nearby soil. In 10 to 14 days, the moth emerges, mates, and dies within a few days. The adult moth will not eat solid food.

Integrated Pest Management Strategies

1. Select webworm-resistant grass cultivars or tall fescue. The Kentucky bluegrass varieties, ‘Windsor’ and ‘Park’, show some tolerance for webworm. For chronic problems, select an endophytic grass (grass that contains beneficial fungi within their tissue).

2. Habitat management. Reduce the thatch level if it is thicker than 1/2 to 1 inch in depth. Webworms also like areas that are hot and dry during the day, which may be caused by soil compaction. Aerate the soil to bring back thicker growth, and the damage will be less noticeable. Water the lawn during periods of drought.

3. Conservation of native biological controls. Encourage the natural enemies of the webworm by planting a diverse landscape. Some of the webworm predators include four species of ants, especially Pheidole tysoni, and the mite Macrocheles, all of which feed on the webworm eggs. The robber fly, Efferia aestuans, as well as spiders, vespid wasps, native earwigs, carabid beetles, rove beetles, and birds feed on the webworm at various stages of its life cycle.

4. Use an insecticidal soap. Spray the infested and surrounding area with a solution of 2 tablespoons of liquid soap or detergent in 1 gallon of water. Once the worms make their way to the surface, rake them into a bucket of soap solution.

5. Bacillus thuringiensis (Bt) is a naturally occurring bacterium that only affects caterpillars by acting as a poison once ingested. It is available at nurseries and hardware stores. The best time to apply it is 2 weeks after seeing a large number of moths, especially during hot and dry weather, in order to stop the new and hungry hatchlings.

6. Natural pyrethrum and pyrethrins and synthetic pyrethroids are effective against the sod webworm, although they also kill beneficial insects in treated portions of the lawn.

7. Use chemical insecticides. Pesticides recommended for use include carbaryl (Sevin), halofenozide (Mach 2), imidacloprid (Merit), spinosad and trichlorfon (Dylox).

Organic Strategies

Strategies 1, 2, and 3 are strictly organic approaches. For organic approaches to Strategies 4, 5, 6, and 7, consult the Organic Materials Review Institute (OMRI™) for appropriate insecticidal soap, Bacillus thuringiensis (Bt), pyrethrum/pyrethrin and spinosad products.

More images:

Click for larger image
Adult sod webworm, a species of crambid moth, Pediasia trisecta (Lepidoptera). J. Lawrence, Eurofins Agroscience Services, Bugwood.org