Distant: Small patches (2-4 inch diameter) of closely-cropped turf that usually turns brown in a few days. Small patches may coalesce into larger patches. These patches may be irregular in shape (Images C & D).
Up close: Grass blades pruned off at the crown, green sawdust-like pellets that turn brown in a few days (Image E).
When damage would be found: May to October
Host: All species
Site: Usually on low maintenance, non-irrigated turf, but sometimes a problem on irrigated turf, including turf around golf course greens.
Description of damaging stage: Webworm larvae are 1/4 to 1/2 inch long, ivory-colored caterpillars with dark brown or black spots. They are difficult to find because they hide in silken tunnels in the thatch during the day. Webworm larvae have a brown head capsule, 6 short thoracic legs, and the last few abdominal segments narrow at the posterior end (Images F & G).
Sampling: Use the hands and knees method when damage is observed. Push through any damaged areas with your fingers and look for patches of turf that are pruned off just above the crowns. Look for small sawdust-like, green pellets (frass) that turn brown in a few days. Larvae can be brought to the surface by drenching infested turf with a soapy water solution made by stirring 1 oz of liquid dishwashing soap into 3 gallons of water. Pour this solution over a strip of turfgrass 1 foot wide by 6 feet long. Watch for larvae to come to the surface within 2 to 10 minutes after applying the drench.
Preventing Pest Injury to Turfgrass: Home lawns, recreational turf and golf course roughs can be managed to avoid insect injury without insecticides by using proper fertility, mowing at a height of 3.0 to 4.0 inches, and watering during dry periods. This will build a dense stand of turf and a diverse community of insects that will keep turf pests under control. All lawns in Michigan will typically start off lush and green in the spring and will go dormant and turn brown during periods of little rain. Information on the use of insecticides to treat turf pest problems is only meant to be used for the unusual cases where turf damage from pests becomes unacceptable. Even then, insecticides should only be used in areas of a lawn with severe turf damage, and only until a dense turf is re-established.
Using insecticides: Use insecticides judiciously, temporarily, and only in response to unacceptable turf injury. Many insecticides are poisonous if ingested, and therefore must be stored in places where they cannot be found by children. Use the rate recommended on the label for the pest you are attempting to control. Wear rubber gloves, safety glasses (or goggles), rubber boots, long pants and long sleeved shirts when mixing and applying insecticides. The most caution is needed when mixing a concentrated insecticide in water, because skin contact with the concentrated insecticide in the original container is more dangerous than contact with the diluted insecticide after mixing. Water the turf immediately after applying the insecticide to wash it down to the base of the plants or into the soil and allow the turf to dry before allowing access by people and pets.
Chemical Control of Webworm: Synthetic pyrethroid insecticides such as deltamethrin, permethrin, cyfluthrin, bifenthrin and lambda-cyhalothrin will work very well. Safe alternative products include Bt (Bacillus thuringiensis) and insect parasitic nematodes.