Watch for large numbers of starlings, cowbirds, red-winged blackbirds, robins, and other insectivorous birds feeding on turf for several days straight. They may be feeding on sod webworm larvae. Early webworm damage appears as indistinct brown areas in well-drained, sunny locations. Close examination reveals that many of the grass blades are gone, allowing the thatch to show. Frequently, there will be small, roundish balls of green webworm feces at the base of the turf.

You can use a disclosing agent to reveal the larvae as well. Mix 1 teaspoon of 5% pyrethrin or 1 tablespoon of dishwashing detergent in a gallon of water. Apply the mixture with a watering can or other application device to 1 square foot of turf. The slender sod webworm caterpillars with brown spots should come out onto the turf surface within 2 to 3 minutes of the application. Full grown larvae are about 1 inch long. Two or more larvae per square foot are enough to cause damage.

Once identified, the larvae are easily controlled with spinosad (Conserve), carbaryl (Sevin), diazinon, and many other insecticides. Apply as a spray and allow to dry on the foliage. If a granular formulation is used, apply less than 1/4 inch of irrigation to activate the insecticide but not wash it down into the root zone.