Symptoms: Irregular dead spots caused by small gray or tan caterpillars feeding on grass blades. You may also see zigzagging moths at night. Confirm the presence of webworms by soaking a small area with soapy water (two tablespoons of dish soap in a gallon of water). They'll come to the surface in about 10 minutes.
Solution: Aerate to reduce thatch. Treat with appropriately labeled insecticide. Bacillus thuringiensis (Bt) is an effective organic control.