Sod webworms are the larvae of lawn moths. They live in the root level of your lawn and munch up the grass leaves. They can kill an entire lawn in a matter of days. When the weather turns hot, patches of your grass may start to turn brown. If you see little moths flying above your grass at dusk, and the brown patches start to get larger, you could have sod webworms.
Look for saucer-sized brown patches where your lawn is driest. The centers of the patches may have been eaten away and replaced by weeds. At root level, you'll see small white tubes made of silky web. At dusk, you might even see the worms themselves. They're about 1/4 - 3/4 inch long.