Lawn grubs are a major pest.
These larval forms of various destructive beetles-including June bugs, Japanese beetles (photo below), scarab beetles, and chafers-chew on the roots of grasses, weakening the turf so that it browns and dies.
If you see brown patches of dead grass, or notice lawn damage by birds, armadillos, raccoons, skunks, or moles (all of which feed on grubs), your lawn probably has grubs.
It's hard to prevent them, because adult beetles fly into grassy areas to lay their eggs. If you check your lawn and find just a few grubs per square foot, you're probably okay. More than five, however, and you'd better apply treatments.
Biological treatments include:
Beneficial nematodes, which attack grubs in the soil.
Neem oil, which disrupts beetles' ability to grow and reproduce.
Milky spore. It takes several seasons for the spores to build up to a point of effectiveness, but thereafter you should have these grubs under control for a long time.
Check with your county extension agency to find the best time to apply treatments in your area.