Grubs are larvae that will grow into beetles. They often grow beneath your lawn, and can cause rings of dead grass to form. When the grubs mature, the adult insects can then move on to eat and kill your plants.
Grubs commonly found in the United States are typically either Japanese beetles or Oriental beetles, although other species have been discovered in small patches. Grubs are typically white, and are easy to spot as they lie in the soil in a C-shape. Gardeners may notice that they have a grub infestation when the lawn begins to die back in large patches. Gardeners can pull back the dead grass in a roll with one big pull, as the grubs typically eat the roots of the grass while leaving the tall leaves alone. Once the grass is pulled away, the C-shaped grubs are easy to spot. Gardeners may also notice that other pests such as raccoons and skunks are frequenting their yards at night, digging up the lawn to eat to the tender grubs.
The best way to deal with a grub infestation is to count the number of grubs you find in a 1-foot-square patch and take one grub sample to your local extension office. The experts there can identify what sort of grub is infecting your yard and advise you on what sort of pesticide to use to rid your yard of the pests. Insecticides must be applied in the fall, when the grub eggs are being laid. There are pesticides available to help you control adult pest populations before they're able to lay more grub eggs in your lawn. Your extension office can advise you on this as well. The only true way to prevent a grub infestation is to let your lawn die back completely in the summer. Beetles won't lay grub eggs in a brown lawn.