White grubs are the larvae of the May beetle or June beetle. The beetles lay their eggs in wide areas of the lawn in the summer, and the larvae will start feeding in the late summer and fall. If a large number of beetles have been in the area during the summer, it is a good idea to look for signs of grubs in the late summer. The beetle sighting doesn't mean grubs will appear because beetles can fly and may have found a better spot to lay their eggs in some other area.
Often, the unexpected presence of skunks and raccoons are the first indication of a white grub infestation, according to Colorado State University Extension. Signs of the nighttime predators digging up the grass when animal pests have never been a problem before can be a good sign that they have been lured into the area by the tempting chance to dine on white grubs. Mole tunnels and holes are not necessarily an indication of grubs, but if they suddenly appear in conjunction with other signs, the moles may also have been attracted by the grubs.
There are many reasons for patches of the lawn to become stressed and die that have nothing to do with grubs. If other signs of the insect pest are present, dead spots may be another indication of a problem. The larvae of grubs will feed on the tender roots of grass. As the grass loses its source of nutrients, uneven patches of the lawn will wilt and turn brown. Cut a strip of sod and peel it back to see if grubs are the cause of dying areas. According to the University of Illinois Extension, a concentration of 10 grubs per square foot of lawn can cause the grass to start dying.
Sometimes finding evidence of grubs in the lawn is as simple as turning over a rock. Grubs can shelter under shade giving rocks, garden debris, boards and other material lying around the yard, and are easily expose when the shelter is exposed.