Lawns that have grubs will show signs such as holes, but the holes are not made by the grubs. They are made by predators searching for the grubs for food. Predators dig holes 1/2 to 1 inch in diameter and 1 to 3 inches deep to find grubs.
Grubs are the larvae of a beetle that lays its eggs in the moist grass in June or July. When the eggs hatch into grub larvae, the grub is less than 1/2 inch long. The C-shaped white grub burrows under the soil to a depth of 1 to 3 inches. These minor holes fill back up quickly with lawn watering or rain.
White grubs will live just under the surface of the lawn, feeding on the grass roots. As the grubs feed, their growth continues until the grub reaches a mature length of up to 2 inches depending on the type of beetle that laid the eggs. Grubs feed on the roots until the soil temperature starts dropping in the fall. The grubs burrow deeper into the soil for the winter months.
Nocturnal predators will search for grubs in the soil during August and September, when the grub is still close to the soil surface. Predators that will dig perfectly round holes in the soil are raccoons, skunks and moles. You won’t see just one hole; you will see many holes throughout the lawn. Wherever you see such a hole, a grub was living in the soil.
Successful treatment of a grub infestation will stop predators from digging holes in your lawn.
Treatment for the most part should be done in the fall. An exception is halofenozide, a hormone mimic used for controlling Japanese beetles, which is applied when the mature beetles are active in June and July. Milky spore disease, a bacteria that eats the Japanese beetle grub from the inside, is an organic method of treating a Japanese beetle white grub infestation. As the grubs eat the roots and the milky spore disease, they die. Once the grub dies, the larva explodes, releasing more milky spore disease, so there is a continuous cycle to kill all grubs. Milky spore disease lasts in the ground for up to 20 years and is not harmful to pets and humans. Insecticides used in the fall to control grub infestations include trichlorfon or carbaryl (Sevin) applied as directed on the package.