White grubs are the larval stage of June or May beetles. Grubs mature during a period of 2 to 3 years. White grubs live under the surface of the soil. As the larvae mature, they feed on the roots of the grass, killing large sections slowly when their number grows large. If there are more than two or three grubs in a 6- by 6-inch sample of grass, treatment is needed.
According to the University of Kentucky, excessive moisture in June and July when beetles lay their eggs encourage the growth of white grubs. Cutting back on irrigation slightly will curb the growth of white grubs. Heavy fertilizing in the spring and summer will stress grass and will make grub damage more prominent in the fall. If a lawn is heavily damaged by grubs, renovating the lawn with Kentucky bluegrass, perennial ryegrasses and tall fescue may fill in stressed areas and are more tolerant to grub damage.
Apply preventative insecticide before grub growth becomes a problem. Insecticides used for prevention are flexible in their application time periods and require less time sampling grub populations in the lawn. Some preventative treatments allow application two to three months before grubs hatch. The main drawback of preventative care is that application is required before the extent of grub damage is known. Only a small percentage of lawns actually require an insecticide treatment.
Apply curative Insecticides when grub damage has appeared and an infestation is known. Fall treatments prevent the largest amount of damage done by grubs in the spring, says the University of Rhode Island. Grubs are weaker in the fall as their shell is not as thick and is easier to penetrate with pesticides. Recommended treatment time is between Aug. 1 and Sept. 15.
Use nematodes, or roundworms, for the control of grubs. Nematode strains such as Heterorhabditis bacteriophora applied at 1 billion per acre prey on the grub as food, killing them off before they do damage. The soil must be at a median temperature of 70 degrees F for application, and the nematodes require watering into the soil with 1/4 inch of water.