Lawn grubs are the larva stage of Japanese beetles and the northern masked chafer grub. Grubs' bodies are white and soft with a brown head and six legs. Full-grown grubs are a little over 1 inch in length and are resistant to many pesticides. Control at the correct time is essential to killing grubs and preventing damage to your grass.
Most beetles and chafers that lay grubs have a 12-month life cycle, according to Penn State University. Adult beetles lay their eggs in late June or early July. The grubs hatch in August and start feeding on the roots of the grass overhead. Cold weather in the fall drives the grubs down 2 to 8 inches to protect themselves from injury due to frost. Grubs come back to the surface during the spring and feed until they enter the pupal stage. In mid-June or early July, the life cycles starts over.
Grubs destroy the grass roots in the lawn, causing browning, patches or dead and decaying grass. The presence of skunks, moles or other insect-hunting animals in the garden or lawn is another indication of grub damage. Over-fertilization and lack of irrigation of a lawn causes similar symptoms as grub activity. Pulling on the grass to inspect whether it pulls up easily due to lack of strong roots distinguishes between other types of damage.
Inspecting the infestation rate of the grass determines whether chemical control of grubs is necessary. The University of Rhode Island suggests using a spade to cut 6-inch by 6-inch samples of the lawn, going down 3 inches. Chemical control is necessary if there are more than 2 to 3 grubs per sample. Take several to determine the extent of the damage throughout the lawn. Send grubs off to your local University Extension to determine the grub variety.
Application of insect nematodes will control grubs. Two are available, according to Penn State University--Steinernema spp. and Heterorhabditis spp. Nematodes require application to the lawn according to the instructions on the label. Because nematodes are living creatures, it is important to apply them before the expiration date on the label. Grubs require moist soil to lay their eggs. Reducing irrigation during June and July to destroy existing eggs and to prevent the laying of more eggs is a cheap, effective control.
Chemical control of grubs is most effective in July and August when the grubs are young and vulnerable. Application of pesticides in the spring when the grubs rise in the soil is less effective as their skin is further developed and less receptive to poison absorption. Grub pesticides are available at many garden centers. Most require irrigation, according to the label instructions to activate. Preventive chemical controls are also available. Lawns that have a history of grub infection are the best candidates for a preventive application of pesticide. Preventive application is best applied in early June or July, a week or two before eggs hatch.