Article: White Grubs in Lawns

Created on: Monday, June 09, 2003

White grubs are among the most destructive soil insects in New England, causing serious damage to home lawns and golf courses by feeding on the roots of grass plants. Often grubs weaken the turf so much that the sod can be rolled up like a carpet. Sometimes raccoons or skunks will tear turf to feed on the grubs. This tearing of the lawn is often more damaging than the feeding of the grubs themselves. Grubs are the larval stage of several species of scarab beetle, such as the Japanese beetle and June bug. Grubs are cream-colored or white, and usually curled into a distinctive C-shape. When feeding actively, grubs will be found anywhere from the thatch to about 4 inches into the soil. They range from 1/4 to 1 inch long. As cool weather approaches in September and October, the grubs migrate downward throughout the soil, remaining below the frost line. Grubs hibernate for the cold winter months and as the soil temperature rises again in the spring, they move upward again until they reach the root zone and resume feeding. Grubs feed for 4 to 6 weeks in the spring. They then go into a "resting" stage, during which many changes take place internally. About a week later, the adult beetle emerges. These beetles mate and repeat the cycle. About 10 months of the year are spent in the grub stage, so these insects can weaken or damage turf, particularly in May and June and again in September and October. Biological Control There is a bacterial disease (called "milky spore disease") which attacks Japanese beetle grubs. Spores of this organism are available commercially and can be applied directly to the lawn. The spores are ingested by the grubs and interfere with the digestive system, eventually killing the grubs. Milky spore disease has many advantages, being virtually non-toxic to people and pets and with long-term control when it works. The disadvantages are that it takes two or more years to get established and usually some grubs survive each year. Chemical Control Grub populations can be reduced with beneficial insecticides, such as Merit®, which with one application gives season-long control. Contact insecticides with dylox, dursban, or diazanon control the insects while they are below the ground, or on the surface. These insecticides should be applied in early June and again in mid-August, when the grubs are most susceptible to chemical control. Any grub insecticide application should always be watered in very thoroughly, with AT LEAST a half-inch of water. This water will drive the insecticide through the thatch and into the soil, where it can come into contact with the grubs.