White grubs are the larvae of various species of scarab beetles, and cause lawns to turn brown by feeding on the grass roots. The larvae of the Japanese beetle (Opillia japonica) and the northern masked chafer beetle (Cyclocephala borealis) do the most damage to lawns in colder areas of the United States and Canada. The larvae of the southern masked chafer beetle (Cyclocephala lurida) are often found in warmer climates. Other scarab beetles may also produce larvae that eat grass roots.
The details of white grubs vary with the species of beetle, but in general, they are less than an inch long and are white or dirty white in color with soft bodies and six legs. When they are exposed they lie in the shape of a C.
Adult beetles of most species lay their eggs in the ground during the summer; when the grubs hatch they begin feeding on grass roots until the autumn cold makes them dig two to eight inches deeper to spend the winter. When the weather warms in the spring, they move back up and resume feeding on the roots. They form pupae from May through early-June, and emerge as night-flying adults in mid-June through mid-July.
Checking for White Grubs
The best time to check for white grubs is in the late summer when they are active. Grubs are counted in a 6-inch by 6-inch square of lawn cut 3 inches deep. The sod is then replaced and watered. Treatment is needed if there are more than two to three grubs in the sample. A dozen or more samples should be taken throughout a lawn, depending on its size.
Horticulturalists at the University of Rhode Island recommend contacting your local agricultural extension office to determine the identity of the beetle that is producing your grubs. This will help you choose the best insecticide to use.
The grubs are mature in the spring and difficult to kill. Insecticides containing the active ingredients imidacloprid or halofenoxide should be applied between the first of April and the middle of August before beetles lay their eggs.
The best time to apply insecticides is in the late summer when the grubs are young and active. Insecticides including the active ingredient trichlorfon or carbaryl act quickly and effectively. Those containing trichlorfon degrade rapidly in soil or water with a high pH. If the pH is 9, half of the active ingredient will be lost in half an hour.
The ground is watered with 1 inch of water before application, followed by another ½ inch after application.
Two species of parasitic nematodes, Steinernema spp and Heterorhabditis spp are marketed to suppress white grubs. Nematodes are small, unsegmented round worms. Horticulturalists at Penn State University say Heterorhabditis works better; horticulturalists at the University of Rhode Island say studies of the effectiveness of nematodes to suppress white grubs are inconclusive.
Rhode Island horticulturalists say data are also mixed on the effects of milky spore disease, a bacterium used to control the grubs of the Japanese beetle, but not other beetles. Infected grubs release more bacteria, taking from three to five years to eliminate white grubs in an area with a cool climate. This is not a solution for lawns infected heavily by white grubs.