White grubs are the larvae of beetles, most often those of the Japanese beetle that commonly infests zoysia grass, a favorite turf grass in warmer climates of the U.S. Japanese beetles lay their eggs in July; by late July the eggs hatch and emerging white grubs feed on the roots of zoysia grass for two months. Several insecticides have recently been taken off the market, but there are several treatments to suppress or kill the grubs. These are ordinarily applied in early August; preventive insecticides are applied earlier.
The two main biological treatments for white grubs are bacteria; these are used by organic gardeners and others concerned about possible adverse effects of chemical pesticides.
Bacillus thringensis is a bacterium that occurs naturally in the soil and on the dark surface of plants and is found in the guts of caterpillars of certain butterflies and moths. It is commonly called Bt and is sold on the Internet and at many garden supply centers. Instructions for using Bt are on the commercial label.
The USDA developed milky spore, a soil-dwelling bacterium (Paenibacillus popilliae, formerly Bacillus popilliae) to kill Japanese beetles in the 1930s. White grubs die in one to three weeks after eating milky spore; each dead grub releases billions of new spores as it decomposes. Ten pounds per acre are applied in a grid. Milky spore will completely eliminate white grubs in two to three years in warm climates and three to five years in cold climates. Once established, milky spore will control white grubs for 15 to 20 years.
Nematodes, another choice for the organic gardener, are microscopic round worms that eat white grubs. Nematodes labeled for control of white grubs may be bought in garden supply centers in packages that may contain up to 50 million nematodes. Nematodes will die if the temperature is not at least 60 degrees F. They are applied by a sprayer at the end of a hose. The zoysia grass needs to be thoroughly soaked to get the nematodes to the grubs.
Preventive Chemical Treatment
Horticulturalists at the University of Connecticut recommend preventive applications of pesticides containing the active ingredients clothianidin, halofenozide or imidacloprid before the beetle eggs hatch, roughly from June 15 to July 15, the earlier the better.
Curative Chemical Treatment
University of Connecticut researchers recommend curing white grubs from zoysia grass with an insecticide containing the active ingredient trichlorfon. The grass should be watered heavily before application to drive the grubs to the surface. After application, the lawn should be watered heavily once more to deliver the toxin to the grubs.