We have had several reports from turfgrass professionals about large white grubs being found in turf. May beetles, Japanese beetle, and masked chafers all overwinter deeper in the soil as larvae. These white grubs migrate upward when the temperature in the turfgrass root zone rises to 50░F. Japanese beetle and masked chafer grubs pupate in early to mid-June, so their feeding will cease soon. True white grub larvae in their second year will continue to feed through the growing season.
These large white grubs are difficult to control with typical white grub insecticides such as imidacloprid (Merit), halofenozide (Mach 2), and trichlorfon (Dylox). Of the three, trichlorfon is likely to provide the best control; but 30% or less control is typical rather than the 95% control that these three insecticides typically provide on young grubs in late summer. Heterorhabditis bacteriophora and other cruising nematodes should provide about 60% control on these older grubs. The nematodes need to applied within a couple of days of arrival, applied in late afternoon to wet turf, and watered into the turf before the turf dries.
Generally, the number of overwintered Japanese beetle or masked chafer white grubs is below or just above the 10 to 12 grubs per foot square typically necessary to cause turf damage. If the grub numbers are marginal for grub damage, generally the best option is to increase irrigation to keep the turf growing roots faster than the grubs can eat them for the few weeks before these grubs pupateŚrather than making an insecticide application.