Preventive treatments of white grubs to avoid turf damage should be applied during July. Eggs of Japanese beetle, northern masked chafer, and southern masked chafer are laid primarily from the last week of June through the third week of July. These eggs will be hatching from late July through early August. The insecticides used at this time of year, imidacloprid (Merit) and halofenozide (Mach 2), provide about 3 months of residual activity, so application before the eggs hatch is acceptable.
All three species of white grub tunnel down into the soil to lay their eggs. This causes them to be attracted to moist soil. There is also evidence that they may be attracted to green grass in which to lay their eggs, which is also associated with moist soil. Prior to the last couple of weeks, the dry weather appeared to have us set up for grub egg-laying to be concentrated in irrigated turf. When that occurs, the resulting high white grub populations can cause severe damage to turf. However, the state has received above-average rainfall over the past few weeks, turning all of the turf green and making it all attractive to egg-laying grub adults.
Under these higher moisture conditions, eggs are typically laid over such wide turf areas that most areas do not have enough grubs to cause injury. This allows one to save the cost of preventive grub insecticide applications. Pockets of damage-causing grub numbers will show up in August. Those areas can then be treated with trichlor-fon (Dylox) to control the grubs within 3 days. An exception to this is in northwestern Illinois. Historically, even in years with heavy late-June and early-July rainfall, the areas between and around Monmouth to Peoria have had widespread grub damage. Based on that, preventive grub treatment would still be in order for that area.
We generally recommend that insecticide applications to control white grubs be watered in with at least 1/2 inch of water. This is definitely true for Merit and Dylox. Mach 2 is more water soluble than the others, so that immediate irrigation is not essential. As long as at least 1/4 inch of rain or irrigation occurs within 3 days after application, the Mach 2 washes off the grass blades and thatch into the root zone where the grubs feed. However, research shows that Mach 2 remaining on the turf surface for more than 3 days suffers a reduction in grub-controleffectiveness, probably due to breakdown from the ultraviolet light in sunlight. Ultraviolet light radiation is intense even on cloudy days.