The following is a good article on grub control in turf from Rutgers University.
Soon they will be flying again, the adults of those pesky white grubs, like oriental beetle, Japanese beetle, masked chafers, Asiatic garden beetle, European chafer, and green June beetle. Adult oriental beetles typically peak in abundance from late-June into early July, followed by Japanese beetles about mid-July. Peak adult activity means peak egg-laying which is followed 2-3 weeks later by peak hatching off the eggs.
The young first stage larvae are the stage that is the most susceptible to insecticides, and numbers of this stage peak from mid-July to early August. If an applicator chooses to preventively treat turf for white grubs, the ideal time to do an application would be somewhat earlier than the time of peak first larval stage density to allow the insecticidal active ingredient to reach peak concentration in the target zone, i.e., the thatch and top 1-2 inches of the soil. How much earlier depends on how fast the active ingredient will move through the thatch. Generally, the more water soluble an active ingredient is, the faster it will move downwards into the soil with rain and irrigation water.
However, the application window of most of the newer white grub insecticides is rather wide and many of them can be applied from May into August with little differences in control rates. Reasons for these wide windows are that (1) the active ingredient has a long half-life in the soil allowing earlier applications, (2) the second larval stage of the white grubs is generally still very susceptible to the active ingredient allowing later applications, and/or (3) the recommended application rates are high enough to compensate for 1-3 months of slow active ingredient breakdown in the soil or the lower susceptibility of the larger white grubs.
Imidacloprid (Merit) and thiamethoxam (Meridian) seem to be very effective when applied from May through August, clothianidin (Arena) even from May into September, halofenozide (Mach2) from June through August, and chlorantraniliprole (Acelepryn) from April at least into mid-August.
An interesting new finding is that chlorantraniliprole (Acelepryn) actually tends to be more effective when applied earlier. Due to its very low water solubility (> 5 ppm) chlorantraniliprole takes around 50 days to reach peak concentration in the thatch and 100 days to reach peak concentration in the soil. However, chlorantraniliprole clearly is effective enough to compensate for this slow movement. Averaged over dozens of research trials for each application timing, chlorantraniliprole at 0.1 lb active ingredient per acre applied in April, May, June, July, and August has provided 96%, 94%, 91%, 88%, and 79% control, respectively, of all the major white grub species. However, even the 79% control in August is at least as good as what one can expect with an application of trichlorfon (Dylox) and certainly good enough to suppress white grub damage.
As always, keep in mind that repeated application of the same insecticide class year after year could lead to insecticide resistance and/or enhanced microbial degradation of the compound. Enhanced microbial degradation would be particularly problematic if the compound is applied several weeks to months before the presence of the target (i.e., before July). Therefore regularly rotate insecticides from different insecticide classes. Earlier applications are also more likely to interfere with naturally occurring predators and parasites of white grubs and other turfgrass insect pests. Applications before July or even June should therefore be reserved for cases where there is some additional benefit such as suppression of other insect pests with damage potential.
Reprinted from "Timing of White Grub Treatments in Turfgrass" by Albrecht M. Koppenhöfer, Ph.D., Specialist in Turfgrass Entomology in the June 11, 2009 edition of the Plant and Pest Advisory, Landscape, Nursery, and Turf Edition from Rutgers University http://njaes.rutgers.edu/pubs/plantandpestadvisory/2009/ln061109.pdf