June is the month where you should start thinking about white grubs.
Most white grubs have similar life cycles. The adults are generally present for about a month in the summer. This is quite obvious in the case of Japanese beetles which can be seen feeding on various ornamental plants during June and July in North Carolina depending upon your location. Others, such as masked chafer are less obvious. When the beetles are observed, it provides us a bit of an early warning that they may be laying eggs in nearby turf which can be subsequently infested with white grubs (the larvae of these beetles). The larvae feed the rest of the summer and early fall, go deep in the soil to overwinter, feed awhile in the spring and then complete development into the adult beetles in the late spring. A grub infestation can only be determined by digging down into the upper 4 inches of soil.
Some of the older products had a fairly large window of application. In others words you could apply them in June through September and get good control. With some of the newer products such as Merit and Mach2 the timing window is a little more narrow and best results are obtained when the products are applied during egg laying and egg hatch in June in July. The only real curative product we have on the market today is Dylox which acts very quickly even on larger grubs later in the summer. Knowing the life cycle of the pest and how to best use each product is very important. While a product like Dylox is quick acting and can work against larger grubs late in the season, it provides little residual control.
Control of white grubs is difficult and should only be a last resort in situations of severe damage.
For more information on grubs go to the following link: http://www.turffiles.ncsu.edu/insects/white-grubs