Masked chafer adults have essentially finished their flight for this year. As adults they do not feed, so they mate and lay their eggs within the 2 weeks or so that they live in the adult stage. Japanese beetles are still numerous, but their numbers seem to already be dropping off. Many of their eggs will have already been laid into turf.
With much of the egg laying complete, the question arises as to how many white grubs are likely. Both species are attracted to soft, moist soil to lay their eggs, and probably the Japanese beetles are more attracted to these areas than the masked chafers. When flight started in late June and early July, soil moisture was high, so initial egg-laying should have been widespread. However, in east-central Illinois and much of the rest of the state, we have had very little rainfall in the first 2 weeks of July. Nonirrigated turf is starting to go dormant, and turf watering is becoming more noticeable. Under these conditions, the beetles will concentrate their egg-laying in irrigated turf, and these areas will have damage if untreated. Nonirri-gated turf will probably have enough grubs to attract skunks, raccoons, and birds, and some isolated spots will likely have enough to cause direct turf injury.
Applications of imidicloprid (Merit) or halofeno-zide (Mach 2, Grubex) at this time to irrigated turf should be effective in preventing grub injury. Because both products take about 3 weeks to kill the grubs, applications should be completed by early August to avoid damage later in the month. You can take a wait-and-see option for unwatered turf by scouting for grubs in the first half of August and spot treating infestations of 10 or more grubs per square foot with trichlorfon (Dylox) or diazinon. If wildlife populations are high, even nonirrigated turf can be treated preventatively to avoid skunk, raccoon, and bird injury. Applications of any grub insecticide are more effective if watered in with at least 1/2 inch of water.