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.