Japanese beetle numbers continue to climb as adults emerge from the soil over the extended period of late June to mid-July. As homeowners face increasing plant defoliation as described last week a common query is to ask about "treating the ground now to prevent beetles from coming back next year."
The question is logical but unfortunately, the world, or at least the world of insects, does not work this way. Your lawn may not be capable of raising all the beetles you find on your trees, shrubs, flowers, and vines. Japanese beetles adults are very strong and capable fliers and may travel long distances from where they developed as larvae in fence rows, ditch banks and other grass sites, to where they are feeding. You are likely to have beetles next summer whether you treat the grub stage in the soil or not. The Japanese beetle adults are capable of flying in from other areas. Controlling one life stage will not preclude potential problems with the other.
This does not mean you should not treat the turfgrass. Certainly, high value turf should be protected from the root-feeding larvae, if desired, by treating the soil with a preventive insecticide such as Merit, MACH2, Arena, Acelepryn or Meridian in a timely fashion and according to label directions. Most of these products must be applied before mid-August to be effective as preventive treatments. Treating for grubs will protect the turfgrass. It will not prevent beetles from feeding in your landscape the following year.