Question:
Will treating my lawn for white grubs reduce the population of Japanese beetles on my plants next summer?Answer:
It would be nice if life could be that simple, but the relatively small area you can treat (compared to the grassy sites in the surrounding area) will not have any impact on the following year’s adult population. Japanese beetle adults are very strong and capable fliers and may travel long distances from where they developed as larvae in fence rows, roadside ditches and other grass sites, to where they are feeding. You are likely to have beetles next summer whether you treat the grub stage in your lawn or not. Controlling one life stage does not preclude potential problems with the other.
This does not mean you should not treat the turfgrass. If desired, high value turf can be protected from the root-feeding larvae by treating the soil with a preventive insecticide in a timely fashion and according to label directions. Most of the available insecticides 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.