There are not many good options for controlling Japanese Beetle, Popillia japonica Newman, in your situation. In MN, the beetles usually first arrive around July 4th.  Next year, pick off and destroy the first scouts so they cannot attract additional  adult Japanese beetle. The adults are less active in the early morning or late evening. Dropping them into a container of soapy water can kill them.
      Japanese Beetles are not controlled with milky spore disease (bacteria Bacillus popilliae) in MN.  The trade names for bacteria Bacillus popilliae products are Japidemic Doom and Milky Spore. Recent trials have not reduced Japanese beetle grub numbers in turf.
       If you are finding many adults on your plants it does not mean that they will lay eggs on your grass. They prefer irrigated lawns.        
     Insect parasitic nematodes are commercially available. Products that contain strains of Steinernema carpocapsae (Biosafe, Biovector, Exhibit, Scanmask) have been marginally effective against white grubs in turf. Preparations containing Heterorhabditis spp. seem to be more effective. Apply the nematodes when the white grubs are small. Irrigate before and after applying the nematodes.
 The MN paper below explains how to identify the different types of grubs that are commonly found in MN.  Before applying an insecticide for grubs, make sure you have a large infestation of Japanese beetle grubs. Look for areas of brown turf and search in adjacent green areas for grubs and pupae.  Irrigating after applying an insecticide improves its insect control. Japanese beetle grubs are best controlled when they are small and actively feeding near the soil surface, usually late July to mid-August.   http://www1.extension.umn.edu/garden/insects/find/japanese-beetles/ http://ohioline.osu.edu/hyg-fact/2000/2001.html  

We live in the country and have several yard lights that are on all night.  Earlier in the season, the ground under the lights was littered with TONS of June Bugs (May Beetles).  Is it possible that the grubs are their larvae?  I haven't seen many Japanese Beetles this season in my gardens.  Thanks for your expertise.

      Many insects are attracted to outdoor lights. If you turn off or change your exterior lights you will reduce the number of insects attracted to your home. Close your blinds and curtains.  Reduce lighting around doors, windows and other potential entry sites. For exterior lighting, use sodium vapor yellow lights.  Yellow lights attract fewer insects compared to white, neon, halide, or mercury vapor lights.  The grubs in your lawn may be might be the larvae of May / June beetles.  White grubs are identified by their raster pattern. Grub ID Pictures of white grubs are found at the following site. ohioline.osu.edu/hyg-fact/2000/pdf/2510.pdf‎

Thank you again for your expertise.  I am considering using nematodes to help keep the grub population under control.  Will the nematodes survive our winters or will I need to reapply them annually?  I need to check the PDF to determine the species I'm dealing with.  (I only check email once a week or so.)

 Most insect parasitic nematodes packaged for sale have a shelf life of several weeks or even months if refrigerated. Effective use of nematodes for insect control depends on environmental conditions during and after application. Commercially available strains of nematodes originated from the southern United States or other mild climates. Those that reproduce can produce multiple cycles of infection during the growing season.  It is likely that extremes of cold, drying or high summer temperatures will kill off nematodes.  The following site has the most current information on nematodes. http://www.ext.colostate.edu/pubs/insect/05573.html http://www.ext.colostate.edu/pubs/insect/05516.html