Sevin is about the most effective quick kill insecticide for the Japanese Beetles. All the garden centers carry the 5% dust. Wear a dust mask when applying it. It's not that toxic, but you don't want to inhale the fumes. Wash or shower thoroughly after done and wash your longsleeved clothing separate from other clothes.
There are many other things you can use, but two ot three dustings of this should get you through the season, and it's cheap.