Japanese beetles are a short term problem, a few weeks at the most (in the beetle stage)... not worth chemical control. My short term solution is to hand pick. I use a wide mouth plastic jar with rubbing alcohol in it. That way they die quickly and don't stink.
They have very distinct escape strategies. They usually roll onto their back and slide down hill with the gravity or fly up and slightly to the side. Place jar beneath and a hand (ready to swat down) above. They typically don't move until disturbed so plan your best angle of approach carefully before disturbing any foliage. Get the ones on the perimeter first then go for the ones in the interior that require more movement of foliage to reach.
They like to aggregate in groups of 2 to many more than 2....mating and herd protection perhaps? Their smell attracts buddies.
Do this routine as often as you can manage. It not only directly reduces their numbers, it also prevents them calling their friends into your lovely buffet via pheromones. I prefer early morning, dusk and midday patrols.