Liquid Seven drops japanese beetles in no time. Look for Carbaryl 4L at your local farm-n-home store.

I've had a relatively minor infestation this summer. They show up, I spray. Couple days later they're back, so I spray again.
I put a pheremone trap over the pond a couple weeks ago, but haven't seen a single bug buzzing around it. It's one that I
found under a stack of stuff in the potting shed so it may just need a fresh lure wafer installed.