I saw my first Japanese beetles of the season a few days ago. They were happily feeding on my hibiscus flowers, but there were only two so I pulled them off and squashed them. The next day I found two more on the hibiscus, plus another one on a canna flower. Repeat catch-and-squash. Today I came home to find one on the hibiscus, four on various cannas, and, worst of all, three on my Abyssinian banana plant.
Japanese beetle feeding damage on an Abyssinian banana leaf
It's war.
I broke out the sudsy water and started drowning beetles. I should have done this from the start, as drowning them will remove the congregation pheromone produced by Japanese beetles (I also scooped up the remains of two of the squashed ones I could find and dumped them in the water). I'm probably going to start applying an insecticide to the banana plant. Generally, I try to avoid using insecticides on flowering plants out of respect for the pollinators and minimally-damaging insects, but the Abyssinian banana isn't going to bloom which makes treating it low risk for most other insects.
Japanese beetle grubs overwinter (and feed!) in turf, so in fall I'll start scouting for damage in my lawn (though my lawn is a mixture of clover, violets, dandelions, creeping charlie, mock strawberries, and the occasional patch of grass, so it's probably not the most enticing place for grubs to dine).
For more information about Japanese beetles, including how to scout your lawn for the grubs and control methods for both larva and adults, read the factsheet from the University of Illinois Extension here.