Just some other trains of thought for how i've tried to manage them
1) neem oil spray for little more natural insecticide, sprayed on the leaves they eat and is passed onto their offspring, so any eggs laid by adult will be infected and die. also not only does it affect Japanese beetles, but controls a number of other pests.
2) milky spore powder if you have large enough area. some of my orchards are semi-large, so over the years i've saturated soil w/ milky spore powder. it is ingested by the grubs and kills them and the spores are released multiplying and stays in the soil for years wrecking havoc on generation after generation. Now granted if they have another viable spot to lay their eggs nearby that isn't treated won't do much good, but if you are a beetle and you could choose this nice orchard w/ apple trees right overhead, so your kids don't have to travel far to eat or the forest floor w/ no nice grassy roots to munch on what would you choose?
3) placing beetle traps AWAY from orchard....i might put them out in the middle of the forest couple hundred feet away from orchard to try to divert beetles elsewhere. this is a catch 22 method as it works to kill them but also attracts many more beetles to your area than what would without it.