I have a problem with those bettles also and although my husband would spray some type of medicine, they still come, so I go outside during the evening and catch them and drain them in water.
Here's what I had found on the internet, that maybe you could try-
Japanese Beetle Traps: Uses patented BIOLURE® control release pheromone system to lure insects into the trap and disrupt the mating cycle.
* Japanese Beetle Traps
Control release system maximizes the useful life of the pheromone and protects thUnique formulation combining effectiveness of insecticidal soap with our popular sulfur based fungicide.
Contains a patented formula to help reduce injury to most plants
Multiple purpose formulation which controls mites, insects, and fungus
Avaliable on-www.byebyebettle.com.