Just like with slugs and snails, the best method for success? Be P r o a c t i v e ! If the beetles are everywhere and eating everything before you take any action, then it will be an up hill battle all the way. Two things will help you tremendously:
1) Use an organic foliar spray starting in early June that the Beetles cannot digest when they attempt to eat your plants.
Photo credit: Nicholas Erwin via VisualHunt.com / CC BY-NC-ND
2) Along with #1 (or if you don’t like using any sprays) be on-your-toes aware of the first beetles to arrive. Knock them into a container of soapy water (dish detergent) to kill them.
Photo credit: stevendepolo via VisualHunt / CC BY
The mission of the Japanese Beetle is to mate and eat. They send out a powerful pheromone signal for others to follow. Getting rid of the first ones – the “scouts”, will help keep the population down.
I’m sure that you’ve noticed their populations rise and fall over and over. Like many insects’ natural life cycles, they are effected by weather and the environment. I have had years of no sightings in my garden and my friends across town have gotten eaten alive. The best defense is to be aware and prepared.