You are right. Japanese beetles originate as grubs, so a first line of defense would be to treat your lawn for grubs each year if infestation is present. Second, do not use so-called Japanese beetle traps. they consist of a pheromone designed to attract the beetle to the trap, and studies have shown they simply attract more of them to your area. I believe they only way to control them is to go over your target plants daily to inspect them for the beetles. Don't simply shake the plant to get them away. They'll just come back later. If you find them, simply squish them between your fingers. If you can't bring yourself to do that, just drop them in a cup of soapy water which will kill them.