"Handpicking Japanese beetles can be an effective control on smaller plants such as roses. But on a tree, you would need to spray with a general insecticide such as Sevin. It would be most effective to spray the trees as soon as you see the beetles. Once they start feeding, they attract additional beetles.
Now that the trees have been defoliated, the best thing to do is make certain that the trees suffer no additional stress this year. That means giving them supplemental water during dry periods the rest of the growing season. It is quite likely that they will push out additional growth yet this summer. Withhold fertilizer. Fertilizing in late summer can push growth late into fall and promote winter injury."