These beautifully iridescent beetles were accidentally introduced into this country in 1916. In 1933 government entomologists isolated a bacteria called Milky Spore Disease that produces a fatal blood disease in this grub. It brings about an abnormal white coloring in the infected insects.
Beneficial nematodes will also help to control them.
Online source of Milky Spore. Check with your local feed stores and plant nurseries for Beneficial Nematodes. They should be sold from a refrigerated storage area.
Chuck Martin: Milky spore disease once heralded as cure-all for Japanese beetles - observations from an entomologist (and tips to be gleaned from his remarks). For example, apply the Milky Spore early - once you see beetles, it won't work. It's meant to treat the grubs only. Much the same as with beneficial nematodes.
Scroll down for instructions on how and when to apply Milky Spore.