From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156715-911238
Question: How do I control the Japanese beetles that are eating all my
roses?
Answer: Japanese beetles are a difficult pest to control. Your best
bet is a two-prong approach: one to deal with the larvae, one to deal
with the adults. Japanese beetle larvae are the white, C-shaped grubs
you find in the soil. They are best controlled by spraying beneficial
nematodes on the lawn and garden area. The microscopic, worm-like
nematodes harm only the grubs in the soil and not plants, animals or
humans. Spray them in spring when the temerpatures are above 55F and
you should see a difference this summer. For adult beetles, try
handpicking combined with a neem oil spray. Avoid using Japanese
beetle traps. In some cases, it appears that they actually draw
beetles to them from the surrounding area -- and you don't need to be
attracting extra beetles!