I'm having some issues with the beetles as well, I'm using Sevin 5% dust on them, very effective at killing them, but must be applied dry, and needs 48 dry hours to work well.
For the long term, which you probably should have done in your first year, is spray
milkey spore bacterium which is a Japanese beetle specific biological control. It not only lasts for years, it actually spreads with time. I'll spray all of my open ground, and maybe 100 ft. or so on the neighboring pasture to get them a ways out. Over the next three years the beetles should virtually disappear.
There are chemical controls that are faster acting, but they require a licensed applicator do the spraying, and that's not cheap.
See also:
http://www.pueblo.gsa.gov/cic_text/housing/japanese-beetle/jbeetle.html