I like to keep things organic when I can, but I didn't get my J-beetle problem under control until I started using grub treatment on my lawn. After I got that added into my lawn program my Japanese beetle problem disappeared the next season.
Neem oil works for the ones you have now, but you need to apply it often (1-2 times a week, plus after every rain). You should see a reduction in damage, but it's certainly not a total cure.