I applied milky spore twice 2 years ago and once last year. I will apply it again this year. I buy the stuff that comes in a bag that can be used in a spreader. Do it when there is absolutely no wind.
It is best if neighbors do it to, but it still helps. I also apply heavily along the property line. It eventually will end up in the neighbor's lawn, because when the grubs eat this stuff, the spores multiply inside the grub. When the grub dies, it eventually bursts open from the increased spores. Then you have more spores! These eventually get to the neighbor's yard if you are patient. You can also toss a little over there (when neighbor is not looking
) Every little bit helps!
You can also educate your neighbors if the topic of Japanese beetles comes up. Sometimes they don't know what's eating their plants
If you have a place at least an acre away from your garden, traps help too. Traps placed any closer will attract them to your garden and make the situation worse. I put one in a wooded lot at the end of our road - with permission of the owner. I still have beetles, but about half as many as last year. I'm hoping that each year will improve.