The best method I've found is to just go out and knock them off by hand. I carry a ziplock bag and knock them into that then throw it in the trash. There are nematodes you can apply to the soil to kill the grubs which may help reduce the population. Sprays aren't effective because you have to actually spray the beetle (they're tuff little buggers) and then the residue kills helpful insects. Since you have to be there to spray I figure why bother, just catch them and dispose of them.

People have been trying to kill off these wee buggers since the mid 1950's. I can remember the research people from Michigan State College asking Mom if it was okay to put one of those beetle traps in the yard and afterwards haveing the beetles where we had not seen them before that trap was placed. Stores sell you all kinds of stuff to "control" them including poisons to kill the grubs that become the adult beetles, usually without telling you that those poisons will also kill off your earthworms.
Do not put any Japanese Beetle traps in your garden.
Knock as many as you can into a bucket of soapy water, as often as you can.
If they are too big a problem to hand control start with the least toxic means of control you can, usually a pyrethrin based dust or spray. The broad spectrum poisons, Carbaryl, Malathion, etc. have been found in the honey in the bee hives and is strongly suspected in making the bees more susceptible to what ever is killing them off, and if you use any honey at all you are eating more of that poison yourself.
There is no one single means of control, except to look carefully at your soil to be sure it is good and healthy so it will grow strong and healthy plants that are less susceptible to insect pest attacks.

My mother cannot stand to even see these beetles. So, we gave up all the flowers they (and we) love, well not totally. We just grow a few they love mixed in with those they don't care for. We've discovered poppies, mums, begonias, lillies and yarrow to name a few. We mix in the geraniums, but only dark-colored blooms. The beetles seem to perfer light colored blooms. We stay away from roses, hibiscus, dahlias, petunias and impatiens. We are experimenting with a dark flowered hollyhock this year, just two plants mixed in with the lillies and mums. The mums have been fantasic, so glad we found them. We garden in whiskey barrels and large pots on a deck.

I envy those west of the Mississippi. If I ever have to relocate, it will be westward, not eastward!

One of things I would suggest is to figure out what they really like and then get a large glass jar..like the glass jars that restaurants purchase pickles in or even a plastic huge mayo jar...put what they love inside of it and puncture holes in the lid..they can get in but can't get out...I have never tried this becuase we don't really have the problem this year..the winter was too hard and cold for them. Cold kills them if they don't find a place to hybernate. On a whole different note...I use the same method to get rid of flys....except put fish guts in the jar about a quarter full...after a few days the whole jar will be black with flies..it smells sort nasty so put them on the outskirts of your yard. It doens't reek but it is certainly not pleasant.

Guinea fowl will eat them along with any ticks you might have in your yard. The pheramone traps will bring them in from far and wide and only catch a portion of them, so if you have low densities it will help reduce damage and the next years population, if you have a lot you will get more damage but still reduce the next years population.

I am intrigued by the giant jar idea. I just might try it. We tried cannas this year, the dang beetles love them. I had bought one of those insect vacuums, go ahead and laugh. The dang beetles hang on pretty good so the vac didn't work. I thought they were supposed to fold up their legs and fall off the plant. Oh well, I don't recommend the bug vac. I have resorted back to using the jar with soapy water. I'm wondering what to use in the giant jar. I remember hibiscus and dahlias being a big favorite of theirs, geraniums too when we had a mass of them.

Now I think I have a katydid problem with my mums!!!! Can't beat these things. I refuse to use poison in my backyard. I feel so bad about the bee population. I'm leaving all the clover in my lawn. Soon I will have all clover, but that's OK because the bees love it. (Now I'm babbling.)

If you have the time, by all means pick em off. I can think of other things I'd rather spend my time on. Traps will attract beetles. I use one to simply see when they are out and when they are gone or in reduced numbers. IPM is all about finding alternative controls to chemicals, but nowhere does IPM say don't use chemicals.