Japanese beetles have been terrible, voracious visitors to my garden. In years past, I've watched them destroy so many beautiful blooms that I actually considered pulling out all of my rose bushes. Two years ago I found a two pronged solution to the problem.
Catch Japanese Beetles Early
In most areas, Japanese beetles emerge at the end of June. You will see a few tentative specimens checking out your garden, and that's when you should act. Be on the look out for them, because catching them when they first emerge can be the solution to your Japanese beetle problem.
These first visitors leave scents behind that others (many others) will follow. It's like the old depression era hobo story. Hobos would leave a specific symbol on the fences of homes and farms where they were given a hot meal as a signal to other hobos that this was a good place visit. But unlike the hobos, Japanese beetles are never welcome visitors. They are too greedy.
Trap and kill these first beetles and place them in a bucket of water to which you have added a couple of tablespoons of dish soap. The rotting beetles will send another scent indicator to other beetles: STAY AWAY, and the soap will discourage mosquitoes and other pests from using the water as a breeding ground. Keep this up every day for a week or so.
If you do this early, the word gets out and Japanese beetles will leave your garden alone for the season. I think this explains why one garden will be plagued with the little @#!* while the garden next door is spared.
This works much better than traps, which can even start attracting beetles if you're not careful.
The emergence of Japanese beetles every year is pretty predictable. If you don't know when to expect them, start keeping watch at about the end of the second week in June. Your local horticultural society will probably be able to pinpoint it pretty closely. If you still need guidance, check out your local Cooperative Extension Location. You can find the online link here: Cooperative Extension Office Locator
Remember, it's important to catch them when they first start scoping out your garden.
Use Special Plants to Fight Japanese Beetles
I've referenced this in my herb blog: The Herb Gardener, and it works great. Planting rue and garlic together near your tastiest plants will discourage Japanese beetles from munching on them. Rue is an attractive plant, and only grows about 12" high, so it won't detract from your beauties.
This has worked beautifully for me. I think it will for you too.