So far the Zinnias are winning the battle of the beetles.
I go out each day to the garden and knock the Japanese Beetles into a bowl of water with dish soap in it. They usually fall as their protection, so it works fairly well just holding a bowl under the leaves and help guide the little creatures into their water and soap bath. I then leave the bowl out in the garden to ward off other beetles. It seems to work quite well. Crunching the little guys in between my finger tips has become a regular practice too. Although this tends to make most a bit squeamish, my hard heart towards these little imps wins out. Grandpa T. (he was about 80 years old then) taught me this on potato plants and potato bugs when I was a teenager. Applying the same techniques with the beetles seems to work very well.
One year, I got on the band wagon of the traps. They worked exceptionally well for calling in ALL the Japanese Beetles from miles around.
I do not use them now!
These pests can be very destructive. Don't ignore them. Counter attack! Pay the 8 year old down the street to go out and get 'em. Give the brave kid a nickel for each one squished or in the water bath. Your garden will thank you.