I suspect they've been around for awhile, but I really didn't notice the Japanese beetles in our area until two years ago. That was when I noticed the leaves at the top of the cherry tree seemed to be turning brown in the middle of summer. I'd seen other trees suffer that fate, but since they weren't in my yard, I ignored whatever had caused the devastation. When MY cherry tree seemed to get worse every day, I finally investigated. I was horrified to find hundreds, perhaps thousands of beetles slowly eating the life out of my tree!
My research that year convinced me to spray the tree with Sevin, especially since all the cherries had been harvested weeks earlier. It was satisfying to see all the bugs lying dead on the ground just days later, but I realized this war would probably have many battles over many years.
The next year, I noticed the beetles were attracted to several plants in our yard, specifically: the cherry tree, raspberry plants, only one of our three maple trees, and hops bines. I sprayed all the affected plants except the maple with Sevin, and was generally pleased with the results, but something about spraying chemicals on food plants made me uncomfortable.
Because these traps attract Japanese beetles, they shouldn't be located right next to a plant you're trying to protect. Spectracide recommends at least 30 feet separation, and our trap is about that distance from the cherry and raspberry plants. Those seem to be protected pretty well this year, although I do occasionally find beetles on both plants. I remove those bugs when I see them, which tends to prevent others from congregating there also. One source I read claims the beetles can smell a leaf that is being eaten, so they tend to gather together on the same plants. Recently, I noticed a few dozen beetles on our small basil plant, and once I removed them I haven't seen any others on the plant since.
Criticized for drawing more beetles to your property, I think this style trap comes out far ahead in removing pests from your plants. The damage I see on our plants this year is probably comparable to what I saw last year when I sprayed with Sevin at the first sign of infestation. I'll continue to monitor and empty the trap faithfully, as I'm interested to see if the trap continues to be effective through the rest of the summer.