Thankfully, Japanese beetles aren't a problem in California. In fact, I don't think they are anywhere near California. Whew!
I built a similar looking trap, though much smaller, for attracting and killing fruit flies that have been a problem near my worm bin. The bait is vinegar. They fly through the funnel and then can't fly out.
Did you grow hot peppers and tomatoes this summer? Making homemade salsa is sooooooo easy and delicious.