I saw this same phenomenon in Orland Park, IL yesterday multiple times. Even while the flies were mating, one had a Japanese beetle in it's grasp.
It's a robber fly, in the genus Promachus. There is another genus in northern IL that I've seen, Machimus, but they're smaller, so much so that I doubt they'd be able to handle a Japanese beetle. If you could get them to hang out in your yard, it'd be a pretty fun way to control some of the beetles.
(The picture is from Japan, but we have pretty much the same flies here in NE IL)
Edit: The beetles will be highly attracted to the following plant genera:
I think if you plant those on the other end of your yard, you'd be set to keep them off your hops. I swear they have nasty orgies on these plants in the fall.
The flies like large open areas, like the 500+acre prairie in Orland Park. They won't bite unless you really mess with them a lot, and they love hanging out on your shoulders/hats/etc. They seem friendly even. Once you start seeing them in the wild, you won't stop seeing them, they're everywhere right now.