On the Japanese beetles, the traps work really well to attract them.......but then what? How many of those things can a bird eat with no harm to them? I know on the grubs, it's unlimited. I had a RIR helping me dig in the garden last fall and she must have downed 50 of them and was not slowing down. If there was a way to get the grubs to surface....maybe water in some cayenne pepper?........the chickens would clean them up as fast as they appeared.
We have swarms of the JB's and last year I trapped buckets of them....I was dumping 2 to 3 bag fulls per day. If a person had enough birds to deal with it, they could put the traps inside the coop or run and draw them in for the birds to eat. Don't know what that would do to their health or eggs.
I also have a theory on using the traps near a pond stocked with fish. Tie a trap with bag (bottom cut out of the bag) on a long pole to dangle the bottom of the bag at the surface of the water. JB's come to the trap, fall down into the bag......fall through the bag.......hit the water and the fish eat them? I don't have a pond, although there is one in a cattle pasture a few hundred yards away. Downside to that is it may be bad form to put a trap on a neighbor's property to draw the bugs to them.
Rule of thumb is on the JB traps........they work really well, so buy a dozen of them for each of your neighbors to put up and let the bugs go to them.