You don't want zapper that people use around their yards. It frys the bugs making them useless to the fish. I have the more expensive version made by Fresh Flow (I think it's called a Will 'o' wisp) that attracts with a circular black light and throws the bugs onto the water via a spinning fan not a spining cord. It does attract feeding bluegills, and I had one last year that was king of the pond right underneath it. Any bluegills that came near his area was forced away. I'd love to have another one to see how my trout would react to it in their pond.
I have noticed that their are peaks to when the bugs are abundant which is just after dark for a few hours then bug activity drops off dramatically. As far as worth it, I would say they will not make a lot of difference in your pond, but I will have an esoteric use for mine.
I have a low density of bluegills in my pond as I am attempting to grow large ones. It seems to attract about 50 to 100 6 to 7 inchers (The really big bluegills don't feed on the surface much), and I will be harvesting these by hook and line and crowding them into a floating cage. They will only get floating feed in the cage and once they are well trained I will cull out only the males and separate into another small pond. Then I will attempt to grow very large bluegills on feed with no reproduction to prevent stunting and no need for predator fish to put them off the feed (In one of my ponds the feed trained bass are so agreesive they push other species out of the way).
If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.