On the ponds and smaller lakes around here, we get sparse hatches of big yellow mayflies sometimes as big as size 8. When these guys are coming off, EVERYTHING is after them. I've caught gills, bass, crappie, and cats on the same evening on the same fly, usuaslly a foam bodied hex dry does the trick, but I've also done well with a yellow hopper pattern in the right size with the kicker legs clipped off. I seldom catch the hatch right, and it's always both sparse and localized, but I'm never without one or two of the flies just in case, and I'm always looking out for them when I'm on the water.
If it swims and eats, it'll eat a fly.