The neat thing about fly fishing is that bigger isn’t always better. Most fisherman would debate that big flies catch big fish, and no doubt the bigger fly hatches tend to bring the bigger fish to the surface….hatches of brown drakes, green drakes, sulphurs, etc.
But, I can tell you from experience that Ron Hill and I have been fishing on Pine Creek many times with green drakes and slate drakes floating on the water. Fish were rising, but the trout were completely ignoring the drakes and turned on to small blue winged olives in the size #18 or #20 category. Ron and I swear that trout like the way BWOs taste.
When mid-summer hits and the bigger hatches are over, tricos and midges are hatching and big fish can be sipping these flies with consistency and will completely ignore anything bigger than a size #24, #26 or #28…..and unfortunately a #28 is a “giant midge” compared to the real fly so trout may ignore your small offering. But, there is nothing more rewarding than landing an 18″ brown trout on a griffith’s gnat or a gray midge on 7X or 8X tippet.
It also works the other way too. I’ve fished the upper stretches of the Yellow Breeches or Slate Run with no apparent hatches, and no fish rising, but the wild fish will take will take a #14 attractor fly that seems as if it is bigger than the fish’s mouth.
So, pay attention to the hatches, what the fish are really rising to, and ofcourse the drift, and when the trout are ignoring your offering, go smaller.
Note: The general rule of thumb is to divide the fly size by 3 to determine your tippet size (#18 hook = 6X tippet). So, when midges are the necessary fly of choice, make sure you have 7X & 8X tippet in your vest.