Orvis publishes a very helpful hatch chart here. In particular, there is a suggested pattern for each bug’s life stage.
I’m struck by how often the old reliable, the Pheasant Tail, comes up as a pattern. I don’t know who invented this pattern centuries ago, but that person deserves a medal.
My suggestion is to use it, but to use a very small size. I’m talking #20 or smaller. When I stomach pump trout, there’s the occasional good-sized larva or pupa. But, nearly all the bugs are really, really small.
And, these small flies work. As they say, elephants eat peanuts. Here are pictures of an 18″ brookie I caught and the fly that duped it: it’s a #20 Pheasant Tail.
Every angler has a set of “confidence flies.” These are the patterns that you use and stick with when there’s nothing clearly hatching or when trout are picky. My short list includes the small PT. It’s simply tough to beat, as it mimics so many bugs.
Small flies are tough to tie, but here’s a Tim Flagler video on tying small PTs. He uses some very ingenious techniques to keep clear the hook’s eye. Worth watching.