Knowing when to use stonefly dries or nymphs is not really rocket science is it fellas? I mean a lot of it comes down to what are you observing on the stream, at the moment you are fishing? IMO, the simple act of observation is extremely important to the competent fly angler. Jkilroy, yes in streams where they occur stonefly nymphs are widely available year round as they have long life cycles. Wherever they're in abundance the nymphs are an important food source throughout the year. It's important to carry several sizes of blacks and browns especially from large #6 to tiny #16 or 18 (for little blackstonefies in late February/March.) I tend to carry golden stones in mostly bigger sizes like #6, #8, & #10.
Most stoneflies emerge after dark, or on very over cast days at times. Stonefly nymphs crawl out of the water to emerge on rocks and shoreline so emerging adults aren't often heavily available to fish, but in deeper runs they may emerge in the water. Stripping nymphs slowly towards the shore can be deadly. Very slowly dead drifting a big black or brown stonefly nymph down deep in winter can also be very effective. However, adult stoneflies are poor flyers and when they return to lay their eggs on the water they become much more available to trout. If you're on the stream & begin to observe numerous adults "crash-landing" on the stream its a good time to switch to a Stimulator, Madam X, etc. when the adults return to the water the trout will often respond very aggressively and it can make for some very exciting and somewhat "easy" dry fly fishing. It's not like a steady, reliable dry fly hatch but when you're lucky enough to be on the water when it happens the fishing can be incredible, and often big fish too. What's more is that presentation is a cinch, no delicate casts. You actually want to really splash that stimulator onto the surface when you cast it... and be ready! My best day ever on Big Fishing Creek was when the large golden stonefly adults were returning to lay eggs and I caught it just right one late June afternoon/evening. For several hours I splashed big #6 & 8 yellow stimulators & golden stone dry patterns and caught wild brown after wild brown. The smallest fish being an 11" and all the others being 14" or better. I've had really good days with stoneflies, nymphs and dries (usually at different times for each), on the Lehigh and Brodhead too and several other streams and rivers in the Northeast & New England.
Stoneflies are probably on of my favorite food sources to fish. I'm particularly fond of the golden stones.
