The caddis larvae imitations are probably the best year-round nymph patterns. Most mayflies are burrowers and clingers and are really only available to the trout during the emergent stage. Even the free swimming mayflies and stoneflies do most of their moving at night when fishermen aren't on the water.
The caddis nymph, in either the cased or free swimming variety, is much more active, on the move and available to the trout in the absence of a hatch, and trout bellies are generally full of them. The trout gorge on even the cased caddis or rock rollers, case and all. Their stomachs are able to pass the very small leaves, rocks, sticks or mud which the caddis worm uses to enclose its soft body.
The "Peeking Caddis' is a great pattern to imitate the cased caddis floating to a new neighborhood in his mobile home, with his head and legs "peeking" out of the case. The "Chamois Caddis" is a great imitation of the caddis worm when it leaves its case. This is my favorite pattern on the Provo, in the absence of a hatch. Free swimming caddis larvae are found in most rivers and streams in Utah and are easily imitated.
Hook: Size 12 to 16 nymph hook (Mustad 3906 or equivalent).
Thread: Black, brown or olive, monocord or 6/0.
Case: Very rough rabbit fur dubbing, with or without gold wire wrap, 2/3 of shank length. Lots of materials could simulate the case. Use your imagination.
Body: Cream or pale green dubbing with peacock herl, black or dark brown fur head.
Legs: Soft hackle in full wrap just extended below the body.
Hook: Size 14 to 18 nymph or dry hook (Mustad 3906 or 94840), or a bent
shrimp or grub hook. (I bend my own Mustad 94845 barbless to shape).
Thread: Dark green or black, monocord or 6/0.
Body: Chamois leather strips, cut very thin.
Head: Peacock herl
Legs: Brown hackle extended below body to hook point. (Optional)
Hook: Size 12 to 18 standard nymph or dry fly hook (smaller sizes). A
bent grub or shrimp hook adds even more life to this pattern.
Thread: Brown on Green, monocord or 6/0.
Body: With 800 species of caddis, many shades of green, olive, brown, tan or grey dubbing will work. Collect samples! Bright green or pale olive are my favorites.
Ribbing: Hackle stems, gold wire, or monofilament.
Head: Peacock herl, black or dark brown dubbing, or just use tying thread.
Legs: Brown hackle, extended below body to hook point.