Little Yellow Stoneflies
By Rick Hafele and Dave Hughes
Little yellow stoneflies thrive in the numerous cool mountain streams of the West. Streams like the Lochsa, Kelly Creek, or St. Joe in Idaho, McCloud, Rising River, and Hat Creek in California, and the Big Hole and Rock Creek in Montana are just a few examples of Western streams with excellent populations of little yellow stoneflies .
This is one of the more complex groups of stoneflies. The family Perlodidae is the most important to fly fishers. It is divided into two subgroups or subfamilies: Perlodinae and lsoperlinae.
The Perlodinae subfamiliy contains numerous genera (over 20) and approximately 50 species. They are mostly moderately-sized stoneflies (1/2 to 3/4 inch) with richly colored yellowish-brown nymphs and yellow or orange adults. They hatch in good numbers on most cold Western streams from early spring to mid-summer.
The other subfamily, Isoperlinae, contains only one important genus, Isoperla, which contains 21 Western species. The Isoperla are small (1/4 to 1/2 inch) light tan to bright yellow stoneflies that hatch from late spring through late summer.
There is one other group of stoneflies very similar in habits and appearance to the little yellow stones: the little green stones of the family Chloroperlidae. Adult chloroperlids are almost indistinguishable from the small yellow species of little yellow stones. For the flyfisher, precise identification isn't necessary since similar patterns and techniques will work for both types.
Habitat and Life Cycle
The nymphs of this group have adapted to a wide variety of habitats. They most often occur in areas with a gravel or rocky bottom and moderate to fast currents. Most species are more abundant in cool mid-to-high elevation streams than in warm low-elevation rivers.
Little yellow stone nymphs are common in stream drift, making them available and important food items for trout. As they reach maturity, they begin migrating to shoreline areas in preparation for emergence. This further increases the number of drifting nymphs, and it is an excellent time to fish nymphal imitations
Emergence usually occurs in typical stonefly fashion: the nymphs crawl out of the water onto shoreline rocks or vegetation where the adult then escapes the nymphal shuck. However, a few species actually emerge in open water in the surface film, just like many mayflies. This is unusual behavior for a stonefly, and drifting emerger patterns become just as important during this activity as during similar mayfly hatches.
After emergence, adults hide on streamside foliage. A quick way to check if adults are present is to simply look on the undersides of leaves of streamside shrubs and trees.
The most obvious and important part of the little yellow stone's life cycle is egg laying. This is when swarms of adults congregate over shallow riffles and runs. After a short flight to gain altitude (10 to 20 feet), egg-laden females set their wings in a shallow "V" and glide gently to the water. As their abdomen breaks the surface tension, a cluster of eggs is released. The females then quickly lift off the water, only to repeat another gliding descent to the surface.
Those lucky enough to escape surface-feeding trout will drop five or six times until all eggs have been laid. They then lie spent on the surface where they die. Most egg laying activity occurs in the late afternoon and evening.
Patterns and Tactics
Fishing methods are simple. Before adult activity becomes heavy, a small nymph fished dead-drift through riffles and runs will often prove effective. Try a size 16 or 14 Gold Ribbed Hares Ear or the Little Yellow Stone nymph developed by Polly Rosborough. If you find adults emerging in the surface film, try a Partridge and Yellow Soft Hackle or a Floating Nymph fly pattern in pale yellow. Fish these near the surface with a down-and-across presentation so the fly is presented dead-drift, then swings downstream with a slight rising motion.
The most fun comes when adults form egg-laying swarms over the water. A good pattern is a pale yellow Elk Hair Caddis in sizes 18 to 14. Polly Rosborough has also developed a dry called the Little Yellow Stone just for this hatch. Fishing the dry dead-drift over rising fish will generally produce well. Sometimes, however, adding a few twitches to suggest a female rising off the water will bring a vicious strike. The little yellow stonefly produces the kind of fishing that makes a long summer evening seem way too short.
I could see trout feeding heavily on a broad tailout above a big riffle on a Western stream whose name I decline to reveal here. I was using a dry fly. It was consistently refused. Frustrated, I tied on a Light Cahill wet fly, fished it on the swing through the rises. The results were instant and puzzling. Trout whacked the wet on almost every cast.
It made no sense until I recalled that some species of these insects emerge in open water, unlike other stoneflies. They are much more likely to be taken beneath the water as nymphs or as emergers than to be taken on top. Trout feed selectively with what seems like surface rises, but they're really taking their food subsurface.
This happens often on Willamette Valley and Yakima Valley streams, some small, some the very largest. You see adults in the air. You see splashes on the water. Once in a while you see an adult insect afloat; strangely enough, trout often ignore it.
I've experimented with feather-wing wet flies and nymphs with emerging wingcases, but so far must confess I've done best with the old standard Light Cahill, and with Polly Rosborough's Little Yellow Stone nymph.
Rick Hafele is a professional entomologist and fly fishing writer living in Gresham, Oregon. His most recent book is Nymph Fishing Rivers and Streams. Rick's good friend Dave Hughes is fly fishing's most prolific author, with over 30 books to his credit including Trout Flies. Together they are the authors of Western Mayfly Hatches.