Epeorus Mayflies

By Jeff Morgan

These yellow mayflies are important on many streams throughout the West. They are not a fashionable insect: hatches are not prolific and don't occur on flat "technical" water. However, Epeorus (yellow quills) are distributed (sparsely) throughout most swift streams, and trout see enough of them to recognize them as food.

Their need for abundant oxygen and pollution-free waters restricts them to pristine conditions. You won't find this insect in the lower Willamette or Sacramento rivers! Schwiebert compares Epeorus to brook trout in regards to their intolerance of pollution. Their sensitivity to pollution is acute: following a test-dispersal of sulfuric acid, Epeorus nymphs undertook 13 times the normal drift rate, and much more than any other aquatic insect, in an attempt to flee the polluted area.

Nymphs

Nymphs are dark-hued clingers that are easily distinguished by their two tails, a physical characteristic shared by no other mayfly nymph. Many people confuse Epeorus nymphs with another mayfly clinger, the Pale Evening Dun. My theory is that this confusion arises from the fact that the later has a fly named after it while the former lacks that honor. This confusion is easily resolved: simply looking at the tails of the nymph (two for Epeorus) will resolve any uncertainties.

Epeorus nymphs, like all mayfly clingers, are well-adapted to their fast-water environment. Their body, head, and legs are all flattened to hug the rock and minimize the force of the current pushing on them. I often pull rocks out of fast water, and the only mayflies that survive the quick trip through the fast current are Epeorus.

Epeorus nymphs are terrible swimmers (perhaps the worst, considering they don't even migrate or rise prior to emergence!), and if dislodged can drift long distances before regaining contact with the bottom. Despite their pitiful swimming skills, however, they rarely lose their grip, and are seldom available to trout.

Emergence

When preparing to emerge, an Epeorus nymph will move to the bottom of a rock, where current is calmest, and split from its nymphal shuck on the bottom. Then the dun is buoyed to the surface by a self-generated gas envelope under the wings and legs.

This shimmering bright yellow insect, rising quickly to the water's surface--like the trout equivalent of a butterscotch disk--is too "loud" for fish not to take notice. Trout focus most of their feeding on Epeorus during the short ascension of the dun. Because of this unusual emergence style, there are very few (none, that I have seen) emergers and stillborn duns that make it to the surface.

The adults rarely stay on the surface for a long time like other mayflies (pale morning duns , for example), but trout will feed on them when they are available. The adult is a usually a true yellow hue, though there can be some variation in color between a light brown and a light tan, and E. albertae (the Pink Lady or Pink Albert) can carry a pinkish hue among the females. I have never seen, nor even heard of, a very good Epeorus spinner fall, and considering the types of water they inhabit imitations of this stage would be quite difficult.

Western Species

Some of the most important species of Western Epeorus include:

E. albertae (Gray Winged Pink Quill, Pink Lady or Pink Albert). This is a widely distributed emergence in the West. Schwiebert claims to have seen large hatches on rivers in Wyoming and Colorado. The nymph of this insect is 5/16" long and mottled brown in color, while the dun is about the same size and is a yellow to pinkish-yellow in color. This is an afternoon and evening emergence, extending between early July-August.

E. hesperus. This species is common in the Pacific Northwest in fast, cool streams. It emerges earlier than other Epeorus species, between Memorial Day and the Forth of July. This emergence can be profound on the McKenzie River in Oregon, as well as many other small streams in the West. This species is about 1/2" long, with a light yellow body on the dun, and a mottled grayish-brown nymph.

E. youngi. This 7/16" long mayfly has a range that extends between Oregon and Montana. It emerges in the late afternoons and early evenings throughout the summer. The dun of this insect is pale yellow, while the nymph is yellowish-amber, often with profound mottling. This is another important small-stream mayfly in waters with sparse insect populations, such as the streams of Glacier National Park.

Tactics

Epeorus is perhaps the least-imitated mayfly genera, probably because of the swift-water habitats it calls home. Most of the time, you don't need an exact imitation of this insect, but an accurate approximation can help out when you find some picky mountain trout

My favorite nymph imitation for Epeorus is a size 10-16, olive or olive-brown version of Oliver Edward's Heptageniid Nymph. However, an exact imitation of this insect is rarely necessary and a flat-bodied olive-brown Hares Ear usually will suffice. It is important to imitate the flat-bodied nature of this insect whenever possible. The flat body can be achieved by wrapping an underbody of lead wire, then flattening it with pliers. The flat body will create a realistic tumbling motion that trout seem to respond favorably to.

Regardless of the nymph pattern you choose, it is important to keep it near the bottom and confine its use to fast-water habitats. Since these insects don't undertake a pre-emergence migration, there is no prime time to fish the nymphs, except near dusk, as with all other nymphs.

Imitating the rising dun is the most important stage for anglers to concentrate on. Some anglers would simply use a yellow Soft Hackle, but I prefer something with a little more flash. Any dun that emerges on the bottom and rises without any swimming motion, must rely heavily on trapped gasses to buoy it to the surface. To imitate these trapped gasses, I use a wet fly with a pearl bead in the thorax, sparkle yellow dubbing, and CDC under a Raffia wing. This pattern has sufficient flash and action to trigger strikes from even passive trout. When imitating the rising dun, dead-it drift under an indicator with some little lifts of the rod tip to make it look like the fly is coming to the surface.

After it arrives at the surface, the dun rides on the surface a very short distance before flying away. A high-riding pattern like the Grizzly Wulff is a good choice for the fast water where these insects emerge. Since hatches are often sporadic, the Grizzly Wulff can be a great searching pattern: effectively matching an insect trout may be responding too, and hooking trout that will rise at anything that drifts by.

The spinner stage of this insect is a mystery, and I haven't ever fished a noticeable fall. Considering the bug lives in such rough water, I can't imagine too many trout leisurely sipping Epeorus spinners.

Jeff Morgan has written many articles for Westfly, mostly on entomology and fly tying. He is the author of An Angler's Guide to the Oregon Cascades and Small Stream Fly Fishing.