Fly Fisherman Throwback: A Primer of Stream Entomology-III

Editor's note: Flyfisherman.com will periodically be posting articles written and published before the Internet, from the Fly Fisherman magazine print archives. The wit and wisdom from legendary fly-fishing writers like Ernest Schwiebert, Gary LaFontaine, Lefty Kreh, Robert Traver, Dave Whitlock, Al Caucci & Bob Nastasi, Vince Marinaro, Doug Swisher & Carl Richards, Nick Lyons, and many more deserve a second life. These articles are reprinted here exactly as published in their day and may contain information, philosophies, or language that reveals a different time and age. This should be used for historical purposes only.

This article originally appeared in the August-September1972 issue of Fly Fisherman magazine. Click here for a PDF of the print version of "A Primer of Stream Entomology-III."

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Part Three of a series of articles designed to familiarize fly fisherman with the practical aspects of stream­ side entomology. The series will cover the favorite fare of trout, smallmouth bass and even bluegill in all of their habitats, slow water and fast, from farm ponds to high mountain lakes. Mr. La Fontaine is both a scientist and fly fisherman who goes where the fish and the hatches are.

Spring Creek is thirty miles north of Missoula, Montana. It runs next to Highway 93 at one point, with a roadside rest and picnic table at the stream edge. This stretch is stocked heavily with small rainbows for the summer travelers, but there is a large self-propagating population of brown trout. An eight­pound brown is the largest fish known to the author that has been caught from the stream on a fly. [This stream should not be confused with Armstrong Spring Creek, a chalk-type stream near Livingston, Mont. Ed.]

Spring Creek is not famous and the fish are small by the standards of Montana trout waters, but it's my favorite stream, and since I know it well, I know why it is a trap for so many fly fishermen.

Left to nature, Spring Creek is a small, gin-clear pasture stream. It is not affected by snow melt and it runs low and clear early in the season, but by June the valley ranches begin returning irrigation water to the stream. This water is laden with nutrients that alter the composition of the stream fauna, and instead of the mayfly, the caddis fly is the predominant order of aquatic insect. Nearly every day of the open season there are at least two species of caddis flies emerging. On the evenings that the pupae of the caddis emerge the trout bulge and slash as they feed and a dry fly, even a downwing imitation of the adult, is a poor fish-catcher because the fish are chasing the rising pupae. How many times I flogged the moving fish with dry flies, the classic American angler in love with the mayfly, not even seeing any insect emerge as the caddis pupae popped quickly from the water. The blindness that traps the fly fisherman is the forgetting that there are aquatic insect beside mayflies.

Even in a typical freestone stream the caddis fly is important. Below is a chart from Paul Needham's Trout Streams, indicating food preference.

The caddis fly leads a complete life cycle (egg, larva, pupa, adult) and as the pupa cuts itself out of the cocoon and rises to the surface buoyed by a translucent sack full of air, the insect is at the most vulnerable stage of the cycle.

In late June on Spring Creek there is a large emergence of Glossosoma alascense. I fished a matching pupal imitation down one riffle, casting down and across and letting the fly swing taut to swim to the surface. Rolling casts next to the opposite bank as I moved, I caught three rainbows at the top end, missing two strikes. I hooked a 12½-inch cutthroat and a 13-inch brown in the deeper section, and at the wide curl of water just above the pool, where I expected a big brown, I hooked a 17-inch whitefish that bulled downstream and tore all over the pool. The grab bag of fish species is typical. Every insect feeder in the stream shows up for the party.

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Because the adult caddis is not a strong flier, there are times when a dry fly is effective. I was fishing on August 31 last year when the winds were gusting to 40 miles an hour. As the adult of three species of caddis were shaken from the trees, the wind knocked them to the water. I let my line unfurl and my greased wet fly, a # 14 Perkin's Pet (silver floss instead of tinsel for the body), smack the surface. The fish were in a frenzy and the fighting trout did not spook them. From the single pool, without moving, I caught browns up to 14 inches and slid them back over the weed bed as I released them.

Spring Creek is a special environment resulting in an abundance of caddis flies, but the environment is not a rare situation. Spring-born rivers, natural lake outlets, and dam outlet streams have concentrations of caddis flies, and known trout streams like the Clearwater of Montana, the Muskegon of Michigan, and the Metolius of Oregon are big caddis fly streams.

Every trout stream contains species of caddisflies and, like the mayflies, there are super-hatches of caddis that occur over sections of the country. While every major mayfly hatch is known and an imitation is commercially tied, few of the caddis flies have been individually patterned by the American angler-entomologist (not so with the English who tie the sedge series). There is a special need for a line of pupal imitations, the silhouette of the fly being distinct. Below is an imitation for Glossosoma alascense. The pattern, in varying sizes, is good for other species of the genus Glossosoma.

• HOOK: Mustad-Viking 9671; #12, 14
• BODY: Covering rear 2/3 of hook–underbody of tan floss (silk shade darkens when wet). Picked out natural otter fur, palmered over the floss body. Do not completely cover the floss. When the fly is wet the fur should mat thinly over the floss. Dubbing silk should be 6/0 white.
• WING CASES: thin slats of dark grey mallard extending to the rear of the body, tied at the sides and slightly under the fly.
• HACKLE: Sparse wisps of grouse bearded short under the fly.
• REMAINING SHANK: Wind stripped peacock quill to the eye of hook.

The pupae of Glossosoma alascense will come off the swifter riffle sections of the stream. I seem to get the most strikes fishing the fly down and across. If I do not get a hit as the fly pulls taut and kicks to the surface, I release and pull the fly a few times before stepping downstream to recast. Whenever I fish downstream I try to stay out of the water.

A pupal emergence of Glossosoma is easy for the angler to foretell. The turtle-shell cases of the insects at the start of pupation are sealed completely onto the rocks of the riffles. Within ten to fifteen days the caddis pops from the water and disappears into the streamside brush. Because the hatch may be sporadic it is sometimes hard to see the emerging insect. But if the trout are showing with the slash-form rise a pupal fly is a probable answer.

On any stream, do not fall into the trap of thinking mayfly when the fish are thinking caddis fly.