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Black Flies: Part I

By Jeff Morgan

If Bob Dylan were a fly angler, he'd have sung about black flies. And after you learn about this depised, neglected, and rejected insect, you'll be singing too.

ob Dylan sometimes uses images of society's rejected and grotesque. He sings of one-eyed midgets, the Hunchback of Notre Dame, a motorcycle black Madonna two-wheeled gypsy queen, and screaming gray-flannel dwarfs.

These images depict the dark, hidden recesses of society. They speak of those whose value is unquestioned, whose existence is undeniable, and whose neglect is unreasonable.

If Dylan fly fished, he would surely have mentioned some ugly, neglected, but nonetheless valuable creatures, such as aquatic worms, craneflies, water boatmen, and black flies. These teeter on the margins of angler awareness--outcasts in spite of their standing with fish. They don't fit into the aesthetic of the mainstream angling world, where primacy is given to pretty bugs that drift sacrificially on meandering currents.

Among the many insects that fly anglers ignore, perhaps none is more important to trout than the black fly (order Diptera, family Simuliidae). Let those people with aesthetic hang-ups keep doing their thing. The rest of us will let the trout dictate what flies to tie.

Most Important, Most Ignored

In D.M. Davies' essay "Predators upon Black Flies," he cites 71 different studies prior to 1981 that discuss salmonid predation on black flies. I went to the Oregon State University library and located nearly twenty other studies where blackflies were, at least for a month or two, by far the most important food resource for trout. In addition, there were over a hundred studies that showed black flies to be more important than every mayfly except Baetis!

In a study by Dr. J. David Allan, one of the premier stream ecologists in the country, black flies were consistently consumed by trout at a much higher rate than their proportion in the drift. While simuliids composed about 10% of the drift, they made up nearly 70% of the trout diets in June and July!

If that isn't enough for you, consider that on the same stream, Baetis--the orthodox angler's security blanket--made up 70% of the drift and contributed a mere 20% of the total trout diet, while other common drifters, Ameletus and Ephemerella mayflies, were nearly unscathed.

Are black flies important on "real" trout streams? On California's famous McCloud River, black fly larvae were the most consumed summertime aquatic food among rainbow trout that were two or more years old.

Roger Crosskey, in The Natural History of Blackflies, says:

The most important fish predators [of black flies] are the species of Salmonidae with very high commercial and sporting value in Europe and North America, the trout (Salmo trutta), the rainbow trout (Oncorhyncus mykiss) and the Atlantic Salmon (Salmo salar)--all three consume prodigious quantities of immature blackflies (the reed smut of the flyfisherman), and trout especially also consume many adults. The gut of even a small trout can be packed with hundreds of larvae and pupae.

Surely, the angling community does not pay these insects their due.

How Has It Been Missed by So Many?

With this staggering wealth of scientific observation plainly accessible to angling researchers, how could black flies have been overlooked by experts and recreational fly fishers for so long?

Several factors contribute to this neglect. First, most anglers probably mistake black fly larvae for caddis larvae, which look quite similar to the untrained eye.

Also, black flies are in the order Diptera. Thus many well-intentioned writers and researchers who take the time to consult professional papers and science books, often see "Diptera" in stomach samples and connect it with "midge." In the process, they forget that the order Diptera casts a wide shadow and includes midges, mosquitoes, phantom midges, net-winged midges, craneflies, as well as our new friend the black fly.

Another reason is that "kicking riffles"--the preferred sampling technique of most anglers--often doesn't knock many of these larvae off the rocks because they are tough clingers in the fastest currents.

Probably more important than these factors is the fact that there is little dry fly fishing (or even fish rises) during a black fly emergence. This cannot be underestimated; so much angling literature has only valued insects that provided dry fly action.

Finally, black fly activity isn't really a concentrated hatch like most aquatic insects. They will go through 4-14 generations a year, with larval growth, emergence, and egg-laying overlapping between generations. This means that black flies maintain a modest, but constant, role in the year-round diets of most river-dwelling trout.

All these factors go a long way to explain why anglers have overlooked black flies, while trout happily indulge in this abundant prey.

Life Cycle--Larva to Pupa

Black fly larvae are medium-sized insects, from 4-15mm in length. They are a distinctly shaped grub with feathery fan gills, prolegs at the head, and a fat butt (the rear 1/3 of Simuliidae larvae can be almost twice the thickness of the front). Colors range from smoky gray, dirty yellow, creamish, and black. I have often mistaken the small black larvae for microcaddis larvae, which also have a distinguishing fat butt.

The larvae are the most important stage for most trout, especially for trout living in streams with many fast riffles. Larvae attach themselves to rocks with the hooks of their posterior disc, then trail their heads and fans downstream to filter the current for food. To move short distances, black fly larvae excrete a strand of silk and "rappel" downstream, in much the same manner as some caddis larvae.

We often think of black flies as lovers of stagnant waters, and while some black flies like slower water, far more can be found in swift streams. Besides bouldered riffle water, black flies can be extremely abundant on bedrock slabs (one of the only insects that are), where their silken pad and anal hooks give them excellent holding power against strong currents. Larvae are also unusually visible at night, when they are heavily fed upon by both fry and adult trout. This feeding may be because the light-colored, semi translucent larvae are easy to detect.

After 3-10 weeks of larval development, black flies undergo pupation, which lasts anywhere between 2 and 8 days. Unlike many other aquatic insects, larvae don't migrate to pupate. They just build their cocoon wherever they happen to be at pupation time.

Life Cycle--Emergence

Black flies have a unique and fascinating method of emergence, a method that is vitally important for fly anglers to understand. Fully formed adults burst from their pupal casing and are shot to the surface enclosed in a bubble of air. This is a similar process to caddis emergence, with two major differences. First, the shimmering "air bubble" effect is much more pronounced for black flies. Secondly, a caddis rides to the surface as a pupa, while the black fly rides to the surface as a fully developed adult. If you do a stomach sample that reveals adult black flies, it is virtually impossible to tell if the trout captured the black fly when it was rising or floating. Emergence occurs in the morning from 6:00 a.m. to noon, and rarely occurs at night or in the evening. Emergence accounts for heavy feeding action throughout the water column, even when no surface action is visible.

By the way, there is usually no visual hint of emergence (i.e. rising trout); the air bubble keeps their bodies and wings dry, so the adults arrive at the surface and instantly fly off. The lack of surface activity is undoubtedly a reason this insect has been overlooked by so many for so long.

Life Cycle--Adult

After emergence, adult black flies are easily recognized. They have black bodies (surprise!), clear to smoky wings, short dark legs, and red eyes. They can range in size from 8 to 15mm in length. I have never had fish be selective to black fly imitations, although I carry patterns from size 12 through size 18.

The adult male black fly, while having a primarily black body, has silvery marks on the abdomen and front legs, while other legs can have yellow or white markings. These may be of significance for the fly tier to consider during prime terrestrial conditions.

The adult black fly is no stranger to the angler or anyone else who spends much time near the water. Adults can often be found on riparian plants, and gusty winds can knock them into the water. On heavily fished waters, where a large number of wading anglers slap and kill a few dozen flies per hour, it is not unreasonable to expect fish to encounter a disproportionate amount of drifting adults.

Stomach samples on Yellowstone's Slough Creek and Lamar River, where anglers and black flies revel in mutual abundance, shows that the majority of trout (62%) had at least one adult black fly in their gut.

Life Cycle--Egg Laying

I have found that an adult black fly pattern can be a superb searching dry fly, especially on heavily fished waters where fish are more circumspect about the standard line of terrestrial patterns.

Adult female black flies must return to fast riffle water in order to lay eggs and regenerate the life cycle. Oviposition usually occurs at sunset, or within an hour or two of sunset, and all species lay eggs best when the air is still. These females will either lay eggs on rocks or vegetation just above the water's surface, or deposit them directly on the water's surface like a stonefly.

Some species are substrate-specific, and will lay eggs on lighter substrates when possible, possibly because the larvae will be better camouflaged amongst the lighter rocks.

Female blackflies can swarm and will concentrate egg laying on a relatively small section of stream. One 10m section of stream had 3 million eggs, the result of about 3750 females laying eggs synchronously!! It is safe to assume that more than a few trout had a hearty dinner that night!

While there is no real black fly "spinner fall" per se, as they don't die within minutes of laying eggs, there is no reason to believe that some females don't get stuck in the surface film and are fed on by trout as they drift helplessly after laying eggs.

Put It in Perspective

Would I say that black flies are the most critical food sources for trout? Probably not, but I think they are more important as a family than many of the species of insects that we fly anglers fuss over.

Simuliids are significantly more important, in terms of annual consumption, than salmonflies, hoppers, and even the much-lauded midge. Even where salmonflies are extremely abundant (Deschutes, Madison, and a handful of other waters), they compose only about 4% of the annual diet of trout, whereas black flies rarely contribute less than 6%. It would be safe to say that simuliids, on average throughout the United States, contribute more to a stream trout's diet than any single species of mayfly except Baetis. When you consider that there are large Simuliidae in the water for the entire year, while most mayflies and caddis are too small to feed respectable trout for half the year, this leap of faith towards black flies makes sense.

For the fly patterns, see Black Flies--Part II.

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. Jeff is currently a graduate student at Stanford University, where he is finishing his PhD in History.

Uploaded 04/23/2002.

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