Posts Tagged ‘what are these trout eating’

Most anglers ask what are you using today as you are laying waste to some really nice fish.  One of the more obscure answers is a Black Fly Larva.  The next response is usually “what the hell is a Black Fly Larva?”   This question has been asked more than once here in Colorado on the famous Chesman Canyon Section of the South Platte.

Blackflies teeter on the margins of angler awareness–outcasts in spite of their standing with fish. black fly larvaThey don’t fit into the aesthetic of the mainstream angling world, where priority is given to prettier 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. Let those anglers with aesthetic hang-ups keep doing their thing. The rest of us will let the trout dictate what flies to fish!!

In a recent study, black flies were consistently consumed by trout at a much higher rate than their proportion in the drift. While Blackflies 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 Ephemerella (PMD) mayflies, were nearly unscathed.

Are Black flies important on “real” trout streams? In the Chesman Canyon Section of the South Platte River, black fly larvae were the most consumed aquatic food among rainbow trout that were two or more years old during the summer months.

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 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 of the Black Fly:

Larva to Pupablack fly larva from Umpqua feather merchants

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 olive, dirty yellow, creamish, and black. Most mistake 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.

Besides bouldered riffle water, black flies can be extremely abundant on bedrock slabs 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 the Black Fly larvae don’t migrate to pupate. They just build their cocoon wherever they happen to be at pupation time.


Black flies have a unique and fascinating method of emergence 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.

There is usually no visual hint of emergence with trout rising as 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.

Final Thought for you the Angler to Consider

Are Blackflies the most critical food sources for trout? Probably not  but 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. 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 numbers of Blackflies available to trout for the entire year while most mayflies and caddis are too small to feed a descent sized trout for half the year, putting your faith in a blackfly larva makes alot more sense.