No other insect symbolizes fly-fishing in streams more than the mayfly and seeing those little iconic sailboats floating downstream can get any angler excited. Even the earliest writings and drawings on fly-fishing tell of anglers catching fish with mayfly imitations. This is no surprise as mayflies are one of the major food groups for trout. Mayflies are on the menu daily and year round, and can be up to 25 per cent of a trout's food intake.

Life of a mayfly

Mayflies need as little as a few months or may take up to two years to produce the next generation. For example, Blue Winged Olives can have up to three generations (some refer to them as broods) a year, Western Green Drakes need a full year and Brown Drakes need up to two years for one generation.

Mayflies do not go through pupation and therefore have an incomplete metamorphosis. Their life cycle consists of four stages: egg, nymph, and two adults stages called dun and spinner. The nymphs molt up to 30 times, and with each molt produce a slightly different looking instar. Once the nymph has reached maturity, it hatches and emerges from the water as a flying adult, known as the dun. The dun in turn hatches again into the mature adult known as the spinner.

Duns and spinners look similar to one another, but there are some differences. The dun usually has opaque wings with more hair on its body whereas the spinner has clear wings, darker coloured body and little to no hair. Mayflies are the only insects with two adult life stages and they live very briefly, typically no longer than three days.

Anglers often talk about emergers and cripples, especially when talking about mayflies. These are not actual life stages but rather terms used to describe what anglers see on the river. The emerger is that step when the mayfly crawls out of the nymphal shuck to the time it appears on the water surface as a dun. A cripple is the result of a damaged wing in the emerging process and sometimes the insect itself gets stuck in the nymphal shuck. Unable to fly these crippled mayflies float helplessly downstream and become easy prey. Many anglers tie flies specifically to imitate emergers and cripples.

Trout feed on nymphs and both adult stages. Nymphs become prey when they are either washed off the river bottom sporadically, during behavioural dispersal drifts, or during emergence. Behavioural dispersal drifts occur when mayfly nymphs migrate downstream in search of new habitat. They do so by simply letting go of the bottom and are carried downstream by the current. These drifts peak in the hour before dawn and the hour after sundown. Naturally trout also feed on mayflies during the hatch. The nymphs and the adult stages can become food for trout at any point during this hatching process.

Mayflies hatch using one of three different strategies. The nymphs of most mayflies (Western Green Drakes, Blue Winged Olives, Pale Morning Dun and Mahogany Dun) swim to the surface. These duns crawl out of the shuck onto the water surface where they drift for meters down river while inflating their wings. The nymphs of other species (Brown Duns and Gray Drakes) will crawl out of the river and then the dun hatch on land. In the third strategy some mayflies hatch while the nymphs are still on the bottom, the duns then swim to the surface, and at the surface these duns fly away almost immediately (some Pale Evening Duns).

How many mayflies do you need to imitate?

In British Columbia there are currently 93 mayfly species, but only a handful are relevant to fly-fishing and many species can be imitated with the same fly pattern. To effectively match the hatch, focus on these mayflies:

1. Blue Winged Olives

2. Western Green Drake

3. Pale Morning Dun

4. Mahogany Dun

5. Pale Evening Dun

6. Brown Drake

7. Western March Brown

8. Tricos

The common names of mayflies can be very confusing. Some names refer to a specific species, such as the Western Green Drake for Drunella grandis. Whereas other names like Blue Winged Olives refer to many species within the Baetidae family. Sometimes it depends on the geographic location. In BC, a number of species belonging to the Heptagenia Genus are called Pale Evening Duns, but on the East Coast of North America Ephemerella dorothea, which is not found in BC, is called Pale Evening Dun.

Mayflies spend in excess of 95 per cent of their time as nymphs, so you need to be effective when fishing nymphs. When imitating nymphs keep it simple as nymphs are generally shades and combinations of olive, brown, gray and black, and are easily imitated with flies tied onto hooks from size 20 to as large as size 10, with the majority tied on sizes 16 to 12 hooks. Mayfly nymphs do vary in shape though and when tying flies, keep these shapes or morphology types in mind.

Mayfly nymphs evolved into four morphology types (clingers, crawlers, swimmers, and burrowers) that allow them to occupy different habitats in rivers. Clinger nymphs have flattened heads and bodies that allow them to cling to rocks in the fast moving water of riffles. Crawlers have strong stout legs allowing them to crawl on the river bottom in riffles and runs without being swept away by the current. Swimmers have torpedo-shaped bodies and can swim in a variety of habitat including riffles, runs and pools. Burrowers are long and slender, and dig U-shaped tunnels in sections of rivers with soft sandy and silty bottoms.

1. Blue Winged Olives (swimmer nymphs, and 2 or 3-tailed duns and spinners)

The name Blue Winged Olive (BWO) is a common name given to several species in the Baetidae family. BWO mayflies are named for the dun stage as it has an olive body and opaque blue-gray coloured wings. As BWOs are small, most anglers avoid imitating the nymphs and focus on the adults using dry flies tied on 16 and 18 hooks.

Blue Winged Olives are arguably one of the most prolific hatches across North America. They hatch year round with peaks in the spring and mid summer, with another small peak in the fall. Occupying mostly runs, the nymphs will swim to the surface where the dun will crawl out. The duns often struggle to break through the surface of the water and end up floating long distances, exposing them to trout.

2. Western Green Drake (crawler nymphs, and 3-tailed duns and spinners)

Western Green Drake (Drunella grandis) nymphs are crawlers with robust looking bodies and legs. Both nymphs and adult imitations are tied on size 10 hooks. The dun has a brown to olive-coloured body with distinctive yellow banding and dark gray opaque wings. The spinners look very similar to the duns, except the wings are more translucent.

Hatching in the late afternoons from June to July, nymphs migrate to slower moving parts of the river. From here the nymphs perform false charges to the surface before they finally ascend to hatch. At the surface, the duns crawl out and float several meters downstream as they need time to inflate their wings. Given their large size and fairly good size hatches, trout gorge themselves on the duns and cripples.

Later in the season in August, a mayfly that looks like a smaller version of the Western Green Drake hatches. Called Flavs, after Drunella flavilinea, this mayfly looks and acts very similar to the Western Green Drake. This hatch is very productive especially on East Kootenay streams.

3. Pale Morning Dun (crawler nymphs, and 3-tailed duns and spinners)

Pale Morning Dun (PMD) mayflies include two species (Ephemerella excrucians and E. infrequens). The nymphs and adults are imitated with flies tied onto size 12 and 14 hooks. The nymph is effectively imitated with a beadhead pheasant tail nymph. The dun has a light cream to yellow body and light gray opague wings. Sometimes you may find the odd orange to amber bodied PMD among the yellow ones. The spinners have clear wings with pale yellow to reddish-brown bodies. And as the name suggests these mayflies will hatch in the morning to mid-afternoon.

Most PMD nymphs live in faster moving riffles and runs but migrate to slower and less turbulent water to hatch. PMDs hatch in similar fashion as the Western Green Drake. They also struggle to break through the surface film and often damage their wings in the process. As a result, emerger and cripple fly patterns are very effective in imitating emerging PMDs. Once on the surface the duns float down river to allow time for inflating their wings.

4. Mahogany Dun (crawler nymphs, 3-tailed duns and spinners)

The name Mahogany Dun (Genus Paraleptophlebia) is a collective name for a few species in British Columbia. Imitations of Mahogany Duns are tied on hooks ranging in size from 12 and 16 hooks. The dun usually has a dark chocolate brown body with dark gray wings. The spinner has clear wings with reddish brown to dark brown bodies.

Mahogany Dun hatches peak at two times in the season: in the late spring and again in the early fall. These peak times are due to different species and not different generations in the same year. Mahogany Duns hatch in slower moving water, and even those that live in faster water migrate to slower water before ascending to the surface. When it is time to hatch, the nymphs swim to the surface, and like many other mayflies the duns often drift long distances before flying away.

5. Pale Evening Dun (clinger nymphs, and 2-tailed duns and spinners)

Mature PED (Genus Heptagenia) nymphs and adults are imitated with flies tied onto size 12 and 14 hooks. As a member of the clinger nymphs, use nymph patterns with a wide profile to imitate the wide head and flattened body of the PED nymph. The bodies of the duns and spinners vary in colour from a creamy-white to yellow, to reddish-brown. The veins in the wings of the duns and spinners are pronounced. Many anglers will confuse PED and PMD adults. In British Columbia adult PED species have two tails and the PMD species have three tails.

As the name suggests these mayflies typically hatch later in the afternoon to early evening from June to August. The mature nymphs emerge by first migrating to slightly calmer water. The nymphs of some PED species swim to the surface to hatch, but other PED species duns hatch on the river bottom and then swim to the surface. Casting soft hackle or traditional wet flies are effective in imitating these swimming duns. Once at the surface the duns fly away quickly.

6. Brown Drake (burrowing nymphs, and 3-tailed duns and spinners)

Brown Drakes (Ephemera simulans) are large mayflies – the largest of the stream dwelling mayflies in British Columbia. This long and slender mayfly is best imitated by tying imitations onto 3X long size ten hooks. The nymphs have pale yellow bodies, and tan-brownish thorax and legs. The wings of both the duns and spinners are clear with distinctive brown marks. The abdomen is a pale yellow with brown markings on the dorsal and ventral sides. Peaking in the month of July, Brown Drakes hatch in the afternoon and into the early evening. The nymphs swim fast to the surface where the duns break through the surface to fly away quickly.

Brown Drake adults are often confused with Hex mayflies (Hexagenia limbata), a mayfly more associated with stillwaters. The easiest way to distinguish between these two mayflies is to count the number of tails of the adults. Brown Drake adults have three tails and Hex mayflies have two tails.

7. Western March Brown (clinger nymphs, and 2-tailed duns and spinners)

The Western March Brown (Genus Rhithrogena) mayfly is one of the most well known mayflies in BC. It is one of the first big insects to hatch on BC streams. Nymph and dry fly imitations are tied on standard size 12 to 14 hooks when fishing Western March Browns (WMB). As a member of the clinger nymphs, WMB nymphs look similar to PED nymphs but grow slightly larger. WMB duns have dark gray marks or patches on their wings and light brown abdomens with cream to yellow coloured segmentation.

This mayfly starts to hatch in March in the lower third of BC and will continue to do so until May as you move further north. When hatching the nymphs ascend slowly to the surface, and as result the current carries the drifting nymphs for meters, making them easy prey. Once at the surface, the duns need time to inflate their wings, exposing themselves further to trout.

9. Tricos (crawler nymphs, 3-tailed duns and spinners)

The Trico (Genus Tricorythodes) is the smallest mayfly anglers need to consider when matching the hatch. Imitated with flies tied onto size 22 to 26 hooks, anglers who fish Tricos focus on the spinner falls. Female spinners have bodies that are tan to light olive in colour. The male spinners are essentially black with clear wings. During a spinner fall trout key in on the male spinners that fall first and later switch to the females as more females become available. In British Columbia the Trico spinner falls are not as intense as the famed spinner falls on some rivers of Wyoming and Montana, but it is always a good idea to have a few spinner imitations for August and September.

The mayflies highlighted in this article represent the major hatches across British Columbia. You may find a few different mayflies on your local streams, but for the most part these are the mayflies you need to know about. It is very satisfying when to tie a fly for a specific hatch and then have success. Matching the hatch improves your fishing success, but it also makes you more aware of your environment, and the complexity and intricacies of our streams.

Mayflies for breakfast, lunch and dinner

by Danie Erasmus

First published in              July/August 2018