The Fly Angler's Big 3 Mayflies

This article originally appeared in the 2022 Fly Fishing Made Easy special publication.

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I remember my first mayfly hatch as if it happened yesterday. My dad and I stood knee-deep in a boulder-strewn section of the Taylor River just upstream from the small community of Almont. We were watching a pod of rising fish eat surface offerings in a large, glassy pool. At the time, I was 12 years old and clueless on what was actually happening, but it quickly became apparent there was a feeding frenzy going on.

Dad promptly informed me that there was a Pale Morning Dun hatch in progress. He pointed out the classic head-to-tail riseforms that are consistent with fish feeding on mayfly duns. Further inspection and watching trout eat individual insects revealed the fish were indeed focusing their attention on a yellow-olive-bodied mayfly dun.

Dad reached into his fly box and grabbed a size 16 Light Cahill and tied it on to the end of his tippet. Within a few minutes he hooked and landed a 15-inch brown trout. That experience was like watching David Copperfield perform one of his world-renowned illusions up on stage.

From that moment forward, it was obvious that there was systematic approach to matching the hatch. That initial tutorial on basic entomology, riseforms, careful observation, choosing the appropriate fly, and a flawless presentation punctuated that there was a strong correlation between knowledge and success. Four decades later, I’m still convinced there’s a lot to learn; so I always remain open-minded and take it all in.

Imitating Mayflies

Growing up, it didn’t take me long to figure out that mayflies were significant parts of a trout’s diet. I was fascinated with entomology so I began familiarizing myself with the different Western hatches, trying to gain a better understanding of their lifecycle. It became clear that the three stages of a mayfly’s development—nymph, dun, and spinner—were some of the most important basic fundamentals in fly fishing.

Mayflies are members of the Ephemeroptera insect order, which literally means “short-lived winged insect.” Mayflies undergo what is referred to as incomplete metamorphosis, meaning their lifecycle consists of a nymph and two adult stages known as the subimago (dun) and imago (spinner).

Mayfly nymphs can be identified by their two or three tails (depending on the species), plate-like gills on the sides of their abdomens, and a single tarsal claw at the end of each leg. Mayflies exhibit a wide range of behaviors and thrive in a variety of habitats. Entomologists categorize mayflies into four distinct groups—swimmers, crawlers, clingers, and burrowers—based on their unique behaviors.

The next stage of development is referred to as the emerger, but it is not technically labeled by scientists as part of the lifecycle.

Emergence occurs when the thorax or wingcase splits open, allowing the adult (dun) to pull itself out of the nymphal shuck. The wings buds appear first, followed by the thorax, head, and legs. Mitchell’s Split Case series (Green Drake, PMD, and BWO) are proven favorites during the height of a mayfly hatch. A soft-hackle pattern or a Sparkle Wing RS2 is another good option to mimic emerging mayflies.

Mayfly duns can be identified by their two or three tails, slim, slightly curved abdomens, and opaque wings. Newly hatched duns look like miniature sailboats floating on the water, which helps distinguish them from other aquatic insects.

Several variables can affect the density of a mayfly hatch. After emerging from its nymphal shuck, a dun sits on the water until its wings dry, then it takes flight. Bright and sunny days accelerate this process, which allows the duns to leave the water quickly. Overcast or rainy (snowy) days tend to stall a mayfly’s development. The high humidity causes the wings to stay clumped-up and unfold slowly.

When conditions are right (extended periods of rain, snow, or above-average humidity), it’s not uncommon to experience a blanket hatch where the surface of the water is covered with mayfly duns. As you might imagine, the dry-fly fishing under these conditions is as good as it gets.

There are several options to consider when choosing a pattern to imitate a mayfly dun—Catskill dry flies, thorax duns, No-hackles, parachutes, and Compara-duns—all of which have advantages and disadvantages. If I’m fishing on a large river with choppy currents, a traditional dry fly works best, but if I’m targeting fish in a glassy pool or flat, a No-hackle, parachute, or Compara-dun performs better because they sit flush in the surface film.

Duns are not sexually mature until they pass though the final stage of development, referred to as the spinner stage. Once a dun takes flight, it travels to nearby foliage to rest and transform into a spinner. The final stage of development involves the dun molting and shedding its exoskeleton before turning into a spinner. This process can take anywhere from one hour up to two days, depending on the species.

You can identify spinners by their clear, glass-like wings. When they’re at rest, you can see through the veined wings—this removes any doubt that you’re looking at a spinner.

Mating flights typically occur in the morning or evening, followed by the females returning to the water to lay their eggs. After they mate and lay eggs, the spinners fall to the water, spent, and trout take advantage of the situation.

When a spinner fall becomes evident, make sure you swap out your upright-wing imitations for spentwing patterns. Anglers who fail to switch from a dun imitation (upright wing) to spentwing pattern quickly become frustrated by their inability to catch fish.

The Big Three

There are many species of mayflies, but Blue-winged Olives, Pale Morning Duns, and Tricos are by far the most important mayflies for trout and for fly fishers who frequent tailwaters and spring creeks. This group of three mayflies has fueled more dry-fly fantasies than any other group of aquatic insects.

Blue-winged Olives are often referred to by their genus name Baetis, as there are more than 150 species of these insects, everything from Baetis alpinus to Baetis vernus. Most fly fishers can’t differentiate among all these similar species, so we just identify them as some sort of Baetis, or by their common name Blue-winged Olives (BWOs).

Baetis are the first mayflies to hatch in the spring and the last to emerge in the fall, and they are known for producing exceptional dry-fly fishing during these shoulder seasons. They are multi-brooded, which means they produce two or three generations per year. Each succeeding generation of the same year is smaller in size, so adjust your tactics as needed.

Blue-winged Olives begin hatching as early as the last week of March and last to the middle part of May. Spring runoff is often the determining factor in how long the hatches provide consistent dry-fly opportunities. Blue-winged Olives also reappear in late August and continue through mid-November in most watersheds.

Baetis nymphs can be identified by their three tails, slender dark olive abdomens, and prominent wingpads. Prior to emergence, the wingpads darken and have a “bursting-at-the-seams” look. Baetis nymphs are powerful swimmers prior to emergence. Their energetic behavior becomes a trigger to nearby trout, requiring anglers to adjust their tactics to imitate the uplifting behavior. Some of my favorite Baetis nymphs include size 18-22 Mercury Flashback Pheasant Tails, Jujubaetis, and Stalcup’s Baetis.

The duns are characterized by their olive-brown abdomens and slate-gray wings. The best hatches are typically between 1 and 3 P.M. each afternoon. As previously noted, the best hatches often occur during inclement weather. My favorite dun patterns include Mathews’s Sparkle Dun, Cannon’s Snowshoe Dun, Barr’s Vis-a-Dun, Stalcup’s CDC Biot Compara-dun, and Parachute Adams in sizes 18 to 24.

Pale Morning Duns (PMDs), begin hatching in mid-June and continue into September in most watersheds. The two most important species are Emphemerella infrequens and Emphemerella excrucians. Their convenient hatching times, widespread populations, and willingness to hatch in bright and sunny conditions make them among the most popular mayflies.

PMD nymphs are thick and robust in comparison to Baetis. PMD nymphs are medium to dark brown, and poor swimmers. They spend most of their time crawling along the river’s substrate, which makes them susceptible to getting swept into the drift and becoming tasty morsels for opportunistic trout. Some of my favorite PMD nymphs include a #16-18 Barr Emerger, Mercury PMD, and Mitchell’s Split Case. These patterns have proved themselves over and over again prior to the hatch.

The duns vary in color from a yellowish olive to a rusty yellow color depending on the watershed. Pale Morning Duns have three tails, and their wings are light gray. The duns can hatch anytime between 10 A.M. and 2 P.M. Because PMDs hatch during the summer, cripples are a common sight, as many of the duns perish from heat before the emerging process is complete. These ill-fated duns get trapped in their nymphal shucks and offer trout effortless meals. Quigley’s Cripple or a Mathews’ Sparkle Dun are excellent imitations to represent the “stuck-in-the-shuck” mayflies.

Tricorythodes, often referred to as Tricos, provide anglers with some of the most consistent dry-fly fishing during the late summer and autumn. Tricos are multi-brooded so they produce reliable hatches between late June and mid-October.

Trico nymphs are tough little mayflies that can withstand a wide range of habitats, even those with silty substrates and slow-moving currents. The nymphs are typically dark tan to dark brown and have a box-like silhouette because of their triangular gills. I’ve found the best Trico nymph to be a Pheasant Tail in a size 22-24.

Tricos hatches are more complicated than the previously mentioned mayflies, and can be frustrating. During the summer, the males hatch in the evening (they start at 5 P.M. and continue until dark). The females emerge in the mornings around 7 A.M. All these generalizations are highly dependent on the weather.

The males are solid black in color, and the females have olive abdomens and black thoraxes. Mating swarms begin by 9 A.M., then the spinner fall follows shortly thereafter, and sometimes lasts until 11 A.M.

My favorite Trico dun imitation is Shane Stalcup’s CDC Biot Compara-dun. Spentwing Tricos have distinctive shapes, so anglers need to adjust their tactics to imitate the spinners. A traditional Z-lon Spinner, Stalcup’s CDC Spinner, or a Poor Witch are among my favorites during the spinner fall.

Dry-Fly Tactics and Techniques

A 9-foot, 5-weight fly rod is suitable for most of the dry-fly encounters you’ll experience in the Western U.S., while a 4-weight might be a better choice for smaller Eastern streams. I’m a huge fan of a weight-forward fly line in conjunction with a 9-foot leader tapered to 4X to 6X when I’m dry-fly fishing. I typically add an additional 18 to 24 inches of nylon tippet to lengthen my leader and prolong its life.

It’s a good idea to carry several spools of nylon tippet (4X-7X) so that you can make adjustments as needed. The size of the fly and the water type dictate the tippet size. For instance, if you are fishing with a size 16 PMD, use 5X tippet, but if you are using a size 24 Trico, 7X is a better option. Some people use a rule of four, which means dividing the hook size by 4 to arrive at the tippet diameter. So a size 20 hook divided by 4 would dictate a 5X tippet to start. The best advice I can give you is to use the largest diameter tippet you can get away and still get the trout to eat your fly.

I use paste floatant on traditional dry flies, and Frog’s Fanny or Shimazaki Dry-Shake for patterns that are tied with CDC, Z-Lon, or snowshoe rabbit. You can use both floatants on Catskill-type dry flies, Parachutes, and Compara-duns. Putting some Frog’s Fanny or other powder desiccant on small dry flies (it turns the topside white) can be a huge advantage because it helps you locate the fly. I typically put the paste floatant on first, then use Frog’s Fanny or Dry-Shake once my fly becomes waterlogged.

Once a hatch becomes evident, and you have selected the appropriate fly, your presentation varies depending on the situation. Your options include: upstream, downstream, and down-and-across deliveries. In most cases, I recommend a down-and-across delivery with a reach mend. A fly that precedes the tippet (leader) increases your odds when it comes to fooling surface-feeding trout. Keep your false casting to a minimum, and do not cast too far above your target. The farther you cast upstream, the greater the chances that your fly drags and a trout refuses your offering. [For more information on fishing a dry fly with a reach mend, read “4 Simple Strategies” by Paul Weamer.]

Trout rising to Tricos require repetitive casting because they are “gulpers” and come up and grab four or five spinners at once. If your cast is out of rhythm of their feeding, you won’t hook up. Time your cast appropriately by laying the fly down right before you expect them to feed.

The biggest mistake I see is setting the hook prematurely. Allow the fish to eat your fly, then dip its head below the surface of the water before setting the hook. The hook-set should be a gentle uplifting of the rod tip until the leader tightens. An aggressive hook-set can cause a break-off or even spook the entire pool. If you are getting refusals, and you are convinced that you are getting a good drift, you may need to downsize your fly or tippet.

Hatch-Matching Strategies

There are two schools of thought with regard to choosing a fly to match the hatch. Some fly fishers prefer using impressionistic patterns, while others favor fishing with realistic imitations.

If you’re a minimalist, it’s hard to go wrong with a Pheasant-tail  or Hare’s-ear nymphs in a wide range of sizes because you can imitate everything from Green Drakes (a large mayfly) down to tiny Baetis nymphs with just two patterns. If you stock your fly boxes with an ample supply of olive and tan Hare’s-ear Nymphs in #10-20 and Pheasant-tail Nymphs in #16-24 you’ll have all your bases covered.

Hare’s-ears and Pheasant-tail nymphs can be tied with or without beads (brass or tungsten for different sink rates). Flashback versions have Mylar tinsel for wingcases. Sometimes that dash of flash helps separate your fly from the crowd because it helps mimic the “gas bubble effect” that occurs during emergence when trapped air in the thorax produces a mirror-like flash. The addition of a silver-lined glass bead on a Pheasant-tail Nymph is another effective strategy with the same goal in mind.

Pheasant Tails are as versatile as they come because you can bulk them up or slim them down by changing the way you tie them. For instance, if I’m tying to imitate a robust PMD nymph, I use six to eight pheasant tail fibers (for the abdomen, wingcase, and legs) instead of three or four fibers for a thin and sparse body on a Baetis nymph. If I want a full thorax (for PMDs and Tricos), I choose a fluffy piece of peacock herl close to the eye of the tail feather, instead of the base, which is considerably thinner.

When I observe mayfly duns on the water, a Parachute Adams is one of my go-to patterns. I can honestly say that I have fooled trout all over the world with a Parachute Adams. In a wide range of sizes (#10-24) you can imitate a variety of mayfly duns without overthinking things.

On the flip side, modern day innovation has led to the creation of some super-realistic flies. A great example is Shane Stalcup’s Baetis Nymph; it’s the perfect pattern to imitate mayfly nymphs prior to a Blue-winged Olive emergence. At a glance, it’s hard to distinguish the difference between an actual Baetis nymph and Stalcup’s imposter.

Phil Iwane’s Dun (Iwane Dun) is another lifelike pattern that dry-fly aficionados have come to depend on. Iwane’s attention to detail is what separates his pattern from other dun imitations. It includes a forked tail, curved abdomen with prominent segmentation and sheen, and parachute hackle and upright wing for visibility.

The list goes on . . . Mike Mercer’s buggy Green Drake Nymph, Mike Lawson’s beautiful No-hackle, Chris Johnson’s stunning Mimic May, and so on, are all must-haves—but due to space constraints we cannot feature them all. In addition to carrying these proven patterns, I recommend exploring the fly bins at your local fly shop for other flies that might pique your interest. Mixing and matching your fly selection with a variety of patterns no doubt stacks the odds in your favor when it comes to fooling discerning trout.

Honorable Mentions

While I have picked out the three most important mayflies you’ll find on popular tailwater streams, these aren’t the only mayflies you’re likely to see out there, and there should be some honorable mentions.

It’s hard to have a discussion about mayflies without talking about Callibaetis, a popular aquatic insect that hatches on lakes and ponds. Callibaetis emergences typically occur from late May until the middle part of August, depending on the elevation of the fishery.

Like their Baetis cousins, they are multi-brooded, with each succeeding generation being smaller in size. Callibaetis duns can be identified by their two tails, gray-olive abdomens, and a light gray wing with a prominent dark leading edge. Callibaetis typically begin emerging between 10 and 11 A.M. with hatches lasting for hours. Callibaetis nymphs are effective prior to the hatch, especially when they are suspended underneath a strike indicator.

Quigley Cripples are effective during the emerging process. Size 12 to 14 Parachute Adams and a variety of other Callibaetis patterns fool trout keying on the duns. If you notice trout feeding on Callibaetis spinners, make the appropriate fly changes to dial in the hatch.

Two other mayflies of interest are Red Quills and Green Drakes. Red Quills (Rhithrogena undulata) fill an important void during the dog days of summer when the PMD hatches fizzle out and the Blue-winged Olives haven’t begun yet. I recommend using a #14-16 soft-hackle Pheasant Tail for Red Quill emergers, and #16 AK’s Red Quill Parachute for the duns. Rusty Spinners can be deadly in the evening hours when the spinner fall takes place, so make sure you have a few handy.

Every summer, dry-fly aficionados count down the days to the beginning of the fabled Green Drake hatch. Emergence typically begins late June and extends into the first two weeks of August. [For detailed information on Green Drake hatches, read “Western Green Drakes: The iconic summer mayflies of clean, brawling Western rivers.”]

Prior to the hatch, a #10-12 olive Hare’s-ear, Stalcup’s Green Drake Nymph, or Mercer’s Green Drake Nymph are the flies of choice. After you begin to see duns on the water, switch to a size 12 Mathews’s Sparkle Dun (Green Drake), Furimsky’s Foam Green Drake, or a Colorado Hen Wing Green Drake. Sometimes I twitch my Green Drake to imitate the skittering effect that takes place when the naturals are trying to dry their wings prior to flight. Because of their enormous size, it takes them much longer to get off the water in comparison to smaller mayflies.

For as long as I can remember, mayflies have been the heart and soul of fly fishing. I hope you get to enjoy the thrill and excitement of fooling a difficult trout during the height of a mayfly hatch . . . it’s an experience you’ll never forget!

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Pat Dorsey is a co-owner of Blue Quill Angler and has been a guide on the South Platte River for 30 years. His most recent book is Fly Fishing Guide to the South Platte River (Stackpole Books, 2019).

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