Mayflies

Mayfly Flies: Follow the Bug Through Its Life Cycle

There are few sights in fly fishing more likely to make an angler forget dinner plans than a river suddenly coming alive with mayflies.

One trout rises.

Then another.

A few duns begin sailing downstream with their wings standing upright like tiny sailboats, and within minutes the river can look as though someone flipped a switch.

The temptation is to immediately start digging through the fly box for whatever dry fly looks vaguely correct.

Sometimes that works.

Sometimes the trout continue rising three feet from your fly while demonstrating the sort of selective feeding behavior that makes otherwise reasonable adults begin talking to themselves.

The better approach is to ask one question:

What stage of the mayfly life cycle are the trout actually eating?

Mayflies give the fly fisher opportunities from the bottom of the river all the way to the surface. Nymphs live underwater for the overwhelming majority of their lives, emergers swim toward the surface, duns struggle free of their shucks, adults return as spinners, and finally spent insects lie helplessly on the water.

Understand that progression and the mayfly section of your fly box suddenly becomes much easier to navigate.

What Are Mayflies?

Mayflies are one of the primary aquatic insect groups eaten by trout and are found in both rivers and stillwaters.

The immature mayfly nymph lives underwater before emerging into a winged adult.

Once mature, it rises toward the surface and transforms into a dun, the first winged adult stage.

After leaving the water, the dun molts again into the sexually mature spinner.

The adults mate, return to the water, lay eggs, and die.

For the trout, this entire process is essentially a buffet moving through different floors of the restaurant.

The trick for the fly fisher is figuring out which floor the fish are currently visiting.

Mayfly Nymphs: Where Most of the Feeding Happens

Mayflies spend far more of their lives underwater as nymphs than they do flying above the river.

That makes mayfly nymph patterns productive throughout the year, even when there isn't a visible hatch.

Patterns such as the Pheasant Tail, Hare's Ear, Juju Baetis, WD-40, and other slim-bodied mayfly imitations can represent a broad range of immature mayflies.

Fish them:

Under an indicator.

As part of a dry-dropper setup.

On a tight-line or Euro nymphing rig.

Or behind another heavier fly.

When nothing appears to be happening on the surface, don't assume nothing is happening.

The riverbed may be crawling with food.

How Do I Choose the Right Mayfly Nymph?

Start with three things:

Size.

Color.

Profile.

If the naturals in the river are small olive Baetis nymphs, tying on a size 10 brown imitation simply because it is easy to see may be asking a lot from the trout.

Match the general proportions first.

Small mayflies such as Blue Winged Olives may require flies in the upper teens or twenties.

Larger drakes may be matched with size 10–14 patterns.

Exact sizes vary tremendously by species and location, but the principle is consistent:

Get close to the natural insect before worrying about the name printed on the fly-bin label.

Trout don't know what a Juju Baetis is.

They know whether it looks edible.

Mayfly Emergers: When Trout Start Looking Up

As a mature nymph prepares to hatch, it leaves the relative safety of the bottom and begins moving toward the surface.

This is one of the most vulnerable periods of its life.

No rocks to hide beneath.

No wings yet.

Just a small insect swimming upward through a column of hungry trout.

You can imagine how that goes.

Mayfly emerger patterns such as the RS2 imitate insects caught in this transition.

Emergers can be especially effective when trout appear to be rising but refuse fully floating dry flies.

Watch the rise forms.

A trout taking adult duns may clearly break the surface.

A fish feeding on emergers can create subtler bulges, swirls, or dimples.

If fish are working just beneath the surface, put something in the film rather than on top of it.

That tiny difference can completely change the day.

Mayfly Duns: The Classic Dry-Fly Hatch

The dun is the mayfly most anglers picture when someone says “mayfly hatch.”

The insect emerges through the surface and holds its wings upright above its back while they dry.

For a brief period, the dun is trapped between two worlds.

The nymph is gone.

Flight hasn't quite arrived.

And trout understand the situation very well.

Patterns such as BWO CDC Thorax Duns, Comparaduns, Parachutes, Light Cahills, and drake patterns imitate this vulnerable adult stage.

Ascent's current collection includes both higher-riding patterns and lower-riding CDC comparadun designs, allowing you to adjust how the fly sits in the surface film.

That can matter on selective fish.

Sometimes a trout doesn't want a different insect.

It wants the same insect presented differently.

Comparadun vs. Parachute Mayfly Patterns

Both imitate adult mayflies, but they sit differently on the water.

A parachute pattern generally uses hackle wrapped horizontally around the wing post, helping the fly float well and remain visible.

These can be excellent general-purpose mayfly dries.

A comparadun generally sits lower in the surface and has a more understated profile.

That lower position can imitate a freshly emerged or struggling dun extremely well.

If trout are confidently taking your parachute pattern, there is no reason to complicate things.

If they begin inspecting it from six inches away before rejecting it with the contempt of a restaurant critic, try a lower-riding comparadun or emerger.

Fly selection often becomes increasingly subtle as the fish become increasingly annoying.

Mayfly Spinners: Don't Leave the River Too Early

After leaving the water, the mayfly dun molts again into a spinner.

Spinners often have clearer wings, brighter bodies, and longer tails than the dun stage.

They gather above the water to mate before returning to the surface.

That return can create a spinner fall, one of the most overlooked feeding events in trout fishing.

The insects land on the water, often with their wings spread flat, and become extremely easy meals.

Patterns such as Rusty Spinners, Grey Drake Spinners, Trico Spinners, and other spent-wing imitations can be deadly during these events.

This is also why leaving the river immediately after the hatch slows down can be a mistake.

The morning's duns may become the evening's spinner fall.

The trout haven't necessarily stopped eating.

They may simply be waiting for dessert.

What Is a Spent Mayfly Spinner?

A spent spinner is a dead or dying adult mayfly lying flat on the water after mating.

Its wings are typically spread horizontally rather than held upright.

The insect has essentially reached the end of the line.

For trout, this is wonderfully convenient.

No escaping.

No swimming.

No flying.

Just protein drifting downstream.

Spent spinner falls can create extraordinarily selective feeding because the fish may be keyed into an exact profile sitting flush in the surface.

If trout are rising steadily during calm evening conditions but ignoring upright dry flies, look closely at the water.

The answer may be lying flat rather than standing tall.

Blue Winged Olive Mayfly Patterns

Blue Winged Olives, or BWOs, are among the most important mayflies for trout anglers because various species can hatch across a wide range of seasons and conditions.

Small olive-bodied nymphs and adults are particularly important on tailwaters and cooler trout streams.

Useful BWO-style patterns include:

Juju Baetis.

WD-40.

RS2.

BWO CDC Thorax Dun.

BWO CDC Comparadun.

Small parachute patterns.

Rather than carrying one “BWO fly,” carry several patterns covering nymph, emerger, and dun stages.

The insects change during the hatch.

Your fly probably should too.

PMDs, Sulphurs, Drakes, Tricos & Hex Mayflies

Different regions have their signature mayfly events.

Pale Morning Duns (PMDs) are staples on many western rivers.

Sulphurs can create tremendous spring and summer fishing in the Midwest and East.

Green, Grey, and Slate Drakes bring much larger insects to the water.

Tricos can hatch in staggering numbers and demand tiny flies and fine presentations.

And Hex mayflies occupy the opposite end of the spectrum, with massive adults capable of turning evening water into something resembling an insect invasion from a low-budget science-fiction movie.

Ascent's collection spans that range—from small Trico emergers to large foam-bodied Hex and drake patterns.

That breadth matters because “mayfly” can mean a tiny insect you can barely thread onto 6X or something large enough to make you instinctively duck when it flies past your head.

What Size Mayfly Fly Should I Use?

Match the naturals whenever possible.

The current mayfly collection spans approximately sizes 6 through 24, reflecting the enormous size range within the mayfly world.

A practical approach is:

Use larger size 6–12 patterns for Hex, larger drakes, and other big mayflies.

Use sizes 12–16 for many medium mayfly species and larger PMDs.

Use sizes 16–20 for BWOs, PMDs, Sulphurs, and many common trout-stream mayflies.

Use sizes 20–24 for tiny Baetis, Tricos, and technical tailwater situations.

These are general ranges rather than laws.

The bug in front of you always outranks the chart.

How Do I Match a Mayfly Hatch?

You don't need to identify every mayfly to Latin genus and species.

Start with four things:

Size — How big is it?

Color — Olive, cream, yellow, brown, grey?

Life Stage — Nymph, emerger, dun, or spinner?

Behavior — Is it underwater, struggling in the film, riding high, or lying spent?

Get those four characteristics reasonably close and you'll solve far more hatches than someone who knows the scientific name but is fishing the wrong life stage.

If you want to go deeper, use Ascent's Match the Hatch Tools or explore our fly-fishing entomology resources.

Biology should simplify fly fishing.

Not make you feel like you need to pass an exam before tying on a fly.

What If I Can't Tell Which Mayfly Is Hatching?

Choose a proven generalist pattern close to the correct size and color.

A Beadhead Flashback Pheasant Tail can imitate a tremendous variety of mayfly nymphs.

An RS2 and Chocolate Thunder both cover many small emerging mayflies.

A Parachute Adams can represent a broad range of adult insects.

A Rusty Spinner handles many spinner-fall situations.

You don't need 611 different mayfly patterns to catch trout.

You need a thoughtful selection that covers the major life stages in several sizes and colors.

This is exactly why building a fly box by life cycle can be more useful than simply collecting every new pattern you encounter.

Build a Complete Mayfly Fly Box

A good mayfly selection should contain representatives of the entire hatch.

Carry:

Several general mayfly nymphs.

Small Baetis-style nymphs.

Emergers.

Higher-riding dry flies.

Low-riding comparaduns.

Species-specific patterns for important local hatches.

Spent spinners.

Then stock them in several useful sizes.

That system allows you to follow the hatch from beginning to end rather than hoping one pattern does everything.

If building that box sounds overwhelming, Ascent's Biologist Packed Fly Selections can simplify the process with our in-house aquatic biologist matching the insects and life stages on the waters you fish, when you fish them!

There is no trophy for owning 400 patterns if you can't find the one you need when the fish start rising.

Why Buy Mayfly Patterns From Ascent Fly Fishing?

At Ascent Fly Fishing, we approach flies through the eyes of anglers and aquatic biology.

The point isn't merely to give a pattern a clever name and hope you buy six.

We want you to understand what the fly represents, when that insect is available to trout, and where the pattern fits into the hatch.

That is why you'll find patterns covering mayflies from nymph through emerger, dun, spinner, and spent stages.

If you're trying to prepare for a BWO hatch, build a PMD box, match Tricos on a tailwater, or figure out what to carry for a destination trip, start with the insect rather than the latest trend.

Fish have been eating mayflies for a very long time.

Fortunately for us, they show no sign of stopping.

Mayfly Fly FAQs

What are the best mayfly flies for trout?

A strong mayfly box includes Pheasant Tail-style nymphs, Baetis nymphs, RS2-style emergers, parachute or comparadun dry flies, and spent spinner patterns. The best specific fly depends on the mayfly species and life stage present.

What is the best mayfly nymph pattern?

The Pheasant Tail is one of the most versatile mayfly nymph imitations ever tied. Smaller Baetis-specific patterns such as Juju Baetis and WD-40 flies are excellent when matching small mayflies.

What is the best fly for a BWO hatch?

Carry several stages rather than one fly: a small Baetis nymph, an RS2 or similar emerger, and a BWO dry such as a CDC Thorax Dun or Comparadun.

What size flies should I use for mayflies?

Mayfly patterns commonly range from about size 6 for very large species to size 24 or smaller for tiny Baetis and Tricos. Match the size of the natural insects whenever possible.

When should I fish a mayfly emerger?

Fish emergers when mayflies are moving toward or through the surface and trout appear to be feeding just below the film. Subtle rises and refusals of fully floating dries are good clues.

What is the difference between a mayfly dun and spinner?

The dun is the first winged adult stage that emerges from the water. It later molts into the sexually mature spinner, which mates and returns to the water to lay eggs.

When should I fish a spinner pattern?

Spinner patterns are most effective when adult mayflies return to the water after mating, often during calm mornings or evenings. Look for spent insects lying flat on the surface.

How can I tell a mayfly from other aquatic insects?

Mayfly adults generally have upright, sail-like wings and long tails. Their nymphs commonly have three tails, although there are exceptions.

Do mayfly nymphs work year-round?

Yes. Because mayflies spend most of their lives underwater, mayfly nymph patterns can produce trout in every season even when no visible hatch is occurring.

Do I need to know the exact mayfly species to match a hatch?

No. Matching approximate size, color, life stage, and behavior will solve most fishing situations without requiring exact species identification.

Watching a mayfly hatch unfold is like being allowed to read several pages ahead in the river's story.

The nymphs begin moving.

The trout shift position.

Emergers rise.

Duns appear.

Spinners return.

And if you understand what comes next, you can change your fly before the fish force you to.

Learn the life cycle.

Carry the right stages.

Then when the river begins to boil with rising trout, you can spend less time digging through your box and more time making the cast.

Tight Lines, Friend!