What Is a Blue-Winged Olive? The BWO Lifecycle, Hatches & Flies to Fish

What Is a Blue-Winged Olive? The BWO Lifecycle, the Flies to Fish, and Then They Get Eaten

There may be no insect in fly fishing with a better public-relations department than the Blue-Winged Olive.

Mention BWOs around trout anglers and suddenly everyone gets serious. Clouds become exciting. Drizzle becomes "perfect weather." Grown adults willingly stand waist-deep in cold water staring at tiny insects that would otherwise be dismissed as annoying little bugs.

And for good reason.

The Blue-Winged Olive, commonly called a BWO, represents one of the most important groups of mayflies in trout fishing. They can hatch during periods when very little else is happening, trout love them, and understanding the BWO lifecycle can transform a frustrating day of watching rising fish into one of those ridiculous afternoons where you forget your hands are cold.

But here's the first thing to understand:

A Blue-Winged Olive isn't really one bug.

So… What Exactly Is a Blue-Winged Olive?

"Blue-Winged Olive" is a fly-fishing term used broadly for several small mayflies, particularly species within the Baetidae family. Anglers group them together because many share similar characteristics: relatively small bodies, olive to brownish coloration and slate-grey or bluish wings.

Entomologists may enjoy arguing about exactly which species you're looking at.

The trout would like everyone to hurry up because lunch is getting away.

Depending on the water and season, BWOs commonly appear around sizes #16 through #22, although larger and smaller examples certainly exist. They are especially important during the cooler portions of the fishing season and can provide excellent spring and fall dry-fly fishing.

The important thing isn't memorizing Latin.

It's understanding what happens to this insect during its life.

Because trout don't simply eat "BWOs."

They eat different stages of BWOs differently.

And that's where things get interesting.

Chapter One: Life Begins Underwater

Before our little BWO becomes famous, it spends most of its existence largely unnoticed on the river bottom.

BWO nymphs are generally considered swimming mayfly nymphs. They're streamlined, active little creatures with tails and developing wing pads, living among rocks, vegetation and the complex underwater structure of a trout stream.

This stage can last for months.

Imagine spending nearly your entire life underwater, slowly developing toward one glorious afternoon when you'll finally grow wings and experience the world above the river.

Unfortunately, trout are waiting.

During the nymph stage, patterns such as a Pheasant Tail, CDC Pheasant Tail, small olive nymphs and slim mayfly imitations can be extremely effective. The Pheasant Tail remains particularly useful because its slender profile and natural coloration suggest a wide range of mayfly nymphs without trying too hard to become one exact insect.

That's an important fly-fishing lesson.

Sometimes resemblance beats replication.

If trout are feeding near the bottom before a BWO hatch begins, fish these patterns deeper and concentrate on natural feeding lanes. As the hatch approaches, however, the game changes.

Our BWO is about to attempt the most dangerous commute of its life.

Chapter Two: The Great Escape

When conditions trigger emergence, mature BWO nymphs begin moving toward the surface.

This is the moment anglers often miss.

We look across the river, don't see many adult mayflies, and conclude:

"Nothing is hatching."

Underwater, meanwhile, the buffet may already be open.

Emerging insects are vulnerable. They aren't safely hidden among rocks anymore, nor have they become functional flying adults. They're travelling through the water column toward the surface, and trout know exactly what's happening.

This is when CDC nymphs, soft-hackle patterns, slim Pheasant Tails and emerger-style flies become incredibly useful. CDC becomes particularly interesting here because its fibres move subtly in the current and can suggest legs, wing material or the general struggling mess of an insect trying to become something else.

Fish these patterns higher in the water column than you might normally fish a nymph. Depending on the situation, swinging the fly gently at the end of the drift can also imitate an ascending insect.

And watch the trout carefully.

If fish are rising but you rarely see an actual mouth breaking the surface, they may not be eating adult flies at all.

They may be eating the BWO before it ever gets there.

Chapter Three: Welcome to the Surface Film

Our BWO has finally reached the surface.

Things are not improving.

The surface film is an extraordinary barrier for something that weighs almost nothing. The insect must break through it, shed its nymphal shuck, unfold its wings and become what fly anglers call a dun.

For us, this is beautiful.

For the mayfly, this is basically trying to change clothes while crossing a six-lane highway.

Trout understand vulnerability.

An insect trapped in the film cannot escape quickly. That's why trout frequently prefer emergers and cripples over perfectly formed adults.

This is where a tiny emerger suspended just beneath a dry fly can be devastating. We have talked about this setup before at FishFuel: a dry fly on the surface with an emerger or pupa-style imitation hanging only a few inches underneath.

Most anglers think "dry-dropper" means a large Chubby Chernobyl with a tungsten nymph hanging four feet below it.

Not always.

During a hatch, try thinking in inches instead of feet.

A BWO dry with a small emerger hanging two to six inches beneath the surface can cover two stages of the hatch simultaneously.

It's almost unfair.

Almost.

Chapter Four: The BWO Dun

Success!

Our BWO has escaped the water.

It now sits on the surface with upright wings, waiting for them to become strong enough for flight.

This is the stage most anglers picture when they hear "Blue-Winged Olive hatch." Trout rise deliberately, noses appear through the surface, and suddenly every angler on the river begins digging through the smallest compartment of their fly box.

Now a BWO dry fly, small parachute pattern, CDC dry or appropriately sized mayfly imitation becomes the obvious choice.

But don't obsess over colour.

Pay close attention to size, silhouette and drift.

A beautifully matched BWO imitation dragging unnaturally across the current is still suspicious. A slightly imperfect imitation in the correct size, sitting properly in the film and drifting naturally, often gets eaten.

Trout don't carry colour charts.

They are remarkably good at detecting bad presentations.

Why Cloudy Days Can Be So Good

BWO fishing has developed a deserved association with cloudy, cool and occasionally miserable weather.

There is biological logic behind it.

Cooler, humid conditions can prolong emergence and keep mayfly duns on the water longer before they're able to fly away. From the trout's perspective, that's wonderful news. Instead of lunch immediately leaving the restaurant, lunch sits on the table for a while.

Overcast conditions also reduce harsh light penetrating the water and can make trout more comfortable feeding near the surface.

This is why an afternoon that looks disappointing to a golfer can look magnificent to a fly angler.

Grey sky.

Light drizzle.

Tiny mayflies.

Happy trout.

Questionable life choices.

Perfect.

Chapter Five: Adulthood Lasts Approximately Five Minutes*

*Not literally. But philosophically, we're not far off.

The mayfly dun eventually flies away from the river and later molts again into the sexually mature adult known as a spinner.

Yes.

Mayflies molt after developing functional wings, which is unusual among insects.

The spinner's purpose is remarkably straightforward.

Find another mayfly.

Reproduce.

Return to the water.

Die.

It's difficult not to admire the efficiency.

Adult spinners gather into mating swarms, often above riffles and recognizable landmarks along the river. After mating, females return to deposit their eggs.

Then their wings flatten.

Their bodies become still.

And thousands of tiny insects lie helplessly on the surface.

We call this the spinner fall.

Trout call it room service.

Chapter Six: And Then They Get Eaten

The spinner stage may produce some of the most technical and rewarding dry-fly fishing of the entire BWO lifecycle.

A BWO spinner, spent-wing mayfly or very low-riding dry becomes the right tool. Look for flat wings and flies sitting flush in the surface film rather than upright dun profiles.

Sometimes the rises become incredibly subtle. Instead of dramatic splashes, you'll see tiny dimples, noses or barely perceptible disturbances.

That's because the trout doesn't need to chase anything.

The food is dead.

It isn't going anywhere.

After months of growing underwater, surviving currents, avoiding trout, breaking through the surface, escaping the river, growing another set of adult characteristics, finding a mate and successfully reproducing, our brave little BWO returns triumphantly to the water.

And gets eaten.

Nature rarely bothers with sentimental endings.

The BWO Fly Box

If you want to successfully fish a Blue-Winged Olive hatch, don't carry one BWO fly.

Carry the lifecycle.

For the nymph stage, carry slim Pheasant Tails, CDC Pheasant Tails and small olive mayfly nymphs. As insects begin ascending, move toward CDC nymphs, soft hackles and emerging mayfly patterns.

When activity reaches the film, fish emergers and cripples. Once duns begin riding the surface, switch to BWO dries, CDC dries and small parachute-style mayflies.

And when the adults return as spent spinners?

Go low.

Very low.

A spent-wing pattern sitting almost flush in the film may outperform everything else in your box.

This is why carrying multiple stages of an insect matters more than carrying twenty different colours of essentially the same fly.

The Bigger Lesson of the Blue-Winged Olive

The BWO teaches us something bigger about fly fishing.

We spend enormous amounts of time asking:

"What fly are they eating?"

Perhaps the better question is:

"What is happening to the insect they're eating?"

Is it crawling?

Swimming?

Ascending?

Emerging?

Floating?

Dying?

Those questions lead to better fly selection because trout aren't feeding on fly names. They're feeding on opportunities.

The vulnerable nymph leaving the bottom is an opportunity.

The emerger trapped beneath the surface is an opportunity.

The dun waiting for its wings to dry is an opportunity.

The spinner lying dead in the film is perhaps the greatest opportunity of all.

And maybe there's something wonderfully philosophical buried inside this tiny insect's absurdly brief adult existence.

The BWO spends almost its entire life preparing for a moment.

It finally reaches the surface.

It grows wings.

It flies.

It finds another.

It creates the next generation.

And then it falls back to the same river where everything began.

Perhaps that's tragic.

Perhaps it's perfect.

The trout doesn't appear particularly concerned with the distinction.

It just eats it.

And somewhere downstream, an angler sets the hook.