Blue-Winged Olive

Hatch Name: Blue-Winged Olive
Family / Type: Mayfly / Small Minnow Mayfly Group
Scientific Name: Baetidae — commonly the Baetis complex and related genera

How to Identify a Blue-Winged Olive

“Blue-Winged Olive” sounds like one insect. On the river, it's better understood as a group of similar small mayflies anglers fish in much the same way. The classic adult profile is: small, slender, olive-bodied with blue-gray or slate-gray upright wings.

Body Color

The body may range through: light olive, medium olive, brown-olive, gray-olive and darker muted tones. Don't expect every BWO to look fluorescent green. Most don't.

Wings

The name comes partly from the characteristic: blue-gray, smoky gray or slate-colored appearance of the adult wings. Like other mayflies, the dun normally holds its wings upright above the body.

Size

BWOs are generally small. A practical angling range often falls around: #16–#24 with #18–#22 extremely common in many fisheries.

But size varies by taxon, generation, region and season. Look at the natural before choosing the hook.

Tails

Depending on the actual species or genus represented by the local BWO complex, adults and nymphs may show different tail characteristics. That is another reason we don't write this page like one exact species profile.

Nymph

Many important BWO nymphs in Baetidae are slender, streamlined swimmers. They are much slimmer than a robust Hendrickson-style crawler. Common traits include: streamlined body, olive to brown coloration, visible abdominal gills, strong swimming ability,small size.

BWO vs. Hendrickson

Hendrickson: larger, more robust, classic early-spring mayfly.

BWO: generally smaller, slimmer and often important across multiple seasonal windows.

BWO vs. Sulphur

BWO: live body + blue-gray wings.

Sulphur: generally cream, yellow, orange or pale sulfur-colored body.

Lifecycle

The Blue-Winged Olive Lifecycle

BWOs follow the mayfly lifecycle:

Egg

Adult females return to the water and deposit eggs. The young insect eventually develops into the aquatic nymph.

Nymph

BWOs spend most of their lives underwater. Many common Baetid nymphs are active swimmers rather than strictly bottom-clinging insects. That matters. They're capable of moving through the water and can become available to trout even outside a dramatic hatch.

Pre-Emergence

As nymphs mature, activity increases. The insects become more vulnerable as they leave normal holding habitat and prepare to emerge. Trout may begin feeding actively on BWO nymphs before any obvious surface hatch.

Emergence

Nymphs swim upward. Near the surface, the insect must complete the transition into its first winged adult stage. That creates an exposed feeding window:

ascending nymph ↓ emerger ↓ dun

Trout can intercept the insect through all three.

Dun

The dun, or subimago, rides the surface while its wings become functional. Cold, damp and overcast conditions can slow that escape. For the mayfly, that's a problem. For the trout, that's lunch.

Spinner

The dun later molts into the mature adult spinner. After mating, females return toward the water. Spent adults may eventually lie flat in the surface film.

Why This Matters

A BWO hatch is not one dry-fly event. It's a progression:

01 nymph feeding

02 ascending nymphs

03 emergers

04 duns

05 spinners

Your fly needs to follow the fish through that progression.

Where Blue-Winged Olives Live

BWO-type mayflies occur in a wide variety of flowing trout water. Depending on the actual species represented locally, they may inhabit: riffles , runs , moderate current , gravel, cobble, vegetation , stream margins and other productive freshwater habitat.

Swimming Nymphs

Many of the Baetid nymphs anglers call BWOs are capable swimmers. That makes them relatively mobile compared with insects built exclusively to cling beneath heavy rocks. They can enter the drift and become available to trout throughout the water column.

Moving Water

In rivers and streams, look around: riffles , riffle transitions , runs, seams and feeding lanes downstream of productive nymph habitat.

Slower Feeding Water

During a hatch, trout may move into surprisingly subtle water. Small mayflies can collect in: slicks , inside seams, pool tails, slow margins and other areas where surface food accumulates.

What This Means to the Angler

Don't decide where to fish based only on where you see duns. Ask: Where are the nymphs coming from? and Where does the current concentrate the emerging adults? Those two locations may not be identical.

Typical Timing

When Do Blue-Winged Olives Hatch?

One of the reasons BWOs matter so much to trout anglers is that the name covers multiple species and generations capable of producing activity across a broad portion of the season.

Unlike a single short-lived seasonal event, BWO activity can occur in: spring and again strongly in fall with some waters producing additional activity outside those primary windows.

Spring

BWOs can become important during cool spring conditions before many larger insects dominate the calendar.

Summer

Some members of the broader BWO complex remain relevant during summer, although local species and conditions determine whether the fishing is significant.

Fall

Fall is another major BWO period in many trout fisheries. Cooler weather and overcast conditions can create excellent emergence.

Winter

Certain Baetid-type mayflies remain relevant surprisingly late or early in the year where conditions permit. Do not assume: cold = no mayflies.

Cloudy Days

BWOs have a famous association with: cloud cover , mist , light rain and cool, unsettled weather. Those conditions can prolong emergence because duns may take longer to leave the water. That gives trout more time to feed.

Sunny Days

BWOs can still emerge on bright days. The window may simply be: shorter , more concentrated or less obvious.

Time of Day

On cool days, activity often becomes most noticeable from: late morning into afternoon. But timing changes by: species, season, latitude, water, and weather.

The Rule

A gray miserable day that makes most anglers stay home? That might be exactly when you should start watching for BWOs.

Water Temperature

Water Temperature & the BWO Hatch

BWOs are another excellent example of why Trout Eddy should not publish a single magic hatch temperature. The term BWO represents multiple mayflies. Different taxa, seasons and populations emerge under different conditions.

Cool-Water Association

Many famous BWO events occur in relatively cool water. Anglers often see strong activity during: spring , fall and cool, overcast weather.

Temperature Trends

Seasonal warming can help initiate spring activity. Seasonal cooling can coincide with important fall generations. But: the same number does not mean the same thing in April and October.

Weather Matters

Cloud cover and damp weather can have major practical effects because winged adults may remain vulnerable on the water longer. That can create better surface fishing without the thermometer necessarily changing dramatically.

Use the Thermometer Correctly

Use water temperature to understand the seasonal system. Then confirm with: nymph activity , shucks, duns, spinners, rise forms and actual feeding behavior. Don't fish a number. Fish the hatch.

Geographic Range

Where Blue-Winged Olives Occur

The angling category called Blue-Winged Olive is widely distributed across North American trout water. Because the name represents multiple related small mayflies rather than one exact species, BWO-type hatches occur across an enormous geographic range.

They are relevant in: Great Lakes trout streams, Northeastern rivers , spring creeks,western freestone rivers , tailwaters and many other trout fisheries.

This Is Why Taxonomy Matters

The BWO on a Michigan river does not need to be the exact same species as the BWO an angler fishes in Montana. Both may legitimately fall under the angling term.

Trout Eddy Architecture

This national entity owns: how anglers understand and fish the BWO complex. Then: State pages own regional relevance. Water pages own local hatch importance and timing. Supporting Guides can later own narrow searches such as:

Michigan BWO timing or Au Sable BWO fishing.

That is exactly why we do not jam a national hatch calendar into this page.

How Trout Feed During a BWO Hatch

BWO fishing can make perfectly reasonable anglers question whether trout are deliberately trying to annoy them. Usually the fish are simply eating a different stage than the angler thinks they are.

Before the Hatch

BWO nymphs are available subsurface long before visible duns arrive. As mature nymphs become increasingly active, trout may key on: drifting nymphs and swimming nymphs near the bottom and through feeding lanes.

This can make a small olive nymph productive even when the surface looks completely dead.

Ascending Nymphs

As emergence begins, nymphs move upward. Trout may shift higher to intercept them. Watch for: flashes , fish suspending off bottom and feeding without obvious surface rises.

Emergers

The surface film can become the critical feeding zone. Trout taking emergers may produce: bulges, subtle dimples , backs , dorsal fins or barely visible disturbances. If you're seeing that instead of clean noses: the fish may not want your high-floating dun.

Duns

Once enough duns are present, trout may feed directly on the surface adults.Because BWOs are often small and abundant, fish can become highly selective.

They may care about: size, profile, stage and especially drag.

Cripples

Cold or damp weather can increase the number of adults that struggle to escape cleanly. Those insects are easy meals. A low-riding cripple or emerger may sometimes outperform a perfectly upright dry.

Spinners

Later spinner activity can create another feeding opportunity. Spent adults lie much flatter than duns.

The BWO Lesson

If trout are rising and ignoring your BWO dry, don't immediately tie on a smaller BWO dry. Ask: Are they actually eating the dun? That's usually a better place to start.

Lifecycle Stages

BWO Stages Anglers Need to Understand

Nymph

Where: Primarily underwater around productive stream habitat.
What trout see: Small olive-to-brown swimming mayfly nymph.
When it matters: Throughout much of the season and especially before a hatch.
Fishing clue: No surface activity but trout actively feeding subsurface.

Ascending Nymph

Where: Moving upward through the water column.
What trout see: Increasingly exposed moving food.
When it matters: Immediately before visible emergence.
Fishing clue: Trout begin feeding above their normal bottom position.

Emerger

Where: Immediately below or within the surface film.
What trout see: Vulnerable mayfly completing emergence.
When it matters: During active hatching.
Fishing clue: Bulging, soft rises rather than clear surface takes.

Dun

Where: Riding the surface.
What trout see: Small olive-bodied mayfly with blue-gray upright wings.
When it matters: During the visible hatch.
Fishing clue: Consistent clean rises.

Cripple

Where: Trapped or struggling at the surface.
What trout see: Easy food that cannot escape properly.
When it matters: Especially during cool, damp or difficult emergence conditions.

Spinner

Where: Mature adults returning toward the water after mating.

Spent Spinner

Where: Flat in the surface film.
What trout see: Dead or dying adult.
When it matters: After reproductive activity.

Fly Patterns

Flies for Blue-Winged Olive Hatches

The BWO box should cover the whole lifecycle, not twelve versions of one tiny dry fly.

Nymphs

Useful patterns include:

Pheasant Tail Nymph
Baetis Nymph
Olive Mayfly Nymph
small Hare's Ear-style patterns
Perdigon-style olive nymphs

where technique and water justify them and other slim BWO-style nymphs.

Size

Common sizes often fall around: #16–#22 with even smaller patterns sometimes appropriate. Match the insects actually present.

Emergers

Excellent styles include:

RS2
WD-40
CDC BWO Emerger
Baetis Emerger
small soft-hackle mayfly patterns
and other low-riding transitional flies.

For BWOs, carrying a good emerger is not optional if you want to understand the hatch.

Duns

Useful dry patterns include:

Blue-Winged Olive
Parachute BWO
Comparadun
Sparkle Dun
CDC BWO
and similar small olive mayfly imitations.

Cripples

Low-riding BWO cripple patterns can be especially useful during difficult emergence.

Spinners

Useful options include:

BWO Spinner
Rusty Spinner
and appropriately sized spent-wing patterns.

Color

Keep body colors in the neighborhood of:

olive
brown-olive
gray-olive
or whatever the natural actually looks like.

Do not let the word “olive” convince you every fly should glow green.

Trout Eddy Rule

For BWOs: size + stage + drift usually deserve your attention before microscopic color differences.

How to Fish the Blue-Winged Olive Hatch

BWO fishing is often precision fishing. Not necessarily long-cast precision. Feeding-stage precision. Before Surface Activity Fish slim BWO nymphs naturally through likely feeding water.

Approaches can include:

indicator nymphing
tight-line nymphing
Euro-style nymphing
or another controlled dead-drift system.

Keep the imitation appropriately small.

As the Hatch Builds

Pay attention to fish depth. If trout begin feeding higher:

reduce weight
shorten depth
or introduce an emerger into the upper water column.

Don't keep dredging simply because nymphing worked thirty minutes ago.

Emerger Fishing

When trout produce bulging or subtle rises:fish: in or immediately below the surface film. A lightly suspended emerger can be far more convincing than a high-floating dry.

Fishing Duns

Use a clean, drag-free presentation. BWOs are small.

That means unnatural movement can become painfully obvious to a selective trout.

Position Matters

Try to approach from an angle that allows: good drift, minimal line over the fish and accurate delivery into the feeding lane.

Don't Overcast

If a trout is rising predictably: watch it. Find the rhythm. Make one good presentation. Don't throw thirteen panic casts at its head because the bugs are small.