Blue Wing Olive: The Foul Weather Hatch That Saves Winter

Description

The Blue Winged Olive (BWO) is arguably the most valuable mayfly for the year-round angler. Belonging primarily to the Baetis genus (and Drunella for larger varieties), these olive-bodied mayflies hatch in reliable numbers during the shoulder seasons of Spring and Fall, and even throughout Winter in tailwaters. They are famously known as the "foul weather friend" because their most intense hatches occur on cool, cloudy, and rainy days when air pressure drops.

Life Cycle Stages

Nymph

BWO nymphs are slender, streamlined swimmers with three tails and prominent gills along the abdomen. They range from olive-brown to dark gray, typically sizes #16-22 (smaller in winter). Unlike clinger or crawler nymphs, Baetis are active swimmers that frequently drift in the current, making them available to trout year-round. They inhabit the spaces between rocks and vegetation, darting out to feed on algae and detritus.

Recommended Fly Patterns

Pheasant Tail Nymph

#14-#20

Brown, Copper, Olive

Lightning Bug

#16-#20

Pearl, Silver, Olive

Slim Shady

#18-#24

Olive, Brown

Perdigon

#14-#20

Olive, Dark Olive, TB, Brown, Black, Purple, Pink, Butano, Copper

Juju Baetis

#18-#22

Olive, Brown, Gray

Blow Torch

#14-#18

Olive, Brown, Black

Hare's Ear Nymph

#12-#18

Natural, Olive, Tan

Red Dart

#12-#18

Red, Purple, Blue, Black

Prince Nymph

#12-#18

Peacock, Brown, White, Gold

Duracell

#14-#18

Brown, Black

Hare's Ear Jig

#10-#18

Natural, Olive, Brown, Black

Spanish Bullet Perdigon

#14-#18

Olive, Brown, Copper, Black

Frenchie

#14-#18

Brown, Olive

Copper John

#14-#20

Copper, Red, Green, Black

Rojo Midge

#18-#22

Red, Black, Silver, White

Black Beauty

#20-#24

Black

Top Secret Midge

#20-#24

Brown, Black, Gray, White

Zebra Midge

#18-#22

Black, Red, Olive, Cream

Brassie

#18-#22

Copper, Olive, Black

Emerger

This is the most vulnerable and important stage for fly fishers. As the nymph ascends to the surface, it begins to split its shuck and the dun starts to emerge. In cold or humid weather, this process is slow and laborious. The half-emerged insect is trapped in the surface film with the nymphal shuck still attached, wings crumpled and wet. Trout focus heavily on emergers because they are easy targets--unable to escape, drifting helplessly. The RS2 and Barr's Emerger patterns were designed specifically to imitate this stage.

Recommended Fly Patterns

Sparkle Dun

#14-#22

Pale Yellow, Olive, Tan, Gray, Brown, Green

Barr's Emerger

#16-#24

Olive, Pale Yellow, Gray, Tan, Brown

Soft Hackle Hare's Ear

#12-#16

Natural, Olive, Orange

WD-40

#18-#22

Gray, Olive, Brown, Black

RS2

#16-#22

Gray, Olive, Black

Klinkhammer Special

#12-#18

Tan, Olive, Peacock, Black

Rojo Midge

#18-#22

Red, Black, Silver, White

Black Beauty

#20-#24

Black

Top Secret Midge

#20-#24

Brown, Black, Gray, White

Adult

The adult dun has a distinctive olive body (ranging from pale olive to dark gray-olive), medium to dark gray wings, and three tails. They must ride the water surface until their wings dry enough for flight--in cold, wet weather, this can take minutes, making them prime targets. After molting into spinners, they return to mate and lay eggs. Spinners have clear wings and are often rusty-brown or dark olive. Spinner falls typically occur in the evening and can be as productive as the initial emergence.

Recommended Fly Patterns

Parachute BWO

#16-#22

Olive, Gray

Purple Haze

#14-#20

Purple, Gray

Parachute Adams

#12-#20

Gray, Brown

Trico Spinner

#18-#24

Black, Gray, White

Parachute Drake

#10-#14

Olive, Gray, Brown, Tan

Royal Wulff

#10-#18

Brown, Red, White

Parachute PMD

#14-#18

PMD Tan, Olive, Yellow

Griffith's Gnat

#18-#22

Black, Gray

Habitat

Blue Winged Olives (BWOs) are incredibly adaptable and thrive in a wide variety of trout waters, from freestone rivers and spring creeks to alkaline tailwaters. They prefer runs and riffles with moderate current and rocky or weedy substrates. Their ability to tolerate slightly lower water quality than other mayflies makes them ubiquitous.

Behavior

BWOs are famous for hating the sun. The most intense hatches occur on overcast, drizzly, or snowy days when barometric pressure drops—hence the nickname "foul weather friend." On bright, sunny days, hatches are sparse or non-existent. The best BWO fishing happens when most anglers stay home.

In spring and fall, expect peak emergence from late morning through mid-afternoon (11 AM - 3 PM). Winter hatches on tailwaters are concentrated in the warmest part of the day (12 PM - 2 PM) and use smaller flies (#22-24). Evening spinner falls can be equally productive, especially in fall.

The duns must ride the water surface for an extended period to dry their wings in humid conditions, making them vulnerable targets. In cold weather, this can take several minutes. Trout feeding on BWOs often exhibit subtle, rhythmic rises—gentle sips rather than splashy takes. Look for noses poking through the surface or dorsal fins cutting the film.

Nymphs are active swimmers and will drift frequently in the current, even when not emerging. This makes dead-drifted nymph patterns effective year-round, not just during hatches.

Fishing Information

Seasonality Primary hatches: March-May and September-November. Winter hatches (mid-day) utilize smaller sizes (#22-24).

Tactics

Overcast is Best: Target BWOs on nasty weather days when other anglers stay home. Rain, snow, and low clouds trigger the heaviest hatches.
Emergers are Key: The emerger stage is where most trout focus. Use RS2s, Barr's Emergers, or Sparkle Duns in the surface film. Grease your leader to within 6 inches of the fly but let the fly sink just below the surface.
Light Tippet: These small flies demand 6X or 7X tippet for a drag-free drift. Micro-drag will put fish down instantly.
Nymphing: Dead-drift Pheasant Tails or Flashback Baetis nymphs year-round, even when no hatch is visible. BWO nymphs are constantly available.
Double Nymph Rig: Run a larger attractor (Copper John) as the point fly with a small BWO nymph (#18-20) as the dropper.
Match the Size: Winter BWOs are tiny (#22-24). Spring and fall hatches are larger (#16-18). Carry a range.
Spinner Falls: Don't ignore evening spinner falls in fall. Use spent-wing patterns (Rusty Spinner, Pheasant Tail Spinner).
Indicator Nymphing: In winter, high-stick nymph with a small indicator and watch for the slightest hesitation.

Recommended Patterns

Nymph: Pheasant Tail (#16-20), Flashback Baetis, RS2 (olive/gray), Barr's BWO Nymph
Emerger: RS2, Barr's Emerger, Sparkle Dun (olive), CDC BWO Emerger, Klinkhammer
Adult: Parachute Adams (#16-20), Comparadun (olive), Thorax Dun, Hi-Vis BWO
Spinner: Rusty Spinner, Pheasant Tail Spinner, CDC Spent Spinner

Conservation Status

Populations are generally robust and stable. They are less sensitive to sediment than stoneflies but still require clean, oxygenated water. Protecting stream flows and reducing agricultural runoff are key to maintaining healthy BWO numbers.