The Top 12 Blue-Winged Olive (BWO) Flies

Updated November 10th, 2026

At a glance:

• Best nymphs: Juju Baetis, Micro Mayfly and Two Bit Hooker
• Best emergers: RS2, WD-40, Film Critic, Bat Wing Emerger and Mathews PT Emerger
• Best dry flies: Sparkle Dun, 401K Baetis, CDC Biot Comparadun and Last Chance Cripple
• Hook size: 16–22, most often 18
• When: spring and fall, in the warmest part of the afternoon, from about 40°F water up
• Best weather: overcast days with drizzle or light snow
• Where: nearly every trout stream in North America, and especially tailwaters and spring creeks

There are few, if any, insects more synonymous with fly fishing for trout than the blue-winged olive (BWO). From coast to coast of North America’s trout country, the BWO is one of the most common and predictable hatches of the year.

Blue-winged olives are often among the first bugs to hatch in early spring and the last reliable hatch of the fall. They hatch in big numbers under cloudy skies and a bit of precipitation, bringing dormant fish back to the surface in a frenzy. Braving what many consider an uncomfortable fishing day can mean having a prolific hatch all to yourself as rain or flurries dapple the river and trout rise to floating duns.

This article covers the most productive BWO flies, from nymphs to emergers to cripples and duns. Having flies for every stage lets you adapt as the hatch changes and catch more fish.

What Is a Blue-Winged Olive?

Blue-winged olive, or BWO, is the anglers’ name for mayflies in the family Baetidae, known as small minnow mayflies, and mostly in the genus Baetis. The family has about 1,000 species worldwide, but one does most of the work on North American trout water: Baetis tricaudatus, the most widespread and abundant baetid on the continent. Eastern anglers once knew it as Baetis vagans. It lives at almost any elevation, including high-altitude western creeks, and in some places it hatches alongside a similar-looking cousin, Diphetor hageni.

BWO Nymphs

Baetis nymphs belong to the swimmer group of mayflies. They have slim, streamlined bodies, three tails and gills along the top and sides of the abdomen, and they grow to about 3–12 mm long. Most are olive to dark brown. They live on gravel and weeds and are strong, quick swimmers, found throughout a stream but most often in riffles and weedy runs. They’re active all year and a big part of the drift, which is why a small olive nymph catches fish even when nothing is hatching.

BWO Duns

Duns are small, with olive, olive-brown, gray or pale green bodies, drab smoky gray wings and two tails. Their hind wings are tiny, long and narrow, and hard to see without a close look. Males have large, turret-shaped eyes that sit up on top of the head.

Most BWOs are imitated on size 16 to 22 hooks. In the East they usually run 16 to 20. Western BWOs can run a size larger, and Baetis tricaudatus sometimes reaches size 14. If you had to choose one size, 18 is the most common.

The Blue-Winged Olive Life Cycle

How Do You Fish a BWO Hatch?

Pick the right day. Rain, snow and high humidity keep the duns’ wings from drying quickly, so they ride the surface longer and trout get more chances to eat them. On bright bluebird days the wings dry fast and duns leave the water quickly, so the dry fly window is usually shorter.

Start with a small nymph. BWO nymphs are in the drift all year, so a small olive nymph is a good bet before the hatch starts and on days it never does. Nymphs range from size 16 down to size 22. If you aren’t sure where to start, size 18 is a good all-around sweet spot.

Fish slow water with a long leader. BWOs often hatch in slow, flat water, where trout can sip duns, emergers and cripples without much effort, and get a long look at your fly. Guides often fish 12- to 15-foot leaders during this hatch, with 5X or 6X tippet for most situations and 7X for the pickiest fish. Shop leaders and tippet.

Fish from upstream and set gently. Position yourself above the rising fish and quarter your cast downstream, so the trout sees the fly before the leader or line. A reach cast lands the fly softly and gives you a drag-free drift without a disruptive mend. When a fish eats, wait for it to close its mouth, then lift the rod smoothly. A hard set can break fine tippet.

Keep your fly floating and visible. Rub floatant into CDC patterns before your first cast. Under gray skies a white post can disappear in the glare, so a fly with a black post, or a hi-vis wing like the 401K Baetis, is easier to follow.

Read the rises. If trout are rising but you don’t see bugs on the surface, or they keep refusing your dry, they are likely eating emergers just under or in the film. Tie a small emerger a short distance behind your dry: the dry shows you where your flies are, and the emerger often gets the eat. If that doesn’t work, change stages, trying a cripple and then a spent pattern until you match what the fish are taking. Read more in our guide to emergers and cripples and our top 5 fall BWO patterns.

Stick around between waves. BWOs hatch in waves. In the lull between them, which can last a few minutes or more than an hour, watch for spent adults dropping to the water and the slow, rhythmic sips that come with them.

The Best Blue-Winged Olive Nymphs

The BWO nymph is a critical food source in many trout streams. Perhaps the most recognized nymph in fly fishing, the Pheasant Tail, imitates a number of mayflies, the blue-winged olive included. The Pheasant Tail can be deadly, but there are a few other staple patterns to consider.

1. Juju Baetis

Charlie Craven’s Juju Baetis is productive everywhere it goes. The size and profile perfectly imitate a Baetis nymph. A tinsel wingcase reflects just enough light to grab a trout’s attention or suggest an air bubble trapped on the insect, and a coat of UV resin highlights the wingcase and thorax segmentation while adding durability. This should be in your box as a do-it-all mayfly nymph.

Best for: any day on BWO water, before a hatch or when nothing is hatching. Shop the Juju Baetis.

2. Micro Mayfly

Mike Mercer’s Micro Mayfly proves flies don’t need to be complicated to get it done, borrowing many of its traits from the Pheasant Tail. Profile is key: a few pheasant tail fibers at the rear, a slender body of stripped peacock herl and a large olive dubbed thorax. A copper bead gets it down quickly, though tiers can swap in tungsten or glass beads to change the weight.

Best for: ticking bottom or working deeper runs. Shop the Micro Mayfly.

3. Two Bit Hooker

Another Charlie Craven pattern, the Two Bit Hooker is a deadly Baetis imitation. Its most notable feature is two smaller tungsten beads instead of one big bead, which gets the fly down quickly without a bulky head and keeps the profile slim for such a heavy pattern. It’s tied in a variety of colors.

Best for: getting deep fast, as the point fly in a nymph rig or a dropper under a dry. Shop the Two Bit Hooker.

The Best Blue-Winged Olive Emergers

Emergers make easy pickings, and trout often key on them over duns. These patterns ride just under the surface or low in the film.

4. WD-40

The WD-40 imitates both emerging Baetis and midges, which makes it a great fly when you aren’t sure what to tie on. It’s quick and easy to tie, traditionally with mallard flank for the tail and wingcase and dubbing for the thorax. It’s usually tied unweighted, so it fishes just under the surface.

Best for: trout rising to something you can’t see, BWOs or midges. Shop the WD-40.

5. Bat Wing Emerger

The Bat Wing Emerger imitates a number of emerging mayflies, but the original Baetis version is a staple BWO emerger. A glass bead lets it just barely break the surface film, like a BWO transforming into a dun. An olive biot body gives natural segmentation, and the wings are made from the fluffy base of partridge or hen feathers.

Best for: fish eating BWOs just as they break through the film; it can pass for a cripple or spent olive, too. Shop the Bat Wing Emerger.

6. Film Critic

The Film Critic comes in a number of variants for good reason: it’s a killer emerger or cripple. It isn’t the easiest fly to tie, but the reward justifies the effort. It rides low, like a mayfly working free of its shuck in the surface film. Bob Quigley, the original tier, added a large post over the hook eye that rides upright, imitating emerging wings and making the fly easy to track.

Best for: trout eating emergers in faster water, where a low-riding fly is hard to see. Shop the Film Critic.

7. RS2

The RS2, short for Rim’s Semblance 2, is simple to tie but should be in every Baetis box. Colorado tier Rim Chung developed it about 40 years ago, and it still fools fish everywhere, especially on tricky tailwaters and technical spring creeks. Its defining features are two Microfibbets splayed at the tail and a small wing of poly yarn or CDC.

It’s also one of the most versatile flies you can carry. Fish it deep as a nymph, just under the film as an emerger, or dress it with floatant as a low-riding dry.

Best for: picky tailwater trout, at any depth. Shop the RS2.

8. Mathews PT Emerger

Craig Mathews’ PT Emerger is built for selective trout eating mayflies struggling in or just under the surface film. Pheasant tail fibers make up most of the body, trapping tiny air bubbles and adding subtle flash like a real emerger. The thorax is dubbed to match the color of the natural as it works free of its shuck.

Best for: selective risers on spring creeks, slack water and places like the Henry’s Fork. Shop the Mathews PT Emerger.

The Best Blue-Winged Olive Dry Flies

This is what we all think of when discussing blue-winged olives—rising trout gorging themselves on the passing “sailboats” of mature duns streaming downriver. There are a ton of different blue-winged olive dry flies on the market, but these are a few that have yet to let us down.

9. Sparkle Dun and PT Sparkle Dun

Craig Mathews’ Sparkle Dun has been fooling trout for decades. It has only three main parts: Z-Lon, dubbing and deer or elk hair. The Z-Lon tail imitates a trailing shuck still attached to a newly hatched dun, and the splayed hair wing makes the low-riding fly easy to track. It can pass for a dun, an emerger or a cripple.

Mathews’ newer PT Sparkle Dun follows the same idea but uses pheasant tail fibers for the body, which trap tiny air bubbles and add a subtle sheen like a real mayfly’s body.

Best for: trout eating freshly hatched duns in the film. Shop the PT Sparkle Dun or the Sparkle Dun.

10. 401K Baetis

The 401K Baetis rides low but stays easy to see thanks to a hi-vis wing. CDC and a few turns of hackle keep it on top without riding too high, so you can follow it without spooking wary fish.

Best for: fishing at a distance or in low light. Shop the 401K Baetis.

11. CDC Biot Comparadun

Yes, another comparadun, but this one earns its spot. Instead of dubbing, the body is tied with biot, which gives natural segmentation and lets the fly ride a little lower in the film. The CDC wing floats well with floatant and lands as softly as a fly can.

Best for: slow, low or technical water, like spring creeks. Shop the CDC Biot Comparadun.

12. Last Chance Cripple

We’d be remiss not to mention René Harrop’s Last Chance Cripple, designed to imitate a crippled or emerging dun. It comes in many versions, from pale morning duns to bigger drakes. The sparse tail mixes natural fibers and synthetics to suggest both the mayfly’s tails and a trailing shuck. A goose biot body keeps it slim, and hackle with a CDC wing imitates a dun whose wings never fully came free. The result is an easy meal trout rarely turn down.

Best for: soft sips that mean trout are eating cripples. Shop the Last Chance Cripple.

The BWO hatches of spring and fall are some of the most fun of the year, and fishing small dry flies makes you a better angler. After a long winter, seeing bugs on the water lifts the spirits, and in the fall they’re a bittersweet end to the warmer months. As winter sets in they can still show up in good numbers, especially when the weather turns sour. There’s nothing better than a quiet river all to yourself, with plenty of bugs and rising fish.

Frequently Asked Questions

What is a blue-winged olive? Blue-winged olive, or BWO, is the anglers’ name for small mayflies in the family Baetidae, mostly in the genus Baetis. The most important species is Baetis tricaudatus, the most widespread baetid in North America. The duns have olive to olive-brown bodies, smoky gray wings, tiny hind wings and two tails. Most are imitated on size 16 to 22 hooks.

When do blue-winged olives hatch? BWOs are among the first mayflies to hatch in spring and the last reliable hatch in fall. Baetis can produce two or three generations a year, so smaller summer hatches happen too. Hatches can start once the water reaches about 40°F.

What time of day do BWOs hatch? Usually in the warmest part of the afternoon, especially in spring and fall.

What weather is best for a BWO hatch? Soft, overcast days with drizzle or light snow often bring the heaviest hatches of the year.

What size flies do I need for blue-winged olives? Fly sizes range from 16 to 22. If you aren’t sure where to start, size 18 is the best all-around choice. Western BWOs can run a size larger than eastern ones.

Why are trout rising but refusing my BWO dry fly? They are probably eating emergers just under or in the surface film. Switch to an emerger such as an RS2, WD-40 or Film Critic, or trail one behind your dry.

How do BWOs lay their eggs? Many female Baetis land on a rock or stick that breaks the surface, fold their wings, and crawl underwater to lay their eggs on it. Others release their eggs at the surface.

What is the best blue-winged olive fly? There isn’t one best fly, because it depends on the stage trout are eating. A Juju Baetis or Pheasant Tail covers the nymph, an RS2 covers the emerger, and a Sparkle Dun covers the dun.

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