Short answer: BWOs are small olive mayflies from the Baetidae family, hook sizes 16-20 and often smaller, with blue-grey wings. Adults have two tails, nymphs have three. They hatch on cool, overcast days when water sits between roughly 40 and 50°F — and the rise form is a subtle sip.

What a BWO actually is

The Blue Winged Olive, commonly abbreviated BWO, is a collective term for a group of small mayflies from the Baetidae family. They're distinguished by olive-green to greyish bodies and distinctive blue-tinted wings.

Mature BWOs have two tails, but their nymph counterparts have three — an essential difference to note when imitating the various life stages.

Fun fact: "Blue Winged Olive" doesn't refer to just one species. It describes a group of mayflies in the genus Baetis. There are multiple species, and while they share similarities there are differences in size, colour, and hatching periods.

The BWO life cycle

Like all mayflies, the BWO passes through four stages that matter to anglers — and unusually, it moults again after it already has working wings. That extra step is what gives you two separate adult forms to imitate.

Fig. 1 — The BWO life cycle. Stage 2 is where the sipping fish are feeding.

Key to the stages

Nymph

Slim, streamlined swimmer with three tails. Darts between the streambed and the surface.

Emerger

Trapped half out of the shuck. The subtle sip is a fish eating this.

Dun

Olive body, blue-grey opaque wings, two tails. Sizes 16-22.

Spinner

Wings now clear, body rust-toned. Mates above the riffle.

Spent spinner

Flat in the film. The most under-fished part of this hatch.

Why BWO nymphs behave unlike most mayflies

Baetis nymphs are strong swimmers, not crawlers or clingers. They dart actively through the water column rather than clinging to rock, which means a BWO nymph is available to trout all day, every day — not just during emergence. It also means a small olive nymph fished with a little movement, rather than a dead drift, often outproduces a static one.

Habitat and behaviour

BWOs are prevalent in cold-water streams and rivers across North America and are known for prolific hatches. They favour slower water, often found in the surface film of pools, runs, and slower riffles. The nymphs are excellent swimmers and can be seen darting quickly to and from the streambed.

Understanding their habitat is crucial — it tells you where to cast and which stage to imitate.

Fun fact: the BWO life cycle, like most mayflies, includes nymph, emerger, dun (subimago), and spinner (imago) stages. Fly anglers carry imitations of each, because trout can be selective and prefer one stage over another at different times. More on that in Mayfly: Stages of an Insect.

Identifying the hatch

The BWO hatch is one of the easier and more fun hatches to read, and it provokes a genuine feeding frenzy.

BWOs typically hatch during cooler, overcast days, often in early spring and fall. They might even emerge during light rain or snowfall. When water temperature hits the 40-50°F (4-10°C) range, expect to see them.

Here's what makes that interesting: the metabolic feeding temperature for trout is slightly higher, so a hatch coinciding with a rising temperature is a deadly intersection.

Look for rising trout, which indicates active feeding on emergers. The rise form during a BWO hatch tends to be subtle given the insect's small size — the term "trout sip" was likely coined during a BWO hatch. Keen observation is vital.

Fun fact: unlike some aquatic insects, BWOs can have multiple hatches through the year. Depending on region you might see them in spring, late summer, and again in fall, which makes them a significant food source across multiple seasons.

Imitation and presentation

Fishing a BWO hatch demands precision in both fly selection and presentation.

For nymphs, patterns like the Pheasant Tail or Copper John in sizes 18-22 are effective. During the hatch, emerger patterns such as the RS2 or a soft hackle are often the ticket — floating an emerger in the film can be the recipe.

Use thin tippet and silicone floatant to keep it up. If you struggle to spot your fly, tie it off the bend of a slightly more visible dun or cripple and use that as a location beacon.

When the hatch is in full swing and duns are on the water, traditional dry patterns like the Comparadun or Parachute Adams in the right size and colour can be deadly. For the spinner fall, a simple rusty spinner pattern suffices.

Proper presentation means casting upstream of rising trout, letting the fly drift naturally, and ensuring the line doesn't create drag. The three-zone approach is covered in How to Fish a BWO Pattern.

Nymph patterns

Bead head flashback pheasant tail nymph flies, three pack in hook size 16

Flashback Pheasant Tail — Size 16

Bead head gold ribbed hare's ear nymph flies, three pack in hook size 16

Gold Ribbed Hare's Ear — Size 16

Copper John and flashback bead head nymph assortment, ten wet flies in five patterns

Copper John & Flashback Assortment

Bead head pheasant tail nymph assortment, ten wet flies in hook sizes 14, 16 and 18

Pheasant Tail Assortment — 14/16/18

The Bead Head Pheasant Tail Assortment is the one to own if you only buy one — sizes 14 through 18 is exactly the BWO nymph range.

Emerger patterns

Barr's Baetis Emerger BWO wet flies, set of six in hook size 18

Barr's Baetis Emerger — 6 Flies, 18

RS 2 Emerger

Olive RS2 Sparkle emerger fly with flash body and trailing fibres

RS2 Sparkle — Olive

Soft hackle partridge and pheasant tail wet flies, set of six in hook size 12

Soft Hackle Partridge & PT

The Barr's Baetis Emerger is purpose-built for this hatch, and either RS2 hangs in the film about as well as anything tied. These are the flies for stage 2 of the diagram above — the one the sipping fish are actually eating.

Adult patterns

Blue Wing Olive parachute dry flies, three pack in hook size 18

BWO Parachute — 3 Pack, Size 18

Blue Winged Olive classic trout dry flies, set of six in size 18

BWO Classic Dry — 6 Flies, Size 18

Olive Comparadun dry fly with deer hair wing

Comparadun — Olive

Baetis Cripple

The Olive Comparadun sits flush in the film, which is often what a fussy BWO fish wants, and the Baetis Cripple is the one to tie on when they're eating everything except your dun. The Adams Classic Dry Fly is still the great generalist mayfly silhouette if you'd rather carry one pattern.

Spinner patterns

Rusty Biot Spinner

Hi-Vis Rusty Biot Spinner

A spent-wing spinner in the same size, dead drifted in the flat water below a riffle, is the most under-fished part of this hatch. The hi-vis version solves the one real problem with fishing spinners at dusk — seeing them.

Listen to this tip on the Parachute Adams:

Fun fact: one reason BWOs are so popular is their predictable emergence. They hatch on cloudy or rainy days and in cooler temperatures, making them a reliable food source when other insects are inactive. A BWO imitation in the box can be a lifesaver on a tough day.

Tippet and floatant matter more here

Because these flies run size 18-22, fine tippet is not optional — see 5X and the full tippets range, dropping to 6X or 7X for the smallest duns. Keeping a size 20 dry riding high needs both a gel and a powder: Loon Aquel and Loon Dust.

Take the next step and learn how to fish a BWO.

From an ecological standpoint, the presence of BWOs indicates clean, healthy water. They're sensitive to pollutants, so a healthy population often suggests a river in great condition — important for anglers and ecologists alike.

Knowing how to identify a BWO and match the hatch makes a significant difference when chasing trout. Keep a keen eye on the water, always looking for those miniature sailboats floating down and getting gobbled up by giant trout.

More hatches: the caddis, the midge, the Pale Morning Dun, the Trico, and the mayfly hatch.

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