Mayfly · Baetis and related genera
Blue-Wing Olive
Also called BWO, Baetis, Olive.
Short answer
The blue-wing olive is a small olive-bodied mayfly, usually hook size 16 to 22, that hatches in cold, grey, drizzly weather from autumn through spring. It is the most widely distributed mayfly in North America, it hatches in more months than any other, and trout feed on it heavily because it emerges when little else does.
What a blue-wing olive is
Baetis is a genus of small, slim mayflies found in moving water across almost the whole continent, and the blue-wing olive is what anglers call it at every stage. The dun has a grey-blue wing standing upright over an olive body, and the whole insect is small — a size 18 is typical, and a size 22 is not unusual on a tailwater. What makes it matter out of proportion to its size is the weather it likes: Baetis hatch best on the cold, overcast, drizzly afternoons when a dry fly otherwise looks hopeless, and they produce two or more generations a year, so the same insect carries both the autumn and the spring.
- Hook size
- Hook sizes 16 to 22, most often 18 to 20
- Colour
- Olive to olive-brown body, dun-grey upright wings, two tails
The stages trout eat
- Nymph
- A slim, fast-swimming nymph that darts in short bursts. Trout eat them subsurface all year.
- Emerger
- The stage that matters most. Baetis struggle in the surface film and trout key on them there rather than on the dun.
- Dun
- The winged adult riding the surface after emergence, wings up, drifting while they dry.
- Spinner
- The mated adult, wings spent and flat on the water. Often fished at dusk and frequently missed.
When does the blue-wing olive hatch?
| When | What is happening |
|---|---|
| March – May | The spring generation. Peak hatches on cold, wet afternoons; a bright calm day produces almost nothing. |
| June – August | Sporadic. The bigger summer insects take over and Baetis become a background food item. |
| September – November | The autumn generation, and the best of the year. Grey days, falling leaves, and trout feeding hard before winter. |
| December – February | Tailwaters only, on the warmest afternoon of a cold week. Rare, and worth being there for. |
Emergence dates move with latitude, altitude and the weather of a given year. Treat these windows as a shape, not a diary, and check a local report before you travel.
Where it matters
Everywhere there is cold moving water. Baetis are the default mayfly of western tailwaters, Appalachian freestone streams, Driftless spring creeks and Catskill rivers alike. If a state page on this site mentions a mayfly in March or October, it is almost certainly this one.
Fly fishing in Colorado Fly fishing in Pennsylvania Fly fishing in Montana Fly fishing in Michigan Fly fishing in New York
How to fish it
Fish the emerger before the dun. A soft-hackle or an RS2 in the film, dead drifted and unweighted, outfishes a perfectly tied dun on most days because that is the stage the trout have chosen. Go smaller and finer than feels sensible: 6X tippet and a size 20 is a normal rig. On a heavy hatch, pick one fish, work out its rhythm, and cast to that rhythm rather than to the rise you just saw.
- Bad weather is good news. The worse the light and the colder the drizzle, the better a Baetis hatch gets.
- If fish are rising and refusing your dun, you are fishing the wrong stage. Try an emerger in the film.
- Two generations a year means two seasons: spring and autumn. Autumn is usually the better one.
Flies that imitate it
Each of these is a card in the deck, and each has a full page in Our Fly-brary.
Keep reading
- Dry fly vs nymph — which stage to fish, and when
- Why trout refuse your fly — usually the wrong stage, not the wrong pattern
- Best time of day to fly fish — the daily logic behind the table above
- Our Fly-brary — every fly in the deck, explained