The Blue-Winged Olive: The Bookend Mayfly

The Blue-Winged Olive belongs to the Baetidae family, a diverse group of mayflies characterized by their diminutive size and olive-tinted bodies. These insects typically range from size 16 to 22, presenting a delicate profile that triggers predatory responses in even the most selective trout.

Nymphs inhabit river substrates, developing through multiple instars before their dramatic metamorphosis. As water temperatures stabilize between 40-50 degrees Fahrenheit, typically during spring and fall shoulder seasons, these nymphs begin their ascent toward the water's surface.

Nymphs transform into emergers, struggling through the water column, their wings developing and becoming visible—a moment of vulnerability that triggers feeding behaviors in trout that lasts throughout the hatch event.

Adult mayflies live briefly, existing solely to reproduce, with males forming mating swarms and females depositing eggs before their 24-hour lifespan concludes.

Blue-winged olive mayflies are among the most common hatching insects in western trout streams. They are present in Montana rivers, streams, and spring creeks frequented by fly anglers with their most active times of year being the spring and fall. Prolific hatches will last for a few hours on cool, overcast days. Increased humidity that accompanies rain delays the duns from flying from the water’s surface as their wings dry, creating the ideal angling scenario for the dry fly angler.

Montana's Premier Blue-Winged Olive Rivers

Madison River

Located west of Bozeman, the lower Madison River sections offer exceptional BWO fishing. Trout concentrate in slower runs between Warm Springs and Greycliffs fishing access sites where consistent currents create ideal conditions. Anglers should focus on seams and soft edges during mid-morning and late afternoon hatch periods on overcast days during the spring and fall.

Missouri River

The tailwater sections below Holter Dam create a world-class, technical dry fly fishery. Consistent water temperatures and regulated flows produce predictable hatches during low-light periods throughout the spring and fall. Pods of rising trout can be encountered throughout the river, but the best Missouri River dry fly fishing is near Craig, Montana.

Yellowstone River

The Paradise Valley stretches provide remarkable BWO dry fly fishing in April and October. Rainbow and cutthroat trout eagerly feed on these hatching mayflies between Livingston and Yankee Jim Canyon, where meanders and gentle glides create perfect feeding lanes. The Yellowstone River is well suited for wade fishing at these times of the year, making fly fishing this hatch accessible to a wide range of anglers for the few hours when the hatch is at its peak.

Livingston Spring Creeks

Near Livingston, these intimate water systems demand precise presentations during BWO hatches. Selective trout inhabit the paradise valley spring creeks, requiring technical drifts and near-perfect fly selection. Micro-currents and vegetation create complex feeding environments that challenge even experienced anglers. Present throughout the year, peak BWO hatch activity takes place throughout the spring and fall months on mild, overcast days.

Bighorn River

This legendary Montana fly fishing destination is located a few hours east of Bozeman, near the small town of Ft. Smith. A tailwater fishery, the entomological profile of Montana’s Bighorn River is rich in biomass, but less diverse than western freestone rivers. BWO nymphs and duns are important food sources to the river’s wild trout which selectively feed with enthusiasm throughout an emergence.

Other Notable Montana Rivers

BWO hatches can be encountered on just about any Montana river or stream on overcast days in the spring and fall. These widely dispersed insects thrive in a variety of cold water environments. The Bitterroot, upper Madison, and East Gallatin rivers, for example, are home to prolific emergences and wildly different ecosystems from one another.

Guide Tips to Fly Fishing a BWO Hatch

Watch the Conditions

Although BWO hatches occur with regularity in the spring and fall, the best hatches happen on cool, cloudy days when water temperatures are in the 50’s. The best days are accompanied by light rain which keeps the duns on the water longer as their wings need to dry before taking flight

Accuracy Matters

Steadily rising trout move into a feeding rhythm which is easily disrupted with multiple false casts and errant presentation. Make a few practice casts well to the side of a rising trout to get a sense of your casting distance. The ideal presentation will place the fly a few feet ahead of the target with the leader slightly to the side of the fish.

Application Specific Gear

Fishing during a BWO hatch is akin to the classic “match the hatch” fishing that embodies fly fishing for many anglers. Moderate action fly rods like the Scott G Series, St. Croix Technica, or Winston Pure 2 in an 8 1/2 - 9' length 4-weight enable accurate and delicate presentations. Pair one of these rods with an Elite Technical Trout fly line for the ultimate presentation fly fishing outfit.

Watch the Rise

Is there just one fish slowly rising? Are there multiple fish, some rising with their heads followed by a showing of their tails? Is there just an occasional head showing, but mostly just the tips of tails creating disruptions on the water’s surface? Identifying these riseforms helps the angler crack the code between fishing adult or emerger patterns.

Match the Hatch

General fly patterns like a parachute adams or purple haze will do the trick in riffles as long as they are sized correctly. The heaviest feeding behavior during a bwo hatch is in the flat water - downstream of riffles – where feeding trout have time to carefully inspect flies before ingesting them. Match the profile and color to imitate the insects they are feeding on for better results.

Top 10 Blue-Winged Olive Fly Patterns

BWO Dry Flies

1. CDC BWO Comparadun (Size 18-22) : These delicate and realistic dry flies imitate adult duns following emergence. The dark colored wings contrast nicely against the water’s surface glare, aiding the angler during low-light conditions associated with BWO hatches.

2. Sparkle Dun BWO (Size 16-20) : A Craig Matthews original, the deer hair wings and trailing shuck of z-lon keeps this fly buoyant and visible while fly fishing Montana rivers with diverse riffles and pocket water like the Blackfoot, Boulder, Gallatin and upper Madison.

3. Parachute BWO (Size 18-22) : A classic dry fly pattern adapted to local mayfly hatches on rivers and lakes around the world. When fly fishing in Montana during a BWO hatch, guides rely on variations with medium dun hackle, olive bodies, and a hi-viz parachute post.

BWO Emerger Patterns

4. RS2 Emerger (Size 18-22) : An innovative pattern created to “crack the code” for fly fishing Colorado’s front range tailwater fisheries. The classic and highly effective variation has a sparse tail, dun colored body, and uses antron fibers for the wing.

5. Soft Hackle Emerger (Size 18-20) : Montana fishing guides carry these in their fly boxes to fish wet in the surface film either solo or as a dropper. Olive bodied variations with speckled, Hungarian Partridge feathers for the soft hackle are preferred.

6. Sprout Emerger (Size 16-20) : Typically tied on a curved shank emerger or scud hook, this bwo emerger imitation sits low in the water film. A small foam parachute keeps the fly suspended below the water’s surface where trout selectively feed on crippled emergers.

BWO Nymph Patterns

7. Pheasant Tail Nymph (Size 18-22) : An essential trout fly that is a near perfect imitation in both color and profile of mayfly nymphs around the world. Natural or olive colored pheasant tail fibers work well for the abdomen sections of BWO nymph imitations.

8. Split Case BWO Nymph (Size 16-20) : Our Bozeman fly fishing guides use this tungsten beaded BWO nymph imitation during the pre-hatch hours when trout actively feed subsurface. The bi-colored wingcase adds confidence to anglers fishing to selective trout.

9. Micro Mayfly Nymph (Size 20-22) : The key to this Mike Mercer original is in the epoxy bubble added over a bit of flash on the nymph’s wingcase. Beaded for added weight, the fly has a subtle flash to it underwater that has been fooling Montana trout for decades.

10. Little Green Machine (Size 18-20) : The skinny profile imitates nymphs and emerging BWO in both the gray or pheasant tail colorways. Anglers fishing Montana tailwaters like the Missouri and Beaverhead Rivers consider this an essential nymph pattern.

Experience Montana's Blue-Winged Olive Magic

For the discerning angler seeking reliable dry fly fishing on their next Montana fly fishing vacation, Fins and Feathers Guide Service creates customized itineraries around their skills and interests. Although dry fly fishing is largely dependent on weather and water conditions, the BWO hatches of spring and fall provide daily opportunities to target rising trout on many of Montana’s world-renowned trout fishing waters.

If dry fly fishing is a priority, a multi-day Bozeman fly fishing trip in April offers anglers the chance to find rising trout on a variety of rivers in Southwest Montana. Expect the day to start off with a mix of nymph and streamer fly fishing techniques as guide and angler wait for the hatch to commence around mid-day on mild, overcast days.

Contact us to learn more about fly fishing in Montana on a guided trip focused on dry fly fishing opportunities. The spring and fall are terrific times of year for experienced anglers with intermediate to advanced fly fishing skills during the peak times of BWO hatch activity. Reservations can be placed online, via e-mail, or by calling us at 1-406-468-5019.