The “Blue Wing Olive”, fondly known as the BWO, is a Mayfly(Order “Ephemeroptera”) in the Family Baetidae, of which there are about 23 “Genera” of which Genus Baetis is but one. While there are quite a few Genera that fall under the heading of Blue Winged Olive, the Baetis, Subspecies parvus, tricaudatus and propincus are the main ones in Western Colorado. Also included in the Blue Wing Olive fly-grouping of insects are the Paraleptophlibia and Pseudocloen sub species.
From the anglers point of view the BWOs are one of our first significant hatches of the year throughout Colorado, and specifically on the Gunnison River from below Taylor Dam, down through the Black Canyon of the Gunnison and the Gunnison Gorge its a good to very good hatch.
Some anglers just call them “those little greenish things”. OK, just match what you’ve got in your box to it and it could work for you!
Adults hatch out in late March and into April in masses sufficient to wake up slumbering trout, but they many subgenera hang around and continue throughout the summer, and have a resurgence in the fall for an encore performance fatten up prior to a long winter.
Colors can range from the Olive color of their namesake through the greens and grey color palette to a slate grey. The “Dun”(Immature Adults) has the grey wing. Spinners have the clear wing. They have two tails as adults and three tails as nymphs. While in their nymphal state these hardy swimmers hang around the rocky bottom, then when the time is right they “whip-kick” their way up to the surface and use the surface tension to hold them tight until they can split the back of their wing pads and start to wriggle out. This takes a few seconds of drifting downstream, making them vulnerable to feeding fish. Known as the “emerger” phase, it’s not a true stage in the metamorphosis from nymph to adult like the pupal state Caddis and midges go through, but for the knowledgeable angler having an emerger pattern in the fly box can make the difference between catching fish and casting to fish.
While hatches can come off during all parts of the day, they normally pick the nicest part of the day—quite often from about 11 to 3. These mayflies love overcast, cloudy and even snowy days if the wind doesn’t blow them away. One amazing site is to look upstream see “trout snouts” breaking the surface with regularity, as a flotilla of Duns float down stream like miniature sailboats. Listen carefully and you may hear them slurping!
Upon hatching the Duns lift off of the water and dry their wings, then they fly off to stream side foliage to fly and eat and hang out with their friends and relatives. After 7 to 12 hours a molt occurs and the Adult, sexually mature “Spinners” start their mating dance. The males swarm and the females fly into the swarm—“Sex in the Mayfly world” The fertilized female then lays her eggs on the water and the next generation begins. The spent female “spinner” is frequently sipped by hungry trout, which seem to know when their meal isn’t in a rush to fly away. The normal cycle from hatching out to spinner fall and death, is less than 24 hours!
Baetis nymphs are in the “swimmer” category of mayflies. They have thin, long bodies, gills on their abdomen, and variegated legs and three tails. They are found primarily in moving water and rivers in the flats, runs and riffles. They are most active in the mid mornings and early evenings, and just before they emerge.
Their wing pads darken and swell just before emergence, and the nymphal stage and the emerger phase are very important food sources for hungry trout, and even in hatching adult situations the nymphing can still be the most productive way to fish this hatch.