Insect Profile: the BWO, a Winter Trout Fly Fishing Staple


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Do you think you could pick a ‘BWO’ out of an insect lineup? Probably not say trout insect experts, for a number of reasons. The term ‘BWO’ that is commonly used by the winter trout fly fishing community stands for Blue Wing Olive. This term not only really refers more to the trout fly but is a catch-all for many, many species of similarly sized and shaped Mayfly species, some of which may be quite localized. Along with Midges these ‘BWOs’ are often the only insects trout fly fishers actually see when winter trout fly fishing! Notice I didn’t say similarly colored- this large of group of small winter Mayflies actually has a wide variety of body and wing shades, and none of them are really blue!

Here's a 'BWO'- With neither an olive body or 'blue' wings...

Here’s a ‘BWO’- With neither an olive body or ‘blue’ wings…

As always in our blogs we note that we try (TRY) not to wax poetic or take too deep a dive into the details. There will be no Latin insect names (OK perhaps one or two), but rather some basic information that we feel a quick read will provide some immediate benefit to the MonthlyFly.com community, perhaps provide new information not previously known, and help you make the most of your precious time on the water!

The term BWO usually refers to the families Baetidae or Baetis and these Mayfly species body color can vary broadly from shades of browns, greys, and on into the greens and ‘olives’. Wing color varies somewhat less, but a truer description for wing color are shades of light grey if not mostly transparent depending on adult maturity (the wings often lighten the older the adult is). I will NOT at this point claim to have any inkling what a trout sees when looking up at a small winter Mayfly on the surface film, as that’s way beyond the scope of this article!

It’s worth it to do a quick life cycle review with a note or two on behavior when profiling any insect, or group of insects as we’re doing here. The BWO is particularly important to trout as sustenance for a couple of notable reasons- their life cycle behavior makes them more available for trout to feed on and their presence over the winter (at least as nymphs in colder climates) leads to increased availability over some of the chillier and hungrier times of year for trout across significant portions of the country. This makes make them very important to fish survival and health. This group appears commonly as a fly size #18 and smaller, and are not noted much larger than a #16.

From the egg stage the BWO hatches into a nymph which trout will feed on year round. Nymphs then become emergers (think of this as a pre-adult phase) where they attempt to swim their way to the surface to shed their ‘shucks’ and unfurl their wings. This is why swinging your small nymph flies at the end of the drift is often so effective, you’ve lucked into (or figured out) that you’re imitating an emerger trying to get the surface. BWO Emergers take some time to transition into adults and are particularly vulnerable as they float along on the surface, waiting to take flight for the first time. Upon flight the adults will mate and then become spinners. Spinners refer to adults in the very last portion of their life cycle, for the female that means egg laying, for the male, well heck let’s just say he’s ‘spent’. And the cycle repeats. This is a significantly abridged and truncated version. As we said we’re going for familiarity, not deep dives!

A little familiarity with insects and their life cycles can really help to put pretty trout in your net like this one!

A little familiarity with insects and their life cycles can really help to put pretty trout in your net like this one!

So if you’re in an area where you can go out this weekend and chase winter trout on the fly- keep this article in mind. If you’re spotting small mayflies in the air and rising trout dimpling the surface take a moment to try and get one in hand, think about what it is and what it’s doing. What you catch may surprise you, and then do your best to match size, life cycle, body and wing color to tempt more trout to strike!

Have fun, be safe, and catch ’em up!

Josh@MonthlyFly.com

P.S. I must share what prompted this article- as I was compiling some of our subscribers’ monthly packages this past weekend who chose to receive Spring selections as part of our Winter program I was holding some #18 Blue Quills. I thought hmmm… this almost looks more like some of  the BWO’s I’ve seen than some BWO patterns! I wonder… so always keep an open mind when fly fishing! 🙂

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