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Bug of the Month: The BWO

If you check any “hatch chart,” you’ll see the Blue Wing Olive stretch across the page from March through October. This prolific mayfly serves up a consistent diet for trout in Colorado and fills up a good portion of an angler’s fly box, too. Hang around any fly shop, and you’re likely to hear guides say, “BWOs were poppin,’” at some point in the conversation.

The challenge when talking about the Blue Wing Olive is that there is so much diversity in this mayfly family. We can’t really talk about just the BWO adult without talking about baetis in a more general sense. (By the way, sometimes you’ll read “blue wing olive,” sometimes “blue-winged olive.” I’ll just be consistent and call it a “blue wing olive” or just BWO.)

A Briefly Written Overview (or BWO)

I’m going to avoid trying to talk too much about the science of the baetis or use any of the scientific language. However, BWOs are in the mayfly family Baetidae, the most prolific family of mayfly in Colorado with around 20 species, but the count expands to 140 species in North America and almost 1000 worldwide. The South Platte River drainage contains all 20 species of BWOs in and of itself. It’s no wonder that BWOs are one of your best friends as a fly fisher and always on the menu for trout.

Rather than a deep dive into entomology, I’d rather focus on the bug’s behavior and what that means for fly fishing anglers.

Director of Services, Jon Easdon, says that BWOs serve as sort of the “bookends of the fly fishing season. They emerge throughout the year, but we’ll get a great hatch in March and another one, but with smaller bugs, in October.”

With such diversity in the species, its wide range, and the fact they are petty hardy insects, it’s always good to bring a few patterns with you to the river 365 days of the year.

Baetis Lifecycle and Fishing the BWO

A few things to keep in mind: With such a large number of species and variation of size, color, and profile, matching a BWO hatch can be a bit overwhelming. It’s not uncommon for a handful of different baetis species to be emerging simultaneously. For this reason, it’s important to consider the lifecycle as we talk about fishing the baetis

As Gilbert Rowley’s drawing depicts, the transformation from nymph to emerger to adult and to spinner is pretty straightforward. And productive fishing can happen at each stage of the lifecycle. 

Another distinguishing factor is the relatively fast lifecycle of the baetis. From egg to larva (nymph stage), to emerger, to adult dun, the lifecycle may be only a few months compared to other aquatic insects.  That’s why we can have a heavy hatch in late winter and early spring and another in the early autumn. Hatches can happen throughout the year and are heavily dependent on water temperature. 

Fishing the Larva Stage

As the baetis transforms from egg to nymph, it seeks out shelter under rocks for protection.  When the nymph begins its migration, its goal is to reach the surface. Sometimes nymphs will move toward the bank so their swim to the surface is shorter. 

Fish are so keyed in on this movement. Easdon says, “For months their diet has been midge, midge, midge, midge. Trout see these larger bugs in the water just as they are coming out of their slower winter life. And they’re ready to feed on these bigger meals.”

Great opportunities exist as fish move with these bigger bugs from riffles and faster water to slower water. Don’t neglect hitting back eddies or seams where bugs may be making their journey to the surface in the slower water and opportunistic trout are in a feeding frenzy.

It’s important to be dialed in on size and profile with these nymphs. And weight is important, too, as the nymphs leave the river bed and the subsurface vegetation.

Effective Nymph Patterns: Juju Baetis, Rainbow Warriors, Stalcups Baetis Nymph, and the classic Pheasant Tail

“Hopper” Juan Ramirez ties up the Jedi Master Baetis and CDC Comparadun. Check it out!

Fishing the Emerger Stage

As they move to the surface, the larva begin to shuck their bodies.  And it is in this emerger stage that they can become even more vulnerable.  As the water temperature rises, and the mayfly emerges into its adult form, anglers can try some different rigs for a productive day.

If you aren’t seeing many rising fish, try fishing with a nymph as your first fly followed by an emerger.  Adjusting for depth is important because fish may be feeding on larva toward the bottom and emergers up to the surface.

Sometimes the rise can be confused for fish feeding on the surface.  In fact, the fish may be eating emergers beneath the surface.  As the emerger is shucking its body to become a dun, they often times have trouble breaking through the tension on the water’s surface.  “The foam is the home” has meaning! These emergers may get stuck here.

Another tactic may be to drift an emerger behind a dry fly pattern so that the emerger is in that two or three inches beneath the surface.

Color matters. Grey and olive may work early in the day but it may be productive to switch to brown as temps increase.

Effective Emerger Patterns: Mercury baetis, Darth Baetis, Barr’s emerger, RS2.

Fishing the Adult Stage

Of course, the adult baetis in the dun stage, the Blue-Winged Olive, is very identifiable. Once the emerger breaks through the surface, they sit on the water until their wings dry. Their wings look like sails on a boat.

One of the angler’s best friends is an overcast sky. Because of he cloud cover, the adult dry fly, the dun, has to stay on the river’s surface a little bit longer for their wings to dry. Colorado weather, especially as clouds begin to gather in the mid-to late- afternoon, can make for incredible dry fly fishing.

The mayfly has a second adult stage. After they dry their wings and fly, they usually make their way to foliage along the banks. At this point, they evolve to their sexually mature stage. (We’ll respect their privacy here.) Then, the female makes her way to the river to lay her eggs. This begins the spinner stage. With some species, the female will swim toward the bottom of the river to deposit her eggs. Mostly, though, she lays her eggs on the surface that then drift to the rocky bottom.

The BWO is in its dying stage.The bodies are more elongated and in this dying stage, they change colors, often to a more rust color.  Of course, the feeding continues.

Effective BWO Adult Patterns: Neil’s BWO (tied by Angler’s Covey Guide Neil Luerhing), Thorax Dun (Video from In the Riffle), and the CDC Comparadun (tiers, check out this video from Juan Ramirez).

Juan Ramirez (Hopper Juan) discusses strategies for fishing the baetis. Great info about hatches, switching up depth, and a great look at the bug life on the Upper Ark.

Final Thoughts

Choosing the right size and color can make a huge difference on how productive your day is. Observe, collect, and examine these bugs in detail. Seine the river, turn over rocks, take a close look at what is flying in the air, and then pick your pattern accordingly.

Did you know that Orvis has a full library of tying videos? Check these out.

Have you checked out our online store to purchase fly assortments for our local fisheries?  And watch our fishing reports so you know what is hatching, where, and info on up-to-date conditions.


  1. […] of the setting sun, a barely perceptible dip in temperature, and a sudden proliferation of blue-winged olive baetis dapping the broken surface of the riffles is just enough alchemy to produce dozens or more of these […]

  2. David Riotte on March 23, 2024 at 5:10 pm

    Thank you so much for such a detailed article. I am a complete novice and this really helped my basic understanding. One question though, it seems you use the terms larva and nymph interchangeably. Are the same thing?

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