BWO dry fly

A Don’s BWO dry fly showing the “red head”.

What has blue wings, an olive body and sometimes, reddish eyes?

A few years ago about this time of year I was fishing on one of the Big Bend Fishing Easements on the Arkansas River. There was a large thunderstorm building to the Southwest. I thought to myself that I had better hightail it to the Jeep, as the storm came bearing down with lightning and thunder crashing.

As I went to climb out of the water, I looked upstream to see a flotilla of small insects on the surface, drifting down the current. At the same time, a number of large trout appeared and started sipping the insects off the surface of the water. Plans suddenly changed, and for the next hour, I caught a bunch of nice trout on dry flies.

Another time, this time in late April, I had an Arkanglers guest from Santa Fe on the river. It snowed hard all day long, and in spite of that, we had great action during part of the mid-day on the same, small mayflies.

These small mayflies are the Baetis, or Blue-winged olives (BWO’s). They are a common sight in the spring and fall here, and in their own perverse way, they seem to prefer the worst weather possible to appear in great numbers. I’ve seen them in late winter snowstorms and spring thunderstorms, and as we call them a “multi-brooded” mayfly, here on the Arkansas we see two generations per year (spring and fall), but they could be present at almost anytime of year.

While this river was traditionally known for its spring caddis fly hatch, usually known as the “Mother’s Day Caddis” or in some parts of the river the “Tax Day Caddis” because of the time of year that most of them appear, the caddis has declined significantly in importance over the past ten years, and other insects have grown in importance. The story of the caddis on this river is a whole story in itself, and can wait for another time.

The upshot is that even though the spring caddis flies have declined in importance, the water quality has improved dramatically in the Arkansas over the past 30 years. So, the diversity of aquatic insects in the Arkansas has seen a tremendous increase year round. The diminutive BWO mayflies are no exception.

Even this September, even though the weather has been dry and storms few, the BWO’s are present. As I write this, mid-afternoon on a cool, overcast day, I can envision that the hatch is coming off on the river right now, and at least for part of this afternoon, the fish will be rising.

Earlier this week, I was guiding a couple of guests on the river. It was a cloudy day, and we were fishing on a stretch of the Cogan Ranch. There were BWO’s and Red Quill mayflies emerging, and fish were rising from the near bank of the river to the far bank.

The BWO nymphs are always present this time of year. They are tiny, and are remarkably fast swimmers. About mid-morning, every day, the nymphs begin a behavioral drift en masse, darting from the rock that they have been grazing upon, and drifting in the current.

Later, if the conditions are right, they may hatch into adults, but often, they just settle down onto another rock to graze on diatoms. During this drift, they become an important food staple for trout.

A tiny pheasant tail nymph can be an effective imitation fly when they drift.

This time of year, some of them hatch almost every day, even in bright sunlight. But when the storms come, the hatch becomes impressive. Some of the best fishing that I have had on the Arkansas has been during Baetis hatches in stormy weather.

What many anglers don’t know is even when a hatch is occurring, even when trout are rising, that often the emerging insect in the surface film of the water is often the preferred target of trout, over adults floating on the surface.

As they prepare to hatch, the wing pads on the nymph darken. Often aquatic nymphs produce a small bubble of gas under the exoskeleton to help them get to the surface. This might, in part, explain the success of bead-headed flies, particularly silver or “mercury glass” beads used in some imitations.

As the nymph gets to the surface, their small size makes it quite difficult for them to break through the surface tension of the water, causing them to struggle in the surface film. As a result, many of the nymphs become “cripples” in the film, eventually dying.

Over the past few years, a number of flies imitating the insect caught in the surface film have been introduced. These include Barr’s emerger, Dorsey’s mercury Baetis, Brook’s sprout Baetis and Rim Chung’s RS2.

Often, we will fish a double fly rig during this hatch. There are a lot of good dry fly imitations, starting with a traditional Adams dry fly in a size 18 or 20, with an emerger pattern trailing in the film. That could be an Adams, a Don’s BWO, a BWO sparkle dun, or a gulper special for the job.

As for why the BWO’s prefer stormy weather to emerge is still a bit of a mystery. I’ve seen some anecdotal research that indicates that successful emergence of the nymphs was far less in low humidity.

The Baetis have very long, fine tails and may have trouble withdrawing the tails out of their shucks in warm, sunny conditions. Humidity may help.

Other anecdotal evidence might point to excessive UV light levels. I was speaking one day with legendary Arkansas River guide Don Puterbaugh. He pointed out that he had included a red thread head on his own Baetis pattern, Don’s BWO.

This is because the male BWO has specialized long forelegs to help him mate in flight. In addition, the male also has large, divided turbinate eyes to help them spot the females in flight. The upper parts of the large divided eyes are sensitive to UV light and they are red.

Don is now in his 90s and is still a wealth of knowledge from his many years of fly fishing the Arkansas River. He will argue that the red head of his BWO imitation is critical to the success of his fly, as it imitates the large red eyes. It may also give us a bit of insight into why the BWO’s pick the worst weather in which to emerge, as those eyes are sensitive to UV light.

So the next time it is stormy in the spring or fall, it might be a great time to pull out the fly rod, put on a 5X tippet and grab your imitations of the blue-winged olive mayfly and head out on the river.

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