By Kelly Oikawa (Kelly Canada)

Baetis, or Blue-winged olives (BWO) as they are commonly referred, are members of Order Ephemeroptera and family Baetidae. They belong to a group of mayflies called swimmers and as nymphs are quite mobile. Baetis inhabit all portions of the Missouri but seem to favor the numerous riffles where the river shallows and there is some gradient. On the Mo there are two major emergences and the arrival of both Spring and Autumn mark baetis season. Dark overcast days with a light drizzle can produce some of the finest surface action of the season as even the largest trout in the river are either replenishing lost winter mass or preparing for the long Montana winter that is just around the corner.

Because of their active lifestyle baetis nymphs are often available year round as they swim along the bottom scavenging detritus or grazing on algae and plant matter. While many mayfly nymphs burrow into or cling to the substrate and are therefore less apt to become of part of the “drift” that trout rely on baetis are easily swept away and nymph imitations will produce trout day in and day out. In order to facilitate a swimming lifestyle the nymph has a tail that contains two outer fibers with hairs that act as paddles. A third middle tail is shorter and lacks these hairs. Because of this many fly dressing incorporate a heavy tail. They have two well defined antennae as well as abdominal gills to extract precious oxygen from the stream flow. A single marabou fiber reinforced with a fine wire wrap is a good way to mimic these gills. Their bodies also tender to be slender and streamlined to ease movement into the current. The colors of the nymph will vary according to the colors of the river bed and will cover the spectrum of most natural colors. Most Missouri River baetis nymphs tend to be light in color and will darken, especially the wing cases, as they near emergence.

All mayflies exhibit incomplete metamorphosis and because of this there is no pupal stage. In order to facilitate growth they go through several growth stages called instars in which they shed an exoskeleton made of chitin. This is very similar to what snakes and many other invertebrates do. The color of the body can actually depend on where the insect is in its molting cycle.  Of all mayflies baetis tend to occurs in the widest range of sizes. The can range in size from 22-16 with early season bugs tending to be bigger than those frustrating October hatches when small flies and 6X seem to be the only ticket to evoke a take.  During baetis season a common question on the river and in the Trout Shop is “what size of BWO’s are popping today?”

Baetis Nymph

Picture from the Troutnut.com
Jason Neuswanger, the developer of this website has a great website with some of the finest entomology images around. His website is definitely worth checking out. Jason is also very generous in that he allows the use of his images as long as him and his website get credit. Thank You Jason.

When ready to hatch the nymphs swim or passively rise to use surface tension as an anchor to emerge from the nymphal shuck. This migration, as well as the short time in the surface film, can provide trout with an opportune time to feed and they will often key on this stage.  I have observed soft hackle addicts swinging small flies near the surface to mimic the rise of the nymph constantly hooked up at the start of an emergence while traditional dry fly anglers have presentation after presentation snuffed. Emerger patterns fished just below or in the film can produce better than duns in certain situation. Rise forms must be carefully observed so you can determine the exact stage of the emergence most fish are keying on. Dorsal fins and tails as trout porpoise usually suggest activity just below the surface while noses and the white of a mouth indicate something is taken on top.

Baetis hatches can be profuse and the sheer number of bugs means that a large number don’t hatch as nature intends and become stuck in their husk, turned on their side and fatally doomed in the film, or somehow damaged so flight is not an option. As most adults quickly leave the surface film these cripples or knock-downs provide a meal that won’t fly away and waste calories so many savvy trout will only rise to a pattern that suggests this. Successful emergers are called duns or subimagoes. The duns have two tails and wings that are grey/blue dun colored and usually an olive colored body, hence the term Blue-winged olive. The Missouri River baetis tend to have a relatively dark olive body. Males will appear to have a large head when compared to females due to the presence of larger eyes. The colder and wetter the day the longer it will take the duns to prepare for flight and therefore the longer they are available to trout. This may help to explain why hardy well clothed anglers can be successful when inclement weather keeps fair weather folks tying flies or watching Sunday games with a beer rather than a rod in hand. While tradition dun patterns will take trout the Missouri, like most tail waters where trout become very discerning, can require smaller, more delicate patterns, that are exceptional dun representations or mimic a cripple or stillborn.

Some days a size 14 adams will kill them while other days even the most astute anglers are humbled when the bugs are out and the fish are on them.

Baetis Dun

Unlike trico or some caddis hatches the baetis emergence usually follows banker’s hours as the time of year usually dictates temperatures aren’t optimal for hatching when the roster crows or Mother nature is painting the vast Montana sky with her masterpiece sunsets. The duns will go to streamside to molt again and this time they transform into a spinner or imago will have clearer wings and 2 long tails with a lighter body color. This phase of the life cycle is usually not of importance to most anglers as the females actually crawl into the water and deposit eggs before dying. While they become part of the drift and are available to trout they are not as conspicuous as trico or PMD spinner’s falls where it is apparent that spinners are on the surface.  If one wanted to fish the spinner stage a traditional spinner fished dead drift below the surface may work.

Baetis Spinner