Hatch Profile – The Mayfly Formerly Known as Pseudocloeon

Hatch Profile – The Mayfly Formerly Known as Pseudocloeon

Every fall in Yellowstone Country there is a tiny mayfly in the Baetis family that is responsible for some outstanding dry fly fishing. These miniature mayflies range from size 20-26, but what they lack in stature they more than make up for in abundance, and mystique.

Massive emergences of these bugs are commonplace in the fall on legendary rivers like the Henry’s Fork, Madison, Big Horn and Missouri. For generations fly anglers have called this bug Pseudocloeon (Sue-doe-clee-on) or Pseudo for short. However, in recent years the entomology community has re-classified the taxonomy so many times that it is hard for the average angler to keep up; consequently, many anglers have no idea what these little olive mayflies are actually called.

Iswaeon anoka is the current genus and species of the mayfly formerly known as Pseudocloeon edmunsi, Pseudocloeon anoka, Heterocloeon anoka, Baetis punctiventris and Plauditus punctiventris. As is always the case, fish could care less about taxonomy, and old habits die hard. The name Pseudocloeon is so ingrained in fly fishing culture that it is hard to imagine a day when fly bins have the name “Anoka” on them instead of “Pseudo” or even “BWO”. There’s also no telling how many more times this little bug will be renamed.  So, in deference to fly fishing history, and practicality, the mayfly formerly known as Pseudocloeon edmunsi, now known as Iswaeon anoka will be referred to as “Pseudo” for the purposes of this article.

The genus Pseudocloeon was first described by Frantisek Klapalek in 1905 from samplings of mayflies he obtained in mountain streams that drain volcanoes in Java. Similar specimens were found in Argentina, and of course, the American West. The main differentiating feature of these mayflies is the absence of secondary wings. Most mayflies in their dun (subimago) and spinner (imago) stages have large primary wings and small secondary wings. Pseudos are missing these secondary wings all together. Aside from their diminutive size, this is one way to tell them apart from their larger Baetis cousins who are sometimes hatching at the same time in the spring and fall.

Pseudos, like others in the Baetidae family, are multi-brooded. There is a hatch in the spring and one in the fall. Eggs laid in the spring have a shorter gestation period as water temperatures are warmer during the summer, and spawn the fall hatches. Eggs laid in the fall have a longer gestation time as water temps are cold in the winter, and spawn the spring hatches.

Pseudo nymphs are strong swimmers and inhabit a wide range of habitat types from pea gravel to large cobble, and long strands of weeds. These tiny (size 22–26) nymphs have two elongated tails and a body coloration that ranges from light olive to tan with dark brown wing pads. Their aptitude for swimming makes them difficult targets for feeding trout; therefore, nymph imitations fished deep are often unproductive.

As good as Pseudo nymphs are at swimming, they are equally lousy when it comes to emerging. Nymphs ascend quickly from the bottom of the water column then pause just under the surface riding the current for some time before they struggle to break through the water’s surface tension. Small nymph imitations fished in, or just below the surface, although challenging, can be very effective.

The emergence process is a clumsy affair for Pseudo duns too. Many duns have difficulty freeing themselves from their nymphal shuck. Half-emerged duns ride the surface for great distances with their trailing shucks trapped in the water’s surface tension. These vulnerable emergers are an easy meal for feeding trout, and patterns dressed with cdc or deer hair to imitate a partially emerged wing, and synthetic materials for a trailing shuck are a must have for selective fish.

Once emerged, Pseudo duns will display a variety of body coloration depending on location and sex which can range from light olive to a vibrant, chartreuse green. Duns have dusky gray wings with no secondary wing, and two long tails that are a striking, chalky white.

Emergences occur during the afternoon, and unlike other Baetis mayflies, strong hatches do not rely on scuzzy weather. Pseudos are perfectly content emerging in vast quantities on sunny days as well as cool, cloudy ones. Though, on especially warm, sunny fall days emergences might not happen until an hour or two before dark.

Pseudo spinners generally return to the water at dusk, and are often present in the drift with duns and emergers during evening emergences. They too lack a secondary wing, but their primary wings are glassy clear. Body coloration can span the full spectrum of greens, but is many times bright apple-green. Adult female Pseudos lay their eggs under water. After mating the females will routinely land on stream side objects and crawl into the water to deposit eggs.

The mayfly formerly known as Pseudocloeon has undergone massive changes in it’ s taxonomy. Yet, despite several new names and an altogether different genus, it remains the same tiny mayfly that hatches in incredible numbers producing memorable dry fly sessions with trophy trout on many of our favorite rivers in Yellowstone Country.