March Browns

Western March Brown

A male “Western” March Brown Dun


I’d seen my first one a little more than two weeks ago, flying from the Yellowstone River into the cool, early-spring air.  It was trying to clear the rocks near shore, searching for a place to rest in the sagebrush and shrubs to change into a spinner and complete its life-cycle.  I tried to net it with my hat, but I missed, and it slowly motored beyond my reach.  It was a big mayfly, bigger than the midges and Blue Winged Olives (Baetis) that were also emerging.  Because of its size and the time of year it was emerging, I knew what it was, though I had never before seen one in person.  Two days ago, I found a lazier one just sitting on the rocks near shore.  I took it home to photograph.  That’s him in the above picture.

Six months ago (before I moved to Montana), I would have called this mayfly a Western March Brown;  truth is, out of habit, I still sometimes call mayflies  “Western” March Browns or “Western” Green Drakes.  But now that I live in the west, I can most assuredly drop the “western” part of all mayfly common names.  After all, no one back east calls their legendary Green Drakes “Eastern” Green Drakes.  And no one there attaches “Eastern” to their March Brown either.


Eastern March Brown

A female “Eastern” March Brown dun


Western March Browns and Eastern March Browns have a few traits in common besides their angler-given names.  But they are not related, and much of their similarities end at their names.  Eastern March Browns are from the Genus Maccaffertium (species vicarium). Their angler-given name belies the fact that Eastern March Browns usually hatch in May.  So why would a mayfly that hatches in May be called a “March” Brown?  It’s because the first European Americans who witnessed Eastern March Brown emergences thought the bugs looked very similar to a mayfly that lived in their former homeland–England.  This mayfly often hatches in March, but it is from the genus Rhithrogena (species germanica).  The same genus as the Western March Brown (species morrisoni )

Both Western and Eastern March Brown nymphs are classified as clingers.  They are similarly shaped and are most often found holding themselves flattened against submerged rocks in riffles.  But these two mayflies emerge quite differently. Western March Browns emerge on the stream bottom and quickly swim to the surface as fully formed duns.  This is similar to how Eastern Quill Gordons (E. pleuralis) and Pink Cahills (E. vitreus) emerge, but very unlike Eastern March Browns.  Eastern March Brown nymphs migrate from riffles to areas of slack current where they emerge, most often, in the surface film.  This makes emergers tied on curved hooks with trailing shucks quite effective for impersonating Eastern March Brown emergers.  But you’d be better off swinging wet flies if you want to imitate the emergence of Western March Browns.

Both Eastern and Western March Browns are big mayflies, around a size 12, and though I didn’t find fish eating them this week, they will be soon in many rivers and streams throughout the country.  March Brown hatches can be some of the most anticipated of the year, whether you live in the east or the west.





2 Comments so far

Well written and informative article.

[…] King Prince…. You may see mayflies hatching, small baetis (blue-winged olives) and/or bigger March Browns. The March Brown hatch has been spotty, but there were some out on the Bird Float (Grey Owl to […]

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