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3. Size Variation in March Browns

I was fishing yesterday in north central Wisconsin. At the end of the day there was a sparse hatch of mayflies. They resembled the March Browns that hatch later in the year on this same river but they were a good 2 to 3 hook sizes smaller than the March Browns (#14/#16 as opposed to #10/ #12) that come off in early June. Add to that that the season is running a week or two behind schedule and that makes it even harder to reconcile an early emergence of March Browns. The fish did not seem particularly interested in the bugs (they go nuts over the MB hatch in June). Any ideas?

Hi George,

Other than the small size, I can't see anything about your specimen that suggests that it is not a March brown (M. vicarium). That species, as currently described, is notorious for wide variation in size and color, and often has a rather lengthy and spotty emergence. I find sizes for this species (or its synonyms) ranging from 10-18mm in the scientific literature. The lower end of that range is close to what you describe, about a #14. (There is a fair amount of subjective interpretation involved in hook size translations.) Your specimen is also a male, and males run smaller than females. Although some unusual influence may have played a role, I would guess that you witnessed a limited emergence of smallish males.

If this is Maccaffertium (formerly Stenonema), vicarium is an excellent guess. It is a fairly early Maccaffertium. My source suggests a range of 9-14mm for the male adults (10-18 for larvae). The longitudinal striping on the abdomen makes me lean towards vicarium, in addition to the size. I'd thought Leucrocuta earlier (deleted the post) based on the smaller size and shading of the wing veins, but M. vicarium is better, due to coloration, leg striping, etc. Good call from the Bear Swamp.

M. integrum is even smaller, 5-7mm, and has a broken stripe, but it occurs later in the year.

Well, I just picked up a ruler and held it against the same spot I took the second picture and 10mm is just about what the bug would have measured. So mystery solved. Interestingly enough a friend was fishing a river last night 20 miles from where I was Monday and also ran across a hatch of smaller than average March Browns. In his case though the eyes were the moss green that the males have in Jason's photos. I'll post a link to his response and his picture below.

http://www.wisflyfishing.com/cgi-bin/yabb2/YaBB.pl?num=1242735856

IMHO, the wing is dead on M. vicarium. I have seen a lot of size variation as well. Although in early June in the NW corner you get a lot of much larger March Browns. If I remember right, "Konchu and gang" lumped Stenonema fuscum (which is smaller) with Maccaffertium vicarium now.

I don't think Konchu was part of that "gang," John. As best I can reconstruct it, that particular "lumping" went something like this: The former Stenonema fuscum was synonymized under Stenonema vicarium by Bednarik and McCafferty in 1979. That same year, Bednarik also proposed "Maccaffertium" as a subgenus of Stenonema. Maccaffertium was then elevated to genus level by Wang and McCafferty in 2004, leaving only Stenonema femoratum in the genus Stenonema. That combination produced Maccaffertium vicarium as we know it now. (I'm sure Konchu can correct me if I've made any mistakes in tracing that process.)