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Mayfly Species Ephemerella excrucians (Pale Morning Dun)

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For trout (if not anglers), this single species is arguably the most important mayfly in North America. In terms of sheer numbers, breadth of distribution and hatch duration, it has a good argument.

Ephemerella excrucians or Pale Morning Dun usually follows its larger sibling Ephemerella dorothea infrequens with which it shares the same common name. What it often lacks in size by comparison is made up for with it's duration, often lasting for months with intermittent peaks. This close relationship with infrequens has led many anglers to confuse Pale Morning Dun biology with that of the multivoltine (Multivoltine: Having more than one generation per year.) Baetidae species, having disparate broods that decrease in size as the season advances. Sharing the same common name has not helped to alleviate this misconception.

Until recently, Ephemerella excrucians was considered primarily an upper MidWestern species of some regional importance commonly called Little Red Quill among other names. Recent work by entomologists determined that it is actually the same species as the important Western Pale Morning Dun (prev.Ephemerella inermis), and the lake dwelling Sulphur Dun of the Yellowstone area, (prev.Ephemerella lacustris). Since all three are considered variations of the same species, they have been combined into excrucians, being the original name for the type species reported as far back as the Civil War. Angler speculation had simmered for some time that the stillwater loving Ephemerella lacustris was much more widespread, inhabiting more water types then previously thought and could account for many large sulfurish ephemerellids found in still to very slow water locations throughout the West. With the revisions, this discussion is now moot.

Ephemerella excrucians variability in appearance, habitat preferences, and wide geographical distribution are cause for angler confusion with the changes in classification. They can be pale yellow 18's on a large Oregon river, creamy orange 14's on western lakes and feeder streams, large olive green on CA spring creeks as well as tiny sulfur ones in many Western watersheds. Then there's the little Red Quill on small streams in Wisconsin. Yet, all are the same species.

Where & When

Regions: East, Midwest, West

Time Of Year (?): April through October with peaks of a month or more within this period depending on location

Preferred Waters: All water types except warm river systems and infertile high country lakes

Altitude: variable
These ubiquitous mayflies are extremely abundant throughout the West and have a wide range of dates for their emergence. There is considerable evidence that on temperature stable spring creeks they can have asynchronous (Asynchronous: The same generation hatching at different times.) emergences in the spring and fall, not to be confused with many baetids multivoltine (Multivoltine: Having more than one generation per year.) life history (Life history: The detailed life cycle of an organism, including the stages it passes through and characteristic behavior relating to growth and reproduction.). In contrast, Eastern emergences are shorter, smaller and far less significant. Anglers would be wise to consult hatch charts and obtain current local information on specific rivers to time this species. Keep in mind that these charts usually combine them with the often larger and earlier hatching Ephemerella dorothea infrequens as they are very difficult to tell apart.

Hatching Behavior

Time Of Day (?): Late morning and early evening in the West; late afternoon to evening in the East

Habitat: Highly variable, though the greatest concentrations occur in weedy riffles and runs

Water Temperature: Varies with location
As noted with Ephemerella dorothea infrequens, excrucians is a classic surface emerger and often engage in "practice runs" exposing the nymphs to trout during extended pre-hatch periods. The main differences are that their smaller size means they struggle a bit more with the water's surface tension and the warmer weather they usually hatch in also means they spend less time on the water preparing their wings for flight. As a result, the observant angler should look for this and be even more ready to use emerger and cripple (Cripple: In fly fishing, a cripple is any insect which has been injured or deformed so that it cannot escape the water. This may include stillborn emergers or fully emerged adults which have been damaged, often by wind or waves, so that they can no longer fly. Trout often favor eating crippled insects.) patterns if high floating dun imitations prove unproductive. Because of this hatches duration and sheer numbers, the fish become increasingly wary as the season progresses. Towards the end of their cycle on heavily fished water, even the most expert angler will be challenged to the limit. Fishing the 'right" fly is usually eclipsed by the need to avoid micro drag and pickup/deliveries that alert the quarry to the attempted fraud.

Spinner Behavior

Time Of Day: Morning and again at dusk in the West; only dusk in the East, where they're not important

Habitat: See notes
The spinner falls are insignificant in some locations while achieving legendary status in others. They can be so dense at times that they are virtually unfishable. They prefer riffles if they exist. Otherwise they seem to pick specific areas for a variety reasons, most often riparian shelter, depth, and substrate related. See comments on Ephemerella dorothea infrequens hatch page for additional information.

Nymph Biology

Diet: Detritus (Detritus: Small, loose pieces of decaying organic matter underwater.) and algae

Current Speed: Slow to fast in the West; medium to fast in the East

Substrate: All types, but prefer gravel and cobble with weed growth or the edges of weed beds in spring creeks

The nymphs can be important to imitate before and during the emergence. They come in shades of olive or very dark brown, but they also run the gamut of cinnamons like the larger infrequens. Unlike their big sisters though, excrucians nymphs are often patterned with longitudinal stripes and/or a veriagated pattern on the abdominal dorsum. Trout can be selective to these schemes.

Pictures of 22 Mayfly Specimens in the Species Ephemerella excrucians:

Specimen Page:123
Female Ephemerella excrucians (Pale Morning Dun) Mayfly DunFemale Ephemerella excrucians (Pale Morning Dun) Mayfly Dun View 3 PicturesSize: 10mm. At emergence the specimen was a fairly bright olive green and there was obvious difference in color between the forewing (med. dun) and the hind-wing (pale cream). It was really noticeable as they floated by. You can just make these features out in the second photo, but not so much in the first that was taken 24 hours after capture. Total time from emergence to molting - approx. 48 hours.


Edit 2/25/13 - This specimen was originally posted to E. d. infrequens because of its size. It turns out large size doesn't hold up as a way to tell these two apart. This is because excrucians has much greater variability than previously understood. The assumption by anglers that excrucians is always the smaller of the two is apparently not supported by science. There is a lot left to sort out with western Ephemerella species. This may include new discoveries and/or synonyms (Synonym: A former name of a taxon, usually a species. Entomologists frequently discover that two insects originally described as different species are one in the same, and they drop one of the names. The dropped name is said to be a synonym of the remaining name. These changes take a while to trickle into the common knowledge of anglers; for example, Baetis vagans is now a synonym of Baetis tricaudatus.) as well as reportage on new intraspecific variations broadening the descriptions of recognized species. Based on this specimen's Fall maturity, the best guess is that it is an unusual form of excrucians.

As to color, both species duns (nymphs too) demonstrate a tremendous amount of intraspecific variability from pale yellow to bright green with a multitude of sulfur shadings in between, ranging from pale amber, through orange to cinnamon and even dark brown. I've seen wings from pale cream through tannish and almost every shade of dun except the dark shades. Some have pigment stained leading edges matching their bodies, some don't. Most of these variations are undocumented except in angler references and periodicals. It seems a rare year that a new variation doesn't pop up to the notice of anglers.

Bottom line - size is only reliable if the specimens are smaller than size 16, pointing to excrucians. Otherwise, the only fairly dependable way to tell them apart (especially the females) is by timing as infrequens is the first of the two to appear, rarely lasting longer than a couple of weeks or later than the end of June most years. The problem with using timing for determination is it requires knowledge of the hatch sequences as they actually occurred for a given year on a given piece of water. Obviously, this kind of information is seldom available. Without it, determining between the two duns if they are larger than size 18 is speculative at best - at least until very late in the Summer.
Collected October 16, 2011 from the Fall River in California
Added to by Entoman on October 21, 2011
Specimen Page:123

Recent Discussions of Ephemerella excrucians

PMD Spinner - Egg sack color? 20 Replies »
Posted by Wbranch on Jan 26, 2010
Last reply on Aug 18, 2020 by Troutnut
Do any of you entomologist types know the true color of the PMD spinner? Dorothea or excrucians. Where I fish in MT there are huge spinner falls, many spents are on the water in the morning and others fall again at various periods during the day. I'd like to tie some with egg sacks as I saw many in July but forgot what color they were. Thanks.

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