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Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large Brown Trout that only touch streamers for the rest of the year.

Mayfly Species Ephemerella invaria (Sulphur)

This species, the primary "Sulphur" hatch, stirs many feelings in the angler. There is nostalgia for days when everything clicked and large, selective trout were brought to hand. There is the bewildering memory of towering clouds of spinners which promise great fishing and then vanish back into the aspens as night falls. There is frustration from the maddening selectivity with which trout approach the emerging duns--a vexing challenge that, for some of us, is the source of our excitement when Sulphur time rolls around.

Ephemerella invaria is one of the two species frequently known as Sulphurs (the other is Ephemerella dorothea). There used to be a third, Ephemerella rotunda, but entomologists recently discovered that invaria and rotunda are a single species with an incredible range of individual variation. This variation and the similarity to the also variable dorothea make telling them apart exceptionally tricky.

As the combination of two already prolific species, this has become the most abundant of all mayfly species in Eastern and Midwestern trout streams.

Where & when

Time of year : May-June

The invaria hatch begins in early May in Pennsylvania. The Catskills peak in late May and early June, while mid-June is best in the Adirondacks and the northern fringe of the Upper Midwest.

Good fishing from this hatch usually lasts about two weeks in a particular location.

Hatching behavior

Time of day : Flexible based on weather. Generally mid-afternoon at first and later as the season progresses.

Water temperature: 52-60°F

Ephemerella invaria nymphs drift for some time just below the surface as they begin to emerge, and these floating nymphs cause rises to the surface which may fool the angler into thinking the trout are taking duns.

Emergence itself takes quite a while, making emerger patterns more important for this hatch than for most. This importance is heightened by the lower availability of the fully formed duns. These duns take to the air more quickly than most of their Ephemerella kin, but anglers should still be prepared to imitate them if need be.

Spinner behavior

Time of day: Dusk

Habitat: Riffles

Duns typically molt and return to the water as spinners within one day. The females drop their eggs in mid-air and are not exposed to trout until they fall spent--if they fall spent at all.

Knopp & Cormier write in Mayflies: An Angler's Study of Trout Water Ephemeroptera that the males seldom fall spent on the water and the females only fall sometimes. Ted Fauceglia's Mayflies says both genders fall spent on the water. This disagreement mirrors the confusion I've felt about this species on the stream; the spinners are generally unpredictable, but they can provide excellent fishing.

Nymph biology

Current speed: Usually medium to fast, sometimes slow

Substrate: Many types, but gravel is preferred

Environmental tolerance: Unlike Ephemerella subvaria, invaria is very tolerant of a wide range of conditions.

These are among the most adaptable of mayflies, thriving in many types of habitat. They inhabit warmwater streams as well as cold and are found up and down the channel in riffles, pools, and runs. They are most prolific in deep, gravel-bottomed riffles.

In the days and hours before the hatch these nymphs become especially active, so their imitation is especially useful. They clamber to better emergence sites where they rest in plain view of both trout and photographers (see my underwater photos). They also make apparent "practice runs" to and from the surface in the manner of Ephemerella subvaria.

The nymphs are a little bit difficult to recognize because they their base color ranges from brown to olive and they're adorned with many possible color patterns. Anglers should be prepared with imitations in brown and olive, hook sizes 12 through 16.

Ephemerella invaria Fly Fishing Tips

Take extra care during emergence to figure out if the trout are feeding on floating nymphs, emergers, or duns, and what size Sulphur they are taking. The invaria duns can be hook size 14 or 16. The possibility of confusing them with the smaller Ephemerella dorothea adds to the Sulphur challenge. Remember that the solution to this puzzle may vary from fish to fish, and you should switch flies or switch fish quickly if what you're trying isn't working.

Specimens of the Mayfly Species Ephemerella invaria

3 Male Spinners
3 Female Spinners
7 Male Duns
5 Female Duns
27 Nymphs

2 Streamside Pictures of Ephemerella invaria Mayflies:

15 Underwater Pictures of Ephemerella invaria Mayflies:

Recent Discussions of Ephemerella invaria

8 replies
Posted by CharlieBugs on Jan 26, 2015
Last reply on Feb 1, 2015 by Gutcutter
Your post on Ephemerella subvaria brought back some memories that might be of interest to some readers.
I got my master’s degree under Ed Cooper at Penn State in 1966. I studied the impact of low oxygen from Penn State’s sewage plant on the mayflies of Spring Creek. The plant mostly removed BOD (organic matter which causes low oxygen) by oxidizing it in a bacteria rich environment. But at that time the plant did not remove phosphorus (and nitrogen) which fertilized the macrophytic algae and other plant growth. There were far more macrophytes (large plants) in Spring Creek below the sewage plant entrance than above, and essentially no mayflies. What was there in the effluent that killed the mayflies? Mayflies put directly in the effluent did not die over a 16 hour day. But oxygen samples taken over 24 hours in summer showed a much greater variation below the effluent (from 16 ppm (or mg/l), 160% of saturation in late afternoon) to 3 ppm (30 percent saturation just before dawn) (vs 14 to 10 ppm, +- 20 percent saturation above the effluent.
I built a Rube Goldberg machine in the lab that would control the oxygen levels and temperature of control and experimental cages that each held 25 mayflies. The control would keep oxygen near saturation and the experimental one would lower the oxygen over 8 hours (the length of night in summer). Mortality was dependent upon both oxygen level and temperature. Virtually no mayflies would die if the oxygen was above 2 PPM (or mg/l) at 8 C, or at 4.5 at 20 C. Seventy five percent of larvae would die at 1 ppm at 8 degrees and 2.5 ppm at 20 degrees. So the mortality was much more if the temperatures were high. Hence most of the mortality would presumably occur in August when the high temperatures (20C) would increase the larvae metabolism and hence need for oxygen, but the water would hold less oxygen even before the night-time respiration of the macrophytes would reduce it much further (to only 3 ppm). Thus the low nighttime oxygen caused by excessive plant growth was a sufficient cause or the near total absence of mayflies below the sewage plant. Recognizing the aquatic impact Penn State started to use land disposal of its effluent which as far as I knew alleviated, and even stopped, the negative impacts on the mayflies. Can anyone verify this ? The otherwise well done “The Fishery of Spring Creek; A Watershed Under Siege “
By Robert F. Carline, Rebecca L. Dunlap Jason E. Detar, Bruce A. Hollender Has nothing on dissolved oxygen or aquatic insects. In my opinion we need much more of an ecosystems approach for streams (Which we are doing for Little Sandy Creek in N.Y.).

Now back to Ephemeralla subvaria. The title of the paper I published on this project (my first of nearly 300 publications) was:
1. Hall, C.A.S. 1969. Mortality of the mayfly nymph, Ephemerella rotunda, at low dissolved oxygen concentrations. J. Elisha Mitchell Sci. Soc. 85(1): 34-39 (M.S. Thesis, Pennsylvania State University, 1966).

Whoa! Ephemeralla rotunda? This was by far the most abundant mayfly in Spring Creek where I sampled! But I could not even find the name in your list. Also I had my samples verified by Burke, author of the authoritative "Mayflies of Illinois", and he said Well that’s what it keyed out to in my book”. (Talk about scientific ass covering! ) Well to make matters worse (as of 24 hours ago) my next girlfriend, Molly, at the University of North Carolina, loved the name of “Ephemerella rotunda”, the rotund one, which comes to think of it described her as well. I liked it too. But n’exist plus: where had the most abundant mayfly gone? Fortunately upon reading the rest of the post I found “Ephemerella invaria is one of the two species frequently known as Sulphurs (the other is Ephemerella dorothea). There used to be a third, Ephemerella rotunda, but entomologists recently discovered that invaria and rotunda are a single species with an incredible range of individual variation." Ahh neither rotunda nor Molly stood the test of time. So I assume what I called rotunda is still alive and well in Spring Creek as invaria. Again, can anyone verify that?

If anyone wants to follow up on the distribution and abundance of mayfly (or any other species) may I recommend: Hall, C.A.S., J.A. Stanford and R. Hauer. 1992. The distribution and abundance of organisms as a consequence of energy balances along multiple environmental gradients. Oikos 65: 377-390. I can send it if you cannot get it from google, which I think you can. (chall@esf.edu)
Aw Shucks
10 replies
Posted by Martinlf on May 19, 2009
Last reply on Jul 24, 2009 by Martinlf
OK, this is going to seem like a major duh experience for some of you, but the other night I found a sulphur spinner on the door of a bathhouse in a campground I was staying at. Looking for other bugs I then saw a pale nymph shuck on the door. I was totally confused. A nymph this far from the stream? Was this some alien bug? Looking closer I noticed that the shape was too slender for a nymph and that the wing pads were more like little protruding pockets--and it hit me. Spinner shuck. I knew that mayflies molted to produce a spinner, but I had thought the shuck would be more insubstantial--something that would be flimsy and lack form. This was so cool, and at the same time I felt so silly for thinking it could somehow have been a nymph shuck. It's the first spinner shuck I've seen, but I assume that I'll start seeing them everywhere now, like a new word you learn. Anybody else have a spinner shuck story?
Hatching Sulphur
Posted by Martinlf on Oct 14, 2008
Last reply on Oct 14, 2008 by Martinlf
Click on "40 more specimens" and scroll down for one photo.

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