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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
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.

Dorsal view of a Sweltsa (Chloroperlidae) (Sallfly) Stonefly Nymph from the Yakima River in Washington
This species was fairly abundant in a February sample of the upper Yakima.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Genus Ephoron (White Flies)

On certain rivers in late summer the Ephoron mayflies gives new meaning to the words "blizzard-like hatch," because their large white bodies give a true snowstorm appearance to their enveloping swarms. This is the most intense aquatic insect hatch of the year in places, and sometimes the flies are so thick that it's hard to get a trout to find one's imitation among the carpet of real insects on the water.

Ephoron leukon is most important species in the East and Ephoron album in the West. They overlap in the Midwest. These are the only two mayflies of this genus recognized in the United States, but Caucci and Nastasi in Hatches II report inspecting specimens which did not fit the description of either species.

Where & when

Time of year : August through October; best from late August through September

Preferred waters: Lakes and rivers, warm and cool. Alkaline waters produce the best hatches.

Ephoron mayfly populations are very localized, often present in apocalyptic numbers in one river and virtually absent from the next drainage over. It pays to tap into a little local wisdom before trying to find this hatch.

Since these species thrive in warm water as well as cool, they are also important to smallmouth bass fishermen. These mayflies mark an excellent time for topwater fishing, often with Ephoron imitations.

In 55 records from GBIF, adults of this genus have mostly been collected during August (35%), July (35%), September (18%), and June (9%).

In 32 records from GBIF, this genus has been collected at elevations ranging from 618 to 6250 ft, with an average (median) of 3533 ft.

Genus Range

Hatching behavior

Time of day : Dusk

Water temperature: 65-70°F

The nymphs are very fast to emerge once they reach the surface, and the duns quickly take to the air. They typically hatch, molt, oviposit, and fall spent within a couple hours, sometimes less, and trout are partial to all the adult stages. The males hatch first so that they have time to molt into spinners before the females arrive within the next thirty minutes.

Unique among mayflies, the females don't molt -- they mate and die as duns. Because they go straight from the water to the mating flights and back to the water, their legs have atrophied to a weak and nearly useless state. Fred Arbona mentions in Mayflies, the Angler, and the Trout that the males may be so eager that they attempt to mate with emerging females before they even left the water. The amount of commotion caused by their antics makes twitching and skittering one's dry fly an excellent technique.

The Ephoron activity seems to occur everywhere on the river: every riffle, every pool, every run, upstream and down as far as the eye can see.

Spinner behavior

Time of day: Dusk

Male spinners fall spent right after mating, but the females fly upstream to lay their eggs, sometimes for miles, and they fall spent far from where they emerged.

There is a common misconception that Ephoron mayflies molt into spinners in mid-air almost as soon as they hatch. One of my streamside photos shows several male spinners still trailing their dun shucks as they fly around, which explains how the rumor started. It is physically impossible for a mayfly to fly (which requires use of its wings) while molting (which takes its wings out of commission for a while). The males actually molt quickly after landing on streamside vegetation, and they often take off again before the shuck is completely off of their tails.

Nymph biology

Current speed: Slow to medium

Substrate: Silt or clay

Environmental tolerance: High tolerance for temperatures warmer than trout can stand; low tolerance for pollution

According to Knopp and Cormier in Mayflies: An Angler's Study of Trout Water Ephemeroptera , the nymphs leave their burrows and become active on the bottom in the hour or so preceding emergence. They may also drift for a long time below the surface as they begin to hatch, creating good opportunities for hungry trout.

Ephoron Fly Fishing Tips

Some fly shops sell "White Fly Nymph" imitations which look like snow-white Hendrickson nymphs, completely the wrong shape and color for real Ephoron nymphs. A better imitation would look something like a smaller Brown Drake (Ephemera) nymph.

Dun imitations have also suffered a loss of realism due to the common names. The real duns have light gray wings over creamy white bodies, but most imitations have bright white wings. Perhaps this has tested better with the trout, but it seems unlikely.

1 Streamside Picture of Ephoron Mayflies:

Discussions of Ephoron

Posted by Imaxfli on Oct 23, 2020 in the species Ephoron leukon
Last reply on Oct 23, 2020 by Imaxfli
Me too looking for photos or better yet, video of matching. These things seem to pop outa the water like no other ......
Polymitarcyidae Question
1 replies
Posted by Steamntrout on Jun 22, 2017 in the species Ephoron album
Last reply on Jun 22, 2017 by Crepuscular
What size are these nymphs & Sub Imago's?
Fly choice during an Ephoron hatch
13 replies
Posted by Jmd123 on Aug 3, 2007
Last reply on Jan 31, 2016 by Martinlf
Hey folks, what would you recommend as an imitation of these guys? They're hatching furiously in the Huron River in/near Ann Arbor, MI (smallmouth are leaping out of the water to eat them). How about a Light Cahill or White Wulff? Both have been major producers for me on other streams with white mayfly hatches in summer (e.g., Rifle River). If anyone has a particular recipe that works for them, I'd like to know. Thanks!

Potomac White Fly Hatch - in progress
1 replies
Posted by BrettB on Sep 17, 2008
Last reply on Sep 21, 2008 by GONZO
There's quite a big white fly hatch on the Potomac at least around Harper's Ferry. Does anyone know a good nymph or emerger pattern for the white fly hatch? I plan to hit it early, prior to duns coming off and know there is a good deal of nymphal movement and drift beforehand. Maybe I'll collect a few tonight to see color / size / behavior. Any thoughts are appreciated.
Ephoron Leukon nymph photographs
11 replies
Posted by BFornadley on Feb 8, 2007 in the species Ephoron leukon
Last reply on Aug 14, 2007 by Gene
I need some help here. I have been looking all over the web for someone who may have taken some Ephoron Leukon nymph underwater (or out of water) photos.I saw the description here at "Troutnut.com" and advice that a smaller Brown Drake (Ephemera) nymph would be a good natural to use as a tying model but I really want to see the real thing. Does any body have any of these pics or can anyone definitively tell me where to look?

Start a Discussion of Ephoron


Mayfly Genus Ephoron (White Flies)

Genus Range
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