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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


Regions: East, Midwest, West

Time Of Year (?): August through October; best from late August through September

Preferred Waters: Lakes and rivers, warm and cool. Alkaline (Alkaline: Having a pH higher than 7 (opposite of acidic). Moderately alkaline water is ideal for trout because it's better for the growth of phytoplankton, the usual base of the aquatic food chain, and that's good for the growth of everything higher up the chain, including trout.) 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.

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 (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) 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 (Atrophy: A body part which has shrunk or degenerated into a dysfunctional state is said to have atrophied. The mouth parts in adult mayflies are good examples.) 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 (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) right after mating, but the females fly upstream to lay their eggs, sometimes for miles, and they fall spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) 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 (
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
)
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 (
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
)
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:

This nighttime flash photograph shows a bunch of Ephoron mayflies flying around during the hatch.  So many of them fly around with their dun shucks attached that it seems like they molt from the dun to spinner stage in mid-air.  Actually they molt on streamside vegetation like other mayflies, but they sometimes take off to mate before they're completely finished.  In this picture: Mayfly Genus Ephoron (White Flies). From unknown in Wisconsin.
This nighttime flash photograph shows a bunch of Ephoron mayflies flying around during the hatch. So many of them fly around with their dun shucks (
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
)
attached that it seems like they molt from the dun to spinner stage in mid-air. Actually they molt on streamside vegetation like other mayflies, but they sometimes take off to mate before they're completely finished.

In this picture: Mayfly Genus Ephoron (White Flies).
Date TakenAug 14, 2004
Date AddedJan 18, 2006
AuthorTroutnut

Recent Discussions of Ephoron

Potomac White Fly Hatch - in progress 1 Reply »
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.
ReplyFly choice during an Ephoron hatch 11 Replies »
Posted by Jmd123 on Aug 3, 2007
Last reply on Sep 25, 2007 by Gene
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!

Jonathon
ReplyEphoron 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?
Reply

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