Mayfly Genus Ephoron (White Fly)

Where & when

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 July (35%), June (35%), August (18%), and May (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.

Hatching behavior

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.

Egg-Laying behavior

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

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.