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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 dorothea infrequens (Pale Morning Dun)

Ephemerella dorothea infrequens (formerly Ephemerella infrequens), together with its often smaller and later hatching sibling Ephemerella excrucians, make up the most important Western hatches. They go by several common names but are best known as Pale Morning Duns (PMD's). They are rivaled only by the many baetid species that typically bookend them. In terms of availability, consistency and abundance (not to mention their convenient timing and preference for beautiful weather), they have no rival. They can run in size from a large 14 to a small 16 and various shades of illusive yellowish creams, sulfurs, and even yellowish greens, depending on the system they inhabit.

This taxon used to be considered the separate species Ephemerella infrequens, but entomologists now regard it as a subspecies together with the small eastern Pale Evening Dun hatch Ephemerella dorothea dorothea. There is another related listing of significance common in California and the Southwest that has undergone revision. The large (often exceeding 10 mm) Ephemerella mollitia is now considered synonymous with d. infrequens.

While it is not the normal policy of TroutNut to list subspecies as separate taxa, d. infrequens and d. dorothea are so important and distinct from each other in terms of geography, appearence, and angling tradition that they warrant an exception.

Where & when

Time of year : March-August, best in May-June in the coastal states;July-August in the Rockies

Preferred waters: All types, but especially abundant in alkaline spring creeks.

Altitude: From near sea level on some West Coast tailwaters to as high as 8,000 ft.

This Western subspecies hatching cycle is more difficult to predict because of the high local variation in climate and the varieties of habitat including spring creeks, tailwaters and snow melt fed freestones. Hatches generally occur in the Rocky Mountain states from late Spring through the Summer months depending largely on elevation, latitude, and water type. They have been reported as early as March on some West Coast watersheds with mid-May to mid-June being the prime time at higher elevations. Despite the wide range of emergence dates, this hatch is usually prevalent on a given stretch of water for less than a few weeks or so, often mixing with the other Pale Morning Dun species Ephemerella excrucians as it begins to taper off. This often gives the impression of a much longer lived hatch. Because of this mix, some spring creeks and tailwaters in the West can have at least sparse activity of "PMD's" virtually the entire season. Caucci and Nastasi note in Hatches II that they can extend well into October on stable spring creeks. Fred Arbona writes in Mayflies, the Angler, and the Trout that they may appear during the last few weeks of June. Knopp and Cormier report their hatches during May in California.

Hatching behavior

Time of day : Usually midday often moving to evenings during warm weather

Water temperature: variable depending on location

Classic surface emergers with long sedate floats in optimal weather. Have been reported to emerge subsurface in faster flows. They usually engage in "practice runs" exposing the nymphs to trout during extended pre-hatch periods.

Spinner behavior

Time of day: Usually morning and often again near dusk

Habitat: Riffles are preferred in freestone environments. Spring creek dwellers prefer specific locations determined by a variety of factors.

Hatched duns typically return to the stream within two days as spinners. After mating, both genders fall spent on the water.

Females often, but not always, drop their eggs from the air above the stream. When they do end up on the water with egg sacs still attached, trout may become selective to spinners with little bright yellow dots near the tail and upright wings as well. The gravid females are often active and far from "spent". Spent spinners usually cause the most feeding activity. Both male and female are usually similar enough that the same pattern can work for both, though the angler should take note of the differences in size between the sexes as the fish can become selective to one or the other. thogh often seen in the evening, mid- morning spinner falls have achieved legendary status at many locations.

Nymph biology

Diet: Detritus, algae

Current speed: Can be found in all current speeds, but especially near vegetation.

Substrate: Gravel, sand, vegetation

The nymphs display the usual Ephemerellidae habits of high activity in the hours and days before they hatch, and trout claim many of them before they're anywhere near the surface. They can be found in the benthic drift at almost any time of the day but concentrated behavioral drift can occur in the evening. Color is not a good way to distinguish them from other Ephemerellids they often share habitat with. In addition to their larger size, another way to often tell them from their close relative excrucians is their lack of veriagation or striping of the abdominal dorsum (top). With a few less common exceptions, their lack of paired abdominal tubercles (little parallel spikey projections along the top of the abdomen) is an easy way (for young eyes) to tell them apart from other common Ephemerellids. They run in colors from a dark chocolate brown through various shades of rust, cinnamon or tan. They can also be olive hued. The three tails and legs are usually speckled to some degree and similarly shaded as their bodies.

Specimens of the Mayfly Species Ephemerella dorothea infrequens

3 Male Spinners
1 Female Spinner
Female Ephemerella dorothea infrequens (Pale Morning Dun) Mayfly Spinner
I'm calling this one Ephemerella dorothea infrequens because it was collected along with two males that positively ID to that species.
1 Male Dun
Male Ephemerella dorothea infrequens (Pale Morning Dun) Mayfly Dun
This dun was collected in proximity to this spinner.
1 Nymph
Ephemerella dorothea infrequens (Pale Morning Dun) Mayfly Nymph
Habitat: Shallow riffle over cobble; approx. 1 ft. deep
Size: 8.5 mm. Mature specimens have been captured as large as 10.5 mm.
Emergence schedule: Variable - starting as early as mid March and lasting as late as early June, depending on the year. Usual duration is at least several weeks or more.
Dun Association: Body is elusive pale creamy yellow w/ orange highlights, cream legs and tails, and dun wings
Specimen status in photo: Preserved
Collection method: Kick net

Comments: Extremely common in samples taken from this location. It's color in life was very close to as depicted in the photo; except for the gradual darkening of the abdominal segments as they progress posteriorly, which has been accentuated somewhat by the effects of preservation. Adult association is based upon capture of this taxon at various stages of emergence including: darkened wingcases, split thoraxic notums, and partially ecloded or ''stillborn'' specimens.


Recent Discussions of Ephemerella dorothea infrequens

So is Ep Infrequens now known as Ep Dorothea?
20 replies
Posted by Wbranch on Feb 17, 2008
Last reply on Jul 1, 2014 by Crepuscular
These mayflies look more like the Sulfurs I see on the Delaware system than the PMD's I see in Montana. The Montana mayfly has a distinct yellow leading edge to an overall light dun gray wing and the abdomen and thorax have a more light greenish/yellow cloration so how is it that Infrequens is now known as Ep Dorothea Dorothea?

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