Mayflies

Standby Patterns | Mayflies | Caddisflies | Stoneflies | Other

The following guide includes general descriptions of the physical characteristics, flight times, and seasons of emergence for the most important mayflies in the region under the purview of our Trip Planner. Suggested patterns are also given. Individual hatch guides for the most prominent streams in the area, with individualized season information and flight times, are given on the appropriate sub-page in the Our Waters page. In the guide below, I have generally tried to use the common name of each insect listed, if possible. Insects are listed in decreasing order of general importance. Please note that some insects are more important on some streams than others, however. Also note that this guide does not cover insects common in the Paradise Valley spring creeks, which are more varied. Please contact us for more information on area hatches and the flies to match them.

The first portion of the following guide consists of an emergence table and notes. The second portion consists of longer descriptions of each listed insect.

Contact Information
Phone:	(406) 848-7314
Address:	PO Box 196 202 Second Street South (US-89) Gardiner, MT 59030
E-Mail	Richard Parks, Owner Walter Wiese, Head Guide

First Column:	Common or Latin name of the insect
Second Column:	Usual size of the insect
Third Column:	Predominant shades of the dun (subadult) insect
Fourth Column:	Time when duns are typically on the water. Spinners are usually present early AM or late PM.
Remaining Columns:	Usual months of emergence. A capital X signifies a hatch of primary importance, a small x signifies a hatch of secondary importance. No hatch usually occurs in months marked with a blank.

1.	There are numerous grayish to olive large mayflies in the Yellowstone area, meaning identification is hard without a seine and a good entomology textbook close to hand. All identifications above are tentative. Thankfully, the flies needed to match all of them are basically the same.
3.	This is primarily a lake insect. It emerges at different periods depending on elevation. The only river under the purview of this guide where it can be important is the Yellowstone, where it can be important in the "estuary" section between Yellowstone Lake and LeHardy Rapids.
4.	This insect is seldom important in the Yellowstone drainage, but can be crucial in the Madison drainage outside our area of operations.
5.	Provided for reference only. Our waters are generally too broken for this insect to be of interest to either trout or angler, unlike on rivers like the Missouri or Clark Fork

Common Name	Scientific Name (if known)	Discussion
Blue-winged Olive (BWO)	Baetis	Blue-wings are probably the most important Yellowstone-area insect for the angler. Fish will rise selectively to them in every drainage we fish, including steep, rocky drainages like the Gardner and Yellowstone. They are most common in spring and fall, on cloudy, drizzly or snowy days in late spring and early fall and milder days earlier or later in the year. They range from size 18 to 22, and tend to hatch in moderate to slow-speed runs and at the tail of riffles. Anytime you see a small, gray- and olive-hued mayfly that looks like it's wearing camouflage, it's a BWO. There are many suitable patterns for matching a BWO hatch, but my favorites are Baetis Sparkle Duns, Sprouts, and various CDC emergers. Pheasant Tails or other small, dark, patterns match the nymphal form.
Pale Morning Dun (PMD)	Ephemeralla excrucians and E. dorothea; formerly E. inermis and E. infrequens	PMDs are our other extremely widespread insect, though they are usually less important on fast rivers than the Baetis. They are most common in midsummer except on geothermally-heated streams and spring creeks, where June is the most likely time to see them. They are light-colored insects, with pale gray (light dun) wings and either a light dun or very pale yellow body with darker segmentations, and range in size from 14-20 (emphasis on 16-18). Look for them at the heads of pools, from late morning until early afternoon, especially on cloudy days. There are a plethora of patterns to match this insect, but again my favorite is a Sparkle Dun. I also like a new pattern we call a PMD Sparkle Parachute. I also like various emergers. The nymphs are rusty brown with darker segments, making a standard Pheasant Tail the best nymph, often with a flashback to imitate a nymph preparing to emerge. The spinners fall at dusk, and are best imitated by a Rusty Spinner.
Western Green Drake (Summer, Yellowstone drainage)	Drunella doddsi	Based on the best information I can find, the large gray-green mayfly present in the Yellowstone drainage is actually the doddsi species of the fabled Western Green Drake. If this is true, the odds are good that the smaller insect present in most of this water is actually a smaller doddsi rather than a D. flavilinea, as I previously believed. At any rate, all are #10-14 greenish gray insects with prominent dark gray wings with hints of pale olive on their abdomens. They hatch on the Yellowstone primarily in June and in the Lamar drainage in July, August, and early September. Generally, larger flies will be found in warmer (downstream) sections of river first and move upstream, being replaced downstream by the smaller (#14) bugs. All are best matched by our own Soda Fountain Parachute, Gray CDC Emergers, and Rusty Spinners on top or near the top and Wiese's Four Feather and Olive Gold-Ribbed Hare's Ear nymphs. Please note that on our waters the bright green flies commonly marketed as Green Drakes are simply TOO green for the most common Drakes, though they do match the flies that hatch in September (see below).
Fall Drake (Drake Mackeral)	Unknown. Some identify this insect as T. hecuba.	This is an interesting bug whose identification I'm not sure of. In general, they're the most common fall Drakes after the doddsi peter out in early September. A large Quill Gordon or Hare's Ear Parachute is probably your best choice for matching this insect, though Blue Duns and Soda Fountain Parachutes are also good choices. We are working on a dedicated custom pattern (or patterns) to match this fly.
Flav (Little Green Drake)	D. flavilinea	Based on observations by former Lamar district ranger and PFS guide Dave Keltner, I long believed that the #14 gray-olive mayfly present in the Lamar drainage was actually D. flavilinea, though I could find no sign that anyone else thought this. It may actually be the Flav, but I now believe it is a small version of D. doddsi. True Flavs are certainly present in the Gibbon and the upper Yellowstone. Their habits closely match the other so-called Green Drakes, save that they prefer to hatch in the evening. Soda Fountain Parachutes and Gray CDC Emergers are the bugs.
Western Green Drake (D. grandis)	Drunella grandis	Based on my current studies, I believe the rare, large, truly green Drake that hatches in the Lamar drainage in the fall is actually D. grandis, the most fabled of the Green Drakes. It usually hatches on September afternoons, often along with the Drake Mackeral. It is hard to miss when it is present, as it's the largest of the Green Drakes, either a #10 extended body or long shank hook or a #8 short shank. In a confusing twist, the best match for it is a pattern called a Beefcake Hecuba, even though the best information I have suggests the insect noted above is actually the Hecuba. If the fly works, fish it... An Olive Hare's Ear covers the nymph.

Gray Drake	Siphlonurus occidentalis	The Gray Drake is a mayfly of slow, often silty water. It typically crawls to shore to hatch, meaning its dun is usually unimportant. Spinner falls occur in late afternoon or early evening in late August and September, over riffles, and are much more important. Except when fishing Slough Creek, the insect most anglers are referring to when they call a bug a "Gray Drake" is usually either the Flav or one of the Green Drakes, a mistake made due to the distinct gray wings of all three insects. The Gray Drake is not found in fast water, and is a paler gray overall than any of these others. A Parachute Adams matches the occasional dun, while the spinners are a pale, slightly rusty gray. On occasion they hatch in Yellowstone Lake as well as the park's rivers.
Brown Drake	Ephemera simulans	Brown Drakes are creatures of darkness. They can be expected from a half-hour before dusk and on into night. These are large, brown mayflies, typically a #8-10, and are common in the Gibbon and Slough Creek's Lower Meadow. They are burrowers, meaning they are seldom available to trout until just prior to emergence. They emerge quickly, as well, meaning the best tactic is to fish a large Hare's Ear nymph with an active retrieve. If fishing dry, the best flies feature full, traditional hackles to imitate the fluttering of the quickly emerging insect.
Speckled Dun (usually known by its scientific name)	Callibaetis	Callibaetis are the crucial lake mayflies in our area. They are present in all lakes and draw considerable surface action in all of them, as well. They are of moderate size, usually #14-16, and are hard to mistake for anything else. Look for a fly that looks like a living grizzly hackle, for all intents and purposes. To imitate the dun, try a Parachute Adams where fish are less-particular, a Mosquito where they're a bit spookier, or a Callibaetis Thorax where they're spookiest of all. A dedicated Callibaetis Nymph or Four Feather fished with a strip-pause retrieve matches the nymph, while spinners are light gray with notable black veins in their white wings.
Pale Evening Dun	Heptagenia elegantula (Genus certain, species not)	This is something of a sleeper hatch on many of our streams. Though nowhere the most common or most important bug, they can draw selective trout. The insect is slightly larger and yellower than a PMD, but otherwise looks similar. Spinner falls are generally more important than duns.
Great Western Sulphur (usually known by its scientific name)	Epeorus albertae (genus certain, species not)	This is an important insect on the Madison but less common on the Yellowstone. Still, if you stumble upon a hatch of more than a handful and have the right flies, you're in for a treat. I like a Light Cahill Parachute or big PMD Sparkle Dun for this hatch. All flies should be #14.
March Brown	Rhithrogenia	This hatch used to be one of the marquee pre-runoff hatches on the Yellowstone, but it was all-but destroyed by pesticide runoff. It is slowly recovering, but is nowhere near what it used to be. They are brown-tan insects, around a #14. This is the only drake for which I think the popular "Para-Drake" style fly is the most appropriate fly.