Parks' Fly Shop: Guide to Yellowstone Area Mayfly Hatches, Yellowstone Hatches

Guide to Yellowstone Area Mayflies

Mayflies are not actually the most important insects to the fish in most Yellowstone-area rivers, but they have the most mystique and they are probably the insect whose hatches and spinner falls more anglers like to fish than any other. The following guide is not a general introduction to mayflies. I suggest you check out Troutnut if you are entirely new to aquatic insect entomology. Suffice it to say here that when on the water, mayflies look like pretty little sailboats.

The following guide includes general descriptions of the physical characteristics, flight times, and seasons of emergence for the most important mayflies in the region. Suggested patterns are also given for the most important stages of each insect. Individual hatch guides for the most prominent streams in the area, with individualized season information and flight times, are given on the appropriate page in the Our Waters section of our Trip Planning guide. Below I have generally tried to use the common name of each insect listed, if possible, rather than the Latin, though in many cases the Latin name is the common name, or at least a shortened version of it. For the Latin name, again check out Troutnut. Insects are listed in decreasing order of general importance. Please note that some insects are more important on some streams than others, however. Please contact us for more information on area hatches and the flies to match them. Any macro photos of insects would also be much appreciated.

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.

Yellowstone Area Mayfly Emergence Table

Table Key: X=primary hatch period, x=secondary hatch period, blank=no hatch
Name Description Emergence Time Emergence Month
      Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Spring Blue-Winged Olive Small, grayish olive body, gray wings, #18-24 Usually early-late afternoon. x x X X x x            
Fall Blue-Winged Olive Gray with olive hints, light to medium gray wings, #16-22 Usually noon to 5:00pm. Gray days best.               x X X x  
Pale Morning Dun Light gray with hints of creamy yellow, wings light smoky gray Usually mid-morning to mid- afternoon, sometimes earlier and later, especially when rainy/snowy.           X X X x      
Western Green Drake (doddsi) gray with hints of olive and brown, prominent segments, three tails, wings medium-dark gray, #10-14 Usually midday. August hatches may happen in midafternoon, especially in colder streams.           x X X x      
Callibaetis gray with prominently speckled wings, #12-16. Usually midday, sometimes dusk.           x X x x      
Gray Drake medium gray with hints of olive, wings medium gray, two tails Usually midday.           x X x        
Hecuba (Tan Fall Drake, Drake Mackeral) robust tan body with a brown rib, medium gray wings. #12 Afternoon.                 X x    
Epeorus (Pink Lady) two tails, creamy grayish olive, often with a pinkish belly, #14-16 Usually midday.             X x        
Heptagenia (Pale Evening Dun) two tails as dun, yellowish gray with light to medium gray wings. #16 Usually early afternoon, sometimes later.             x X X      
Little Green Drake (Flav) gray and olive, a smaller version of the Green Drake above. #14-16 Usually evening, sometimes midday.             X X x      
Western Green Drake (grandis) Imposingly large, olive green with hints of gray, yellow, and brown. #8-10 Midday to early afternoon in summer, mid-afternoon to early evening in the fall.             X x X      
Western March Brown Long and slim. Brownish tan with a brown rib, wings are well-marked . #12-14 Afternoon.     x X x              
Trico Tiny, black with almost clear wings for the female spinners, which are more important. #20-22 Usually early AM or late PM. Spinner falls AM.             x X x      
Brown Drake Brown and yellow, imposingly large. #8-10 Late evening, right before and after dark.           X X          
Mahogany (Blue Quill) Brownish tan, small, #16-18 Afternoon.               x X x    

Discussion of Insects

Boldface type indicates fly patterns. Italics are used for scientific names.

Blue-winged Olive (BWO) (spring and fall)

Baetis and various species.

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 16 to 24, 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. On faster streams a Parachute Adams works well. Increasingly, we use Purple Hazes and Purple Haze Cripples, especially on faster rivers. Pheasant Tails or other small, dark, patterns match the nymphal form. The spinners are olive, often with a red undertone. They often dive underwater to lay eggs, so a soft hackle is as good as an appropriately-colored spinner.

Pale Morning Duns (PMD)

Ephemerella excrucians and E. dorothea

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, 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 14-22 (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 Parachute Sparkle Emergers and Trina's Extended Body PMDs on top and various subsurface or surface-film 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. Lightning Bugs and Rust Shimmer Nymphs are also favorites. The spinners fall at dusk or early in the morning, and are best imitated by a Rusty Spinner.

Western Green Drake (doddsi)

Drunella doddsi

We believe the Green Drakes that predominate in the Lamar drainage in July and August are in fact this species, rather than the more famous D. grandis. We believe the fall bugs, which are larger and more olive, are the grandis species. They hatch in the Lamar drainage in July and August, in the warmer sections of rivers (downstream) in July, and towards the headwaters (especially of Soda Butte) in August. The smaller (#14) insects which fill in downstream in August and early September, but usually hatch sparsely amidst the PMDs except in cool, wet weather, are either small D. doddsi, D. flavilinea, or Heptagenia. All are best matched by our own Soda Fountain Parachute, Gray CDC Emergers, and Jewell's Slough Creek Spinner (which is also a good cripple) 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).


Callibaetis, various species.

Callibaetis are the crucial lake mayflies in our area. They are present in almost all lakes and draw considerable surface action in most 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.

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.

Hecuba (Tan Drake, Drake Mackeral)

Timpanoga hecuba

Hatches of this insect, which looks much like a March Brown though it's fatter and slightly larger, have been important for quite some time in the Lamar Drainage in September, but numbers have been increasing on the Yellowstone, as well. September and early October 2010 saw regularly fishable numbers of this insect from Gardiner to past Livingston, with extremely good fishing on late September afternoons from Carter's Bridge to 9th Street Bridge, the "town" float. Hare's Ear Parachutes, Tan Sparkle Duns, and our own Fall Drake Cripples work well to watch the duns, while Ben Jewell's new Slough Creek Spinner to match it can work both as a spinner and a cripple.

Epeorus (Pink Lady)

Epeorus albertae

These look like big PMDs with slightly pink/yellow bellies, and are most commonly seen in July and August in the Lamar Drainage and on the Madison. Very occasionally they'll bring fish up on the Yellowstone, especially if we get a rare overcast day before mid-August. Various PMD-type flies are close enough, though having a parachute pattern with a pink belly is a good idea, too.

Heptagenia (Pale Evening Dun or Red Quill)

Heptagenia elegantula

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. We have been seeing increasing numbers of these insects on the Yellowstone in its canyons and just below Gardiner in the last couple years.

Flav (Little Green Drake)

Drunella flavilinea and D. coloradensis

Former PFS guide and Lamar District ranger Dave Keltner identified the #14 gray-olive mayflies in the Lamar drainage as this insect, but now we're not so sure. I now believe this insect is either a small D. doddsi or even a Heptagenia. 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 (grandis)

Drunella grandis

Most Green Drake patterns are medium olive with some yellow, usually a rib. They match this insect perfectly. It appears sporadically in the Lamar Drainage in September, and 2010 saw big numbers on the Yellowstone in July. In fact, one of our largest client-caught fish of the season was a brown that ate an Olive Hare's Ear during a hatch when no fish were rising. The Olive Hare's Ear is the best nymph, while for the duns you should use a Foam Green Drake or a Parachute Green Drake. The spinners are matched by Ben's Slough Creek Spinner in the appropriate color. Note that just outside the Park, the Henry's Fork sees heavy hatches of this insect in June.

Western March Brown

Rhithrogena morrisoni

These insects are FINALLY starting to rebound strongly after nearly being wiped out by DDT in the 1950s-1960s. We've started seeing more than very rare fishable hatches of them in April and early May (occasionally late March) on the Yellowstone, and it's really made spring fishing easier. We'd much rather fish a #12 March Brown Sparkledun than a #18 BWO imitation. Some big fish will rise to these, especially in late April.



Much more important on the Bighorn and especially the Missouri, we'll VERY occasionally see fish (most of them whitefish) rising to the tiny, #22 spinners on the Yellowstone near Gardiner. They're slightly more important east of Livingston, and a lot more important on the spring creeks and Hebgen Lake. Various spinner patterns work for this insect. It's important to note that the males and females look different. Usually the females are more important.

Brown Drake

Ephemera simulans

Late evening drakes that hatch explosively from the river surface, really getting the trout excited. Unfortunately, they're only widespread in our operating area in the Gibbon, the Lower Meadow of Slough Creek, and down on the Bechler. With the new Park regs forcing you to quit at sundown (rather than 10:00PM) it's basically impossible to fish this hatch anymore.


Brown-tan bugs, occasionally important in September.


phone 406 848 7314, address 202 Second Street South PO Box 196 Gardiner, MT 59030 E-mail Walter Wiese E-mail Richard Parks link to PFS Youtube Channel link to PFS Facebook

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