Mayfly hatches are the insect hatches that are most likely to get trout up and rising selectively and consistently for hours on end. It's therefore no coincidence that they're the hatches that most excite fly anglers all over the world. In the Yellowstone area, mayfly hatches are less important from a general perspective than they are in many other places, since our rivers and streams have so many caddisflies, stoneflies, and terrestrial insects to get the fish to rise. That said, there are many times and places when mayflies are by far the most important insects.
In a general sense, mayflies are most important in spring, early summer, and through the fall than they are in late summer. The major exception is on high-elevation meadow streams such as those in the Lamar River drainage in Yellowstone Park (and also the upper Gallatin River), where they are also very important in late summer. With the exception of assorted "drake" mayflies, most of which in our region are one species or another of Green Drakes, most mayflies are small, from size-16 down to size-22 or even size-24. The smallest are the late summer Tricos critical on the Missouri River but usually unimportant elsewhere.
Mayflies can hatch and interest the fish on any type of water, but they are more important on flatter streams than rough ones, or at least flatter sections of steep, fast streams. They are least likely to be important on small mountain streams, whether these are steep and broken or low-gradient. They are most important on spring creeks, tailwaters, and meadow streams, including both freestone meadow streams like those in the Lamar Drainage and geothermal streams like the Firehole River. There are only a few species of mayflies present in area lakes, primarily Callibaetis or Speckled Duns, but these are CRITICAL and often the only insects that will get fish to rise in lakes at all. They are most common on smaller, weedier lakes but can be present much of the season.
The remainder of the page consists of a mayfly hatch chart followed by descriptions and notes about each mayfly. In general I use common names rather than Latin namees of insects, save where the Latin name or a version of it (Callibaetis or Trico) is as well-known as any common English name.
The following table covers important mayfly hatches region-wide, from the most common and important insects to the least. Note that this is a general chart. Crucial insects on one body of water might be rare or even absent in others. Check the appropriate page under the Guide to Area Waters menu above for hatch charts specific to all area waters.
If you wish to print a full-size version of this hatch chart, those covering other insects and food items important when fly fishing the Yellowstone area, and all other charts on this site, please visit this link.
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, though the autumn hatches begin as early as mid-August on the Yellowstone River and in Yellowstone Park. Look for the best hatches on cloudy, drizzly or snowy days in spring and early fall and milder days earlier or later in the year. Cloud cover always helps. The best hatches typically occur from mid-morning through about 5:00PM, clustering in early afternoon on the coldest days. BWO range from size 16 to 24, with the larger insects present in early autumn and the smaller ones earlier and later in the year, sometimes even on warm midwinter days. Regardless of time of year, BWO tend to hatch in moderate to slow-speed runs and at the tails of riffles. Anytime you see a small, gray- and olive-hued mayfly that looks like it's wearing camouflage, it's a BWO. Note that the BWO in our region, especially those on the Yellowstone River and its tributaries, tend to run more gray in overall color tone than they do elsewhere. This is particularly true for the insects that hatch from mid-August through October. There are many suitable patterns for matching a BWO hatch, but my favorites are Baetis Sparkle Duns***, Hi-Viz Gray Baetis, and various CDC emergers. On faster streams a Parachute Adams works well. Increasingly, we use Purple Hazes and my Purple Hazy 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.
For what it's worth, the fall BWO hatches on the Yellowstone River between Gardiner and Big Timber are the single hatch that I most look forward to during course of the year.
PMDs are our other extremely widespread insect, though they are usually less important on fast rivers than the BWO. They are most common in July and early August except on geothermally-heated streams and tailwaters, 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. Very occasionally hatches will last all afternoon or have a resurgence in early evening. This is most common on the Missouri River. 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, PMD BLM Nymphs, and Military Mayflies are also favorites. The spinners fall at dusk or early in the morning, and are best imitated by a Rusty Spinner.
Assorted species of Green Drakes ranging in size from #8 or #10 down to #16 are the marquee summer insects in the Lamar Drainage, and are one of the major reasons the Lamar and its tributaries Soda Butte Creek and Slough Creek are famous worldwide. They are found in lower numbers in the Gallatin River, Yellowstone River, Madison River between Quake Lake and Ennis Lake, and Gibbon River. They are absent or almost absent elsewhere. The hatches in the Lamar Drainage are by far the largest, longest-lasting, and most widespread. Yellowstone area "Green Drakes" comprise many species. They are distinguishable to the average angler only by size. Otherwise, they all live in the same bodies of water and the same substrate. These insects feature a pale olive to gray body, sometimes with darker segmentation or faint yellow striping. Their wings and tails are medium to dark gray and are their most recognizable feature. Note that insects within the Yellowstone area are MUCH GRAYER IN COLORATION THAN WESTERN GREEN DRAKES ELSEWHERE IN THE WEST. Apologies for yelling, but this is a very key feature and most patterns tied to match Western Green Drakes are far too green to work for our bugs. In fact, when most anglers refer to "Gray Drakes" they've seen in the Lamar Drainage or elsewhere in the Yellowstone region, they are actually referring to some type of Green Drake.
The bright green Green Drakes common elsewhere are typically only seen in our area for a brief period in July and again in September. The grayer insects can be found as early as mid-June on the Gibbon River and are common in the Lamar Drainage from early July (or whenever runoff ends) until at least mid-September. In general, larger (#10-12) insects are common in July and September while August and early September bugs are more likely #14 to #16. In July and August, the best hatches typically occur sometime between 9AM and 2PM, with the earlier timeframe more likely when weather and water are warm and the later timeframe more likely when it's cooler. In September, hatches may not begin until midafternoon, particularly when it is cool. Gray skies bring the best hatches, with even moderate rain not necessarily putting the bugs down. Regardless of location or time of day, these insects hatch best in fairly fast water at the tail ends of riffles and heads of pools. You can expect trout feeding on them to move into faster and faster water the heavier the hatch is, with the biggest fish often found in shoebox-sized patches of slow water amid very fast riffles.
Our favorite dry flies to match this group of insects are Soda Fountain Parachutes*** and Furimsky's Foam Gray Drake and Green Drake on the surface. These insects often get knocked over and drowned due either to the fast water in which they emerge or strong winds capsizing them like small sailboats, so patterns fished slightly wet are also killer. My favorites are Slough Creek Spinners*** (really drowned duns) and Gray CDC Emerging Duns. All patterns should run #10 to #16, with #12 and #14 the sweet spot most of the season. Fish the wet patterns as droppers behind one of the dries. Nymphs are usually unimportant. The Slough Creek Spinners*** work to match the occasional morning or evening spinner fall, as do other pale olive or gray spinner patterns. Nymphs for these insects are generally unimportant.
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, occasionally as large as #12, and are hard to mistake for anything else due to their easily recognizable wings and the fact that you usually won't see other mayflies when Callibaetis are present. If you see a mayfly with prominent black splotchy markings on the leading edges of its wings, in a lake, it is a Callibaetis. Their wings are otherwise pale to medium gray. Depending on the body of water and time of year, their bodies range from tan to light olive to pale creamy gray. The best hatches usually occur from around 9:00AM until early afternoon. It is vital that the wind is down; these insects have evolved so that they do not hatch if there's much chop on the water. This isn't a joke. If the wind is fitful, you'll see flurries of insect and trout activity when the wind is down, then nothing while it is up. Dry flies to match these insects must feature something to match the dark leading edges of their wings, either grizzly or speckled hackle or the integration of some sort of dark-speckled material into otherwise gray wings. Good patterns include Parachute Adams, Callibaetis DOA Cripples, and cripple patterns. The real key is integrating a dark color to match the wings. Nymphs are also very important for this insect, as the nymphs are strong swimmers and often visible to the trout. Look for patterns marketed as dedicated Callibaetis nymphs, in gray or peacock-bodied colors. You can also try any other tan-gray-olive nymph pattern provided it has abundant dark speckling and elements that will breathe well when the fly is retrieved.
Drake Mackerals, often called Tan Drakes, Hecubas, or Red Quills, are the most important fall mayfly after assorted Blue-winged Olives. Formerly confined to the Lamar River Drainage, in recent years these insects have become more common on other freestone rivers: the Yellowstone, Madison between Quake and Ennis Lakes, and the Gallatin. They are rare to absent elsewhere. Drake Mackerals still hatch in only modest numbers unlikely to interest the fish to the exclusion of other insects except in the Lamar Drainage, where they often hatch in more abundance than the various Green Drakes. Their main importance where they are less common is for the opportunity they provide to fish a larger fly during BWO hatches. These insects often hatch together, so fishing a larger Drake Mackeral with a BWO on the dropper makes for a good one-two punch. Hatches are most likely from the last week or so of August through September or early October, with the best hatches from early through midafternoon, sometimes extending until early evening. Tan Drakes run #12 to #14 and are generally tan to reddish-tan in color, with medium gray wings. All or almost all have prominent reddish-brown segmentation, so good patterns for this insect all feature a strong rib. Good patterns include March Brown Sparkle Duns, Drake Mackeral Emergers***, and Brindle Chutes.
The Trico (Tricorythodes) is one of the classic mayflies on many spring creeks and tailwaters from coast to coast. This is our tiniest mayfly that still routinely draws interest from fish, typically running #18 at the absolute largest down to #24, with #20 and #22 most important. While they are found in all area streams with the exception of the steepest, most broken creeks and rivers, they are seldom important except in isolated stretches of slow water in most of these streams. If you do happen to find fish rising along eddy currents in slow sections of the Lower Madison, the Missouri, or perhaps the streams in the Lamar Drainage and the Paradise Valley spring creeks on August mornings, odds are they're eating Tricos. Both duns and spinners are important. The duns hatch early in the morning, the spinners fall immediately thereafter.
The Missouri River is a completely different ballgame. They are one of the critical hatches on this river, the only river in the region where this is true. They can hatch anytime from early July through mid-September, always in the morning. On this river they hatch in prodigious numbers, so that the mating flights resemble clouds of pollen in the air. Since the bugs are tiny and the numbers of them are immense, trout will seldom move more than an inch or two from their feeding lane on the Missouri to take these insects. This means precise casts and perfect drifts are required.
On waters other than the Missouri, particularly the freestone rivers, a small Purple Hazy Cripple*** or even an ant pattern works fine to match these insects. On the Missouri, you must be more specific. Since Tricos look like no other mayfly, being black with clear-white wings, particularly when they fall as spinners (duns may be more gray-olive at times), you want to use dedicated Trico patterns, whichever strike your fancy or the fly shop recommends. I seldom fish the Missouri at Trico time, so I'm not the guy to ask. Being small insects that hatch in slow water, choose parachutes, Sparkle Duns, or similar low-riding patterns. You may want to fish these behind something more visible like a tiny Wulff, or Parachute Adams, but this is up to you. Another option is to fish a Griffith's Gnat, Buzzball, or similar cluster pattern, to imitate balls of dead Tricos that have glommed together.
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.
Western March Browns typically hatch from larger rivers (freestone and tailwater) in April and early May, despite their name. These insects are tan in color, slender, and have prominent brown segmentation and speckled wings. They seldom hatch in sufficient numbers to get the fish rising specifically to them, but they often hatch in smaller numbers alongside BWO. Therefore, you can fish a larger March Brown Sparkle Dun alongside your BWO whenever you happen to see these larger, graceful mayflies as well. They also sometimes hatch along with the Mother's Day Caddis on the Yellowstone, but the trout usually prefer the caddis.
When anglers report seeing tiny PMD in late August and early September, this is almost always the insect they're truly seeing. They are tiny, usually #20, and slightly more yellow-cream in color than PMD. They are most important on the spring creeks, where trout may very occasionally rise well and pickily towards them, but otherwise you are more likely to find single trout or small numbers of them eating these insects. You may see them on the Yellowstone or Madison, but they are more numerous in the Lamar Drainage. PMD patterns or patterns marketed as Sulphurs work fine.
These tannish-gray insects are most common on freestone rivers in late August and early September. They often hatch in conjunction with more-numerous BWO, so if you are seeing a few slightly larger mayflies mixed with the Blue-wings, odds are it is these. The best bet if this occurs is to fish either an attractor like my Purple Hazy Cripple*** in #14 or #16 or perhaps a Hare's Ear Parachute or Brindle Chute in the same size as your top fly, with a BWO imitation as the dropper.
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.
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 one of the Green Drakes, a mistake made due to the distinct gray wings of both types of 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.
Late evening drakes that hatch explosively from the river surface, really getting the trout excited. Unfortunately, they're only widespread in the Yellowstone area in the Gibbon, the Lower Meadow of Slough Creek, and down on the Bechler. They almost never hatch before sunset, and often pop as late as midnight. With current Park regs forcing you to quit at sundown (rather than 10:00PM, as was the case until about 2013), it's basically impossible to fish this hatch in our area anymore.
Richard Parks is Montana Outfitter #327 and Yellowstone Park CUA holder #13-037. Parks' Fly Shop operates under his licensure in Yellowstone National Park, the Yellowstone River Drainage upstream of Livingston, and the Custer-Gallatin National Forest. Walter J. Wiese is Montana Outfitter #22001. The shop operates under his licensure in the Yellowstone Drainage downstream of Livingston and in the entire Missouri River Drainage.
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