The Summer Mayfly Hatches of Ephemerellidae

No matter where you live – east, west or in the middle – a few species of summer emerging mayflies produce some of the best mayfly hatches of the year.

Summer has arrived, and as I sit back and reflect on summertime fishing several thoughts come to mind. One is a vision of trout feeding recklessly in the choppy water of a riffle in the fading light of a summer’s day. In this vision I see caddis hatching, and smile at the thought of the easy fast fishing that often occurs during such summer evening feeding sprees. Then another vision comes to mind. I see gentle riffles and long smooth glides. There are trout feeding heavily, but not with reckless abandon. Gentle dimples on the surface mark the regular feeding rhythm of trout taking small mayflies; a bulge of water at the surface, then the nose of a trout breaks through and a small mayfly dun disappears. In this vision I am casting more than playing fish. I see spooked fish swim away as my fly drifts downstream. I see trout follow my fly several feet then turn to take a natural a few inches away. I see a small yellowish-olive mayfly dun with three tails land on my arm. Then I know I’m having visions of a summer hatch of Ephemerellidae.

No matter where you live in North America there are probably streams nearby that offer good mayfly hatches in June, July and August. And no matter where you live in North America it’s more than likely that these mayfly hatches will be a species of the family Ephemerellidae. Which species however will depend on where you live. In the Midwest and East there are four important species commonly called blue-winged olives. These species all belong to the genus Drunella, and include Drunella cornuta, Drunella cornutella, Drunella lata and Drunella walkeri. In the West there are also four important species: Ephemerella inermis and Ephemerella infrequens, commonly called pale morning duns or simply PMDs, and Drunella coloradensis and Drunella flavilinea, usually referred to as slate-winged olives. There are other mayflies that you might encounter, but in June, July and August these species produce some of the heaviest hatches of the year, and the kind of surface feeding worth dreaming about.

Let’s look at the western hatches first. Pale morning duns and slate-winged olives thrive from the Rocky Mountains to the Pacific Coast. Both groups are widespread in all types of trout streams from small mountain brooks to large rivers and spring creeks. Though both groups are in the family Ephemerellidae, they belong to different genera and show distinct differences in size and color. The two species of PMDs (Ephemerella inermis and Ephemerella infrequens) are the smaller species and generally produce the most consistent hatches. Ephemerella inermis and E. infrequens are nearly identical in appearance varying only slightly in size: E. inermis nymphs are 7-8mm long, while E. infrequens nymphs are 9-10mm long. Both are light to chocolate brown with some specimens showing a light stripe down the center of the thorax and abdomen. Nymphs have three tails with a distinct fringe of fine lateral hairs. Of these two species E. inermis is the most common and widespread. Adults of both species range from 8-10mm long excluding tails (hook size 18-16), and range in color from light lemon yellow to pale tan as duns, and pale tan to reddish brown as spinners. Typical of most mayfly duns and spinners, the body is lighter in color underneath than on top. These two species are so similar in appearance and behavior that they can only be reliably distinguished by comparing male spinner genitalia under a microscope. Something most fly fishers will probably decide to skip.

Interestingly the two slate-winged olive species, Drunella flavilinea and Drunella coloradensis, are also next to impossible to distinguish without dissecting male spinners and comparing their genitalia under a microscope. They are however, rather easy to distinguish from the PMDs. Slate-winged olive nymphs and adults are larger – 9 to 11mm long excluding tails – than PMDs. The nymphs are also stouter in appearance with enlarged femur on the front legs, and the tails have a distinct central dark band and no fringe of fine lateral hairs. Nymphs range in color from a brownish-olive to dark brown. Duns are light to dark olive, and spinners are generally a rusty brown. Slate-winged olive duns and spinners are also rather stout and heavy bodied for mayflies, and like all members of the family Ephemerellidae they have three tails rather than the two tails found on most mayfly duns and spinners. Of the two slate-winged olive species D. coloradensis has a preference for colder water – it’s rarely found in streams that warm above 60 degrees – than D. flavilinea. As a result D. coloradensis is most common in streams at higher elevations or those naturally cooled from springs or the release of cold water from bottom of reservoirs at dams.

The four eastern species, all commonly called blue-winged olives, belong to the genus Drunella, the same genus as the western slate-winged olive species. As such, they have much in common with their western cousins. Nymphs have the same stout bodies with the femur of the front legs enlarged like the bulging biceps of Mr. Universe. Nymph colors range from grayish olive to reddish brown. The four species separate nicely into two groups based on size: Drunella cornuta and D. walkeri are the largest ranging from 9 to 10mm long, while D. cornutella and D. lata both range from 6 to 7mm long.

Duns and spinners of the eastern blue-winged olives also show some resemblance to the western slate-winged olives. Duns have slate gray wings with rather robust olive bodies. Spinners have clear wings with dark olive to brown bodies. It has been noted that the duns of blue-winged olives darken from a bright greenish olive to a dark olive color quite quickly upon emergence, even before they fly off the surface of the water. Thus, while the angler generally sees the duns as a dark olive color, trout may be focused on newly emerged bright olive duns, causing some problems if you are using a dark olive pattern. Like the nymphs the duns and spinners fall into two size groups with D. cornuta and D. walkeri larger (9 to 10mm) than D. cornutella and D. lata (6 to 7mm).

These species of mayflies have adapted to many types of stream habitats resulting in their wide distribution. In the West, PMDs and slate-winged olives are abundant in streams from sea level to over 10,000 feet. They thrive in the gravel areas of riffles as well as on roots and vegetation in quieter water, and can be equally abundant in spring creeks and freestone streams. In general PMDs prefer slightly gentler currents with smaller substrate than the slate-winged olives, however, there is considerable overlap in habitats used by both groups. It is common for streams to have good hatches of both PMDs and slate-winged olives, but if one dominates it is usually the PMDs. The Midwest and eastern blue-winged olive species prefer habitats very similar to the western PMDs. Slow to moderately fast riffles with gravel bottoms ranging from small pebbles to cobble size stones provide excellent conditions for the nymphs to hide and feed. Besides falling into two size categories the four blue-winged olive species also segregate regionally with D. walkeri and D. lata most common in Midwestern streams and D. cornuta and D. cornutella most common in eastern streams. This doesn’t mean they only occur within these regions, just that they are more abundant in their preferred region.

The nymphs of these species, like all Ephemerellidae, are known as crawlers due to their habit of crawling over the substrate and generally poor swimming ability. While crawling along the stream bottom the nymphs feed on algae and decaying vegetation such as leaves and wood debris. Nymphs frequently get washed into the currents, and because of their abundance this “drift” provides important food for trout. It also means that nymph imitations are important, especially in the weeks and days prior to emergence. The wide distribution and abundance of these species also means nymph imitations can be successful searching patterns most of the year. On western streams, for example, a size 16 PMD nymph imitation is one of my favorite all around nymph patterns.

Emergence time for these mayflies varies with species and location. Eastern hatches start with D. cornuta, which begin in mid May and continue to early or mid July. Next to emerge are D. lata and D. cornutella, both of which hatch from about mid June to mid August. Finally, D. walkeri emerges from mid July to mid September. Out west the widely varying elevation, weather, and resulting snow melt patterns greatly influence water temperature and thus the timing of these and most other hatches. PMDs begin emerging as early as May in California spring creeks, but may not begin until mid July in high elevation Colorado trout streams. Likewise the slate-winged olives may begin hatching anywhere from mid June to mid August and last until mid September. Overall the PMD hatch lasts longer, often six to eight weeks, and is more reliable than the slate-winged olive hatch.

The emergence of eastern blue-winged olives almost always occurs in the morning, typically between 9 am and noon. Spinner falls then follow in the evening beginning about an hour before dark. The spinner falls can produce some of the best feeding activity of the day and shouldn’t be overlooked. The emergence of the western PMDs and slate-winged olives tends to be an afternoon occurrence, starting as early as 1 pm and lasting until 4 or 5 pm. Hot sunny weather however, will often push the emergence to mid morning. Spinner falls are most prevalent in the evening, though if the weather is hot PMD spinner falls will often occur in the morning between 8 and 10 am. Spinner falls of both PMDs and slate-winged olives can produce excellent fishing, so pay attention for the soft, subtle rises that indicate trout are feeding on spinners.

Effective patterns for these hatches can be simple. For nymphs a dark gold ribbed Hares Ear or pheasant tail tied in the appropriate size (18-14) will usually do the trick. Adding a bead head will help get them to the bottom. Fish them dead-drift along the edges of and below riffles, through pocket water or along undercut banks. Nymph patterns are most effective just prior to emergence when the naturals are migrating to slower water or beginning their restless ascent to the surface for emergence. Weighted flies and split shot may be necessary to get the flies near the bottom during the early stages of the hatch when most nymphs are still on the bottom. Later nymphs can be effectively fished in mid depths without weight. A strike indicator located six or seven feet above the fly, is a great help for detecting strikes when fishing these small nymphs.

Changing from nymph to dun is often a trying task for these mayflies. When everything goes right the nymphs hang in or just under the surface as the wings of the dun escape the nymphal shuck and break through the surface. This is a good time for a floating nymph pattern. Often, however, not everything goes right. Duns with wings partially unfurled get caught in the surface and never get off the water. Soft hackles or flymphs make excellent emerger patterns when this occurs. Fish floating nymphs or flymphs upstream and across with a dead-drift float. I prefer casting to fish I’ve spotted feeding just under the surface on emerging duns. However, even fish clearly taking duns will often take a well presented emerger after refusing numerous dun patterns.

Surface activity can be fast during the often heavy hatches of these mayflies. For this reason a good durable dry fly can save a lot of time from changing flies between fish. Compara-duns have proven themselves very durable and imitative. In recent years, however, I have found “Harrop” duns (originated by Rene Harrop) to equally durable and more effective at fooling selective fish. Fish during these hatches can become ultra selective, especially on heavily fished waters. Patterns from 14’s to 20’s may be needed depending on the local species and conditions. The color of the naturals varies considerably between different locations, even for the same species, so it’s best to collect the local duns and select a pattern according to its size and color.

Delicate presentations and drag-free floats are a must when fishing dries during these hatches. If you are getting refusals with an upstream cast, try a downstream slack-line presentation. If that fails and naturals are seen fluttering on the surface struggling to get airborne, try giving your fly slight twitches.

Finally, watch carefully for spinners in the evening. They can be surprisingly difficult to see in the fading evening light. Rises to spinners are also subtle. A simple hackled fly clipped top and bottom makes a good spinner pattern. A downstream slack-line cast, carefully positioned over the feeding lane of a rising fish, is usually the best approach.

Rick Hafele has been chasing hatches for over 35 summers, and will be looking for these mayflies, and many others, this summer on his home waters in Oregon and other waters throughout the west.