The Order Ephemeroptera: Why Mayflies Matter
There are roughly 630 species of mayflies in North America, and while no angler needs to know them all, understanding the major genera — Baetis, Ephemerella, Drunella, Ephemera, Tricorythodes, and Hexagenia — will unlock most of the dry-fly fishing this continent has to offer. Mayflies are unique among insects in that they undergo an incomplete metamorphosis with a winged subimago stage. The dun, as anglers call it, emerges from its nymphal shuck at the water's surface, rides the current while its wings dry, and then flies to streamside vegetation where it molts one final time into a sexually mature spinner. No other aquatic insect has this two-stage adult life, and it is the reason mayflies provide such prolonged and varied feeding opportunities for trout.
The lifecycle begins with eggs deposited on the water's surface or along the substrate. Nymphs develop over periods ranging from a few weeks (multivoltine species like Baetis) to two or three years (Hexagenia), feeding on algae, detritus, and organic matter. Nymph morphology varies dramatically: clingers like Epeorus flatten against rocks in fast water, swimmers like Baetis dart through moderate currents, crawlers like Ephemerella pick their way across cobble, and burrowers like Hexagenia tunnel into silt. Each body plan dictates where the nymph lives, how it emerges, and what patterns best imitate it.
For the fly angler, the critical insight is that trout encounter mayflies in every stage of their lifecycle — as nymphs drifting in the current, as emergers struggling through the surface film, as duns riding the water, and as spinners falling spent after mating flights. The stage at which trout are feeding determines not just which fly to use, but how to present it. A size-18 Pheasant Tail dead-drifted along the bottom during a pre-hatch is a fundamentally different game than a size-18 Sparkle Dun fished flush in the film during the emergence itself.
Blue-Winged Olives: The Year-Round Hatch
Blue-Winged Olives — primarily Baetis tricaudatus and Baetis flavistriga in the West, with Drunella species contributing larger olives — are the most important mayfly genus for year-round trout fishing. Baetis are multivoltine, meaning they produce multiple generations per year, and their hatches span an astonishing calendar: from February through May in spring, intermittently through summer on cooler streams, and again from September through November. In tailwaters like the South Platte, Frying Pan, and Bighorn, BWO activity can occur in every month of the year.
Baetis nymphs are swimmers — slender, olive-to-brown insects with three tails that propel themselves in short, darting bursts. They favor moderate currents over gravel and cobble substrate, and they are found in enormous densities on productive trout streams. A single square foot of riffle can hold hundreds of Baetis nymphs at various stages of development.
The emergence behavior of Baetis is what makes them so important to anglers. Unlike many mayflies that emerge quickly, Baetis duns often struggle in the surface film, particularly during the cold, overcast conditions that trigger the heaviest hatches. Water temperature plays a direct role: when air temperatures hover in the forties and fifties, the duns' wings dry slowly, leaving them riding the surface for extended distances. This creates a conveyor belt of food that trout exploit with rhythmic, confident rises. The classic BWO day is gray, drizzly, and cold — conditions that keep most anglers at home but produce the best dry-fly fishing of the year.
Western BWO duns range from size 18 to 24, with fall hatches trending smaller than spring emergences. The key to matching them is body color (olive-gray to olive-brown), wing profile (upright, medium-dark), and size. In the Driftless region and across Midwest spring creeks, the same Baetis species dominate the cool-weather hatches, providing technical dry-fly fishing on glassy limestone flows.
BWO Fly Selection: From Nymph to Spinner
A complete BWO box covers three stages. Below the surface, the Pheasant Tail Nymph in sizes 18-22 is the universal Baetis imitation — the natural coloring of pheasant tail fibers is an almost perfect match for the olive-brown nymphs. Fish it on a dead drift with split shot or as a dropper behind a dry fly. For the emergence itself, the Barr's Emerger and CDC Emerger are devastating: both sit in the surface film with trailing shucks, imitating the moment of greatest vulnerability. As a dun imitation, the Sparkle Dun's deer-hair wing and trailing Antron shuck split the difference between emerger and adult, which is exactly what trout see when they look up through the surface film. The Comparadun and Parachute Adams round out the dun category with slightly different profiles for varying water types.
Pheasant Tail Nymph$2.95
nymphbeginner
Barr's Emerger (BWO)$3.95
emergerintermediate
CDC Emerger$3.50
emergerintermediate
Sparkle Dun$3.50
dryintermediate
Comparadun$3.50
dryintermediate
Parachute Adams$2.95
drybeginner
dryintermediate
Pheasant Tail Nymph$3.50
nymphbeginner
Blue Wing Olive$2.95
dryintermediate
Pale Morning Duns and Sulphurs: The Summer Pillars
If BWOs own the cold months, Pale Morning Duns (Ephemerella excrucians and E. infrequens in the West) and Sulphurs (Ephemerella invaria and E. dorothea in the East and Midwest) own summer. These closely related species hatch from late May through August, typically during morning and early afternoon hours, producing some of the most reliable and fishable dry-fly activity of the season.
PMDs are crawler nymphs that migrate from their rocky homes to slower water before emerging. The hatch often begins with scattered rises in the soft water along current seams and gradually intensifies as more insects reach the surface. Duns are a distinctive pale yellow to light orange, sizes 14-18, and they emerge relatively cleanly in warm weather — meaning the window of opportunity shifts toward emerger and cripple patterns when conditions are right. On cold mornings or during early-season hatches, the duns ride longer and dry-fly fishing improves.
In the Midwest Driftless area and across Appalachian limestone streams, Sulphur hatches are the summer equivalent. Ephemerella dorothea (the pale evening dun) produces exquisite evening hatches on streams like Penns Creek and the spring creeks of southwestern Wisconsin. The emergence typically begins around 7 PM and builds to a crescendo at dusk, with spinner falls following immediately after. Trout in these fertile limestone environments become highly selective during Sulphur hatches — matching size and color precisely is not optional, it is required.
A trout feeding during a heavy mayfly hatch is not choosing randomly. It is locked into a search image — a specific size, profile, and behavior — and your fly must match that image or be ignored among thousands of naturals.
Green Drakes and the Big Mayflies
Green Drakes (Drunella grandis in the West, Ephemera guttulata in the East) are the marquee mayflies — the hatches that drive anglers to plan vacations around emergence dates. Western Green Drakes are size 10-12, enormous by mayfly standards, with distinctive olive-green bodies and heavily veined, dark-mottled wings. They hatch from late June through mid-July on freestone rivers, and their size means even the largest, most cautious trout will move to the surface.
The emergence behavior of Drunella grandis is dramatic. Nymphs are crawlers that migrate to slower water along the banks and in backeddies before emerging. The duns are large and conspicuous, and on cold or overcast days they can ride the surface for remarkable distances, drawing explosive rises from trout that would otherwise never reveal themselves. On warm, sunny afternoons, the duns lift off quickly, and fishing shifts to emerger patterns and subsurface presentations.
Water temperature and photoperiod control the timing. Green Drakes emerge when water temperatures reach 50-55 degrees Fahrenheit, which means the hatch progresses upstream through a river system over the course of several weeks as lower-elevation reaches warm first. An angler who understands this gradient can follow the hatch up-valley, fishing fresh water each day.
Eastern Green Drakes on streams like Penns Creek and the Delaware system are equally celebrated. Ephemera guttulata is a burrower — nymphs tunnel into soft substrate — and the hatches tend to be more concentrated and dramatic than their western counterparts. The Coffin Fly spinner fall, when spent Ephemera guttulata fall to the surface at dusk, is one of the great events in eastern fly fishing.
PMDs, Sulphurs, Green Drakes, and Tricos
Pale Morning Dun patterns should include both emerger and dun styles in sizes 14-18. The CDC Transitional fishes beautifully in the film during PMD emergences, while the Sparkle Dun in pale yellow handles the dun stage. For Green Drakes, size matters — tie or buy your Comparaduns and Parachute patterns in sizes 10-12 with olive-green bodies. The Green Drake hatch also produces excellent nymphing before emergence: a Hare's Ear in size 10-12 bounced along the bottom during the pre-hatch period can be extraordinarily productive. For Tricos, you need the smallest patterns in your box: Trico Spinners in sizes 20-24, fished on 6X or 7X tippet during the morning spinner fall. The Griffith's Gnat serves double duty as a midge cluster and a Trico cluster imitation when fish are sipping spent spinners in foam lines. Sulphur Duns in the Midwest and East demand precise color matching — carry both standard yellow and the paler cream versions.
dryintermediate
CDC Transitional Dun$3.95
emergeradvanced
Sparkle Dun$3.50
dryintermediate
Green Drake$3.50
dryintermediate
Comparadun$3.50
dryintermediate
Trico Spinner$2.95
dryadvanced
Griffith's Gnat$2.95
dryintermediate
Hare's Ear Nymph$2.95
nymphbeginner
Sulphur Dun$3.50
dryintermediate
Trico Spinner$2.95
dryadvanced
nymphbeginner
Hex (Hexagenia)$5.95
dryintermediate
Adams$3.50
drybeginner
Reading Rise Forms to Identify the Stage
The single most valuable skill during a mayfly hatch is reading rise forms to determine what stage trout are eating. A splashy, aggressive rise with a visible head and dorsal fin usually means duns — the fish is chasing a moving target that might escape. A subtle sip with only a ring of disturbance means spinners or spent insects lying flush in the film. The most frustrating rise form — a gentle bulge with no break in the surface — means emergers. The trout is taking insects just below the surface film, and your dry fly will be refused no matter how perfect the drift.
When you see bulging rises, switch immediately to an emerger pattern: a Barr's Emerger, CDC Emerger, or Sparkle Dun fished with the body below the film and only the wing and shuck visible. Grease your tippet to within six inches of the fly to keep the connection point on the surface while allowing the fly to hang. This presentation is devastatingly effective during the transition period when both emerging duns and still-submerged nymphs are available.
Also pay attention to where in the water column rises are occurring. During the early stages of a hatch, trout often feed subsurface on ascending nymphs — this is the pre-hatch nymphing window that many anglers miss entirely. Only when enough adults are on the surface to make surface feeding efficient do trout commit to rising. That transition happens faster during heavy hatches and slower during sparse ones.
Tags
mayflyhatch-guidedry-flyemergertroutentomologybwogreen-drake
