While written from an East Coast perspective, these hatches are all active right now in the West, and especially California. Note the hatch timing changes throughout the day and be prepared. Lots of good patterns suggested here as well.
...Rob
Reprinted from MidCurrent, September 17, 2025
MidCurrent full article
September 10, 2025 By: MidCurrent Staff
A beautiful Vermont brown ate a size 20 CDC Olive Emerger.
September is often described as the “second trout season,” which is one of the reasons we launched the “20 Days in September” campaign. The blazing days of summer wane, nights cool, and many anglers relish long days on the water. By this month, many of the big-name summer hatches—green drakes, golden stones, and caddisflies—are gone, but three groups of insects reliably keep anglers busy across the country: blue-binged olives, Tricos, and midges. All three occur widely, all three are important to trout across the continent, and all three require a mix of technical skill and careful presentation to fish successfully.
1. Blue-Winged Olives (Baetis sp.)
Blue-winged olives (BWOs) emerge in both spring and fall. By September, a fresh brood of Baetis begins hatching across nearly every major trout water. In the East and Great Lakes tributaries, BWOs emerge steadily from early September into October, especially on cloudy afternoons. In the West, fertile tailwaters—such as the Missouri, Bighorn, Fryingpan, and Henrys Fork—see prolific Baetis emergences through September and beyond.
Expect hatches primarily from late morning to afternoon when clouds and drizzle prevail. Bright, sunny days may suppress activity until evening, but a sudden overcast spell can spark a blanket hatch. Sizes range from 16 down to 24, but most insects are in the 18-20 range.
Emergence and Spinner Falls
Baetis nymphs are agile swimmers and become especially active as they prepare to hatch. During emergence, they rise from riffles and seams, pop through the surface film, and become vulnerable as duns drifting on the surface. Trout often key heavily on emergers and crippled duns that struggle to break free.
Spinner falls occur on calm September evenings. These are often subtle events, with trout sipping quietly in back-eddies and slicks.
Presentation Techniques
Fishing BWOs is a game of finesse. On limestone streams and spring creeks, trout may refuse all but the most precise drift. Use a 12- to 15-foot leader tapered to 5X or 6X to present emergers in the surface film with minimal drag. On riffled water, bushier parachutes stay afloat and draw attention, but in flats you’ll need flush-riding patterns.
If trout refuse duns, switch to tiny emergers like an RS2 or WD-40 just below the surface. During spinner falls, dead-drift a Rusty Spinner or BWO Spinner across the slicks. Since the naturals are often tiny, fish these patterns in a tandem rig with a more visible dry fly.
Effective Fly Patterns
Pheasant Tail Nymph (sizes 16–20): A universal Baetis nymph imitation for subsurface work.
RS2 (sizes 18–24): A classic emerger for selective fish.
CDC Sparkle Dun BWO (sizes 18–22): Rides flush in the film, perfect for sipping trout.
Parachute Blue-Winged Olive (sizes 18–20): A higher-floating option for riffles and broken water.
Rusty Spinner (sizes 18–22): Covers evening spent-flies.
2. Isonychia
Isonychia mayflies—often called slate drakes, mahogany duns, or simply “Isos”—are among the most reliable large mayflies of late summer and fall. Unlike many hatches that vanish by September, Isonychia actually ramp up again with a second brood in late summer, providing steady hatches into October. In the East, they are a staple on Great Lakes tributaries and northern freestones throughout Pennsylvania and New York. In the West, they are less widespread but still occur in select freestones and tailwaters with the right gravel-bottomed habitat.
September Isonychia are typically sizes 10 or 12, slightly smaller than their June counterparts, and they tend to hatch during afternoons into evening, often from 2 p.m. until dusk. Unlike some tiny mayfly hatches, Iso activity is noticeable: trout rise aggressively to these big morsels, and anglers often describe September Iso time as a “second opening day” because it brings fish to the surface after summer doldrums.
Emergence and Spinner Falls
Isonychia are strong swimming nymphs that often live in riffles and fast water. They drift or actively swim before emerging, making the nymphal stage extremely important. Trout frequently key on these nymphs migrating toward shore or ascending in riffles. Unlike most mayflies, many Isonychia crawl onto rocks or logs to hatch, but plenty also emerge in midstream. The duns are robust—with slate-gray wings and mahogany-brown bodies—and they take flight quickly.
A day or two after emergence, the spinners form swarms at dusk. Their deep mahogany bodies and smoky wings make them distinctive, and the spinner fall can create intense surface feeding near riffles and pool heads.
Presentation Techniques
Fishing Isonychia hatches in September often calls for a mix of subsurface and surface strategies. Because Iso nymphs are fast, active swimmers, trout frequently key on them before the hatch even begins. Swing or use a Leisenring lift with a weighted nymph through riffles, adding short strips or twitches to mimic the naturals’ darting movement. Many trout will strike aggressively at this stage, often slashing or chasing the imitation in broken water.
When duns start to appear on the surface, switch to a flush-floating pattern. Trout tend to rise confidently and even splashily to Isonychia duns, so parachute-style or Comparadun patterns that sit naturally in the film work especially well. Because the flies are larger than most fall mayflies, you can step up to 4X or even 3X tippet without spooking fish, which also helps turn over the bigger patterns in riffled water.
As the day winds toward dusk and spinner swarms begin to form, presentation shifts once more to a delicate, drag-free drift. The mahogany-colored spinners with smoky wings fall spent across calm slicks and pool heads, and trout rise quietly to sip them. Here, careful positioning and accurate drifts are critical. Together, these three phases—nymph, dun, and spinner—offer varied opportunities, but success depends on matching each with the right presentation at the right time.
Effective Fly Patterns
Isonychia Nymph (sizes 8–12): Weighted, dark brown with a pale throat tuft; swim or lift during the drift.
Borcher’s Drake (size 10–12): A Michigan classic designed specifically for Isonychia duns.
Parachute Adams (size 10–12): Originally developed as an Iso imitation; still one of the most effective dries during September hatches.
Mahogany Spinner or Leadwing Coachman Wet (size 10–12): For evening spinner falls.
Dry-Dropper Rig: A bushy Iso dry with a nymph dropper covers both stages at once, especially effective in riffled runs.
3. Midges (Chironomidae)
Unlike mayflies, midges are ever-present. By September, when larger hatches wane, midges often dominate a trout’s trout diet. They are ubiquitous in Eastern spring creeks and tailwaters and equally critical in the West’s fertile tailwaters.
Midges hatch in steady trickles, not blizzards, so trout feed on them consistently throughout the day. Morning and evening activity is strongest, but on cool September afternoons, midges may hatch all day long. Sizes range from 18 to 32, with colors black, olive, cream, and red most important.
Emergence and Adult Behavior
Midge larvae look like little worms, and they come in a variety of colors. Photos: Ted Fauceglia
Midge larvae live in mud and vegetation. Pupae ascend toward the surface buoyed by gas bubbles, making them highly vulnerable. Trout feed heavily on pupae but also sip emergers caught in the film. Adults are tiny two-winged flies, often ignored unless clustered or mating. September is famous for glassy pools filled with rising trout, leaving nothing but tiny dimples. These are almost always midge feeders.
Presentation Techniques
Fishing midges requires delicacy and precision. On stillwaters and tailouts, 12- to 15-foot leaders tapered to 6X–8X tippet are essential. Trout feeding subsurface can be targeted with double-midge rigs: a larger pupa with a trailing larva or emerger. In lakes, suspend chironomid pupae or bloodworm patterns beneath indicators at the proper depth.
For surface feeders, a CDC emerger or Griffith’s Gnat fished drag-free are solid choices. Because these flies are tiny, takes are often subtle; watch for gentle dimples and small twitches at the end of your line.
Effective Fly Patterns
Black or Red Zebra Midge (sizes 18–24): A universal larva/pupa imitation.
Al’s Rat (sizes 22–26): A simple thread midge, deadly on limestone creeks.
Brassie (sizes 18–22): Copper wire body suggesting larvae or pupae.
CDC Midge Emerger (sizes 22–28): For film-feeding trout.
Griffith’s Gnat (sizes 20–26): Classic adult/cluster imitation.
Matt’s Midge or Morgan’s Midge (sizes 20–26): Effective dry patterns on tailwaters.
Taken together, these three hatches shape September into a month of variety and opportunity. An angler might spend the morning coaxing trout with delicate midge emergers, switch to Blue-Winged Olive dries during an afternoon drizzle, and finish the evening with a Parachute Iso or spinner. This rhythm—constant midges, opportunistic olives, and dramatic Isonychia—defines early fall trout fishing from the spring creeks of Pennsylvania to the freestones of the Rockies. Those who prepare for all three will find themselves in step with the trout, no matter which river they fish.
