September Fly Fishing Hatch Guide: Best Flies for Early Fall Trout

Quick Answer: What Flies Hatch in September?

The most important September trout hatches are usually Blue-Winged Olives, Isonychia or Slate Drakes, Tricos, midges, and several late-season caddisflies. October Caddis may begin appearing in September, especially on Western freestone rivers, while hoppers, ants, and beetles can remain productive until the first hard frosts. On some Western rivers, Mahogany Duns and other regional fall mayflies also become important.

September is a transition month. Early in the month, the river may still fish like August: small flies, low clear water, early starts, and terrestrials along the banks. By late September, cooler nights can improve trout activity, shift the best fishing later in the day, and bring stronger BWO hatches during cloudy weather. Carry tiny dries and nymphs, but also make room for larger Isonychia, October Caddis, and streamers.

If you are planning ahead from late summer, start with our August Fly Fishing Hatch Guide, then use the chart below to adjust your box for early fall.

September Fly Fishing Hatch Chart at a Glance

Remember: A hatch chart is a guide to seasonal progression, not a fixed appointment. Latitude, elevation, water temperature, dam releases, rainfall, and local weather can move a September hatch forward or backward. Check a local river report, inspect streamside spider webs, and watch the water before deciding what the trout should be eating.

Blue-Winged Olives: September's Most Important Fall Mayfly

Blue-Winged Olives, often shortened to BWOs or Baetis, are among the most dependable mayflies of early fall. September's new generation is often smaller than the spring brood, commonly around size 18-22. These insects are widespread across Eastern, Midwestern, and Western trout water, with especially strong opportunities on tailwaters, spring creeks, and fertile rivers.

The best BWO weather is often the weather that keeps casual anglers home: cool, gray, breezy, damp, or lightly drizzling. Hatches commonly build from late morning into the afternoon. Look for trout gathering in softer water, current seams, tailouts, deep slicks, and the slow edges below riffles.

How to Fish a September BWO Hatch

Begin below the surface before the first noses appear. A size 16 or 18 Olive Quill Perdigon has the slim olive profile of a small mayfly nymph and sinks quickly through riffled water. In slower or more technical water, a size 18 CDC Pheasant Tail - Tungsten adds subtle movement without becoming overly bright.

When trout begin bulging just under the film, shorten the distance beneath your dry or fish the nymph on a controlled swing at the end of the drift. Once you see upright wings and consistent surface rises, switch to a size 18 or 20 Classic Blue-Winged Olive. If the trout want a lower-riding profile, use a Parachute BWO.

Redd's tip: A BWO hatch is often a stage problem, not a pattern problem. If you are getting refusals, change where the fly rides before changing to a completely different insect. Move from nymph to emerger depth, then to a low dry, and finally to a spinner if the rises become quiet and delicate.

Isonychia and Slate Drakes: The Big September Mayfly

Isonychia mayflies are often called Slate Drakes, Leadwing Coachmen, or simply Isos. They are most important in the East and Midwest, where a second brood can produce excellent fishing from late summer into fall. The Bug Book notes that the fall generation is often smaller than the spring generation, commonly near size 12-14.

Unlike crawler mayflies that mostly stay close to the streambed, Isonychia nymphs are strong swimmers. They live in fast, rocky water and can move through the water column almost like tiny baitfish. Many crawl onto rocks or streamside structure to emerge, but some hatch in the water. Either way, trout recognize the active nymphs and occasional duns as substantial meals.

Best Isonychia Flies and Presentations

Fish the nymph stage through riffles, fast runs, pool heads, and broken pocket water. The Slate Drake Bugger is designed for this exact crossover: it can imitate a large swimming Isonychia nymph while its movement also suggests a small streamer. Dead-drift it first, then let it lift or swing near the end of the drift.

A size 12 or 14 Prince Nymph - Tungsten is another strong September Iso pattern. Its dark peacock body and contrasting wing case provide a useful impression of a large moving mayfly nymph. When trout begin taking duns, a larger Parachute Adams offers a visible, adaptable mayfly profile for riffles and pool heads.

Isonychia hatches are often sporadic rather than blanketing the river. Do not wait to see hundreds of adults. A few large mayflies around the riffle, active nymphs near shore, or confident splashy rises may be enough evidence to fish the hatch seriously.

Tricos: Tiny Flies, Later Mornings

Tricos remain important through September and can continue until the first or second hard frost. Eastern Tricos are often around size 20-28, while Western Tricos are commonly a little larger, around size 18-24. Their small size is offset by enormous numbers, especially during the morning spinner fall.

The biggest September change is timing. On hot August days, the female emergence and spinner fall may occur shortly after sunrise. As mornings cool, the same event can move later - sometimes toward midmorning or even noon. That is welcome news for anyone who enjoys both dry-fly fishing and sleeping past 5 a.m.

How to Fish September Tricos

Look for steady, rhythmic sipping in slow pools, glides, eddies, tailouts, and the softer water below riffles. Fish a black Redd's Favorite Trico or Foam Case Spinner on fine tippet with a drag-free drift.

Not every Trico spinner remains on top. Spent insects can be pulled under in riffles and eaten just below the surface. If trout are rising but refusing the dry, trail a Sunken Trico 12-24 inches behind a visible dry. Watch the lead fly as an indicator and set the hook when it pauses, twitches, or disappears.

October Caddis: The Fall Hatch That Starts in September

The name is slightly misleading: October Caddis activity can begin during September. These large orange or pumpkin-toned caddisflies are especially important on Western freestone rivers, although related fall caddis also occur in the East. The adults may hatch sporadically, often around low light, nighttime, or early morning, so trout can become familiar with them even when anglers do not see a heavy surface event.

That makes the subsurface stage especially important. The October Caddis Nymph - Tungsten imitates the large larval or pupal food trout encounter before the orange adults appear. Fish it deep through riffles, bankside pockets, and current seams, then allow the fly to rise or swing at the end of the drift.

If you see large adults fluttering or returning to the water, try an orange or tan adult caddis profile. A larger Elk Hair Caddis can handle broken water and can be dead-drifted, twitched, or gently skated. Keep the movement controlled; the goal is a struggling caddis, not a wakeboard demonstration.

Other September Caddisflies

October Caddis get the fall attention, but smaller tan caddis, spotted sedges, and other species can continue from spring through fall. Trout can feed on caddis as larvae, pupae, emergers, adults, and egg-laying adults, which makes carrying several life stages more useful than carrying five nearly identical dry flies.

• Before surface activity: Dead-drift a Holy Grail - Tungsten near the bottom through riffles and seams.
• During emergence: Swing a Caddis Wet Fly or let it rise through the water column at the end of the drift.
• For low-riding adults and cripples: Fish an X Caddis in the film with a clean drift.
• For fast water and splashy rises: Use an Elk Hair Caddis and add a small twitch or short skate when a dead drift is ignored.

Terrestrials: September's Warm-Afternoon Option

Do not remove the hoppers, ants, and beetles from your box just because the calendar says fall. Terrestrials often remain productive through early autumn, and trout may continue eating terrestrial imitations for a short time even after the first frost. The best windows are usually warm, sunny, and breezy afternoons.

In the West, continue fishing hoppers along grassy banks, meadow bends, undercuts, and wind-blown edges. In the East and Midwest, ants and beetles may be more dependable than oversized grasshoppers. A Micro Hopper - Natural is a strong small dry-dropper platform for clear September water. A Realistic Beetle is useful beneath trees, beside brush, and anywhere a compact black silhouette looks more natural than a large hopper.

Cast close to cover and let the fly land with a soft plop. Begin with a drag-free drift, then add one slight twitch if the best water goes untouched. If a large terrestrial gets inspected but refused, downsize to a Baby Hopper before abandoning the bank.

Midges: The Reliable September Backup

Midges are available throughout the year, but their importance increases whenever larger hatches become sparse. Tailwaters, spring creeks, slow pools, and stillwaters can produce steady midge feeding in September, especially during calm morning and evening periods.

Fish a black or red UV Zebra Midge Perdigon near the bottom or beneath a small dry. Its slim body and tungsten bead help it reach depth without adding much bulk. When trout are sipping adults or clusters from the film, switch to a Griffith Gnat and concentrate on a drag-free drift.

When the rises are tiny but regular, do not assume the trout are eating Tricos. Check the surface and nearby spider webs. Tricos have upright mayfly wings and tails; adult midges have two wings, no tails, and often appear as tiny mosquito-like insects or clusters.

Mahogany Duns and Other Regional Fall Mayflies

On some Western rivers, Mahogany Duns become important from September into October. These are generally small to medium mayflies, often around size 14-18, with darker reddish-brown bodies. They are not present in the same numbers on every river, so local hatch charts matter.

A Parachute Adams or appropriately sized Dark Mayfly provides a useful adult profile, while a Classic Pheasant Tail covers the nymph stage. Watch slower riffles, glides, back eddies, and calm feeding lanes during cool afternoons.

September may also hold localized PMDs, Western Green Drakes, Callibaetis, flying ants, and late stoneflies. Treat the national chart as a packing list, then let your specific river decide the final fly.

September Streamer Fishing: The First Serious Fall Window

Streamers are not a hatch, but they deserve space in a September guide. Cooler nights can improve trout activity, and low light, cloud cover, or a safe bump in water can encourage larger fish to chase baitfish, sculpins, and leeches.

Start with a compact Tungsten Woolly Bugger in olive or black. It is easy to cast, sinks quickly, and can be dead-drifted, swung, stripped, or jigged through a pool. For a larger natural profile, the Natural Nutcracker suggests a sculpin or small brown trout and works well along banks, deep runs, and structure.

Do not assume faster is always better. Begin with strips and pauses, then vary speed, depth, and direction. A trout that follows without eating is giving you useful information: pause longer, change the angle, or downsize before changing water.

What Is the Best Time of Day to Fly Fish in September?

The best time to fly fish in September often shifts later as the month progresses. Early September may still favor cool mornings. Later in the month, midmorning through afternoon can become prime time as water warms into a comfortable range and fall mayflies begin to hatch.

• Early morning: Midges, October Caddis adults, small nymphs, and streamers in low light.
• Midmorning: Trico spinner falls, especially after cool nights delay the event.
• Late morning through afternoon: BWOs during cloudy weather, Isonychia nymphs and duns, and general nymphing.
• Warm sunny afternoon: Hoppers, ants, beetles, and dry-dropper rigs along banks.
• Evening: Caddis, Isonychia spinners, mayfly spinners, and streamer fishing around low light.

In hot regions, early September water can still reach stressful temperatures. Carry a thermometer and follow current regulations or closures. Trout Unlimited recommends checking stream temperature and moving higher, fishing only during cooler periods, or changing species when the water is too warm.

Four Proven September Fly Fishing Rigs

1. BWO Nymph-to-Dry Rig

Begin with a Parachute BWO and trail an Olive Quill Perdigon below it. Fish the nymph deeper before the hatch. As trout begin feeding higher, shorten the dropper until the nymph rides near the film. Remove the nymph when fish consistently take adult duns.

2. Isonychia Lift-and-Swing Rig

Fish a Slate Drake Bugger or Prince Nymph - Tungsten through the head of a pool or a fast riffle. Dead-drift it near the bottom, then raise the rod slightly or let the fly swing at the end of the drift to imitate a swimming nymph moving toward emergence.

3. Trico Double-Fly Rig

Use a visible Foam Case Spinner or small Parachute Adams as the lead fly. Trail a Sunken Trico 12-24 inches behind it. This covers spent insects both on and immediately below the surface.

4. October Caddis Dry-Dropper Rig

Use a buoyant adult caddis or small foam dry on top with an October Caddis Nymph - Tungsten below. Dead-drift the rig through riffles and bankside pockets, then allow the nymph to rise and swing at the end of the drift. If trout begin striking the dry, remove the dropper and fish the adult with controlled movement.

September Trout Fishing by Region

• Northeast and Mid-Atlantic: Prioritize Tricos in the morning, Isonychia in riffles and pool heads, small BWOs on cloudy afternoons, and terrestrials during warm weather.
• Southeast: Watch water temperature early in the month. Fish higher-elevation streams or cold tailwaters, with BWOs, midges, caddis, terrestrials, and small general nymphs.
• Midwest and Great Lakes: Look for Tricos, BWOs, Isonychia, midges, caddis, and localized late mayflies. Low clear water often rewards longer leaders and careful approaches.
• Rocky Mountain West: Expect Tricos, BWOs, terrestrials, October Caddis, midges, and localized Mahogany Duns or late PMDs. Cooler afternoons can replace the dawn patrol of August.
• Pacific Northwest: October Caddis, BWOs, terrestrials, caddis pupae, and streamers can all matter. Pay close attention to local regulations and species-specific spawning activity.
• Stillwaters: Fish midges, Callibaetis, damselfly nymphs, leeches, and small baitfish around weed beds, drop-offs, shoals, and wind lanes.

What Should Be in a September Fly Box?

Dry Flies

• Classic Blue-Winged Olive in sizes 18-20
• Parachute BWO in sizes 18-20
• Parachute Adams in small and medium sizes
• Redd's Favorite Trico
• Foam Case Spinner
• X Caddis
• Elk Hair Caddis
• Micro Hopper - Natural
• Realistic Beetle
• Griffith Gnat

Nymphs and Wet Flies

• Olive Quill Perdigon
• CDC Pheasant Tail - Tungsten
• Slate Drake Bugger
• Prince Nymph - Tungsten
• October Caddis Nymph - Tungsten
• Holy Grail - Tungsten
• Caddis Wet Fly
• UV Zebra Midge Perdigon
• Sunken Trico

Streamers

• Tungsten Woolly Bugger in olive and black
• Natural Nutcracker
• One compact light-colored streamer for stained water and one dark silhouette for low light

September Conservation Notes: Warm Water and Spawning Trout

Cool nights do not guarantee safe trout temperatures. During early September heat, measure the water rather than guessing from the air. If the river remains too warm, move to colder water, fish during a safer window, or target bass and other warmwater species instead.

As fall progresses, brown trout, brook trout, and some other salmonids begin preparing to spawn. Learn to recognize redds - clean, disturbed patches of gravel that may appear lighter than the surrounding streambed. Do not cast to actively spawning fish, and do not walk through redds. Trout Unlimited offers a useful overview of why these spawning areas deserve space.

FAQ: September Fly Fishing Hatch Guide

What are the best flies for trout in September?

The best September trout flies are usually small BWO dries and nymphs, Isonychia or Slate Drake patterns, Trico spinners, October Caddis nymphs, general caddis dries and pupae, midges, hoppers, ants, beetles, and small-to-medium streamers. The best choice depends on region, weather, water type, and time of day.

What is the main September trout hatch?

Blue-Winged Olives are the most widespread September hatch across North America. Tricos, Isonychia, midges, caddis, terrestrials, and regional fall mayflies can be equally important on specific rivers.

Do Blue-Winged Olives hatch in September?

Yes. September often marks the return of a strong fall generation of Blue-Winged Olives. Look for small olive mayflies from late morning through afternoon, especially during cool, cloudy, drizzly, or unsettled weather.

Are Tricos still hatching in September?

Yes. Tricos can continue through September and into fall until hard frost ends the season. Cooler mornings often delay the spinner fall, so September Trico fishing may begin later than it did in August.

When should I fish October Caddis?

Begin carrying October Caddis patterns in September, especially on Western freestone rivers. Fish large larva or pupa patterns deep during the day, then try adult caddis dries near banks, pocket water, and riffles during low-light activity.

Are hoppers still good in September?

Yes. Hoppers, ants, and beetles can remain productive during warm September afternoons, particularly before hard frost. Fish them close to grassy banks, undercuts, overhanging cover, and wind-blown shorelines.

What time of day is best for September fly fishing?

Midmorning through afternoon often becomes the best September window as nights cool. Tricos may fall during midmorning, BWOs often hatch from late morning through afternoon, terrestrials work on warm afternoons, and caddis or spinners can become active toward evening.

Should I fish dry flies, nymphs, or streamers in September?

Carry all three. Nymph before visible hatches, switch to dries when trout begin rising, and fish streamers during low light, cloud cover, stained water, or periods when larger trout are actively chasing prey. September conditions can change enough during one day to reward every approach.

What size flies should I use in September?

September fly sizes range widely. Fall BWOs are often size 18-22, Tricos about size 18-28 depending on region, Isonychia around size 12-14, midges commonly size 18-24 or smaller, and October Caddis much larger. Match the insects on your river instead of relying on one universal size.

Final Take: September Is the Bridge to Fall

September rewards anglers who are willing to carry two seasons in one fly box. Keep the tiny Tricos, midges, and terrestrials that worked in August, then add fall BWOs, Isonychia, October Caddis, and a few streamers. Start early during lingering heat, but expect the best window to move later as nights cool and fall hatches strengthen.

If you want a compact September starting lineup, carry a Parachute BWO, Slate Drake Bugger, Sunken Trico, October Caddis Nymph - Tungsten, Micro Hopper - Natural, and Tungsten Woolly Bugger. Together, those patterns cover small technical hatches, large fall insects, bank fishing, subsurface feeding, and aggressive trout.

Watch the weather, read the rises, and let the river tell you which version of September has arrived.