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Isonychia / Slate Drake (Isonychia bicolor)
Mayfly, hatching June–September, active in water between 55 and 70°F, and imitated at #10–14. Recorded from the Virginia mountains, where 796 Virginia rivers on this site carry a page.
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A dark mayfly, size 10 to 14, with a maroon-brown body, pale-striped legs and slate wings, and it emerges over months rather than days — June through September, in water from 55 to 70 degrees, spanning cold, cold-transitional, cool and warm classes. That long emergence makes it one of the most useful summer bugs in the book. The nymph is a powerful swimmer with a distinctive pale stripe down its back; it migrates to rocks and logs to emerge, and the shed shucks left on midstream boulders are how the hatch announces itself. Because most crawl out on rock, nymph and wet-fly fishing beats dry-fly fishing more often than not, and the nymph fished tight to the bank in fast water works all summer. The dun is slate-winged, dark-bodied, sporadic through summer rather than concentrated. Evening spinner falls are rusty-bodied and clear-winged. An Isonychia Dun or Leadwing Coachman at 10 to 14, a Prince Nymph 10 to 18, a Zug Bug 12 to 16, and a Rusty Spinner 14 to 20 cover the stages.
- #10–14Hook size
- 55–70°FWater, active
- Jun–SepPeak months
- 4Life stages fished
What it is
Mayfly · Isonychia bicolor · nymph → emerger → dun → spinner
A dark mayfly, size 10 to 14, with a maroon-brown body, pale-striped legs and slate wings. It emerges over months rather than days, which makes it one of the most useful summer bugs in the book.
A strong swimmer that crawls out on rocks and logs. Fish the nymph tight to the bank in fast water; it works all summer.
When it hatches
Peak is June–September. The pale months either side are the shoulder — the app widens a peak by one month in each direction and recommends the bug there, and no further: that widening is bounded so that a bug peaking in May is never offered in August.
peak shoulder not recommended
Water conditions
The band is a water temperature, not an air temperature, and it is the range in which this insect is worth fishing — for a mayfly that is the emergence window; for a scud or a midge it is simply the range trout will still eat one in.
Active from 55°F to 70°F. On this map that is cold, cold-transitional, cool, warm water — the band crosses 3 of the three breaks the river colours use. Breaks at 58, 64 and 70°F.
Stages
What the insect is doing, and what that means for what you tie on.
| Stage | What it is |
|---|---|
| nymph | A powerful swimmer with a distinctive pale stripe down its back. It migrates to rocks to emerge, and the shed shucks left on midstream boulders are how you know the hatch is on. |
| emerger | Most emerge by crawling out on rock, so nymph and wet-fly fishing beats dry-fly fishing more often than not. |
| dun | Slate-winged, dark-bodied, big enough to see. Sporadic through summer rather than concentrated. |
| spinner | Evening falls, rusty-bodied and clear-winged. |
Matching flies
7 patterns in this book imitate the Isonychia / Slate Drake, across its stages. Which one to tie on follows the stage the water temperature says it is in — the app makes that call on a live reading; this page lists the whole set.
- Isonychia Dun dry fly · #10–14 — Slate wing, mahogany body. Trickles off all summer and into September.
- Isonychia Nymph nymph · #10–14 — A dark, slim, fast-swimming nymph with a pale dorsal stripe. Fished with movement — the natural swims hard for the bank and does not drift.
- Leadwing Coachman wet fly · #10–14 — The old wet-fly answer to Isonychia. Swung on a tight line in fast water.
- Parachute Adams dry fly · #12–20 — Universal mayfly dun imitation. Every angler carries these.
- Prince Nymph nymph · #10–18 — An attractor nymph with biot wings. Nobody agrees what it imitates and it does not matter.
- Rusty Spinner dry fly · #14–20 — Almost every mayfly spinner is rust-coloured. This covers the last hour of light.
- Zug Bug nymph · #12–16 — Peacock and a dark body. Older than most of us and still catching fish.
Where to find it right now
The rivers with a page here that sit in this insect's regions, with a USGS gauge reporting water temperature. This list is re-ranked against the reading each gauge is sending now and today's date, so it is live — the fallback below it, if the readings do not arrive, is each river's measured July mean.
- Falling Spring Creek the Virginia mountains · July mean 63.1°F
- Indiandraft Creek the Virginia mountains · July mean 63.1°F
- Jerrys Run the Virginia mountains · July mean 63.1°F
- Cascades Creek the Virginia mountains · July mean 59.2°F
- Left Prong Wilson Creek the Virginia mountains · July mean 59.2°F
- Lick Block Run the Virginia mountains · July mean 59.2°F
- Jackson River the Virginia mountains · July mean 66.3°F
- Biggs Run the Virginia mountains · July mean 67.5°F
- Furnace Branch the Virginia mountains · July mean 67.5°F
- Hipes Branch the Virginia mountains · July mean 67.5°F
Regional presence
Regions, not collection records. This insect is recorded from the coarse regions below; nobody has surveyed which named river it has been caught on, and this page will not pretend otherwise.
- northern lower Michigan MI
- southern lower Michigan MI
- the Upper Peninsula MI
- the Wisconsin driftless WI
- northern Wisconsin WI
- the Virginia mountains VA
- the North Carolina mountains NC
Rivers in these regions with a page here
- Jackson River
- Smith River
- Als Run
- Alum Creek
- Amos Branch
- Anderson Run
- Ararat River
- Auberry Creek
- Back Creek
- Back Creek
- Back Fork
- Back Run
- Baker Branch
- Baldwin Branch
- Baldwin Branch
- Barbours Creek
- Bark Camp Branch
- Bark Camp Branch
- Barkcamp Branch
- Barnard Creek
- Barney Run
- Barterbrook Branch
- Batie Creek
- Battery Creek
- and 772 more
Related hatches
Same family, or overlapping this one's calendar on the same water.
- Little Yellow May Leucrocuta / Heptagenia spp. · Mayfly · June–September
- Trico Tricorythodes spp. · Mayfly · July–September
- Yellow Drake Ephemera varia · Mayfly · June–August
- Light Cahill Maccaffertium ithaca · Mayfly · May–July
- Sulphur Ephemerella invaria · Mayfly · May–July