Isonychia
Imitates: Adult Isonychia bicolor mayfly (Slate Drake)
Quick Reference
- Best Sizes
- #10-14
- Best Season
- June through October (two emergences)
- Best Conditions
- Eastern and Midwestern freestones, riffles into runs, late afternoon
- Water Temp
- 55-68°F
- Recommended Tippet
- 4X-5X mono
How to Rig It
Single dry, or trail an Iso nymph 18" below as a dropper during the emergence window.
How to Present It
Dead-drift along seams and current edges. Isonychia nymphs swim to shore to emerge, so adults appear on the water near banks — fish edges hard.
Why It Works
Iso adults are large (#10-12), reddish-mahogany mayflies that hatch in two waves — June and again in September-October. The fall emergence overlaps the BWO hatch and brings big browns to the surface for one of the last major dry fly opportunities of the year.
History
The 'Leadwing Coachman' was the original Catskill imitation in the 1800s. Modern Iso patterns trace to Pennsylvania and New York fly tiers who recognized that Isonychia bicolor — not a Drake — was the actual bug.
Pro Tip
Watch the banks during an Iso emergence. The nymphs crawl out on rocks and stumps to hatch, leaving distinctive cream-colored shucks on shoreline structures. If you see those shucks, the hatch is on.
Frequently Asked Questions
When does the Isonychia hatch?+
Two waves — late June into July, then again from late August through October. The fall emergence is the more reliable dry fly window.
What does an Isonychia look like?+
A large reddish-mahogany mayfly with dark slate-gray wings and three tails, about 12mm long. Often confused with a small Brown Drake.
Not sure if Isonychia is right today?
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