McCoys Iso Dun Grey is a spin off of an old Michigan Standard, the Roberts Yellow Drake. This fly was tied to imitate the Isonychia Duns that hatch on our local streams in late May through mid July. This pattern provides a realistic silhouette and is a great searching pattern that also fishes well when trout become more selective during a hatch. This has been one of the most productive hatch matching patterns for the Isonychia Mayfly on our local streams.