Isonychia bicolor is commonly referred to as the Mahogany Dun, Slate Drake and Leadwing Coachman. In the past, many books also included species Isonychia sadleri and Isonychia harperi, which entomologists have now grouped as Isonychia bicolor. This is one of the major hatches on the freestone trout streams in the Upper Midwest.
Fly fisherman should carry a selection of fly imitations to match the Mahogany Duns (Slate Drake) throughout the summer months. The nymphs are excellent swimmers and at emergence time most nymphs will crawl out of the water to shuck their nymphal case and become a dun. Look on rocks, logs and grass reeds that are partially submerged in the water along the shallows for shucked Isonychia bicolor nymph cases. At these times dun patterns are not important unless the nymphs are emerging in mid-stream which is usually not the case. In the evening hours look for the returning Isonychia spinners over the riffled waters. They offer the trout a choice meal.
The Leadwing Coachman wetfly, in size #10, is also a good pattern for imitating drowned Isonychia duns.