Isonychia bicolor is commonly referred to by fly fishermen as the Mahogany Dun, Slate Drakes, and Leadwing Coachman. In the past, many fly fishing books included 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. On warm, cloudy days that hatch can happen anytime. During these times, the trout are always on alert for the Iso nymphs in the drift.
Fly fishermen should carry a selection of fly imitations to match the Mahogany Duns (Slate Drake) throughout the summer months. The nymphs are excellent swimmers and during emergence, most nymphs will crawl out of the water to escape their nymphal shuck and become a dun. Look on rocks, logs, and grass reeds that are partially submerged in the water along the shallows for empty Isonychia bicolor nymph cases. Dun fly patterns are not usually 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 trout an easy meal when they land on the water.
The Leadwing Coachman wet fly, in size #10, is also a good pattern for imitating drowned Isonychia duns.