Isonychia bicolor mayfly hatch
Mahogany Dun / Slate Drake / Leadwing Coachman Mayfly

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.

Isonychia sicca

Through aquatic insect sampling during 2023 and 2024 seasons, I have found and identifed numerous Isonychia sicca nymphs on several northwestern Wisconsin trout streams. It seems Isonychia sicca may be more abundant on some of our northern coldwater streams than Isonychia bicolor. To fly fishermen, the is no difference to how you fish the hatch or patterns you would use.

Fishing the Isonychia mayfly hatch

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 shed their nymphal shuck and become an adult, subimago mayfly dun. Look on rocks, logs, and grass reeds that are partially submerged along the edge of the stream for empty Isonychia nymph cases. Dun fly patterns are usually 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 flying over the riffled waters. The egg-laying female spinners 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. Also, a simple, peacock herl nymph pattern can be deadly when the Isonychia nymphs are swimming toward the banks during emergence.