Isonychia (Slate Drake) Hatch in Michigan

Late June through September emergence of the dark mahogany mayfly. Crawling nymphs, evening duns, and swing tactics.

Isonychia (slate drake or mahogany dun) is the long-running summer-into-fall mayfly on Michigan rivers. Chris Izworski covers the unusual crawling nymph emergence, the late-day adult activity, and the swing tactics that turn the hatch into a producer.

The slate drake mayfly

Isonychia bicolor (slate drake, mahogany dun, white-gloved howdy) is a large mayfly, hook size 10 to 12, with a dark mahogany-brown body and slate-gray wings. The species emerges in two waves on most Michigan rivers: a first wave in late June and a longer second wave from late July through September. Hook size and body color stay consistent across both windows.

What makes isonychia unusual is the nymph behavior. Unlike most Michigan mayflies, isonychia nymphs swim or crawl out of the water onto rocks, logs, and bankside debris to emerge. The hatch is therefore not a midstream surface event the way a hendrickson or sulphur hatch is. The duns are usually first seen flying from the bank rather than emerging from open water. Trout key on the nymphs as they leave the substrate and on the spinners returning to the river.

When isonychia emerge

On the AuSable mainstem and the Manistee, isonychia begin in the last week of June with a brief peak through early July, then taper through August. A second wave picks up in the last week of July and runs continuously into September. By August, isonychia is one of the few significant mayfly emergences in Michigan and is often the only mayfly worth tracking after the hex hatch ends.

Daily activity is afternoon and evening. The nymphs become active in late afternoon, crawling toward emergence sites. The duns are most often seen flying from 6:00 PM until dusk. Spinner falls happen at dusk and into dark on warm summer evenings. The peak feeding window is the last 90 minutes of daylight.

Tactics for isonychia

Because isonychia nymphs crawl to emerge rather than swim to the surface, dead-drift nymphing is not the most productive subsurface tactic. A more productive approach is the swung soft-hackle or wet fly: a size 10 or 12 partridge and brown, partridge and mahogany, or a dedicated isonychia soft-hackle, swung down-and-across through riffles and runs in late afternoon, imitates the active swimming nymphs and produces solid eats.

Once duns are on the water in evening, a parachute isonychia or a Mahogany Dun in size 10 fishes well on flat water. For spinner falls, a Mahogany Spinner with reddish-brown body and clear poly wings in size 10 covers the dusk feeding window. Fish often hold tight to the bank during the isonychia spinner fall, so cast accuracy along structure matters more than presentation distance.

Best Michigan rivers for isonychia

The AuSable mainstem from below Wakeley Bridge through McMasters and into the Mio section produces strong isonychia. The Manistee mainstem from CCC Bridge through M-72 holds the hatch. The Pere Marquette flies-only section produces consistent isonychia activity, especially in August. The Boardman, the upper Sturgeon, and the Pigeon all have isonychia populations of varying density.

Look for rocky or gravelly riffle water with rocks and logs along the banks (the emergence sites). The classic isonychia run is moderately fast water two to three feet deep with structure on the bank for the nymphs to crawl onto. Pure sandy or silt-bottomed water rarely produces strong hatches. The presence of round cobble or larger rocks correlates closely with isonychia density.