Blue-Winged Olive (BWO) Hatch in Michigan
Spring and fall Baetis emergence on Michigan trout rivers. Overcast-day duns, fall windows, and small-fly tactics.
Blue-winged olives are the most reliably overlooked hatch in Michigan. Chris Izworski covers the spring and fall Baetis emergences that produce surface activity when nothing else is hatching, especially on overcast days when bigger mayflies stay down.
Baetis species in Michigan
Blue-winged olive (BWO) is a catch-all common name for several species in the Baetis genus, plus a few related small mayflies (Plauditus, Diphetor). On Michigan rivers, the dominant BWO species emerge in two distinct windows: a spring window from late April through May, and a fall window from mid-September through October. Hook size ranges from 18 to 22 in most cases, with some larger 16s in the early spring and fall. Body color is olive to dark olive, wings are slate gray, three tails.
The defining feature of the BWO hatch is its preference for low light. Cool overcast days, drizzle, and approaching weather fronts produce the densest hatches. Bright sunny weather kills the emergence. Many anglers ignore BWO conditions and miss what is consistently the most productive surface fishing of the cool-weather shoulder seasons.
Late April through May
On the AuSable mainstem, BWOs begin in the last week of April and run through May in overlapping waves with hendricksons and sulphurs. The signature spring BWO day is cool (50 to 60 degrees air, low 50s water), overcast, and ideally with light rain. The duns emerge in late morning and continue intermittently through midday, often producing rising fish for two to four hours.
The Pere Marquette, Boardman, and Manistee all hold spring BWO hatches with similar timing to the AuSable. On the upper Sturgeon, the Black River, and the cold-water creeks of the UP, the spring window pushes a week or two later into mid-May.
September into October
The fall BWO hatch is more reliable than the spring hatch and runs from mid-September through late October on most Michigan rivers. Cooler nights drop water temperatures into the 50s, and overcast or rainy fall days produce the same dense midday emergences as spring. Many anglers focus exclusively on streamers and steelhead in the fall and miss what is often the best dry-fly fishing of the season for wild trout.
On the AuSable, the fall BWO emergence peaks in the first two weeks of October. On the Pere Marquette and the Manistee, the same calendar applies. The fall hatch tends to be more concentrated by time of day (12:00 PM to 3:00 PM) than the spring hatch, and the duns are slightly smaller (size 20 to 22 is common). The trout, which have seen fewer flies through the late summer, are often less selective than they are in May.
BWO patterns
Carry parachute BWOs in sizes 18 and 20, comparadun BWOs in 18 and 20, and a few sparkle-dun BWOs in 20 for flat-water selective fish. For the emerger stage (which trout often prefer over fully-emerged duns in BWO hatches), a CDC and elk emerger or a RS2 in size 20 covers the in-film feeding behavior. A small BWO nymph in size 18 (Pheasant Tail variant in olive) covers subsurface pre-hatch fishing.
Tippet matters. With size 18 to 22 flies, 6X is the right tippet diameter. 5X is too thick to get a natural drift on these small flies. 7X works on very selective fish but breaks easily on the hookset. Most BWO refusals happen because the angler is fishing tippet that is one size too heavy.