The Midwest March Brown mayfly hatch is used to describe Maccaffertium vicarium mayfly and it's smaller relative the Gray Fox. Actually, the Gray Fox (Stenonema fuscum) used to be considered a different species than Stenonema vicarium, but in 1979 the entomoligists combined the smaller Stenonema fuscum with Stenonema vicarium. Then later on entomologists changed the Genus from Stenonema to Maccaffertium. Many books (like Hatches) reference Stenonema vicarium which is now called Maccaffertium vicarium.
Note: The Eastern / Midwest March Brown mayflies are a different genus/species than the Western March Brown mayflies which are Rhithrogena. Besides Maccaffertium vicarium, it has been documented by entomologists that Maccaffertium modestum, Maccaffertium mexicanum integrum, Maccaffertium pulchellum, Maccaffertium terminatum terminatum, and Maccaffertium mediopunctatum arwini also exist in our northern Wisconsin rivers and streams.
This is one of the great hatches found on many northern Wisconsin freestone streams and rivers. Look among the rocks in the riffles and faster runs for the flat, clinging nymphs. The mature nymphs will be about 15 - 20 mm long. The hatch generally occurs near the end of May through mid-June depending what stream you are on. Northwest Wisconsin generally sees the March Brown (Maccaffertium vicarium) mayfly hatch in early to late June. The nymphs are known to migrate towards the shallows just prior to hatching, making the the larger Maccaffertium vicarium duns less vulnerable to the trout. The duns usually hatch sporadically all day long and often along with the more dense hatching Emphemerella invaria (Sulphurs) in the late afternoons. At midday, when the March Browns are sporatically hatching throwing a large Grey Fox Variant can be just the ticket to arouse the big trout lurking in the pools. In the cool June evenings the concentrated March Brown spinners can give fly fishermen the best chance of finding some larger feeding trout.
Fly fisherman should carry a selection of size #10 & #8 nymph, dun and spinner patterns to match the March Brown hatch. The clinging nymphs will be migrating towards the shallows prior to emergence and the trout may follow. During these times I have spooked larger trout in the shallow riffled waters probably looking for the large clinging nymphs. Look for the March Brown spinners near dusk over the riffles and faster water sections.