Match the Hatch All Year Long: Your Month-by-Month Insect Emergence Guide for the Au Sable
Photo by Photo by Ben Wicks on Unsplash on Unsplash
If you've spent any real time on the Au Sable, you know the river has a personality. It shifts with the seasons, the water temps, the weather, and — maybe more than anything else — with what's crawling, swimming, and flying off the surface at any given moment. Trout on this river are educated. They've seen a lot of flies, and they can afford to be picky when a genuine hatch is underway.
That's the thing about hatch-matching: it isn't just a nerdy entomology exercise. It's the most practical fishing skill you can develop on a river like this one. When you know what's emerging and when, you stop guessing. You stop cycling through every fly in your box hoping something sticks. You show up prepared, tie on the right pattern, and fish with confidence.
Here's how the Au Sable's hatch calendar plays out, roughly month by month.
April: The Season Wakes Up with Hendricksons
For most Au Sable regulars, the season really begins the moment the Hendricksons show up. These medium-sized mayflies (Ephemerella subvaria) typically start emerging in mid-April, usually in the afternoons when water temps climb above 50°F. The hatches can be dense and the trout response is aggressive — these fish have been waiting all winter.
The Main Stem and the upper Holy Waters see some of the best early Hendrickson action. Standard Hendrickson dry fly patterns in sizes 12–14 are your go-to, but don't overlook the nymph before the hatch kicks off. A Pheasant Tail or an A.K.'s Hendrickson Nymph drifted along the bottom can be deadly in the hour before fish start rising.
Early Blue-Winged Olives (Baetis) also make an appearance in April, especially on overcast days. Keep a few BWO dries in size 16–18 handy.
May: Caddis and the Sulphur Warm-Up
May is arguably the most exciting month on the Au Sable calendar. Caddisflies start showing up in earnest — particularly the Grannom (Brachycentrus) — and when they're on the water, trout go absolutely nuts. Elk Hair Caddis in tan and olive, sizes 14–16, are essential. So is a Soft Hackle Wet Fly fished just below the surface during the emergence itself.
Late May also marks the beginning of the Sulphur hatch (Ephemerella dorothea), which runs into June. These pale yellow mayflies emerge in the evenings, and the spinner fall after dark can produce some of the most exciting dry fly fishing of the year. Parachute Sulphurs in size 16–18 are the standard, and a Comparadun works well when fish are being selective.
June: The Holy Grail — Hex Season
If there's one hatch that defines the Au Sable, it's the Hexagenia limbata — the giant Michigan mayfly, locally known simply as the Hex. These massive insects (sizes 4–8) emerge after dark, typically from mid-June through early July, and they trigger some of the most intense dry fly feeding you'll ever witness.
Hex fishing means late nights, headlamps, and listening for the slurping rise of big brown trout. It also means being patient — the hatch rarely gets going before 10 p.m. Fish a size 6 or 8 Hex dry with a wide foam body and good visibility. The Borcher's Drake is a classic local pattern worth having. Float trips through the Main Stem after dark are a bucket-list experience if you haven't done it.
Light Cahills and Yellow Sallies (small stoneflies) also fill out the June calendar nicely during daylight hours.
July: Summer Doldrums and Terrestrials
Once Hex season winds down and water temps rise, hatches slow and fish go deep. This is terrestrial time. Grasshoppers, ants, beetles, and inchworms become the meal of choice for trout holding under banks and overhanging brush.
Fish a Dave's Hopper or a Chubby Chernobyl along grassy banks in the afternoon. Ant patterns in sizes 14–18 can be surprisingly effective, especially after a windy day that's blown insects onto the surface. Trico hatches also begin in July on some sections — tiny size 20–24 flies that require fine tippet and a steady hand.
August: Tricos and Blue-Winged Olives Return
August mornings on the Au Sable often open with Trico spinner falls, where clouds of tiny black-and-white flies collapse onto flat water and trout sip them quietly. It's technical fishing — 6X or 7X tippet, size 22–24 patterns — but the rewards are worth the challenge.
BWOs also start picking back up as August cools toward September, especially on cloudy days and in the evenings. Keep that size 16–18 box stocked.
September and October: Fall Baetis and the Last Dance
Fall on the Au Sable is underrated. The crowds thin out, the maples turn, and the Blue-Winged Olives come back with a vengeance. Overcast, drizzly September and October afternoons can produce some of the best dry fly fishing of the entire year as Baetis nymphs hatch in huge numbers.
Parachute Adams in size 16–18 works well, as does a CDC Baetis dun. Streamers also start earning their keep in fall as big browns get aggressive ahead of the spawn. A Woolly Bugger or a Zoo Cougar stripped through deeper runs can draw vicious strikes.
Build Your Box Around the Calendar
You don't need every fly ever tied — you need the right flies at the right time. Build your Au Sable box around this calendar, prioritize the hatches that match your planned trips, and you'll fish smarter every time out. Stop into the shop and we're happy to help you fill in any gaps. We stock locally proven patterns and can tell you what's been working on the river right now — because the calendar is a guide, but the river always has the final word.
