One of the most prolific Wisconsin mayfly hatches is Ephemerella subvaria, also known as the Hendrickson mayfly hatch. These mayflies are very common on the northern Wisconsin freestone streams. They generally hatch sometime in late April to early May. I have personally witnessed the hatch as early as April 8th and the Hendrickson spinners at dusk into mid-May. For the Hendrickson hatch you definitely need to tie several different patterns and carry a good supply for when you hit the hatch in full swing.
Hook: TMC 100 #12 & #14
Thread: Uni-thread 8/0 Tan, Gray or Brown
Tail: Coq De Leon tail fibers
Body: Nature's Spirit's Hendrickson or Pink Cahill fine & dry dubbing
Wing: Fine deer hair (comparadun style)
Don't make the body too bulky. You can also use Zelon for tail to make it a sparkle dun. emerger pattern.
Two great Hendrickson soft hackle patterns...
Hook: Mustad 3906 size #12 & #14
Thread: Veevus #16 or Uni-thread 8/0 Burnt Orange
Tail: Grouse or Woodcock fibers
Rib: Fine copper wire
Abdomen: Awesome Possum Rusty Brown or dyed rust colored hare's mask
Hackle: Starling feather
Hook: Hends BL254 size #12
Thread: Veevus or Uni-thread 8/0 Burnt Orange
Tail: Grouse fibers
Rib: Live4FlyFishing UV Ribbing no. #12
Body: Dyed rusty orange Australian possum dubbing
Wing: dyed grey hen feather
Tie the hen feather in first pointing forward. Tie in the tail, ribbing and dubbing. Place the thread at the back of the thorax section and wrap the hen hackle towards the back of the hook shank stopping when you reach the thread. Secure the feather tip with two thread wraps and snip off the tip of the feather. Wrap the thread forward through the hackle wraps up to the eye of the hook then whip finish the fly.
For one type of my Hendrickson nymph patterns I use a barbless jig hook to get the fly down to the bottom fast. I also tie some Hendrickson nymph patterns on light wire hooks (no bead) to fish near the surface during emergence. The Hendrickson nymphs in northern Wisconsin trout streams are a darker rust color and sort of stout. These nymphs are in the drift squirming towards the surface and the trout are looking for something that is the right size, shape and color. Tying on legs and a separate wing case I do not feel is needed.
Hook: Hends Barbless BL120 Jig Hook, Size #14 & #12
Head: Competitive Angler's 3.0mm Slotted Copper Tungsten Bead
Thread: Dark Brown thread of your choice
Tail: Grouse hackle
Ribbing: Copper Wire
Body: Australian Possum dyed rust color
Thorax: Australian Possum dyed dark rust or black color
For a simple Hendrickson nymph pattern that is super easy and fast to tie with just a tail and a symmetrical dubbed body tied on a Mustad 3906 hook, size #12. Start by wrapping copper wire around 2/3 of the hook shank after securing the thread onto the hook shank Then tie in a few dark colored grouse fibers for the tail. After that dub the body with some dark rusty brown Awesome Possum dubbing about 1/2 of the way forward. Lastly, add some black Awesome Possum dubbing mixed with some dark rusty brown Awesome Possum dubbing for the thorax and wing case. Wrap the dubbing tightly all the way to the eye of the hook and whip finish. That is it. Don't bother tying in any legs the trout don't seem to mind.
If you ever look closely at a Ephemerella subvaria (Hendrickson) nymph from a northern Wisconsin freestone streams you will notice the nymphs have a dark rusty brown abdomen and a very black wing case with a white slit running the length of their wingcase when they are ready to hatch. Also, the nymphs are fairly stout so I prefer tying the Hendrickson nymphs with dubbing rather than pheasant tail fibers to add more bulk. It is really about the silhouette, shape and color. Keeping the dubbing tightly wrapped also helps the fly sink faster.