Another one of the dominant mayfly hatches on the freestone trout streams of the Upper Midwest is the Isonychia bicolor hatch, commonly referred to as the Mahogany Dun, Leadwing Coachman, or Slate Drake. The Isonychia nymphs are excellent swimmers and trout are always on the look out for them. Prior to emergence the nymphs head to the shallows and prefer to crawl on a rock, log or grassy bank to shuck their nymphal case and transform into a dun. You can see more on the Mahogany Dun web page on WiFlyFisher.com.
Below are a couple of isonychia nymph patterns that work great. These fast swimming nymphs are relatively wide in the abdomen and their gill plates make them appear even wider. To better appreciate understand patterns below here is a photo of an Isonychia bicolor nymph I took in the Spring of 2011 on a Northern Wisconsin trout stream. (Notice the gills plates on both sides of the abdomen.)
Isonychia bicolor nymph
The first pattern is a recipe from Eric Peper. The pattern is extremely realistic right down to the gill plates on the lateral sides of the abdomen and very effective.
Isonychia bicolor nymph pattern by Eric Peper
Isonychia bicolor nymph pattern by Eric Peper
Here is Eric’s instructions on how to tie his Isonychia nymph pattern:
Two strips of lead approx the same diameter as the hook wire tied parallel to the shank, one strip on either side of the shank.
Recipe:
Hook: Mustad 9671 #10
Thread: Black Monocord
Tail: 3 peacock herls
Abdomen: Peacock herl
Rib #1: Brown ostrich herl clipped top and bottom
Rib #2: fine copper wire
Dorsal stripe: stripped brown hackle stem
Thorax: Peacock herl
Wingcase: Turkey tail segment, overlayed with continuation of dorsal stripe
Hackle: Furnace, clipped on top
Steps:
1. Coat the hook with thread
2. Tie in the lead strips, overwrap with thread, and coat with head cement.
3. Tie in tails, dorsal stripe, rib #2 and rib #1 in that order, just over the point of the barb.
4. Wrap abdomen to about the 55% point on the shank, wrap rib #1, tie in dorsal stripe, wrap rib #2 .
5. Trim off waste from the two ribs and make a couple of thread wraps tying the dorsal stripe back slightly.
6. Tie in wingcase.
7. Continue wrapping peacock herl to form the thorax
8. Bring wingcase over the top of thorax and tie down
9. Bring dorsal stripe over wingcase and tie down
10. Wrap hackle (3-4 wraps) and whip finish.
The second pattern is the one I like to tie. It is very similar to Eric’s but I don’t add the ostrich herl and I like to wrap the furnace hackle before pulling the turkey tail segment forward.
Top view Isonychia nymph pattern
Side view of Isonychia nymph pattern
I prefer to tie in 2 short pieces of 25 lb. monofilament parallel to the hook shank, rather than lead wire. If I need to add weight I add it to my leader on the stream. That way when the Isonychia nymphs are heading to the shallows my fly doesn’t get hung up on the bottom as much.
Recipe:
Hook: Mustad 9671 #10 or TMC 200R
Thread: Black Thread
Tail: 3 peacock herls
Abdomen: Peacock herl
Rib: fine copper wire
Dorsal stripe: stripped hackle stem or barb from white mallard duck wing quill
Thorax: Peacock herl
Wingcase: Turkey tail segment, or very dark, bluish-black feather section from a mallard wing
Hackle: Furnace, clipped on top and bottom
A tying tip for helping make the monofilament stay flat and parallel to the hook shank is to use a pair of needle nose pliers to pinch the mono while wrapping the thread to make sure the mono stays on the sides of the hook shank.
Flattening the monofilament on the hook
Fish the Iso nymphs either dead drift, quarter swing them downstream, or or slowly crawl them along the bottom. Add weight to the leader to get the fly down when necessary.