Iso dun

The Specimen

Isonychia bicolor (Mahogany Dun) Mayfly Dun

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Collected June 14, 2005 from the Namekagon River in Wisconsin
Added to Troutnut.com by on May 26, 2006

The Discussion

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Wbranch	February 12th, 2010, 10:01 am
York & Starlight PA Posts: 2733	In Paul Weamer's book (page 190) "Fly Fishing Guide to the Upper Delaware River" he writes "Some Upper Delaware Isonychias are bright green when they first emerge. They eventually change into their reddish purple to grayish pink color as they are exposed to the air". I'd like to tie some of these but would like to see a decent picture of the adult insect as it first emerges. Anyone have any pictures with the bright green abdomen?
	Catskill fly fisher for fifty-five years.

Oldredbarn	February 12th, 2010, 11:53 am
Novi, MI Posts: 2608	Matt, I'd like to hear more about this from the entomologist "guru's" on this site...Taxon and Gonzo or Konchu, but I believe that Gonzo has been kidnapped and is being held by a Mexican drug cartel! We haven't heard from him in quite some time... I have noticed that the emergence "color" of the "sub-imago" seems to change pretty rapidly. I have written here before about the E invaria I have seen on the South Branch of the Au Sable and how difficult it is to discribe, let alone reproduce, their color...It has a light olive tint to it... Now...I'm from the school that places the representation of color way down the list of importance when tying and fishing artificials...But...I like to satisfy myself sometimes at the bench in terms of trying to make my flies somewhat close to the naturals I've seen on the stream. Good post Matt... Spence
	"Even when my best efforts fail it's a satisfying challenge, and that, after all, is the essence of fly fishing." -Chauncy Lively "Envy not the man who lives beside the river, but the man the river flows through." Joseph T Heywood

Taxon	February 12th, 2010, 1:45 pm
Site Editor Royse City, TX Posts: 1350	Perhaps I'm missing the point here, or just plain wrong, and it would surely not be the first time. However, it is my belief that imitating Isonychia duns would not be very productive, as the nymphs generally crawl completely out of the water onto a rock or plant stem before shedding their nymphal exoskeleton. And as such, it seems the duns would rarely be available to trout, however accurate the coloration of their imitation.
	Best regards, Roger Rohrbeck www.FlyfishingEntomology.com

Taxon	February 12th, 2010, 2:01 pm
Site Editor Royse City, TX Posts: 1350	but I believe that Gonzo has been kidnapped and is being held by a Mexican drug cartel! We haven't heard from him in quite some time... Hi Spence- It's my recollection that Gonzo is usually absent from this forum during the winter months, as he is busy ski instructing.
	Best regards, Roger Rohrbeck www.FlyfishingEntomology.com

Oldredbarn	February 12th, 2010, 2:17 pm
Novi, MI Posts: 2608	Taxon, But what about the change of colors we mentioned? Is there a process taking place here...Or do the flies really need to moult to change color...Matt is talking about Iso's but how about other flies in general... In terms of Iso's...I have run in to them in good numbers when fishing for bass, but I can't really say I remember them actually hatching...I usually have them out-and-about near Labor Day and they come back as spinners just after the sun has passed the tree line and through till dark... Taxon...I am sometimes suspect when it comes to the how's and when's of aquatic bugs hatching...Not that I doubt you in any way, I have read all that in the books as well, but I have seen some stoneflies that were supposed to do as the Iso's you mentioned hatching right in mid-stream. I sometimes think of this as maybe when it's time it's time...Sort of like when I have to piss...Always in the middle of a good hatch! Common sense would say that once these bugs were getting ready to go that they may travel to shallower water...But their timing could be off...Does this make any sense? I have heard of certain bugs that emerge from the bottom etc...Could this be because they have actually matured before they could get themselves to where they would normally hatch? I have seen E. dorothea hatching right out of really shallow water...the beach or shore really, and I have thought that they had crawled to near there to hatch... Thanks! Maybe I'm over thinking this! Won't be the first time! Spence
	"Even when my best efforts fail it's a satisfying challenge, and that, after all, is the essence of fly fishing." -Chauncy Lively "Envy not the man who lives beside the river, but the man the river flows through." Joseph T Heywood

Martinlf	February 12th, 2010, 2:57 pm
Moderator Palmyra PA Posts: 3233	Hi Roger, I've read that Isos don't hatch in the water, but we do get them riding the waves here in PA. Several folks, among them an entomologist or two, I believe, note that they sometimes hatch midstream. I never see many of them on the water at one time, but in the fall the fish are looking for them, and if one floats the right pattern through a riffle (I fish an emerger) the fish will smash them. I typically use the Caucci/Nastasi mahogany brown dubbing blend for the emergent thorax, and, so far so good on the Little Juniata. But it might be improved on, or another color might be best on the Delaware. I haven't fished this hatch there, but I'd be very interested to hear the answer to Matt's question. Gonzo would be a good person to provide it, if he weren't spraying powder down the slopes these days. I wonder if anyone knows Weamer well enough to ask . . .. I think he's in PA these days, perhaps at TCO State College.
	"He spread them a yard and a half. 'And every one that got away is this big.'" --Fred Chappell

Taxon	February 12th, 2010, 3:19 pm
Site Editor Royse City, TX Posts: 1350	Spence and Louis- Aquatic insects tend to be lighter in body color immediately following the shedding of their exoskeleton, and darken during the following minutes (or hours). In the case of the subimaginal lifestage of some mayfly species, a change in color seems to be much more pronounced than in others. The emergence behavior of mayflies varies greatly, not only by species, but also by environmental circumstance. For example, a species of mayfly nymph which (under normal circumstances) crawls out of the water to shed it's exoskeleton, may do so at the water's surface during flood conditions. While some mayfly species tend to depart their nymphal exoskeleton at or near the bottom, probably the vast majority of species tend to do so at or near the water's surface. Generally speaking, I would expect any given species to have evolved with an emergence strategy which most favors their successful propagation.
	Best regards, Roger Rohrbeck www.FlyfishingEntomology.com

Wbranch	February 13th, 2010, 4:57 am
York & Starlight PA Posts: 2733	Hello Roger, "it is my belief that imitating Isonychia duns would not be very productive." That may be the popular belief and it is often written to reflect that in literature. However on the Delaware, and it's two main branches, we often see not only nymphal shucks on bank side rocks but also hundreds of shucks floating in the water. I frequently see spatterings of duns all day long floating in the current. Ocassionally I have seen, on overcast days, hundreds of the sub imago emerging and floating along and being gorged on by the trout.
	Catskill fly fisher for fifty-five years.

JAD	February 13th, 2010, 10:02 am
Alexandria Pa Posts: 362	Hi All This ISO thing has always befuddled me. On some waters the nymph cases are laying all over the rocks, on other streams not a one to be found. I can only talk about my own experiences. I think each stream has some determining factor(??) which way they hatch. On the Little Juniata the ISO'S hatch in the water (mostly) , some times in the main current sometimes in the side currents, wet flies in 2X size 6 or 10 are very effective at times along with floating nymphs and drys of course. Color,of the flies I gather for patterns are a gray brown mix, for dries a stripped peacock or suitable biot will do for emergers a brown thread touch dubbed with muskrat does a fine job. Spinners, in ten years of fishing after retirement I have never been present for a spinner fall, I read the books :) and every year their are always ISO's so their are spinners but I never see them---so I presume they return after I leave the stream. Well that's all I know about Isonychia Bicolor--Except I like them. Best JAD
	They fasten red (crimson red) wool around a hook, and fix onto the wool two feathers which grow under a cock’s wattles, and which in colour are like wax. Radcliffe's Fishing from the Earliest Times,

Patcrisci	March 8th, 2010, 2:55 pm
Lagrangeville, NY Posts: 119	I had always thought that Iso duns were a waste of time because I'd never really encountered a situation where fish were taking them. I'd kill 'em on a nymph or wet fly fished near the bank or around rocks. This summer and through most of the fall with the water relatively high for throughout the season, thanks to abundant rainfall, I found the Esopus, in particular, was chockablock with isos floating mid stream. I guess it was because of the higher than normal water levels at emergence, but not sure. at any rate, i was not prepared and missed out on a lot of action the first trip. a couple of days later, armed with iso duns, i got into em good. first time i ever saw this with isos. you can teach an old dog new tricks.
	Pat Crisci

Patcrisci	March 8th, 2010, 2:58 pm
Lagrangeville, NY Posts: 119	as for color, i tend not to get too hung on it, but i too have noticed isos can have a greenish cast to them. is it the freshly emerged dun that is this color? and do they darken to deep ruddy red? I have seen some that are even more brown in color. that's mother nature for ya.
	Pat Crisci

RleeP	March 8th, 2010, 5:39 pm
NW PA - Pennsylvania's Glacial Pothole Wonderland Posts: 398	I'll chime in with the guys who have found fishing an Isonychia dun imitation worthwhile during the main hatch periods of the 3 or so species we have in PA. In my experience, while it isn't very often you'll run into large numbers of these bugs on the water, they tend to have a fairly long emergence period and there are usually enough of a trickle of duns available to the fish to make fishing an imitation in a speculative manner worthwhile/ They are prolific, but they are pretty constant..

Tonyz	May 15th, 2010, 8:55 pm
Northern MI & Central PA Posts: 1	Hi guys. Saw the posts from the MI and PA folks here, so thought I'd chime in. I recently relocated to PA from MI. The Iso hatch on the Au Sable South Branch has been my favorite in MI for years. I've not seen them hatch from the water there, but have caught lots of trout with a parachute dun pattern. Probably more with a rabbit's foot emerger. While kayaking down the Conodoguinet in PA today, I saw a number of large nymphal shucks in the water and Iso duns flying. I found this water-hatching a little odd so I snapped a couple shots of one to ensure it was an Iso and not some PA fly that I wasn't familiar with. Its an Iso without a doubt... and it was at least a size 10. I also noticed that it was indeed very green, but it turned a more rust-color. I've seen this in Michigan, too. Anyway, I have a very good picture of this, but don't know how to post it (created an account only minutes ago... somebody let me know and I'll post).

Martinlf	May 16th, 2010, 6:41 pm
Moderator Palmyra PA Posts: 3233	I too saw a big Iso recently, in the Little J. I'd love to see the photo, and there are some here who can explain how to post it for sure. I'm not one of those, though. :(
	"He spread them a yard and a half. 'And every one that got away is this big.'" --Fred Chappell

GONZO	June 5th, 2010, 11:44 am
Site Editor "Bear Swamp," PA Posts: 1681	Due to Shane's inquiry about Isonychia on another thread, I thought I'd bump this one back up. 'Tis the season, and I missed this thread when it was first posted. Just to try to confirm a few things from the earlier posts: Isonychia often has a fairly flexible emergence. Sometimes you will see most of them crawl out to emerge; other times most will emerge instream; sometimes they will emerge both ways. Weather and water conditions probably play a role. Low water sometimes seems to promote crawling out, and high water seems to promote instream emergence, but I'm not sure there are really hard and fast rules about this. (In some streams, low water seems to trigger instream emergence.) Under "normal" conditions, I often see a mix of both, slightly favoring crawling out. Paul Weamer is right, and the "green" color is not exclusive to the Upper Delaware. I have often seen greenish or olive-bodied Iso duns. In my experience they have always been females, so I have a bit of a hunch about the green color. (Strictly a hunch--as soon as I see a green male, I'll have to revise my thinking.) The color of the underlying eggs often influences the body color of female mayflies. This is especially true of (usually) lighter-colored species like Epeorus vitreus (the females are sometimes called "Pink Ladies" due to the underlying egg color) or some Stenacron species (the females are sometimes called "Salmon Spinners"). The egg color could also show through on recently emerged ("teneral") duns of normally darker-colored species. The egg color of Isonychia bicolor spinners (probably the most common species in much of the East and upper Midwest) is usually greenish. (Some other Iso species have orange or yellowish eggs, and I'm not at all sure that egg color is always constant within a species.) I think that the greenish color of recently emerged (female) bicolor duns might be due to the color of the underlying eggs. Just a thought. Although I can't provide the photo of a green Iso dun that Matt requested at the beginning of this thread, Jason does have two photos that show somewhat "olive" (female) Iso duns that might have darkened from a more greenish color: http://www.troutnut.com/specimen/217 http://www.troutnut.com/specimen/216 It's a shame that we didn't get to see Tonyz's photo--perhaps it was a male. :)

Dryfly	June 5th, 2010, 2:22 pm
rochester mn Posts: 133	The ones I saw had green egg sacks. FWIW, The one Hex I saw had orangish eggs. This underlieing egg idea is seen in tricos too. The female duns can have a pale olive abdomen, that is if your there early enough to see the duns.

Vinlflyfish	June 5th, 2010, 5:21 pm
northern cambria Posts: 42	it wold be tied as green
	trout; a mans best friend

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