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Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Mayfly Species Isonychia bicolor (Mahogany Dun)

This is by far the most important species of Isonychia. Many angling books once split its credit with the species Isonychia sadleri and Isonychia harperi, but entomologists have since discovered that those are just variations of this abundant species.

See the main Isonychia page for more about these intriguing mayflies.

Where & when

Time of year : Late May through October

This species has two distinct emergence peaks, once in late spring or early summer and again in the fall. It may be found on the water in lighter numbers at any time in between.

The first peak begins in the freestone mountain streams of Pennsylvania in early to mid-June. It reaches the Catskills in mid-June and continues through early July. The Upper Midwest and the northern ranges of the Appalachians peak from late June through mid-July.

The second peak usually comes in September, and those flies are more likely to emerge at midday than their Summer brethren.

Isonychia bicolor is only abundant on certain rivers, even in ideal regions. One river can be fantastic while the next drainage over shows no sign of Isonychia.

Hatching behavior

Time of day : Usually concentrated in evening with a peak near dusk, especially in Summer; may be sporadic all day

Water temperature: Below 70°F

The duns are known for emerging by crawling out onto shore, and one of my streamside pictures shows the nymphal shucks they left behind on a rock. But they may also emerge on the surface in standard mayfly style. Many books attribute this to high water submerging the convenient places to crawl out of the water, and others attribute it to the size of the stream, but I think it is more likely a combination of these things, current speed, and geographic region. My home river in Wisconsin seems to have entirely midstream emergence, and some Catskill rivers display both.

When the duns emerge midstream, they're clumsy and make excellent targets for trout. Their heavy bodies ride low in the water, and they often make several failed attempts at flight before succeeding. Low-riding dry flies are good imitations of the placid duns, and heavily hackled patterns may be twitched and skittered to imitate the takeoff commotion.

I've had some of my best dry-fly fishing during sporadic hatches of Isonychia bicolor duns. Trout have an unusually ferocious appetite for them compared to other mayflies of similar size and profile, and they will often smash an imitation when no real duns are on the water.

The duns have pale off-white fore tarsi, which they often hold up on the air when they're resting. This feature has given them both the scientific name bicolor and one of their common names, the "White-Gloved Howdy."

Spinner behavior

See the main Isonychia page for spinner details.

Nymph biology

Isonychia has some of the most interesting mayfly nymphs; see the main page for the genus for details, which do not really differ between species.

Some books have made a big deal over the strength of the stripe running down the back of Isonychia bicolor nymphs, because it was a confusing identification characteristic used to separate them from Isonychia sadleri. Because the two species are now synonyms, the confusion is no longer needed--they're all Isonychia bicolor. However, anglers tying nymph imitations should collect some local specimens before putting bright white stripes down the backs of all their flies. In some populations the stripes are only a little bit lighter than the rest of the body and don't run its whole length. In others, the stripes are indeed vivid enough to warrant the prettier imitation.

Isonychia bicolor Fly Fishing Tips

The size and color of Isonychia bicolor adults may vary throughout the season on a single river, so one should catch specimens from time to time and check them against their imitations.

Specimens of the Mayfly Species Isonychia bicolor

4 Female Duns
1 Male Spinner
Male Isonychia bicolor (Mahogany Dun) Mayfly Spinner
I got several really nice pictures of this spinner. I also collected a female on the same trip.
1 Female Spinner
Female Isonychia bicolor (Mahogany Dun) Mayfly Spinner
I collected this female together with a male.
13 Nymphs

5 Streamside Pictures of Isonychia bicolor Mayflies:

1 Video of Isonychia bicolor Mayflies:

Isonychia nymph swimming around

These nymphs may be the best swimmers of all North American mayflies.

Recent Discussions of Isonychia bicolor

1 replies
Posted by JMV on Sep 21, 2006
Last reply on Sep 21, 2006 by Troutnut
Great site, I'm an Iso. fanatic... JM

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