Color is the least reliable way to tell a Blue-Winged Olive from a Mahogany Dun, even though most guides sort them that way. The mark that holds up is tails on the adult, two for a Blue-Winged Olive (Baetis) and three for a Mahogany Dun (Paraleptophlebia), backed by size, since the fall mahogany runs two or three hook sizes larger. Both hatch on the same cool, gray fall afternoons, so on a day when both are up you need to know which one the fish are eating before you tie anything on.
How to tell a Blue-Winged Olive from a Mahogany Dun
Count the tails on the winged insect. A Baetis dun shows two tails and a small, plain hind wing. A Paraleptophlebia dun shows three tails and an oval hind wing held upright, and that pairing is the call to trust.
The tail count works on the dun, not the nymph. Baetis nymphs commonly carry three tails, and the archetypal fall species, B. tricaudatus, is named for exactly that. The middle tail is reduced when the insect emerges, which is why the adult shows two. Count tails on a bug in the water and a three-tailed Baetis nymph will point you at the wrong family, so make the call on the winged insect. Underwater, the gills separate them: Leptophlebiidae are the prong-gilled mayflies, with forked gills along the abdomen, where Baetis carries simple oval plates.
Size is the next mark. In the East and Midwest, fall mahoganies run about size 14 to 18 and the Blue-Winged Olives sharing the water run 18 to 24. In the West the mahoganies run mostly 14 to 16, with fall olives around 18 to 20. A reddish dun clearly bigger than the olives coming off with it is almost always the mahogany. Use color only as a tiebreaker. A dark reddish-brown mahogany next to a clearly olive Baetis is easy; when both sit in the brown-gray middle, color tells you nothing.
One naming trap: “Mahogany Dun” also gets applied to Isonychia (the Slate Drake), a much larger size 8 to 12 mayfly with cream-colored middle and hind legs. Tails won’t sort that one out, since Isonychia duns carry two as well and would key as an olive. Size and those pale legs are what rule it out.
When and where each hatches in fall
Both hatches favor cool water and low light. Baetis tends to get going once water reaches the low 40s Fahrenheit, with the Bighorn Trout Shop putting the number close to 43°F, though it moves around by river. In fall the hatch peaks from around midday into the afternoon rather than at the edges of the day. The hatch can come in waves and blanket the water.
Paraleptophlebia peaks in September across most of the country, with adults into October and, on western rivers, into early November. In the East it comes off earlier in the day, mid- to late morning into the early afternoon. On much of the western water the two overlap in one midday block instead, roughly 11:30 to 3:00. Either way it runs sparser than the olive hatch but lasts longer, and its duns ride the surface a long time.
When both are out, they tend to hold different water. Baetis spills out of riffles and faster runs; Paraleptophlebia comes off slower margins, back eddies, flats, and seams. Fish working the fast lines are usually on the olives; fish tucked along soft, silty edges are usually on the mahoganies. It isn’t a hard rule, since the fall mahogany P. debilis tolerates faster water better than its relatives, but it holds on most rivers.
How to match each
For the Blue-Winged Olive, the emerger often produces best in fall: an RS2 or CDC emerger in size 18 to 22, or a low-riding Sparkle Dun or Last Chance Cripple, a cripple being a fly tied to sit like a dun stuck half out of its shuck in the surface film. Carry a beadhead Pheasant Tail in size 16 to 20 for the nymph, a parachute or Comparadun in size 18 to 22 for the dun, and a rusty spinner in size 18 to 22.
For the Mahogany Dun, size up and go reddish-brown. A Pheasant Tail around size 16, a hook or two larger than the olive version, covers the nymph. For the surface, a mahogany Comparadun, Sparkle Dun, or brown parachute (size 16 to 18 East, 14 to 16 West), plus a Quigley Cripple for the film. Keep the body slim.
When both hatch together, a double dry, a mahogany on point with a smaller Blue-Winged Olive trailing, covers both silhouettes in one drift. Match size and silhouette first, color second. If a fish refuses a good drift, drop a hook size and switch to a low-riding emerger before you change color. Fall water on a lot of rivers runs low and clear, so fish a drag-free drift on a 9- to 12-foot leader and 5X or 6X tippet, down to 7X on flat, glassy water. Tailwaters and irrigation-fed rivers can come up instead once headgates close, so check flows before you count on it.
FAQ
What’s the difference between a Blue-Winged Olive and a Mahogany Dun?
The most reliable difference is tail count on the adult: a Blue-Winged Olive (Baetis) has two tails, a Mahogany Dun (Paraleptophlebia) has three, along with an upright oval hind wing. The mahogany also runs two or three hook sizes larger. Both are angler nicknames rather than single species, and both hatch on cool, overcast fall days.
Can you tell them apart by color?
Not reliably. Baetis bodies range from olive to gray to brown and pale out when freshly emerged, while Paraleptophlebia runs reddish-brown to nearly black depending on the river. The two overlap enough in the brown-gray middle that color alone will cross them up, so lead with tails and size and use color only as a tiebreaker.
What size fly should I use for a fall Mahogany Dun?
Use size 14 to 18 in the East and Midwest and size 14 to 16 in the West. Reddish-brown Comparaduns, Sparkle Duns, and brown parachutes in those sizes cover the dun, and a size 16 Pheasant Tail works for the nymph. Keep the profile slim, since a bulky body puts fish off.
Do Blue-Winged Olive nymphs have two or three tails?
Usually three. The two-tail rule that identifies a Blue-Winged Olive applies only to the winged dun, which loses or reduces its middle tail at emergence. The nymph commonly shows three tails, so identify the insect as an adult rather than trying to key it as a nymph.
What flies should I carry when both hatch at once?
Carry both a Blue-Winged Olive and a Mahogany Dun in dun and emerger versions, then fish a double dry, a mahogany with a smaller olive trailing, to cover both. Match size and silhouette before color, and if fish refuse, drop a size and try a low-riding emerger or cripple in the film.
