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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

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.

Lateral view of a Onocosmoecus (Limnephilidae) (Great Late-Summer Sedge) Caddisfly Larva from the Yakima River in Washington
This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Species Baetis brunneicolor (BWOs)

This is the largest common species of Baetis on our trout streams, and it can hatch in incredible numbers, drawing impressive rises of selective trout.

Anglers may have read in books about Baetis hiemalis, which is now a synonym of Baetis brunneicolor. It appears to have been a name for the fall-hatching brood of this species, which was reported to prefer slow water and weedy habitat instead of the gravelly riffles of the early summer brood.

Where & when

Time of year : June through mid-November; best in early summer and again in the fall

Baetis brunneicolor is most often praised for the action it creates in the Midwest, but it is locally abundant in parts of the East and maybe in the West as well.

In 28 records from GBIF, adults of this species have mostly been collected during July (29%), June (21%), May (18%), October (11%), and March (7%).

In 16 records from GBIF, this species has been collected at elevations ranging from 3 to 3500 ft, with an average (median) of 500 ft.

Species Range

Hatching behavior

Time of day : Sometimes all day long; best in late afternoon to evening

The duns drift a long distance on the water before taking flight, making them excellent dry-fly insects.

Nymph biology

Current speed: Any

Substrate: Gravel, vegetation

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Body length: 6 mm
Wing length: 6 mm

Abdominal tergites of male imago deep brown; genitalia of the Baetis intercalaris type, the tubercle on the inner margin of the first forceps joint well developed.

Turbinate eyes large, deep brown in color; the stalk rather short, and shaded with yellow. Head and thorax deep blackish brown. The lateral anterior margin of the mesonotum, the pleural suture and the lateral extensions of the mesosternum shaded with paler brown. Rear portion of the mesothorax with slight reddish brown markings. The median dorsal projection of the metathorax is cream-colored. Legs pale yellowish brown, the fore legs deeper in color, with smoky shading at the apex of the tibiae. Wings hyaline, venation pale. Strong granulations between the cross veins of the stigmatic area of the fore wing. Intercalaries of the first interspace much longer than those in the second space; all intercalaries well developed. Hind wing large and broad, the third vein well developed. Between the second and third veins are two marginal intercalaries; often another intercalary is present between veins 1 and 2.

Abdominal tergites 2-6 deep brown with faint ruddy tinges, and obsolescent pale submedian dashes anteriorly. Tergites 7-10 similar to the basal ones; sternites pale yellowish brown. Forceps of the intercalaris type, the inner tubercle of the first joint well developed.

Discussions of Baetis brunneicolor

Big Green River, Wisconsin, late September
1 replies
Posted by Admiralb on Sep 28, 2013
Last reply on Sep 30, 2013 by Entoman
I hosted two visiting delegates to the T.U. National Convention in Madison, WI. on September 25, 2013, taking them to the Big Green River in Grant County near Fennimore. There was a high overcast in the morning, and these mayflies [which I merely called "BWO's" and imitated with #16 parachute dries - thin olive bodies, gray wing posts and dun parachute hackle] - were active. Both of my guests raised browns fishing a gray nymph behind a #16 parachute dry BWO. As long as my flies catch fish, I don't need to be a detailed entomologist. Nevertheless, I do sincerely appreciate the detailed scientific info, because it helps me focus my flytying and fly selection. Thank you.

Start a Discussion of Baetis brunneicolor


Mayfly Species Baetis brunneicolor (BWOs)

Species Range
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