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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 Clostoeca disjuncta (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Drunella lata (Small Blue-Winged Olives)

When Selective Trout was first published in 1971, Swisher and Richards included Drunella lata (Small Blue-Winged Olive, Slate-Winged Olive) as a Midwestern "superhatch." Although it can also be found in many Eastern trout streams, it is probably more important to Midwestern anglers. Typically a morning emerger, this species often competes for the attention of trout with more abundant Tricorythodes and small baetids during parts of July and August. For this reason, the authors of Selective Trout considered the concentrated evening spinner falls to be more important than the somewhat sporadic morning emergence. From an angling standpoint, this situation is nearly the opposite of the earlier Drunella cornuta emergence in the East, where the morning emergence is usually the main event and spinner falls are often of little consequence.

Currently, Drunella lata shares its name with another mayfly, the former D. longicornis. That mayfly can be important in mountainous areas in the Southeast, but they are larger and the nymphs lack the distinctive pale markings mentioned in the Juvenile Characteristics section. (The information on this page does not describe D. longicornis)

Where & when

Time of year : End of June through August, depending on location

Preferred waters: Usually most abundant in larger streams and rivers

In 7 records from GBIF, adults of this species have been collected during July (57%), August (29%), and June (14%).

In 3 records from GBIF, this species has been collected at elevations of 981, 1909, and 2625 ft.

Species Range

Hatching behavior

Time of day : Usually mid-morning, but variable based on weather and water type

Habitat: Some may emerge in fast water areas, but they seem to prefer somewhat slower water.

Water temperature: Emergence often commences as temperature approaches 62 degrees.

Emergence can be scattered and sporadic, though more concentrated emergence tends to happen early in the morning during heatwaves.

Spinner behavior

Time of day: Twilight

Habitat: Riffles

Female spinners often drop their eggs above the water. Action can be fast when large concentrations of spinners fall at twilight, but the spinner fall often concludes in darkness.

Nymph biology

Current speed: Medium

Substrate: Gravel to cobble, often with some weed or algae

The coloration of the nymphs varies from nearly black or very dark olive to grayish or reddish brown. Most have distinctive contrasting pale areas on the pronotum, femora, and towards the tip of the abdomen. These pale areas can be bright green on blackish-olive nymphs and yellow or orange on brownish nymphs. The pale area near the tip of the abdomen sometimes takes the form of three small spots in a triangular arrangement.


To determine whether a specimen of Drunella belongs to Drunella lata, use the Key to Species of Drunella Nymphs.

Drunella lata Fly Fishing Tips

Standard "BWO" patterns in #16-18 can work well for fish that rise to emerging duns. Freshly emerged duns are light olive and darken rather quickly to a dark dull olive or olive brown. Spinners are a very dark olive, often nearly black.

Fishing an imitation of the nymphs can be productive before, during, or after the emergence. Smaller trout often focus on more numerous Tricorythodes or Baetidae mayflies, but some of the larger trout may prefer the lata nymphs.

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

Described in Needham et al (1935) as Ephemerella lata
Body length: 6-7 mm
Wing length: 6-7 mm

A species of the fuscata (now a synonym of Drunella walkeri) group; rather closely allied to E. cornutella (now a synonym of Drunella cornutella), from which it may be separated by the more inflated second joint of the forceps, which is likewise rather sharply constricted near the middle.

Head and thorax deep blackish brown. Pleura and lateral areas of the mesosternum shaded with dull olive brown. Fore femur and tibia rather dark brown, about equal in length; coxa rather olive brown; tarsus pale, whitish. Middle and hind legs paler, the femora pale olive brown, the tibiae and tarsi lighter; an indistinct reddish longitudinal streak dorsally on the femora. Hind femur about as long as tibia and tarsus combined. Wings hyaline; venation pale, the main veins of the costal margin tinged with yellow; a small brownish spot at the base of the fore wing. Abdomen somewhat paler blackish brown dorsally, almost unicolorous. Indications of a pale mid-dorsal line on the anterior tergites. Sternites paler, dull olive brown. Second joint of forceps strongly inflated at each end and sharply constricted near the middle; penes united almost to the apex, the apical notch shallow and rounded (see fig. 153). Tails dull whitish.


Described in Needham et al (1935) as Ephemerella lata

Nymph brownish black; prothorax and 8th tergite usually conspicuously white, but the amount of pale color seems to be variable. Frontal horns present but very short, not reaching to the margin of the frontal shelf. Anterior margin of fore femur with teeth or spines; posterior margin rather widely expended. Tibial ‘thumb’ stout, almost straight, extending about to the middle of the tarsus. No dorsal spines (although mentioned in the original description, the type material and all other specimens examined show no such spines). Tails light brown; basal joinings dark brown.

3 Streamside Pictures of Drunella lata Mayflies:

Discussions of Drunella lata

Midwest Lata Emergence
20 replies
Posted by DarkDun on Mar 4, 2007
Last reply on May 2, 2007 by Taxon
The D.Lata emerges in Michigan waters at 10 AM on the dot and stops at noon from about June 25 thru July 10. I have fished this hatch avidly for years and find it very punctual on moderately overcast days. Sunny days make it much shorter duration, about 30 minutes. A size 14 imitates it perfectly with dark dun wings, bright olive green body and med dun tails and legs at emergence. The body color does change to dark green after a while.
The D.Lata also is significant in PA Northern Streams in Mid May.
I have not encountered it in the South Appallacian streams as yet. I fish some smaller BWO (#16-20) in NC but none so large as D. Lata.

Start a Discussion of Drunella lata


Mayfly Species Drunella lata (Small Blue-Winged Olives)

Species Range
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