Blue-Winged Olives

"These little critters supplant the importance of many other well-known mayfly hatches."

-Fred Arbona in Mayflies, the Angler, and the Trout

Arbona did not overestimate these critters. Their great numbers and multiple broods each season make up for their size, which is rarely larger than size 16 and often smaller than size 20.

Hardly mentioned in angling literature prior to the middle of the last century, baetids have become increasingly important to anglers, rivaling any other family of mayflies in this regard. This is largely due to the extension of fishing seasons that now include the early and late periods when this family’s species usually dominate hatching activity. Another important reason is the tremendous improvement in tackle allowing more practical imitation of these little mayflies. The dramatic ecological changes in many of our watersheds and the subsequent impact this has had on the makeup of taxa populations is also a factor.

Taxonomically speaking, this is a most unruly family. The entomological community seems to be perpetually reclassifying its genera and species to the chagrin of many anglers. These changes are not capricious. The reason is older nomenclatures haven't provided the taxonomic flexibility required as more becomes known about the complexities of baetid relationships. Classification of this family’s genera and species is very much a work in progress. The changes have been so extensive that it is beyond the scope of this hatch page to track the taxonomic history effectively without interjecting even more confusion. If you are frustrated by the inability to find some of the old familiar names, you're not alone. Rest assured these popular hatches are listed here, just under the latest classifications. The old famous names are referenced in their hatch pages.

Common baetid hatches with a national distribution are the species Acentrella turbida, Baetis brunneicolor, and Baetis tricaudatus. In the West, Baetis bicaudatus and Diphetor hageni are also common. In the East and the Midwest, look for Baetis intercalaris and Plauditus dubius. The species Iswaeon anoka is important in both the West and Midwest.

Stillwater anglers are likely to run across Callibaetis ferrugineus ferrugineus in the East and Midwest. Western anglers will find Callibaetis californicus and Callibaetis ferrugineus hageni to be very important.

Streamside identification of these mayflies to specific and often even generic level has always been difficult. This is now even more so as taxonomic revisions have made hind wing conformation (or lack of hind wings) and other features less dependable as ways to tell them apart. Many of the lesser-known species probably produce excellent local hatches but have not caught enough attention to be properly recognized by anglers. The lesson is that we should not assume anything about the identity of many Baetidae hatches we come across; they may not even be in the Baetis genus, let alone familiar species.

This genus contains most of the important Blue-Winged Olive ephemerellids that make up some very important hatches across the country.

In the East, there are the morning hatches of Drunella cornuta (Large Blue-Winged Olive) followed as the season progresses by the smaller Drunella cornutella and Drunella lata species, both commonly called Little Blue-Winged Olives. These hatches usually begin in late spring and can continue well into midsummer on many Eastern and Midwestern streams.

In the West, the Drunella genus is even more prominent. Besides having abundant Blue-Winged Olive hatches represented by the common species Drunella coloradensis (Little Western Green Drake), Drunella spinifera (Western Slate-winged Olive Dun) and Drunella flavilinea (Western Blue-winged Olive or "Flav") the region also has the Drunella group of famous Western Green Drakes made up of the curious flat-faced nymph species Drunella doddsii and the three subspecies (Subspecies: Entomologists sometimes further divide a species into distinct groups called subspecies, which have two lower-case words on the end of their scientific name instead of one. The latter is the sub-species name. For example, Maccaffertium mexicanum mexicanum and Maccaffertium mexicanum integrum are two different subspecies of Maccaffertium mexicanum.) of Drunella grandis. Coloration ranges widely by region, going from dark brown to bright green. Most commonly they will be in the olive range. The wings too can range from dark slate to medium dun, often with yellow or olive pigment in the basal (Basal: Close to the base. For example, your armpit is at the basal part of your arm.) costal region (lower leading edge) of the forewings.

There have been many changes in taxonomic nomenclature affecting the Ephemerellidae over the years with few impacted more than the species of this genus. Anglers interested in "connecting the dots" with entomological information provided in fishing literature of the past need to take note of this.

Though not as well known as the Hendrickson and Sulphur hatches of the Ephemerella genus, Drunella cornuta (Olive Morning Dun or Large Blue-Winged Olive) is nonetheless an important Eastern hatch.

Lloyd Gonzales, in his book Fly-Fishing Pressured Water, notes that this attractive olive-green mayfly can provide good-to-excellent morning fishing in the faster sections of many streams. He also mentions that it can hatch in surprising numbers and usually faces less competition for the trout's attention than many of the spring or evening-emerging ephemerellids. Yet, cornuta and its sister species have largely escaped the limelight. The renowned twilight hatches of big Ephemera drakes and many other popular mayfly species command fly fisher's attention at this time of year. As the days lengthen, fishing all of the available mayfly hatches would require a pretty long day astream, so most anglers choose to focus on the late afternoon and evening activity.

Nature seems to have several recurrent color themes, and one of these is gray-winged/olive-bodied aquatic insects. Perhaps this explains why the name "Blue-Winged Olive" is often held up as the poster child for common name confusion. It has been freely applied to a multitude of mayflies in various families, genera, and species. Prior to having a well-established common name, this mayfly was referred to by Gonzales as the Olive Morning Dun. However, it has already been added to the long list of "BWOs" on many Eastern/Midwestern hatch charts.

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)

Although by no means a superhatch, this species can be important. Authors who discuss it lament the general lack of credit it receives for the fine hatches it produces on some streams.

This intriguing species has received a lot of attention in past angling books. Recent authors suspect that much of this credit was a case of mistaken identity, with Attenella attenuata receiving praise for the hatches of Drunella lata and Dannella simplex. Much of the credit was legitimate and accurate, but this species is no longer thought to be on par with its most popular cousins in Ephemerella and Drunella.

I have several specimens listed under this species, but I'm not positive the identification is correct.

Few anglers can claim to have fished to hatches of this little olive mayfly, even though the species has been mentioned in popular angling entomologies like Caucci and Nastasi's Hatches or Knopp and Cormier's Mayflies. On many of the larger Eastern freestones, their emergence happens after the waters have warmed and dedicated anglers have turned their attention to streams that remain cold: headwaters, spring creeks, or tailwaters. Because good populations are usually not found in tiny streams, it is on some the Eastern tailwaters, like the branches of the Delaware, that fly fishers take notice of this species.

Drunella cornutella looks like a Mini-Me version of Drunella cornuta. In streams where the two species cohabit, size is the only easy way to tell them apart. Although there is some slight overlap between the largest cornutella and the smallest cornuta, the average difference is usually pretty obvious. Both share very similar coloration and morphology (Morphology: The form and structure of an organism, or the study of the form and structure of organisms.) in all stages, even down to the little curved horns coming out of the frontal shelves of the nymph's heads.

Ernest Schwiebert had this to say about the hatches of this chunky Eastern Drunella (Blue-Winged Olive) species:

An imitation is rarely required, but notes show that when it is needed it is needed badly.

See the Drunella and Drunella cornuta hatch pages for additional information.

A cult following is something to which few insects can lay claim, but the tiny Tricorythodes mayflies certainly qualify. Their widespread, reliable, heavy hatches draw impressive rises of ultra-selective trout which demand the most of a technical dry-fly angler's skills.

It is surprising that such a great hatch took so long to come to the attention of fly fishermen. The Tricos were first introduced to anglers in a 1969 Outdoor Life article by Vincent Marinaro, who misidentified them as Caenis. By the early 1970s the identification had been corrected but Swisher and Richards still wrote in Selective Trout, "Few anglers are familiar with these extremely small but important mayflies." The next wave of publications boosted Tricorythodes to its current fame. I suspect their early dismissal was due in part to tackle limitations; anglers in the 1950s had no means to effectively tie and present size 22-28 flies.

For anglers, this single species is arguably the most important mayfly in North America. In terms of sheer numbers, breadth of distribution and hatch duration, it has a good argument.

Ephemerella excrucians or Pale Morning Dun usually follows its larger sibling Ephemerella dorothea infrequens with which it shares the same common name. What it lacks in size by comparison is made up for with it's duration, often lasting for months with intermittent peaks. This close relationship with infrequens has led many anglers to confuse Pale Morning Dun biology with that of the multivoltine (Multivoltine: Having more than one generation per year.) Baetidae species, having disparate broods that decrease in size as the season advances. Sharing the same common name has not helped to alleviate this misconception.

Until recently, Ephemerella excrucians was considered primarily an upper MidWestern species of some regional importance commonly called Little Red Quill among other names. Recent work by entomologists determined that it is actually the same species as the important Western Pale Morning Dun (prev.Ephemerella inermis), and the lake dwelling Sulphur Dun of the Yellowstone area, (prev.Ephemerella lacustris). Since all three are considered variations of the same species, they have been combined into excrucians, being the original name for the type species reported as far back as the Civil War. Speculation had simmered for some time that the stillwater loving Ephemerella lacustris was much more widespread, inhabiting more water types then previously thought and could account for many sulfurish Ephemerellids found in locations throughout the West. With the revisions, this discussion is now mute.

Ephemerella excrucians variability in appearance, habitat preferences, and wide geographical distribution are cause for angler confusion with the changes in classification. One could fish a hatch of creamy 18's on a Calfornia tailwater, an olivaceous 16 on an Idaho freestone, a sulfur 14 on a Yellowstone lake, and a little Red Quill on a small stream in Wisconsin, yet all are the same species.

The Flavs pick up about a week after the closely related but larger Western Green Drakes (Drunella grandis and Drunella doddsii) finish hatching on most Western waters.

Their hatches may be complemented by simultaneous hatches of two less prolific species, Drunella coloradensis and Drunella spinifera.