BWOs:
It must be spring
The Blue-winged Olive


What you need to know are the stages of a hatch and the proper imitation to use to match each stage. While the names of individual species and their placements in genus A or genus B are changeable and artificial, the life cycles mayflies go through are fixed in stone.

All Baetis-type BWO nymphs are streamlined, swimming-type mayflies. They live in moderate-to-fast current, usually on rocks or gravel, but also on aquatic plants or submerged woody debris. They swim with quick, darting bursts and they drift freely in the water's current, particularly just after dark in the warmer months. They can, therefore, be imitated with a specific nymphal imitation or with the old reliables, the Gold-ribbed Hare's Ear and the Pheasant Tail.

Emergence to the dun stage begins when the mature nymph swims to thewater's surface and penetrates the surface film, which is no small task. Its exoskeleton splits along the back and the dun pulls itself out of a nymphal shuck. Blood is pumped into the nymph's wings to fully expand them, and during this time the dun floats helplessly on the water. Throughout its emergence the Baetis is vulnerable to predation.

And with any major change in life, all sorts of things can go wrong. The nymph or emerging dun may become entrapped in the surface film or the tip of a wing or leg may not completely release from the nymphal shuck. If so, the hapless mayfly becomes a "cripple," struggling vainly to fly away from danger.

If all goes well, however, the dun flutters on the surface of the water and eventually flies away where it may be attacked by swallows, flychatchers, and dragonflies. Life, as they say, is hard.

The various stages of a mayfly's emergence--mature nymph at the surface, emerging dun, cripple, dun and, later, returning spinner--create a number of choices of imitations for the angler . The presence of rising trout gently sipping something from the water's surface with no duns visible on the water generally means that the trout are taking the suspended nymphs and emerging duns.

Use a two- or three-inch square piece of window screen to capture floating nymphs or emergers to get an idea of size and color of the emerging insects.

Is it Baetis pygmaeus or Acerpenna pygmae? Who cares? Is it a #16 or an #18 or a #20, and is it brown or olive? That's what matters and not whether Baetis is a polyphyletic mish-mash. Baetis is trout food, polyphyletic or not. The trout could care less.

Trout also could care less if it's a Baetis or an Ephemerella that is emerging. A mayfly is a mayfly; food is food. But here the old homonym business returns to haunt us, for several Ephemerella-type mayflies have olive bodies and blue wings. Consequently, several Ephemerella types of mayflies, by no means closely related to Baetis species, also have received the generic BWO label.

Remember Ephemerella? That rather stocky, crawling-type mayfly nymph? The guys that bring you those magnificent hatches of Dark Hendricksons, Light Hendricksons, sulfurs, and, out West, the PMDs and Flavs? It used to be so simple -- gills located on the top side of four or five abdominal segments.

Entomologists long have recognized that Ephemerella, while obviously a pretty closely related batch of species, was (with over 80 species recognized in 1953) getting pretty unwieldy and, with several clear species groups, was ripe for some good splitting into more homogeneous, manageable genera. So subgenera were elevated to genera and the old Ephemerella was restricted to species like subvaria, dorothea, inermis, and others.

The newly elevated genus Drunella, in the East and Midwest, has several species of what we call BWOs and they can be a factor in our Midwestern fishing. In the West, other species of Drunella are known as Green Drakes and Small Green Drakes.

While the descriptor Blue-winged Olive may imply that the adults of some species of Baetis and Drunella are very similar, the nymphs of these two genera are anything but similar. Both species groups inhabit moderately to swiftly flowing water as nymphs, but Baetis-type nymphs are streamlined
swimmers; Drunella-type nymphs look like body builders on steroids.

D. cornuta has been documented in Michigan, Wisconsin and Minnesota. The nymph of cornuta is fairly large (9-10 mm) and, with two projecting spines on the front of its head, fairly recognizable. In the new text Mayflies by Knopp and Cormier, the authors claim "significant hatches on Midwestern waters," with emergence from 9 a.m. to noon, but they don't relate personal experiences or cite others for the basis for these statements.

The authors of Hatches II, Caucci and Nastasi, on the other hand, relate personal fishing and collecting experiences with cornuta, and they extol this species as "on par with the more famous and celebrated mayfly hatches." For the Midwest, they found hatches from very late May through early July.

First, find out if cornuta is present on your favorite streams. Turn over some rocks, lift out large woody debris and look for the nymphs--their two horns should be visible with a small hand magnifier. Look for dark wing pads, signs that the nymph is nearing maturity. Finally, get to your favorite stream in the morning, not in the evening, and hope that everything clicks--that weather, insect hormones, and your skill at fly tying come together in one of those magical hours on a stream when trout are rising to a known mayfly that you have come prepared to imitate.

Cornuta hatches have been noted as lasting from an hour to an hour and a half in the morning, with the water temperature at 50 to 60 degrees. It's a good idea to record hatches on a particular stretch of stream. Such data can prove invaluable for better fishing in the coming years.

The spinner activity of both Baetis and the Drunella-type BWOs leaves something to be desired for the trout angler. Female spinners of Baetis often crawl down a twig, rock or any debris protruding from the water and lay their eggs underwater. I remember seeing a Baetis female clinging to a submerged tree limb I had lifted from the water to examine for nymphs and larvae. "What are you doing here?"

I incredulously wondered out loud. Several years and several similar experiences later, I read about the underwater egg-laying activity. The female Baetis spinner was actually crawling about on some submerged debris with a very definite purpose in mind--laying eggs.

Females of D. cornuta, on the other hand, are reported to drop their eggs onto the stream from above the water and are thus not as readily available to the trout as the more classic spent spinners.

By Dean Hansen

By Dean Hansen

It should be enough to know that the flies we call Blue-winged Olives are almost universally present and bring on some of the best and most consistent fishing of the year in the Midwest, but it isn't -- at least, for most of us.

The reports we hear from the streams and rivers our friends fish are at best inconclusive and usually of little help:

"I wasn't sure what was going on--looked like little olives, Blue Wings, probably. Or.... "

"They sure were little, and I never did catch any, but the fish were eating them like crazy."

What are the flies we call Blue-winged Olives or BWOs, these small mayflies (usually) that are so much a part of our angling lives in the Midwest? It should be simple enough: The body of the imitation is olive and the wing is blue (more accurately, a shade of gray). The confusion comes from oversimplification: Blue-winged Olives, or BWOs, imitate some (but not all) species from such diverse mayfly genera as Baetis, Ephemerella and Drunella (and others).

Sometimes there are advantages to generalizing. One can, for example, use some sort of BWO imitation and catch trout throughout the fishing season. That's a plus. The disadvantages, however, are glaring and numerous: one BWO emerges in the morning, another at dusk; one is a #12, another is a #22; one emerges in April, another in November. Such diversity can obviously throw a monkey spanner in one&Mac245;s carefully laid match-the-hatch plans.

One way to understand a complex situation is to examine its components separately and then try to see the whole again. Let's look at these diverse genera one at a time, starting with Baetis.

In the "old days," Baetis was described as being a rather small, swimming-type mayfly nymph. Baetis nymphs had single gills along the sides of the abdomen, minute hind wing pads and a middle tail that was shorter than the outer ones. Simple? Here's the catch: Entomologists long have recognized that Baetis was a bit of an artificial mish-mash of sometimes distantly related species. They recently rearranged things and created new names (Diphetor, Labiobaetis) and obliterated one old friend, Pseudocloeon.

There are a couple dozen species of Baetis (in the old sense) recorded in the central U. S. alone. Two collectors in Wisconsin found 14 species in that state in the 1970s and 35 years ago nine species were found in Michigan. While the bodies of several of these species (e.g., B. brunneicolor and intercalaris) are more brown than olive and the imitations are sometimes called Iron-blue Quills and Slate-winged Brown Quills, they traditionally have been lumped with their more definitely olive-bodied cousins the Blue-winged Olives.The first of these species to emerge is the old Baetis vagans, now more properly called B. tricaudatus. While they don't blanket the waters during the winter months, I have seen B. tricaudatus duns in Minnesota and Wisconsin in January and February. The sight of fish rising to take tricaudatus duns on a west-central Wisconsin river on New Year's Day remains a vivid memory for one angler I know.

Sometimes there are advantages to generalizing. One can, for example, use some sort of BWO imitation and catch trout throughout the fishing season. That's a plus. The disadvantages, however, are glaring and numerous: one BWO emerges in the morning, another at dusk; one is a #12, another is a #22; one emerges in April, another in November. Such diversity can obviously throw a monkey spanner in one&Mac245;s carefully laid match-the-hatch plans.

One way to understand a complex situation is to examine its components separately and then try to see the whole again. Let's look at these diverse genera one at a time, starting with Baetis.

In the "old days," Baetis was described as being a rather small, swimming-type mayfly nymph. Baetis nymphs had single gills along the sides of the abdomen, minute hind wing pads and a middle tail that was shorter than the outer ones. Simple? Here's the catch: Entomologists long have recognized that Baetis was a bit of an artificial mish-mash of sometimes distantly related species. They recently rearranged things and created new names (Diphetor, Labiobaetis) and obliterated one old friend, Pseudocloeon.

There are a couple dozen species of Baetis (in the old sense) recorded in the central U. S. alone. Two collectors in Wisconsin found 14 species in that state in the 1970s and 35 years ago nine species were found in Michigan. While the bodies of several of these species (e.g., B. brunneicolor and intercalaris) are more brown than olive and the imitations are sometimes called Iron-blue Quills and Slate-winged Brown Quills, they traditionally have been lumped with their more definitely olive-bodied cousins the Blue-winged Olives.The first of these species to emerge is the old Baetis vagans, now more properly called B. tricaudatus. While they don't blanket the waters during the winter months, I have seen B. tricaudatus duns in Minnesota and Wisconsin in January and February. The sight of fish rising to take tricaudatus duns on a west-central Wisconsin river on New Year's Day remains a vivid memory for one angler I know.

Such occasional hatches on warm winter days become more reliable in March and April when B. tricaudatus appears around midday. Note the word "around" &Mac220; predicting the hour of a Baetis hatch is like predicting next weekend's weather. Baetis hatches depend on water temperature, cloud cover, and whatever unknowns go on in their very tiny brains. They do, for instance, like adversity--a snow squall, drizzle, or heavy overcast may be more to a tricaudatus dun's liking than a bright afternoon with temperatures in the 80s.

At 8 millimeters (mm) long, the duns of these spring hatches of B. tricaudatus are best imitated by a BWO tied on a #16 hook. A second brood of B. tricaudatus emerges in June, while a third brood follows in late July, August, and throughout September. Like most multi-brooded insects, the adults of these warmer-weather mayflies are both smaller and more pale than their spring parents, and a BWO tricaudatus imitation in June or August would be tied on a #18 or #20 hook. These later broods also emerge much later in the day than do the March and April generations. A few weeks ago I discovered tricaudatus duns emerging at dusk on a nearby stream.

The early B. tricaudatus hatch is followed in the Midwest by hatches of two smaller species, B. intercalaris and flavistriga. In B. tricaudatus nymphs, the three tails are fairly uniformly colored. In B. intercalaris and flavistriga, however, each tail has a definite, broad dark band. Both of these species are smaller than tricaudatus. Duns measure 4 to 6 mm long--sizes #18 to #22.

B. intercalaris emerges in Wisconsin from June to early October, an emergence duration that I assume exists in Minnesota and Michigan. B. flavistriga has an even longer recorded emergence period--late May to November. Conceivably, then, anglers could find one, two, or even three species of Baetis hatchingon a summer's day. Does it make a difference which one is emerging? As an entomologist, I spend a lot of time behind a microscope, peering at gill shapes or mouth parts of nymphs and going through journal articles. I do this because I like it, not because I must do it to catch fish. Frankly, 300-plus-page books on mayflies seem a bit intimidating, even to an entomologist. What do you really need to know about BWOs to enjoy a couple hours on a stream? That Baetis pygmaeus is now Acerpenna pygmae?

You don't need to know these minutiae. What you can be assured of is that there probably will be a Baetis of some sort or another emerging, weather permitting, on any Midwest trout stream from January to November. They may not be emerging by the thousands, but they are emerging none the less.
Classic blue-winged olive artificial
The Drunella lata is another midwestern BWO. At 6-8 mm long, it is one or two hook sizes smaller (#16 or #18) than its cousin cornuta (9-10 mm long, #14 or #16). Emerging sometime from early July to mid-August in the Midwest, lata hatches follow the larger cornuta. But emergence time is also in the morning, generally between 7 and 10 a.m. In fact, lata may be on the water when the prolific Trico hatch is at its peak or winding down and an angler may have to observe carefully which species is eliciting rises.

Blue-winged Olive--yes, the name refers to several distinct yet similar species of mayflies. With their amazing spread of emergence times--January to November--one of the BWOs can provide fishing excitement throughout the entire season. The challenge is to anticipate and match correctly the distinct insect with its similar imitation. That's the challenge, and that challenge is part of what makes this sport so interesting.

Dean Hansen has a Ph.D. in aquatic entomology. His column appears regularly in Midwest Fly Fishing.
copyright Midwest Fly Fishing Magazine 1999