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> > Midwest Lata Emergence

This topic is about the Mayfly Species Drunella lata

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) Read more...

The Discussion

DarkDunMarch 4th, 2007, 9:40 am
Posts: 16The 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.
TroutnutMarch 4th, 2007, 10:42 am
Bellevue, WA

Posts: 2737
Thanks for sharing! As I wrote in the description, I think local advice like yours is even more important for this species than for most others.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
KonchuMarch 7th, 2007, 4:52 am
Site Editor

Posts: 505
How easy is it to recognize the hatches of the old species combined under Lata? Are these mainly local differences? I've never been able to sort them out very well at any one place.
TaxonMarch 7th, 2007, 11:21 am
Site Editor
Royse City, TX

Posts: 1350

The former Drunella species now considered to all be Drunella lata are: D. cornuta, D. cornutella, D. longicornis, and D. lata. As mature nymphs, and under magnification, their frontal horn differences are quite distinctive. As duns and spinners, they would be extremely difficult to reliably differentiate.
Best regards,
Roger Rohrbeck
KonchuMarch 7th, 2007, 8:08 pm
Site Editor

Posts: 505
My question was more about the behavior, emergence time, color, ANYTHING about the spinners and duns.
TaxonMarch 7th, 2007, 10:55 pm
Site Editor
Royse City, TX

Posts: 1350

Okay, Konchu, I’ll take a crack at answering your question (as clarified), but please understand, Drunella lata doesn’t reside in the Pacific NW, so I have no experience with them, and am strictly relying on the considerable book knowledge to which I’ve been exposed. Hopefully, Jason, Gonzo, Louis, and others can provide some knowledge based on first hand experience.

Anyway, with regard to the Drunella lata (subordinate species name equivalent) duns, Knopp/Cormier lists their dates of emergence as follows:

D. cornuta – 1st week of May through 2nd week of July
D. longicornis – 1st week of June through 2nd week of July
D. lata – last week of June to mid August
D. cornutella – 1st week of July through 1st week of August

The time of emergence starts around noon in early May, and as the season progresses, and morning temperatures rise, starts at 5 AM by mid July, at which point it reverses itself, and starts progressively later in the morning, 8 AM by early September.

In preparation for emergence, the mature nymphs are said to “excitedly” crawl to slower water, often loosing their footing and becoming vulnerable to feeding trout as they drift helplessly with the current. Trout are said to show consistent preference to emergers over duns, both actuals and imitations.

D. cornutella and D. longicornis duns are colored light olive green when emerging, and medium olive when aged, have gray to dark slate wings, and legs with ambur with an olive cast. D. lata duns are colored light olive when emerging, and medium olive when aged, have dark gray wings, and legs are ginger with an olive cast. D. cornutella duns are colored chartreuse when emerging, quickly turn to light olive, medium olive when aged, have bluish gray wings, and legs are olive.

With regard to spinners, mating swarms form an hour before dusk, and female spinners return to the water for egg laying, sometimes resulting in impressive spinner falls (for about a half hour before dark). The spinner falls are said to be of minimal interest to the trout, other than in mid July, when other mayflies are absent.
Best regards,
Roger Rohrbeck
MartinlfMarch 8th, 2007, 5:44 am
Palmyra PA

Posts: 3233
I only have experience with Cornuta locally here in PA. Roger's general observations fit my experiences with one exception. On sunny days I have seen Cornutas emerge from about 8:30 or 9 until around 10, though on drizzly days I've watched them emerge almost all day long. I have noted hatches from May 25 to June 3. I believe that hatch times vary by location, and time of year, though. Gonzo knows a lot more about Cornutas than I do, and can shed more light on other Lata emergences also.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
KonchuMarch 8th, 2007, 10:26 am
Site Editor

Posts: 505
Sometimes I've seen hatch DATES vary with altitude in eastern Tennessee and parts of North Carolina.
TroutnutMarch 8th, 2007, 2:59 pm
Bellevue, WA

Posts: 2737
Konchu that's a big can of worms you opened. :) I have no idea to what extent the behavioral variability within the species is actually associated with the former species boundaries. However, I do have LOTS of notes from anglers' books about them.

This will give everyone a little peek into how I wrote all the articles about all the hatches last summer. I read and highlighted relevant passages from 8 different angler-entomology books, then I typed ALL of the highlighted material (with a few modifications / notes-to-self) into a private section of this site's database (which I can't make public because it's copyrighted information). From those organized notes I combined all the information from those books and my own experiences to write the articles.

The point of that little aside is that I'm going to post all my notes for Drunella lata to answer Konchu's question. These are mostly just lists of direct quotes of copyrighted material, but since I'm blockquoting and attributing it I think I'm ok here. These are private notes so there are probably lots of typos, the result of my typing them into my database at 110 words per minute while not looking at the screen.

Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.

(refers to lata itself)

(Note: Caucci & Nastasi write a bit about how lata is a midwestern species only, and cornuta and others account for the eastern reports. So I can take these notes to refer to its midwestern behavior.)

Lata is a first-rate hatch int he midwest. General emergence in MN, WI, and MI is from the end of June to mid-August. Best hatching on northern rivers, like Michigan's Au Sable and Wisconsin's Wolf, is from mid-July through the first week in August.

Unlike the more feeble-legged Ephemerella nymphs, these flattened nymphs live in faster stretches, especially those with gravel bottoms. The emergence characteristics of these nymphs are typical of the entire genus. They wiggle enticingly to the surface, often making several attempts to break through the surface tension. In the Drunella subgenus, they have a tendency to hatch 6 to 12 inches below the surface and float to the top, buoyed by gases previously released by the nymph.

In early July, before the hatch is in full swing, emergence may occur in the afternoon, especially on cloudy days. During this period, there is little other activity during the day, so trout may feed on the sparse emergence of duns if they float by convenient lies.

In mid-July when the water gets warmer, good hatching normally takes place in the mornign whent he water is still cool. During this time, emergence can occur anytime between 5am and noon, but the best hatching and feeding activity will usaully take place between 7 and 10am.

Lata duns are sporadic emergers which usually hatch over a four-hour period.

The best time for these ideal fly-fishing sessions is generally the 3rd and 4th weeks of July.

Due to their frequent subsurface emergence, the hatched lata duns normally float complacently, drying their wet wings before takeoff. This characteristic, and the fact that htey are often funneled into pools and backwaters having smooth surfaces, makes them ideal for the Compara-dun application.

The spinner flights are both impressive and productive. About an hour before dusk, the male spinners will usually appear over the riffles. On Midwestern waters, these spinner-falls usually precipitate some of the best feeding activity of the year. The spinner-fall and subsequent feeding activity will last about an hour; usually between twilight and dark.

A riffle with good trout-holding capacity and one that also has good lighting conditions (preferably to the angler's rear) is of primary importance. If the trout seem to be slashing at the ovipositing imagoes we often start out with full-hackled spinner patterns and manipulate them to imitate the dipping spinners.

(refers to syn. cornuata)

Cornuta hatching takes place on northern and westenr PA streams between mid-May and early June, but the best activity occurs between May 25 and June 3. Catskill emergence lags by a week or two. We photographed Beaverkill specimens on June 15th while we camped at Wagon-Tracks pool.

Northern streams, like those in the Adirondacks and northern New England, continue hatching throughout June. General emergence in the Midwest is from late May to the beginning of July, while emergence in northern MN, WI, and the uP of MI usaully continues until the second week in July.

Although cornuta nymphs are found in slower waters, their flattened profile, which is typical of the Drunella subgenus, extends their habitat to runs and riffles and eddies of pocket-water.

Cornuta nymphs live in medium-to-fast riffles having gravel, pebble, or medium-sized rock bottoms. During emergence the may migrate to slower sections and eddies. As do most Ephemerella nymphs, they make several trips to the surface during emergence.

The nymphs, in most cases, seem to split their shucks several inches below the film or right in or beneath the film. The trout key in on the emerging nymphs or freshly emerged duns.

Unlike most Ephemerella species, the body of the dun changes its color drastically once it hits the atmosphere. As the duns hatch in or beneath the surface film, their body is a pale greenishyellow color (almost chartreuse). Within seconds after emergence (when the duns are floating on the sruface) the bodies turn a medium olive. By the time they fly to the nearest shrubbery, they are a dark dirty olive.

They emerge in the morning, at a time when the peak of other mayfly activity occurs during the evening hours and at dusk.

Finally, we believe that cornuta's obscurity was due to a simple case of mistaken identity. We are convinced that even the most serious experts have mistaken the activity of cornuta for that of Ephemerella attenuata.

Our experience with cornuta indicates that they ill hatch anytime between 8am and midday, that emergence usually lasts from 60 to 90 minutes and that the best hatching activity takes place when water temp is between 50 and 60F.

The cornuta duns hatch in riffles and medium-to-fast runs as well as in eddies and back-waters adjacent to these areas. AFter emergence, they ride the current momentarily before they become airborne.

The most effective imitation for this hatch is a sparsley tied deerhair compara-emerger with a greenish yellow body (almost a chartreuse) fished in the surface film.

The glassy-winged imagoes are still somewhat of a mystery to us although we have sighted them many times over the riffles about an hour before dusk. Thus far we have found their significance questionable, as most spinners seem to drop their eggs a safe distance above the stream.

Arbona, Fred Jr. 1989. Mayflies, the Angler, and the Trout. Nick Lyons Books.

(described for cornuta)

They emerge during midsummer mornings in eastern and midwestern streams.

The nymphs of E. cornuta and those of their closely related species inhabit medium- to fast-running portions of streams. They may also be found in the sluggish margins of deep pools, clinging to dthe debris and organit detritus that collect in them.

Daily emergence of E. cornuta and all species that follow is from nine in the morning until noon. As a general rule, spinner falls of these species appear to be of limited importance to anglers with the exception of E. lata, an extremely common midwestern species that emergers during July and August.

Their large size and early seasonal occurrence in June serve to separate them from E. cornutella, which is almost morphologically identical though noticeably smaller, and which follows it in emergence.

(described for longicornis)

Good populations of E. longicornis are found in the trout streams of North Carolina and Tennessee, and the species is capable of causing fair BWO hatches for the Smoky Mountain angler. Its seasonal emergence takes place from the early part of June until the end of July, with daily appearances between ten and one o'clock in the afternoon.

(described for cornutella)

SEasonal emergence takes place from late June through August, with daily occurrences during the morning from 9am to 11am. The best hatches are in moderate-flowing streams common throughout the Allegheny mountain ranges.

(actually described as lata)

Some rivers in the East, such as the Beaverkill and Saranac, support fair to good populations. Morning hatches can be expected to occur from late June until the end of August. Immense spinner swarms gather at dusk. The spinner falls are very impressive, and usually entice the frenzied feeding of trout.

They usually exhibit bright, crimson dashes on their legs, thorax, and abdomen; however, the species is known to have distinctive and different color phases.

Leonard, Justin W. and Fannie A. Leonard. 1962. Mayflies of Michigan Trout Streams. Cranbrook Institute of Science.

Habitat: Nymphs occur widely in gravel riffles in steams of all sizes

Emergence: June 25-August 13

The evening hatches stimulate trout feeding activity for several weeks after the majority of our mayflies have completed their adult life.

(about cornuta)

Habitat: Nymphs inhabit gravel- and stony-bottomed streams in moderate to fast current.

Emergence: July 1-8

Knopp, Malcolm and Robert Cormier. 1997. Mayflies: An Angler's Study of Trout Water Ephemeroptera . The Lyons Press.

Emergence: First week May through second week August

(described for lata and synonyms cornutella/longicornis/cornuta, as well as walkeri)

The eastern BWOs appear on stream from early May to mid-August, with eastern species, influenced by a more temperate climate ,producing hatches several weeks before the same species on midwestern waters.

Several days before the duns' emergenc,e the nymphs begin a migration from fast open waters to slower currents.

Emergence may take place randomly anywhere from the riverbed to the water's surface. Subsurface emergers are buoyed to the surface by tiny gas bubbles trapped between the folded wing tissue to arrive at the surface with their wings in a wet and bedraggled state, whereas surface emergers are more prepared for flight. Because of such random emergences, many duns, especially those emerging during spells of cool weather, drift considerable distances downstream befor etheir wings are sufficiently dry and rigid for flight.

Eastern BWOs usually emerge during the morning hours as the river is warming from its overnight lows.

When the nymph, emerger, and adult stages of the eastern BWOs are presented to the trout as a food source, a definite preference is directed toward the emerging dun.

The dun's body darkens rapidly on exposure to air and continues to darken during its downstream drift and while maturing on streamside vegetation. Such color transformations are often confusing to the uninformed fly fisher and may result in the sue of a darker eastern BWO imitation than what the trout are actually feeding on.

The actual importance of any eastern BWO hatch is determined by the absence of other mayflies on the water.

(discussed specifically)

Male D. lata spinners gather in swarms about an hour before dusk to court the females over riffly sections of the stream. AFter completing the mating act and resting, the females begin egg-laying just as dusk approaches and oftne produce impressive spinner falls that last for about thirty minutes. The spinner fall is especially important for the fly fisher during the last days of July, when other mayfly species are absent from the water.

Fauceglia, Ted. 2005. Mayflies . Stackpole Books.

(discusses the lata synonyms together with walkeri)

Common name: Eastern Blue-Winged Olives

With few exceptions, large populations of all five species inhabit the gravelly bottome,d fast-water sections of both rivers and streams.

On those rivers and streams where Eastern BWO thrive, hatching activity begins in May and continues into early September.

They get within inches of the surface before suddenly stopping short and then slowly drifting back down to the bottom. After several failed attempts, the nymphs finally reach the surface.

The trout take the helpless nymphs as they swim toward the friend and greedily feed on the nymphs as they are emerging, suspended below the surface film.

In typical mayfly fashion, Eastern BWO spinners return to the stream within twenty-four hours after emergence. Normally they reach the stream at dusk. But depending on conditions, the spinners may not hit the water until late evening, well after dark. Eastern BWO spinner falls are not hit-or-miss affairs. For the most part, they do not generate much of a response from the trout.

Swisher, Doug and Carl Richards. 2000 edition. Selective Trout. The Lyons Press.

Emergence: July 1 to August 10
Nymph habitat: Gravel riffles in streams of all sizes

Emergence can begin as early as 6:30am and continue until noon, with the peak period occurring from 7 to 9am. The duns appear sporadically over an extended period, rarely hatching in large numbers at one time.

The nymphal imitation is sometimes effective before any surface activity is noticed. As the hatch commences, an emerging pattern is productive when floating in the film dead-drift.

The spinners of Drunella lata return just before sundown and fall for almost an hour after dark, providing tremendous evening fishing.

The spinners are far more important than the duns for two reason. First, the duns hatch sporadically over a 4- or 5-hour period in the morning, resulting in a low-intensity emergence and slow feeding activity. Second, the duns must compete with other more numerous species.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
KonchuMarch 8th, 2007, 9:24 pm
Site Editor

Posts: 505
Troutnut: you ARE nuts! (We're not worthy, we're not worthy)
Where were you when I was trying to take notes in college?

TroutnutMarch 9th, 2007, 5:23 am
Bellevue, WA

Posts: 2737
Probably taking notes in college. :)
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
MartinlfMarch 9th, 2007, 6:30 am
Palmyra PA

Posts: 3233
Wow. This is amazing. And I didn't notice any typos in my quick scan through.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
TroutnutMarch 9th, 2007, 8:03 am
Bellevue, WA

Posts: 2737
Must have been a very quick scan!
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
KonchuMarch 9th, 2007, 9:37 am
Site Editor

Posts: 505
Finally, we believe that cornuta's obscurity was due to a simple case of mistaken identity. We are convinced that even the most serious experts have mistaken the activity of cornuta for that of Ephemerella attenuata.

This got my attention. What does everyone think about this speculation? Has anyone followed up on this? It seems possible to me...
TaxonMarch 9th, 2007, 11:28 am
Site Editor
Royse City, TX

Posts: 1350
Finally, we believe that cornuta's obscurity was due to a simple case of mistaken identity. We are convinced that even the most serious experts have mistaken the activity of cornuta for that of Ephemerella attenuata.

This got my attention. What does everyone think about this speculation? Has anyone followed up on this? It seems possible to me.


It certainly sounds plausible, but I believe the author of that passage was talking about mistaking the emerging duns of Drunella cornuta for those of Attenella attenuata, rather than mistaking identification of immatures of the respective species. Further, I believe that “the most serious experts” wasn’t necessary a reference to professional taxonomists specializing in mayfly classification. In any event, the full pertinent passage reads as follows:

Finally, we believe that cornuta’s obscurity was also due to a simple case of mistaken identity. We are convinced that even the most serious experts have mistaken the activity of cornuta for that of Ephemerella attemuata. Both are typical of the Blue-Winged Olive varieties that hatch in the morning hours during the season. To get to the base of the angler’s identification problem, let’s scrutinize these similarities. To the untrained eye, cornuta and attenuata look alike. Yet, up close, they are physically different and they have different emergence traits. The cornuta dun is larger, averaging about 9 mm, while attenuata averages around 7 mm (a difference of one hook size). Although the wings are identical in color, the olive body hues are each different.
These differences along with the contrasting subsurface emergence traits of attenuata and cornuta causes trout to exercise extreme selectivity to the respective naturals. This selectivity probably explains the lack of success most anglers report during alleged attenuata hatches. Our stream research indicates, surprisingly, that cornuta is much more prolific on Eastern streams than is the more publicized attenuata. Therefore, in most cases when anglers believe that trout are rising to attenuata duns, they are in reality probably taking the larger and darker cornuta mayflies.
Best regards,
Roger Rohrbeck
KonchuMarch 9th, 2007, 2:18 pm
Site Editor

Posts: 505
absolutely it wouldn't be the nymphs

lata is relatively more common, so their statement seems to hold true; i was curious about historic/present confusion of hatches of the two

regarding who's an expert, that's usually someone at least 50 miles away from home... ;)

PS, Thanks, Taxon, for the complete quote.
GONZOMarch 17th, 2007, 3:10 pm
Site Editor
"Bear Swamp," PA

Posts: 1681
Konchu and Roger-

I noticed this thread last week during a break from skiing, but I didn't have time to reply. The former D. cornuta is a personal favorite of mine, and I have more than twenty years of notes that reflect my pursuit of this hatch in the waters of PA and NY. Historically, the new D. lata synonyms make for a fascinating study and are the source of much confusion in the fly-fishing (and scientific) literature.

As for the supposed D. cornuta/A. attenuata confusion, you are right that this probably pertained only to adults. (The nymphs would be pretty hard to mistake for one another.) Early fly-fishing entomologies like Schwiebert's Matching the Hatch and Flick's Streamside Guide (2nd edition) refer to attenuata, but make no mention of Drunella (then Ephemerella) species like cornuta, longicornus, cornutella, or lata. Hence, a casual encounter with any of these morning-emerging olives might easily have been attributed to attenuata.

The original version of Swisher and Richard's Selective Trout (1971) describes the Midwestern activity of lata, but continues to perpetuate the myth of the significance of attenuata by describing it as an Eastern "superhatch" (with no mention of cornuta). Schwiebert's later volume Nymphs (1973) includes A. attenuata as well as D. cornuta, D. longicornus, and D. lata (at the time, all were under the genus Ephemerella). Curiously, Schwiebert attributes the "extremely important hatch in June" on the Brodheads to lata. This was mistaken identity at the time (it would have been cornuta under the former taxonomic divisions), but now is strangely correct.

The original Caucci and Nastasi Hatches (1975) was the first angling entomology to properly attribute and describe the cornuta activity in the East. While they discounted the presence of lata in the East, this morphotype is also found in many Eastern waters, although its prominence usually doesn't approach what is typically accorded to it in the Midwest. (And while the authors mention attenuata as a species of local importance, I'm reasonably sure that their photograph of an "attenuata" dun is really that of a Drunella dun.) Around that time, cornuta came to be recognized as the most important morphotype in the East, and lata was considered the most important type in the Midwest (though both of the former species were recorded by scientists in both regions).

Upon reviewing the citations that Jason has included above, I find the Caucci/Nastasi and the Arbona information to be the most accurate based on my experience and observations. Later books (like Knopp's or Fauceglia's) mostly parrot this information. Thomas Ames' recent Hatch Guide for New England Streams, which is otherwise excellent, describes the Eastern Drunella activity as primarily a midsummer hatch of small (#16-18) flies occurring around noon. This is a very limited assessment and doesn't account for the earlier hatch of larger and (usually) more important flies (the former cornuta). I can only assume that his experience is limited to the later lata hatch in cold tailwaters (hence, the atypical emergence timing).

As mature nymphs, and under magnification, their frontal horn differences are quite distinctive.

Roger, while these differences in horn (frontoclypeal projection) length were often used as a character to differentiate between the former species, Jacobus and McCafferty (Revisionary Contributions to Drunella [Ephemeroptera], J.N.Y.E.S. 112[2-3]: 127-147, 2004) say that this is a variable trait: "Based on specimens we have examined...the frontoclypeal projection characterizations attributed to D. lata, D. cornutella, D. cornuta, and D. longicornus appear arbitrary." They do say, however, that "Frontoclypeal projection development is usually consistent within samples taken from a single population...."

Other morphological characters that have been used as a means of differention are also revised as follows:

Epicranial sutures--consistent across morphotypes of former species.

Median ocellar spine--relatively well-developed, but variable in terms of bluntness or sharpness.

Third genital forceps segment (adult males)--generally about three times or more as long as wide, but variable within that range among populations and individuals.

In addition, Koss (1968) and Studemann and Landolt (1997) provided equivalent egg data for D. cornuta, D. cornutella, and D. lata. And equivalent, highly variable adults were reared from each larval morphotype.

The best attempt I can make at answering Konchu's original question is to relate the information from my own notes, which reflect the sequential activity of three of the synonyms as found in my favorite streams--the former D. cornuta, D. cornutella, and D. lata. Keep in mind that this information is specific/anecdotal to the Poconos and Catskills (though similar activity occurs in much of the East), and that the former species are distinguished here (for my purposes) primarily by size, color, and emergence period. Here's how the annual activity shakes out in a favorite Pocono watershed:

Olive Morning Dun (formerly D. cornuta).

This is the main event and the heaviest Drunella hatch of the season. The large (9-11mm) nymphs are somewhat concolorous, ranging from tannish olive to dark olive-brown. Some display brownish banding on the legs and others do not. The smaller male nymphs are somewhat darker, have large eyes, and the developing genital forceps are visible on mature specimens. Males seem to dominate the early part of the hatching period. On lower mainstem waters, the hatch usually commences in late May--earliest date on 5/21 and latest on 6/9. Because this same hatch extends well into the small cold headwaters, hatching there can be as late as early July (latest headwater date, 7/11). Under ideal conditions, the peak hatching occurs at a water temperature of around 58 degrees F. When the weather is either too cold or too warm to reach this range, the hatch occurs when the water temperature makes the greatest movement toward the ideal. This usually translates into a morning peak somewhere between 8:00-10:00 am. Hot spells concentrate hatching closer to daybreak, and unusually cold weather may cause sporadic hatching well into the afternoon. (The latest daily peak I have experienced was around 2:00 in the afternoon on a very cold and rainy day.) In a given stretch, the hatch lasts for 4-7 days, with females becoming more prominent during the latter part of the hatching period. In the Poconos, this is one of the most reliable, predictable, and productive mayfly hatches.

Little Olive Morning Dun (formerly D. cornutella).

This smaller hatch (in both size and concentration) follows the former cornuta, and in all respects is a miniature version of that activity. The nymphs are virtually identical in appearance except for their size (6-8mm). Due to warmer weather, the hatch usually occurs earlier in the morning and is typically finished in the lower mainstem water by sometime in mid-July. (The latest date I have found the nymphs in the mainstem is 7/20.) Because of their extreme similarity, I was not surprised to find that they are considered to be a diminished component of the earlier hatch. Interestingly, I have not found this hatch to follow immediately on the heels of the earlier hatch. Instead, there seems to be a lag of a few weeks before the smaller synonym gets started.

Dark Olive Morning Dun (formerly D. lata).

This is the most curious and puzzling of the three components of the new lata hatch. In size, it usually matches the former cornutella at maturity (6-8mm), but its coloration (in Pocono waters) is very different. Descriptions and photographs of the former lata larval morphotype often have contrasting bright or light colors on the pronotum, femurs, and sometimes bright spots or streaks near the rear of the abdominal tergites. The contrasting colors can be dark brown with orange or crimson accents, or very dark olive (almost black) with bright green accents. In the Pocono streams, the latter color combination predominates. Because hatching typically occurs in July heat, the daily timing is often close to daybreak. This is the smallest and least significant of these synonymous hatches. It usually peaks in mid- to late July, and the latest that I have found these nymphs in the mainstem is 7/26 (although they may extend into August in some years). While all of the Drunella duns are a fairly light green immediately upon emergence, this one is typically darker and ripens into a very dark olive/grey dun in a short time.

In summary, I find these Drunella olives to be very important in many (if not most) Eastern trout streams. They are found in both acid and alkaline waters (the acidic Tobyhanna Creek and the alkaline Penn's Creek are two contrasting examples with good populations). They thrive in large mainstem waters as well as in tiny tributaries. Only streams without fastwater stretches seem to lack populations. (I have never found a good population in flat, weedy limestoners, while limestone streams with extensive fastwater stretches--like Penn's Creek--usually have good populations.) I have fished them in streams as diverse as the Delaware River, the Beaverkill, the Esopus, the Brodheads, the Tobyhanna, the Lehigh River, the Yellow Breeches, Mountain Creek, Penn's Creek, Fishing Creek (both the one in Columbia County and the more famous one in Centre/Clinton County), and even in Logan Branch and upper Spring Creek. They are a favorite hatch because the trout love them, and they have been very rewarding to study. The main reason that they are not better known or understood is that the heaviest hatches occur on mornings when most fly-fishers are pursuing the famous evening hatches--Sulphurs, Yellow Quills, March Browns, Light Cahills, Slate Drakes, Green Drakes, and Pale Evening Duns. This period (late May through early June) constitutes "prime time" for mayfly hatches on many Eastern waters, but the morning hours are often neglected by dedicated hatch-matchers. This situation allows the former D. cornuta to hide in plain sight. Personally, I'm quite content with its relative obscurity.

KonchuMarch 30th, 2007, 6:58 pm
Site Editor

Posts: 505
Thanks for the treasure trove of information. I hope to spend more time with it in a few days, in order to sort some things out. I think your encyclopaedic dedication may match that of Troutnut.
-- konchu
GONZOMarch 31st, 2007, 8:20 am
Site Editor
"Bear Swamp," PA

Posts: 1681
Thank you, Konchu. But Jason is a true "phenom." It's scary to think what he'll become in ten or twenty years. The fish will have to be warned! :)
DarkDunMay 2nd, 2007, 5:36 pm
Posts: 16 I never realized my original post would draw so much attention, but this is a serious hatch in every part of the US. Lots of new info for me such as the Cornuta hatches of PA which I have fished several times and assumed them to be Lata. Also the Longicornus of NC, a new insight for me into our local NC hatches.

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