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Insect Factory: Why the South Holston has the best Sulphur hatch in America

Insect Factory: Why the South Holston has the best Sulphur hatch in America
Because of the underlying Knox dolomite formation, the South Holston is rich with aquatic plants and heavy hatches of Ephemerella invaria. Photo | Drew Bennett

In the early 1950s, Vincent Marinaro drove from his humble home in Mechanicsburg, Pennsylvania to Charlie Fox’s fishing cabin on the LeTort’s big bend. He had come to tempt the Letort’s brown trout with innovative flies that tested the willingness of selective trout to feed. Soon after, fly fishers in Pennsylvania’s Cumberland Valley began to popularize the practice of identifying spring creek insects to imitate and mimic their different life stages.

Early work published in Outdoor Life by these Letort regulars strengthened the notion that understanding insect life histories leads to a greater understanding of predator behavior and increased fishing success. One of these insects, the Sulphur mayfly, Ephemerella invaria (then considered to be Ephemerella rotunda), was part of a suite of insects responsible for the Golden Age of match-the-hatch fly fishing.

Early June emergences of this yellow mayfly drove fly-fishing icons like Ernest Schwiebert to visit and write about Letort Spring Run in Carlisle and other nearby limestone spring creeks. Although the Sulphur hatch was popularized by the great fly-fishing authors of that era, its predictability, spectacular spinner falls, and importance as a food source for trout placed its emergence into an event category all to itself.

That Was Then,
This Is Now

While at one time, limestone spring creeks provided the best habitat for Ephemerella invaria, tailwater rivers, the products of man-made reservoirs with deep coldwater hypolimnetic releases, have taken their place. One of these, the South Holston River in Bristol, Tennessee, is home to perhaps the largest population of Sulphur mayflies in North America and provides some of the best trout habitat in the Southeast, with approximately 11,000 brown trout and 2,000 rainbow trout per river mile.

A geologic bedrock layer known as the Knox dolomite formation causes both the conductivity and pH of the tailwater to skyrocket. Dolomite is a sedimentary carbonate rock that is typically composed of 70 percent magnesium and 30 percent calcium carbonate.

Combine dolomite bedrock, caves, springs, and creeks with a constant cold water source, and you have the ingredients for a large artificial spring creek filled with fertile beds of Fontinalis sp. (common river moss), Chara sp. (musk grass), and Elodea sp. (water weed). This perfect storm of aquatic plants, cold water, and dolomite-rich bedrock and sediments make the South Holston both a mind-boggling trout fishery and an insect factory.


The Sulphur mayfly is a member of the order Ephemeroptera (mayflies) and is found within the family Ephemerellidae (spiny crawler mayflies) and the genus Ephemerella (which also includes Sulphurs, Pale Morning Duns, and Hendricksons). There are a number of different species of Sulphurs in North America which include Ephemerella dorothea (Eastern Little Sulphur Dun), Ephemerella dorothea dorothea (Western Pale Evening Dun), and Ephemerella dorothea infrequens and Ephemerella excrucians (Western Pale Morning Duns).

Aquatic entomologists generally agree that the true Sulphur, Ephemerella invaria, is currently recognized as a “species complex” where multiple cryptic species likely exist within multiple populations. There are no Ephemerella dorothea—the Little Sulphurs of spring creek lore—on the South Holston. However, they are extremely abundant in the nearby Watauga Tailwater in Elizabethton, Tennessee.

Sulphur mayfly nymphs in the South Holston are larger than those in nearby granite-bottom freestone streams. Of course as they develop, you’ll find instar nymphs of all sizes from 4 mm to 12 mm (size 16 to 22 hooks).

Late instar nymphs are most important to fly fishers and are solid black with golden or yellow spots and stripes, gray gill plates on the dorsal (top) side of their abdomen, and are 12-15 mm long (size 14 to 16 hook). The thoraxes of these late instar nymphs often appear to be bulging and they eventually split open during emergence exposing a bright yellow thorax and light blue/gray wings.


One developmental stage some South Holston regulars fail to consider is the second-to-last instar nymph. During this stage, the nymphs are bright yellow with dark, grayish-blue wingpads. Although there is little size difference between this nymphal stage and the final nymphal instar stage, the bright yellow color sometimes acts as a significant color cue for feeding trout.

Male Sulphur duns have light blue-gray wings, yellow abdomens, amber thoraxes, and large orange bulging eyes. Photo | Paul Weamer

The color of South Holston Sulphur duns (subimago) varies between sexes. Female Sulphurs are typically a little smaller than the males (14 to 17 mm), have stout thoraxes, light bluish gray wings, yellow bodies, and black, beady eyes. The larger males (17 to 22 mm) have more slender thoraxes, grayish wings, amber bodies, and large orange, bulging eyes. A size 16 dry-fly hook is fine for imitating both females and males.

Sulphur spinners (imagos) are slightly larger than the duns. The only major morphological differences between the duns and spinners are the development of clear wings, longer forelegs in the males, and longer tails. Spinners are generally 1 to 3 mm longer than duns, and I frequently find that a size 14 hook is easier for both fly fishers and trout to see as daylight fades.

Male and female spinners display strong color differences. After molting, male spinners begin with olive-black and olive-gray bodies and lighten to light amber before mating. Female spinners begin with bright yellow bodies and darken to amber before mating.

Habitat Requirements

Sulphur nymphs inhabit the quick riffles and runs of the South Holston and also feed in thick beds of river moss. Pull a few strands of Fontinalis from the river and you’ll find hundreds of Sulphur nymphs, yet the same insects are barely present in Chara (musk grass) in slower water where scuds and sow bugs make their home.

Fontinalis is a aquatic moss composed of long dark green strands with a reddish tint and is extremely abundant in the riffles where Sulphurs live. This strong affinity for Fontinalis is likely linked to the plant’s ability thrive in flowing water and trap the type of detritus that Sulphurs find most palatable. It’s therefore important to focus your nymph fishing in riffle and run habitats where you see Fontinalis, as compared to slower flats with musk grass bottoms.

Sulphur nymphs generally partition their habitat needs as they develop. My field collections reveal that nymphs of all developmental stages live in Fontinalis grass, but the majority of nymphs you find in small to medium cobbles are larger and further along in their development. So, in open, rocky riffles, you should use larger Sulphur nymphs to get more strikes.

Year-Round Hatch

In the lower river—from Weaver Pike Bridge (TN 358) downstream—Sulphurs hatch from eggs by October, develop into late instar nymphs by early April, and emerge into adults from late April to late May.

This timing mimics the hatch schedule found on most freestone streams and spring creeks in the East, but the density of the hatch is likely much greater than what you may be used to.

In the upper river—from the powerhouse down to Hickory Tree Bridge (TN 44)—Sulphurs are multigenerational and show year-round development, with nymphs hatching into adults almost every day from January to December. Although the winter emergence is relatively weak, there are still fishable Sulphur emergences close to South Holston Dam on sunny, 60-degree days in December and January.

Fishing the Emergence

Like most mayflies, Sulphur nymphs are most available to trout just prior to emergence. To mimic their increased activity, and capture the attention of trout, in the mornings before the hatch begins, I use a two-fly rig in riffles that showed significant dry-fly activity during the previous afternoon. This rig is commonly a black soft hackle and a bright yellow Second-to-Last Instar Nymph. I tie the Sulphur Soft Hackle to the 5X tag end of a blood knot, 22 inches from bottom fly, with two #6 split-shot between the two flies.

Second-to-Last Sulphur Instar. Photo | Matthew Green. Hook: #16 Tiemco 9300. Thread: Yellow 6/0 UNI-Thread. Tail: Mallard flank. Abdomen: Yellow 6/0 UNI-Thread. Wingcase: Gold Mylar tinsel. Thorax: Yellow 6/0 UNI-Thread. Legs: Mallard flank feather fibers.

With this rig, I use a wet-fly technique passed on to me by Watauga River guide Theo Copeland. To begin, position yourself upstream and a little to the side of where you think the trout may be holding.

Positioning is everything. You must be in proper casting range of the fish so that the fly drifts into the feeding lane of the fish at the right time. Making the wet-fly cast from too great a distance brings the fly to the surface too quickly and too far in front of the holding trout. If you’re too close, the fly comes to the surface too slowly and too far behind the holding trout.

To orchestrate the wet-fly swing, cast 45 degrees across-stream and mend upstream immediately after the fly hits the water so that the fly has a chance to sink. As the flies drift downstream, the line gradually tightens and at the same time, you’ll raise the rod tip up and lift the fly toward the surface to induce a strike.

You can also squeeze the cork of the rod and gently twitch the flies and actively lift them to the surface. Although historically the “Leisenring lift” technique was coined for caddis emergences, I have found that it also adds enticing lifelike action during Sulphur emergences.

At about 11 A.M., South Holston Sulphur nymphs begin to migrate to the surface and shed their final nymphal exuvia to become duns. This is where the real magic happens. Unlike most mayflies that take only a few seconds to shed their exoskeleton, Sulphur nymphs can hug the surface film for five or ten seconds or more before they emerge.

On the South Holston, the hatch often takes three different forms. Small Sulphur emergences consist of emerging mayflies flying into nearby riparian vegetation with a few crippled and deceased individuals floating downstream. During a weaker hatch like this, dun patterns catch the most fish as trout are less likely to be choosy given the lack of adult insects on the surface.

However, most Sulphur emergences on the South Holston are more intense and last from three to eight hours during peak power generation. And for a few days a year, Sulphur mayflies can blanket the South Holston in a super emergence that lasts most of the day.

In the two heaviest hatch scenarios, fishing soft hackle imitations on a tight line is the most effective at catching fish. There are simply too many insects on the surface during this time for your dun pattern to garner any attention, and swimming emergers is a far better tactic. On lighter hatch days, fishing a dun pattern is a more enjoyable and productive tactic.

Male duns emerge first followed by females, and the fish often show selectivity based on gender.

Some years ago, I created a dry fly named the Downwing Dun that I use with regularity to imitate the Sulphur duns of both sexes by simply changing the color of the abdomen. It came from the lineage and influence of MarJan Fratnik’s F Fly, Terry Melvin’s Dawayne, and Cam Cantwell’s Puff Diddy. [For more detailed background information on these flies, see “Make Room for Puff Diddies,” by Jim Dean at The Editor.]

Downing Dun (Male). Photo | Drew Bennett. Hook: #16 Tiemco 100. Thread: UNI-Thread Yellow 8/0. Abdomen: Yellowish tan (male) or Pale Morning Dun (female) turkey biot. Wing: Two married Wapsi Tan 041 CDC wings of equal lengths and widths. Thorax: Hareline Dubbin TBQ388 (male) or 277 (female). Note: The fly shown is the male dun version. The tails of Sulphur duns are barely visible after emergence, therefore tails on the imitations are unnecessary.

Sulphur duns often make an initial first flight failure that results in the insect moving just a few feet upstream and then falling back to the surface. Another attempt ensues where the insect moves completely off the water into streamside vegetation. This hopping, skating movement is sometimes important, and the CDC of the Downwing Dun and a liberal coating of Frog’s Fanny allows you to skate and twitch the fly on surface without completely drowning it.

Super Spinner fall

I am not sure if anyone has coined the term “super spinner fall,” but the term fits the South Holston, where early June spinner falls are so impressive that in the morning, bridges are often littered with the shrunken bodies of female and male spinners from the evening before.

At 8 P.M. when an early summer fog begins to form on low water, and the sun sets behind the million dollar farms, female imagos bring South Holston anglers the best dry-fly fishing of the year. When this happens, you’re on the “Henry’s Fork of the South” or merely just in Sulphur heaven during an hourlong spinner fall.

The Sulphur spinner fall brings some of the largest trout in the river to the surface, and offers the best opportunity to catch 20-inch+ brown trout on dry flies.

In the early summer, duns emerge until 5 or 6 P.M. with about a two-hour break before the spinner fall occurs. In the late summer, duns often hatch right up until the spinner fall occurs. Sulphurs’ mating ritual begins in the river’s riparian zone and can progress to riffle egg-laying sites if the males are not finished copulating. The females make significant egg-laying flights to shallow riffles and oviposit or lay their eggs on the river’s surface.

Sulphur female spinners are large insects, with some measuring 17 mm or larger. They have clear wings, long tails, an erratic flight pattern (similar to that of a paper airplane falling), and display orange egg masses at the end of their abdomens.

South Holston brown trout average 10 to 16 inches, but there are also many 20-inch and larger trout that feed on Sulphurs. Photo | Drew Bennett

When fishing Sulphur spinners, it is a good idea to fish females and males differently because each gender ends up in a different stream habitat. Female Sulphurs lay eggs in the riffles and are quickly eaten by trout staged in the riffles or just below. Male Sulphurs abandon the females soon after mating and the trout find them mostly in the downstream flats.

Timing and accuracy are everything during the spinner fall, and you should isolate and target a single fish, and plan to drift your spinner imitation over a trout exactly when you think it might rise again. Sometimes this means watching a fish for a while to determine its feeding pace, then timing your cast to intercept it.

When the spinner fall is almost complete (indicated by successional decreases in spinner activity), I fish a spent spinner pattern that imitates shriveled male and female spinners that have darkened slightly and formed into a perfect J shape.

This Curved Spent Spinner is especially effective for the largest trout that hang back in the slower flats ­downstream of riffles, and feed on spent spinners that the smaller trout have missed. These big risers are perfect test dummies for your best selective trout spinner patterns, not to mention your best casts. South Holston spinner falls are something special to behold, and I believe that one day they will be legendary among all fly fishers.

Curved Spent Spinner. Photo | Drew Bennett. HOOK: #14 curved scud, Tiemco 2457. Thread: UNI-Thread orange 6/0. Tails: Sheet organza, stripped.Body: Wapsi Life Cycle Dubbing Sulphur Orange LCN129. Legs: Sheet organza, stripped.Wings: J:son Realistic Wing Material RWM M3. Hackle: Whiting Farms grizzly hackle, gauged to size 18.

This is the upper echelon of dry-fly fishing, and it does not disappoint for anyone who enjoys the challenge and the opportunity of match-the-hatch dry-fly fishing to super selective trout. When you see this hatch you won’t want to leave, and it’ll leave you tossing and turning in your bed and fretting at your vise until next year when you can again step into one of the greatest rivers of all time.

*Matthew Green is a graduate research assistant in the Department of Biology at Appalachian State University in Boone, North Carolina. His research focuses on how temperature influences aquatic insect life histories and development. More recently, he has been studying how the invasive diatom Didymosphenia geminata affects aquatic insect diversity, distribution, and abundance in tailwater rivers.

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