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Tricorythodes: The History and Challenge of the Tiniest Mayfly

Tricorythodes: The History and Challenge of the Tiniest Mayfly

I have often wondered how such diminutive mayflies can cause so much excitement. When Tricos appear, fly fishers plan their days around them. These little mayflies get anglers up at the crack of dawn, make them skip breakfast to arrive on time for the hatch, and they’ve kept me on the water until after dark on more than a few occasions, squinting into the glare to see my tiny dry fly.

My first experience with Tricos was on Pennsylvania’s Susquehanna River. I was 11 or 12 and fishing with my father. My mom and dad ran a fishing tackle store in Berwick, Pennsylvania, just 10 minutes from the river. Dad was an avid angler, and we often fished for smallmouth bass for a few hours before the shop opened.

On one outing I remember asking my dad about the clouds of small bugs hovering over the water. He called them gnats, and I said that if they started biting us, we were dead. He laughed and told me they’d soon go away. Sure enough, the snowstorm of insects disappeared, and soon after I noticed the river was full of them. The minnows were feeding on the dead bugs all along the edges of the river. That was a long time ago, but the clouds of tiny insects left an impression on me that I never forgot. Today those misidentified “gnats” constitute my favorite hatch.

Tricorythodes stygiatus and Tricorythodes minutus are the proper names for these tiny mayflies, but most fly fishers simply call them Tricos. Stygiatus is important for fly fishers in the East, while minutus is prominent in rivers of the West. Trout really don’t care what we call them, they simply know these insects as a smörgåsbord of food. East or West, Tricos are well-documented, multi-brooded hatches that provide fantastic dry-fly fishing from early summer until late fall.


It’s been written that male duns hatch during the night and therefore are not important to fly fishers. I’ve heard many fly fishers repeat this misinformation but have to disagree. Over many years of fishing the Trico hatch on Montana’s Bighorn River, Cathy and I have had fantastic evening dry-fly fishing when the male Trico duns hatch just as the light leaves the river. If you can find an angle where there is still glare on the surface, you can pick out the sipping fish.


The male duns are a bit larger and darker than the females, which have lighter olive abdomens and a darker thoraxes. We have also found males hatching first thing in the morning before the females, and the trout most definitely get on them.

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Female Trico spinners have black thoraxes and pale olive abdomens. Male tricos have all black bodies. After mating, the males immediately fall to the water in a spent position, and the trout can become selective to them. As the females lay eggs and fall to the water, the trout may switch their preferences, so be prepared to change flies.

My first trout experience with Tricos was on the Little Lehigh near Allentown, Pennsylvania. I was still a teenager and drove my old Land Rover, which had a top speed that might have been 45 MPH . . . downhill. Eventually I got to the parking area, and as another angler walked by, I asked him about the fishing. He hurriedly answered that there was a great hatch going on, and advised me to hurry. Gearing up as fast as I could, I nearly ran to the river and found rising fish everywhere.

I took a close look on the surface of the water, and there they were . . . the insects that I saw years before on the Susquehanna. They had white spentwings and black bodies, and I guessed them to be size 22 or smaller.

I knew I was cooked, because there was nothing that small in my box. I did have some size 20 black fur ants, so I tied one on and started to fish. For the next half hour my ant got lots of looks, but no takes. When the hatch ended, I was hooked. I like a challenge, and this certainly was a good one. I knew I would be back, and I promised myself I would learn about the hatch, and return with flies that looked like the spent insects I saw on the water. That was the start of a lifelong love affair with the Trico hatch.


In about 1968 Vincent Marinaro, at the request of Outdoor Life editor Joe Brooks, was working on a story for an insect hatch that he witnessed on Pennsylvania’s Falling Springs. Marinaro called me and invited me to fish the hatch with him. The next day I drove to Mechanicsburg (yes, same old Land Rover) and picked him up before dawn. He told me that we needed to be on the stream early, because this was a morning hatch and would only last for a few hours. We arrived to see balls of insects still in the air, and a few rising fish.

There were no other cars in the parking lot, but as we geared up, two cars with fishermen showed up. When we got to the water I could plainly see the same insects that I had encountered on the Little Lehigh. I asked Marinaro what they were, and he said they were Caenis spinners. A few months later Outdoor Life published his story about the Caenis hatch on Falling Springs, and it was not long before another writer pointed out that Marinaro had misidentified the bug, and the correct name was Tricorythodes.

The following season Marinaro and I went back to fish the hatch again, and this time when we arrived there was not a parking spot to be had. Marinaro was mumbling something about too many people. Smiling, I reminded him that he wrote about this place in Outdoor Life, so what did he expect? He saw no humor in my perspective.


While the males hatch in the late evening, at night, and very early in the morning, female Tricos hatch in early morning or midmorning. Once their wings are dry they fly to shore, quickly molt, and return to join the swarms of mating male and female spinners. They can be high in the air, and often look like swirling dust or snow. They are sometimes difficult to see. I’ve watched many Western guides looking toward the sky, shielding the glare of the sun with their hands or hat to find the balls of spinners.

This airborne mating activity can be a long or short event, depending on the air temperature and the wind. Wind (that other four-letter word) can ruin the fishing opportunity simply because the insects are unable to connect, mate, and fall into the water. They simply get blown away. Smart fly fishers look for areas sheltered from the wind where the spinner fall may still take place.

The Flies

Imitations for this hatch seem to be endless, and over the years fly tiers have come up with some really crafty patterns, especially for the spinners. Tying a fly that looks just like the spinners isn’t difficult, and much of the effort has gone toward tying a fly you can see on the water. There have been times when Cathy and I have faced pods of trout rising in acres of spent spinners, and finding your fly in that mass of insects can be frustrating.

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To be successful during a Trico hatch you must be extremely accurate, and you’ll need to anticipate the rhythm of each rising trout so your fly appears in the feeding lane at exactly the right moment. These skills are crucial when you are targeting a single trout, especially a large one, within a pod of rising fish.

Marinaro tied a spent spinner with split tails, and a sparse black dubbed abdomen. Then he wound a white hackle at the thorax, followed by a sparse amount of black dubbing he would also figure-eight wrap around the white hackle, creating a stubby black thorax with spent white wings. This pattern still works today.

Pennsylvania fly tier Ed Shenk—who passed away in April of this year—first showed me a double-wing Trico spinner shortly after polypropylene yarn came out. Putting two sets of wings on one body was a dynamite idea, especially when there were a lot of spinners on the water.

In the early ’70s I worked with fly tiers Hank Leonard and Bill Lazardo to introduce Poly Fluff, a synthetic material that worked better than polypropylene. It eventually was marketed as Hi-Vis wing material from L&L Products, and today it still works perfectly for spinner wings.

My favorite pattern is tied with white Hi-Vis stacked at the thorax and spread out in a Compara-dun fashion. I split dun-colored Microfibetts for the tail, and dub a thin body up to the wing and then build a black dubbed thorax around the wing. I find this design stands out better than a low-profile spentwing, and I can see it.

I use the same idea for duns but dub a medium olive abdomen, a black dubbed thorax, and a light gray Hi-Vis wing. If visibility is a real issue, I mark the center of the upright wing with a red waterproof marker. [To see exactly how Barry Beck ties Trico dun and spinner patterns, visit the Fly Fisherman magazine YouTube channel and search for the author’s last name. The Editor.]

For the nymphs I use a Pheasant-tail Nymph tied with black-dyed pheasant tail fibers. It’s also easy to just take a black waterproof marker and color a regular Pheasant-tail Nymph. My last go-to pattern is a Sunken Spinner. Bighorn guides Matt McMeans and Jared Nelson got Cathy and me started on using a Sunken Spinner under a larger indicator dry when the fish are super selective. The Sunken Spinner has saved more than one day for us. Just use a heavier-wire hook and a few wraps of lead under the dubbed body.

The size of the fly can be very important, and Tricos in the East are generally smaller than in the West. Eastern patterns are generally tied on size 22 and 24 hooks, while Western imitations can be as large as 18 in some instances.

More Than Just Trout

While the Tricorythodes hatch brings large trout to the surface, they aren’t the only fish that get triggered by mats of floating insects. On more than one occasion we have found carp in the back channels on the Bighorn, and in the afterbay itself, going crazy eating Trico spinners.

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Last August, Cathy and I spent several mornings fishing Trico spinner falls on the afterbay. The carp (and some trout) were up feeding on the surface every morning, and there was almost no angler pressure. Carp can be quite challenging, but it was exciting to see all those heads up, and to land some of the gold ones on 5-weight rods. We found that when carp are feeding on Tricos, accuracy and gentle presentations are every bit as important as when we’re fishing to trout. Whether for carp or trout, Trico hatches are addictive—they excite, frustrate, reward, and bring you back for more. 

Fishing Techniques

Trout feeding on Tricos can be very selective. I’ve watched more then a few fly fishers walk away from a Trico hatch completely frustrated. With Tricos, there are often enormous numbers of just one type and size of insect, and if your imitation isn’t the right size, it sticks out like a sore thumb. If the spinners or duns are a size 22, your imitation needs to be a 22.

Tippet size can also be critical, especially on heavily fished rivers. A good example is Pennsylvania’s Little Lehigh. This river sees a ton of angler pressure during the Trico hatch, and 7X tippet is the norm here. I have friends who even insist on 8X. On Western rivers with a lot of weeds—such as the Missouri and the Bighorn—I never go lighter than 5X, and I often try to get away with 4X. I use longer 12-foot leaders on calm days and shorter 9-footers if there is a breeze. Try to limit your false casts, and above all, do not line the fish.

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With Tricos, there are often enormous numbers of just one type and size of insect. Your fly should be exactly the right size, but you also need to be able to find it on the water. Barry Beck’s Compara-dun style Trico stands out on the water so you can see it.

There are usually a lot of flies on the water, so the most accurate casters almost always catch more fish. This is also a hatch where you often need to time the feeding rhythm of the trout, and get your fly there at the precise moment the fish rises. Just getting in the feeding lane isn’t always enough.

You may find a pod of fish feeding on the spinners, and one or two trout in the group might be substantially bigger. It takes a good angler to pick out and target the larger trout.

One of the best anglers I know is René Harrop. Harrop needs no introduction. He is a world famous fly tier, author, and a trout hunter. I’ve watch Harrop pick out a single, super-selective Henry’s Fork rainbow from a pod of trout, and he makes it look easy. His tactics are simple—move slowly, be observant, and make the first cast count. If you’re going to target a specific fish like this, you need to be able to see it. Polarized sunglasses are a must in these situations. I use the Smith Guide’s Choice with copper-tinted lenses for sunny days, and Smith low-light lens for cloudy days.

Tricos are well distributed throughout the U.S., but Western tailwater fisheries like the Bighorn and Missouri rivers are famous for their Trico hatches. Idaho’s Silver Creek is also well known among Trico aficionados. The Madison where it enters Hebgen Lake can be a great choice, as is chasing “gulpers” in Hebgen Lake itself.

In the West, wind can be an obstacle at times, but it’s not by accident that the spinner fall normally occurs during calm mornings before afternoon winds pick up.

In the East we have had some great days fishing Trico hatches on the West Branch of the Delaware. Most Eastern limestone streams have fishable Trico hatches, and wind is rarely a problem. Eastern Trico hatches are like clockwork.

If push came to shove and I had to make a choice to fish just one river, it would be easy. Cathy and I have fished Tricos on the Bighorn for the last 30 years. In general, 2019 was a tough season for the ’Horn. High water and lower fish numbers made fishing difficult through the early season, but when we arrived in late August the flows were reduced. We saw fewer fish, but on the bright side, the fish we caught were large and in great shape from the high flows. I personally look at the Bighorn as a quality-over-quantity river, and that’s fine with me.

Tricos seem to prefer a silty bottom and plenty of weed growth. I have found them elsewhere, but Tricos seem to enjoy weedy environments in tailwaters and spring creeks. They are very dependable hatches and, weather permitting, can give you some exciting fishing from summer until late fall.

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Barry and Cathy Beck have been Fly Fisherman contributors for more than 30 years and are also travel hosts for Frontiers Travel.

*Barry and Cathy Beck have been Fly Fisherman contributors for more than 30 years and are also travel hosts for Frontiers Travel.

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