Short answer: Tricos are tiny black-and-white mayflies in hook sizes 20 to 24 that hatch in the cool of the morning and fall spent an hour or two later. The spinner fall, not the hatch, is the event. It happens on a clock you can almost set your watch by, it happens in August and September when nothing else is going on, and it will humble you.
This is the technical hatch. Tricos are what anglers graduate to after they've learned PMDs and BWOs, and they demand more of every part of your game — smaller flies, finer tippet, better drifts, and enough patience to fish to a single rising trout for twenty minutes.
Get it right and you'll have hundreds of fish feeding steadily in front of you at eight in the morning while the rest of the river sleeps.
What a Trico actually is
Trico is short for Tricorythodes, a genus of tiny mayflies found across North America. They're distinctive enough that once you've seen one you won't mistake it.
- Size — hook sizes 20 to 24. These are genuinely small flies; a size 20 is on the large end.
- Colour — the males have black bodies, the females olive to pale cream with a black thorax. Both have pale, almost white wings, which is why a Trico swarm catches the light like smoke.
- Tails — three, and on the male spinner they're comically long, up to twice the body length.
- Behaviour — they emerge overnight and at first light, then swarm in enormous mating clouds above the river before falling spent, all within a few hours.
Fig. 1 — The Trico cycle. Stage 5, the spent fall, is the one that matters.
Key to the stages
Nymph
Tiny silt-dweller, sizes 20-24. Rarely fished — too small to be worth imitating.
Emerger
Overnight and at first light. Mostly over before you're on the water.
Dun
Brief. Female olive with black thorax, male black. Off the water fast.
Spinner
Swarms in clouds overhead. Male tails twice the body length. Watch the swarm drop.
Spent spinner
The whole event. Thousands lying flat in the film. Fish feed steadily for an hour or two.
Notice that stages 2 and 5 are both marked in olive rather than gold. With most mayflies the emerger is the stage that matters. With Tricos it's the spent spinner — because the emergence happens overnight and is largely finished by the time there's enough light to fish.
The clock
Tricos run on temperature, and the whole thing is compressed. Duns emerge before dawn. The swarm forms over the river shortly after sunrise. The spinners drop somewhere between 8 and 10am. By late morning it's over. In the height of summer the whole sequence shifts earlier — by September it can push toward midday. Arrive before you think you need to.
When it happens
Tricos are a late-summer and autumn insect, and they hatch daily for weeks on end — which is the great virtue of the hatch. Once it starts on your river, it happens every single morning until the first hard frost.
| Region | Jun | Jul | Aug | Sep | Oct |
|---|---|---|---|---|---|
| Montana — Missouri, Bighorn | On | Peak | Peak | On | |
| Utah — Green, Provo | On | Peak | Peak | On | |
| Colorado — South Platte, Frying Pan | On | Peak | Peak | On | |
| Pennsylvania — limestone spring creeks | On | Peak | Peak | On | |
| Midwest — spring creeks | On | Peak | Peak | On | |
| Arkansas — White River system | On | Peak | Peak | On |
Peak Active
Tricos favour tailwaters and spring creeks — stable temperature, silty margins for the nymphs, and the flat water that makes a spinner fall visible. Freestone rivers hold them in far smaller numbers.
How to fish it
Find the flat water. Spent spinners collect in slicks, back eddies, and the smooth tailouts below riffles. That's where fish set up. Fast water disperses the fall and the fish won't bother.
Fish to one fish. When thousands of naturals are on the water, blind casting is hopeless — your fly is one of ten thousand. Pick a single rising trout, learn its rhythm, and put the fly on its nose at the right moment. This is the hatch where the three-zone approach earns its keep.
Go to 6X or 7X. Not optional. A size 22 fly on 5X hinges badly and lands like a brick. See our 5X and full tippets range — and carry more than you think, because you'll be re-rigging in poor light with cold fingers.
Use a visible partner fly. You cannot see a size 22 spent-wing at thirty feet. Tie it 16 to 20 inches off the bend of something you can see — a small parachute with a white post — and watch that instead. Any rise near it, set.
Set slowly. The takes are soft and deliberate. A fast strike on 7X breaks fish off. Lift, don't snap.
Consider a spinner just under the film. Many spent Tricos sink. A drowned spinner fished a few inches down catches fish long after the visible activity ends, and almost nobody does it.
Common mistakes
Arriving too late. The most common Trico error by a distance. Turning up at 9:30 for a fall that started at 8:15 means watching the last twenty minutes.
Fishing too big. A size 18 looks small until you hold it against a natural. If you're getting refusals from steadily feeding fish, drop two sizes before you change anything else.
Blind casting the swarm. The cloud overhead is mating adults, not food. Fish don't eat them until they fall.
Giving up when the rises stop. Fish keep working drowned spinners well after the surface goes quiet.
Flies to carry
A spent-wing pattern in sizes 20, 22 and 24 is genuinely most of what you need, plus a small parachute to hang it from.
The Postfly First Cast Beginner Dry Flies Assortment and the Fly Rod Chronicles Dry Fly 12pk cover the surface spread, and the Adams Classic Dry Fly works as the visible partner on top. Browse all fly pattern assortments.
Because Tricos overlap with midge season on most tailwaters, the Basics Essential Bead Head Midge Nymph Assortment is worth having in the same box for when the fall finishes and fish drop back down.
Floatant on a size 22 is a real problem. Gel mats tiny wings. Use Loon Dust as your primary and keep Loon Aquel for the larger partner fly.
This is also the hatch that makes people take dry fly fishing seriously — my Dry Fly Tactics for Selective Trout eBook is written for exactly this problem.
Where to fish it
Tailwaters and spring creeks. In our state guides that means the Utah tailwaters, the Colorado South Platte, and the Montana Missouri and Bighorn. The White River in Arkansas is a superb and underrated Trico fishery.
Because the window is so tight, a guide who knows which slick the fall collects in is worth a great deal — see our Certified Guides & Lodges, or plan a trip through Book a Trip with Frontiers International Travel.
Fly Fishing Insider Podcast has a referral partnership with Frontiers International Travel. If you book after being introduced through us, we may receive compensation at no additional cost to you.
The bottom line
Tricos reward the angler who shows up early and fishes carefully to one fish at a time. It's the most technical dry fly fishing most of us will do, it happens every morning for weeks, and it comes at a time of year when the rest of the river has gone quiet. Set an alarm.
More hatches: the Pale Morning Dun, the Blue Winged Olive, the salmonfly, terrestrials, and the midge.
By Christian Bacasa
Host of the Fly Fishing Insider Podcast
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@flyfishinginsiderpodcast
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