There are two hundred real Tricos per square foot on the water and one of yours. That’s the entire fly-selection problem in one sentence, and it explains why a hatch with only one meaningful insect stage produces more box-rummaging than any other event on this river.
This page is the box: which patterns, which sizes, which silhouette, and in what order I reach for them. Where to stand and when it happens are their own pages, because on this hatch they’re their own problems.
The stage that matters, and the three that mostly don’t
Tricorythodes runs the standard mayfly sequence — nymph, emerger, dun, spinner — and on the South Platte the fish care about roughly one of them.
Spinner. The spent adult, wings flat to the sides, lying flush in the film with no ability to move. This is what trout key on and it’s what almost every fly in this article imitates. The silhouette from below is a small dark body with two translucent horizontal wings and three long tails, and it does not sink, twitch, swim, or do anything else interesting. Your fly should be equally boring.
Dun. Real, but brief. Duns come off at first light and clear the surface quickly, so you’ll only be fishing them if you’re standing in the river before the sun is. Worth one or two flies in the box, not worth a strategy.
Emerger and cripple. The insurance policy. When fish are rising steadily and refusing a correct spinner, they are often eating stillborns and half-emerged duns hung in the film rather than clean spent adults.
Nymph. Tiny, dark, and genuinely eaten in the hour before the fall. Not the reason you set the alarm.
Size: smaller than you think, and smaller than most lists say
Here’s the number that reframes everything: the naturals are equivalent to a size 24 to 28. They get fished on #20–24 hooks because that’s what you can reasonably tie, thread and hook a fish on, which means even a correctly-sized Trico fly is usually a touch big.
So when you see #18–22 recommended for this hatch — and you will, it’s on plenty of lists — treat it as the top of a range rather than the middle. The naturals I’ve pinched off the surface at Deckers and on the Dream Stream are consistently smaller than a #20.
My working rule: start at #22. Drop to #24 when the water is clear and flat, which on a Trico morning it usually is. Size up to #20 only when the light is genuinely poor, the water carries any color, or you have lost visual track of the fly badly enough that you’re fishing blind. #26 exists for low, glassy water and should be a last resort, not an opening move — mostly because threading 7X through a #26 eye at seven in the morning is an experience I’d describe as character-building.
When you get refusals, drop a size before you change the pattern. That order matters. Anglers change flies because changing flies feels like doing something, and size is the variable more likely to be wrong.
Male or female? The call that costs you refusals
The two sexes look different enough on the water that the fish notice, and this is the detail most Trico writing skips.
Males are all dark — black body, black thorax, clear spent wings. They fall first, and a black-bodied spinner is the workhorse silhouette that covers the majority of the fall.
Females carry a pale abdomen — olive-cream to whitish — under a dark thorax, with the same clear spent wings. They fall after the males, once they’ve dropped their eggs, and late in the window the film can be substantially female.
The practical version: fish the black male body first, and if the fish are up, feeding, and refusing it in the back half of the window, switch to a pale-abdomen female. That single change has turned around more Trico mornings for me than any pattern swap. It costs you two minutes and it’s the first thing to try when a fly that was working stops working without anything else having changed.
Carry both. They are the same fly with two bodies and there’s no excuse for owning only one of them.
The six patterns, in the order I reach for them
I’m not giving you a list of ten. This is what performs on pressured South Platte fish, ranked by when it comes out of the box.
1. CDC Trico Spinner / CDC Biot Spinner (#20–24) — the first fly on. CDC fibers lie flat and translucent in the film in a way poly does not quite manage, and the silhouette from underneath is the cleanest of anything I’ve fished. Wing orientation is the whole point: flat, to both sides, not upright and not parachute-style. Black dubbed body, fine white or light-dun CDC fanned horizontally. This is the pattern for glassy water and difficult fish, and it’s what I default to unless I have a reason not to.
2. Poly-Wing Trico Spinner (#20–24) — the baseline. Black abdomen, white poly or Z-lon wings tied spent. Less delicate than CDC, floats longer between dressings, and catches fish whenever the drift is right. If the fall is heavy and the fish are less than surgical, this is honestly all you need.
3. Parachute Trico (#20–24) — the visibility compromise, and the most-used fly in my box regardless of what the purist half of me thinks. The white post buys you a fly you can actually see; the cost is a slightly higher profile than a true spent wing. On water with a little texture, in flat light, or at any distance past about thirty feet, that trade is worth making.
4. Stalcup’s CDC Black Trico Comparadun (#22–24) — a bit more visible to you than a flush spinner while staying low in profile. It’s Pat Dorsey’s go-to for Cheesman during the fall, which is a decent endorsement for a fly that splits the difference.
5. CDC Trico Emerger / Film Critic (#20–24) — the refusal fly. When trout are rising on a metronome and eating nothing you own, they are usually on cripples and stillborns stuck in the shuck rather than clean spent adults. These fish in the film rather than on it. This is a confidence pattern for the specific moment when everything looks right and nothing is working, which on this hatch happens often enough to plan for.
6. Triple Trico (#20–22) — three flies clustered on one hook, imitating clumped naturals. It earns its spot on visibility during dense falls when fish are less discriminating about individual accuracy. Not my first choice, but there are mornings when the film is so thick that a single #24 is simply invisible to both of you.
Also worth a slot: a black RS2 (#22–24). Not a spinner imitation at all, but the transition fly — during emergence, before the fall proper, an RS2 fished just in the film picks up fish that are rising without being locked on spent wings yet.
My take
The CDC spinner is the best fly and the parachute is the one that catches me the most fish, and I’ve stopped pretending that’s a contradiction.
I’ve had mornings with the fall going off, fish eating every four seconds, and a CDC spinner somewhere out there in a film covered in identical naturals that I genuinely could not pick out. A fly I can’t see is a fly I can’t fish — I don’t know when to set, I don’t know if it’s dragging, and I’m essentially guessing on a hatch that punishes guessing. Switch to a parachute, take the small hit on profile, start catching.
Imitation you can’t monitor isn’t imitation, it’s hope. Buy or tie both and don’t let pride pick the fly. On this hatch, your eyes are the weakest piece of gear you own, and they don’t improve with age — mine certainly haven’t.
Quick reference
| Pattern | Hook | Reach for it when |
|---|---|---|
| CDC Biot Trico Spinner (black) | #20–24 | Opening fly. Flat, clear water, selective fish |
| CDC Trico Spinner (pale abdomen) | #20–24 | Late in the window, or refusals on the black body |
| Poly-Wing Trico Spinner | #20–24 | Heavy fall, standard conditions, longer float |
| Parachute Trico | #20–24 | You’ve lost the fly. Distance, texture, flat light |
| Stalcup’s CDC Black Comparadun | #22–24 | Selective fish in slow water, some visibility needed |
| CDC Emerger / Film Critic | #20–24 | Steady risers refusing a correct spinner |
| Triple Trico | #20–22 | Dense fall, less selective fish, nothing else visible |
| RS2 (black) | #22–24 | Pre-fall emergence, rises with a bulge under them |
What to actually carry
Quantity beats variety. A dozen of your two confidence patterns in two sizes will out-fish two of each of eight patterns, every time. You will lose flies on this hatch — the tippet is 0.004 inches and the hooks are tiny — and running out of the one thing that’s working at 8:15 is a bad way to spend the rest of a fall.
Segregate the box. Trico flies at #22–24 disappear into a general dry-fly box and cannot be found at seven in the morning with cold hands. A small dedicated compartment, males on one side, females on the other, parachutes together. This sounds fussy right up until the first time you fumble the box open over moving water.
The knot is a real variable at this size. The eye on a #22–24 hook wants a knot that passes cleanly and doesn’t crowd the eye — an improved clinch or a Davy works. Check it after every fish, because 7X does not warn you before it gives up. If knots are the weak point in your game, the three knots I actually use is the place to start.
Dress the CDC properly, or not at all. CDC and standard floatant paste do not get along — the paste mats the fibers and kills exactly the property you bought the fly for. Use a dry powder or a CDC-specific treatment, and false-cast a soaked fly dry rather than reaching for the gel.
Buy or tie, but have them before August. Trico spinners at #24 are not a fly shop staple everywhere and they’re a fiddly tie. The Fly Fishing Place stocks Trico patterns through #24, and our link gets you 15% off. Ordering the week the hatch starts is how you end up fishing a #18 Griffith’s Gnat and telling yourself it’s close enough. It is not close enough.
When the right fly still gets refused
At some point you will be fishing the correct pattern in the correct size, on the correct sex, to a fish that is eating everything except yours. Change one more thing and then stop changing flies.
The answer is almost always the drift, and no fly in this article fixes a drift. A trout inspecting hundreds of identical insects in one lane is rejecting yours for how it moves, not for how it looks, and that rejection is invisible from where you’re standing. It’s a presentation problem and it’s a separate skill from anything in your box.
Frequently Asked Questions
Q: What size fly for Trico spinner falls on the South Platte? Start at #22 and drop to #24 in clear, flat water. The naturals are equivalent to a #24–28, so even a correctly-sized artificial runs slightly large — which is why the #18–22 you’ll see recommended elsewhere is the top of the range rather than the middle. Size up to #20 only for poor light or colored water, and treat #26 as a low-water exception.
Q: What’s the best Trico spinner pattern for South Platte tailwaters? A CDC Biot Trico Spinner in #22–24 on flat, clear water — the CDC wings lie flush the way real spent wings do, and the silhouette from below is the cleanest available. A standard poly-wing spinner and Stalcup’s CDC Black Comparadun are both reliable alternatives. Carry a parachute version too, because a fly you can’t track is a fly you can’t fish.
Q: Should I fish a black Trico or a pale-bodied one? Black first. The all-dark male silhouette covers most of the fall and is the workhorse dressing. Switch to a pale olive-cream abdomen with a dark thorax — the female — when fish start refusing the black body late in the window, which is roughly when the spent females start hitting the water in numbers.
Q: Why won’t trout eat my Trico even though it’s the right size? Usually because it isn’t drifting like the naturals. On flat water with hundreds of real insects in the lane, micro-drag is instant rejection and it’s invisible from thirty feet. Before you change flies a fourth time, try dropping one hook size, then switching sexes, then fixing the drift — in that order.
Q: Do I need to fish the dun? Rarely. Trico duns come off at first light and leave the surface quickly, so unless you’re standing in the river before sunrise, the fish will already be on spinners. Carry a couple of CDC duns in #22–24 for the mornings you beat the sun to the water.
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Next: where to stand, and the South Platte hatch calendar for where the Trico window sits in the year.