A flying ant fall is the one event in this section that no hatch chart on this site can predict, and it would be dishonest to pretend otherwise. None of the 92 river matrices carries a flying ant line. The Green River page lists a Black Ant in 16-18 under "Terrestrials" for August and September, and that is the closest the data comes. What anglers know is this: on some warm, humid afternoon in late summer, usually after rain, an ant colony somewhere upwind sends out its winged reproductives by the thousands, a share of them end up on the river, and for an hour or two every trout in the water eats ants and nothing else. If you are there with the right fly, it is the easiest fishing of the year. If you are there with a hopper, you will get refused by fish rising all around you.
What is actually happening
Ants are Hymenoptera, family Formicidae. The workers you see on the bank all summer never fly. Once a year, on a day the colony picks by temperature and humidity, it releases its alates: winged males and winged virgin queens, which fly up to mate in a swarm. Neighboring colonies of the same species tend to fly on the same afternoon, which is why the event is so concentrated. After mating the males die and the queens drop their wings, and a great many of both come down on the water. Flying ants are poor fliers and easily blown, so a breeze off a hillside or a meadow toward the river is what delivers them.
Species vary in size and color. The ones trout key on in the West run from a size 14 down to a size 20, and they are black, cinnamon-red, or a two-tone of the two. The body is the giveaway: two distinct bulbs with a pinched waist, which no mayfly or caddis has, and a pair of clear wings that lie flat over the abdomen once the ant is in the film.
Why trout go crazy for them
Nobody has a settled answer, and the honest thing is to say so. The usual explanations are that ants taste of formic acid and trout like it, that an ant on the water is helpless and never leaves, and that the sheer number of them during a fall trains fish to lock on. Whatever the reason, the behavior is consistent and well documented by anglers on every kind of trout water: fish that were sipping size 18 Baetis or ignoring size 8 hoppers switch completely to ants within minutes of a fall starting, and they take them with a soft, confident rise that is almost lazy. The fall is also one of the few times a large, wary trout in a spring creek will feed steadily in the open in bright afternoon sun.
When and where to hope for it
Late July through September, on warm humid afternoons, especially the day after rain or when a thunderstorm has just passed. That is the window, and it is as specific as anyone can honestly be. Ant falls happen on every kind of water, but they matter most on rivers where trout feed on the surface in slow water and can be selective: the flat meadow water of Silver Creek and the Henry's Fork, the Missouri and the Bighorn, the Middle Provo and the Green below Flaming Gorge, and the Yellowstone Park meadow streams like the Lamar and Gibbon. Those are not rivers with a documented ant fall on this site; they are rivers where the fish behave in the way that makes an ant fall matter.
The signs on the water: fish rising steadily with quiet, unhurried rises in water where nothing seems to be hatching; winged insects with a pinched waist stuck in the foam lines and the eddies; swallows and gulls working low over the water; and a warm, sticky, breezy afternoon. When you see all four, stop what you are doing and change flies.
Patterns
Ants are simple to tie and simple to buy. Carry these in 14 through 20, in black and in cinnamon:
- Flying Ant, a fur or foam ant with a pair of flat hackle-tip or synthetic wings. Black is the first choice.
- Parachute Ant, the same body with a white post, for seeing it on broken water or in poor light.
- Foam Ant in 14 to 16, which floats all afternoon and takes a small dropper.
- CDC Ant in 18 to 20 for the flattest spring-creek water, where the fly must sit in the film and not on it.
- A sunk ant, a fur ant with no wing, fished a few inches under the surface. More ants drown than float, and a drowned ant behind a dry catches the fish that are not showing.
The fly must ride low, in the film, not on top of it. A high-floating hackled ant gets refused. If your dry is being ignored during an obvious fall, trim the hackle flush on the bottom, or grease only the leader and let the fly sit in the surface.
Fishing the fall
Ant-eating trout are easy to fool and easy to spook. Use 5X or 6X, a leader of at least twelve feet, and a downstream or across-and-down presentation so the fly reaches the fish before the tippet does. Pick a single fish and put the fly two feet above its nose. The takes are soft; set with a lift. The fall may last twenty minutes or two hours, and when it ends it ends completely, and the fish go back to whatever they were doing before. Fish the drowned ant for another half hour after the last rise. Then go back to the hopper.
Be ready, not hopeful
You cannot plan a trip around an ant fall. What you can do is carry a dozen ants in two colors and three sizes in the same box as your hoppers every day from mid-July through September, on every river above, and know the signs well enough to recognize the first five minutes. That is the whole strategy. The hopper page covers the rest of the late-summer terrestrial afternoon, and the same rules on the same rivers apply, so check the state fish and game agency for the water you are on.