Trico is the common name used by fly fishermen to describe the genus Tricorythodes mayflies that occur on streams and rivers all over the USA. They are often mistaken for the Blue Wing Olives because they both resemble the small size and olive abdomen coloration as duns and at times can emerge during the same periods. The easiest way to tell them apart is by looking at the dun or spinner's tails. Tricos have 3 tails and BWOs have 2 tails. Also, Trico mayfly adults have no hind wings. There are three common Trico species: Tricorythodes stygiatus, Tricorythodes allectus, and Tricorythodes minutus, but there is no need to define these tiny mayflies to the species level.
For most fly fishermen when someone mentions Tricos we think of the white clouds of tiny mayfly spinners in the air and clusters of spent Trico spinners all tangled together and drifting along the surface of the stream. Times like these can create a feeding frenzy for the trout and it is common to see the trout gulping in mouth fulls of tiny black and white spent mayflies. During this time the trout are usually on the move as they casually swim back and forth and left and right just under the surface feeding on the tiny spent morsels.
Before the spinner-fall there is the emergence of the tiny Tricorythodes duns. I have read the male Trico duns hatch at night and molt into a spinner before dawn. While the female Tricorythodes emerge in the morning hours. The tiny Trico nymphs and emergers in the film are nearly impossible to see and often puzzle fishermen. The emergence of the female Trico duns is generally an early to mid-morning hatch depending on the water temperatures. The Tricorythodes female duns after hatching go off for a short while to molt before coming back over the water to mate. After mating the males fall spent on the water while the Trico females need to go and push out their tiny green eggs which turns their abdomens white in color. Finally, the female Tricos lay their eggs and fall spent on the river in great numbers.
Female Trico duns have an olive-colored body and blackish thorax. Male Trico duns are mainly black. The male Trico spinners are black, but the Trico female spinners change from olive to a creamy/white abdomen after pushing out their eggs. See the photo at the bottom of this page.
Depending on the stream you may have Tricos emerging daily from July through October. Out West, I have fished the Trico hatches through September. In July, the Trico hatch may start early, around 7:00 am. As the season progresses and water temperatures cool off the hatch will start later in the morning hours. The Trico mayflies range from size #20 to #28 depending on the river.
Note: From my calculations, the wings are about 20% longer than a Trico's entire body length. Don't be afraid to make the wing longer than the shank of the hook.
My preferred hook as of late is a TMC #2488 because it is 2x short and 3x strong with a straight eye. A size #20 through #26 pheasant tail nymph (PT nymph) works fine for imitating the nymphs. Carry one or two tiny short-winged emerger patterns to imitate the transition from a nymph to a dun. The dun imitation I prefer is a thorax pattern with a light gray poly wing and a grizzly hackle clipped underneath the hook shank, or just a CDC wing. This type of pattern floats well and is easier to see. For Trico spinners, white-colored split tails (or none at all) with a body of either black, olive, or white thread, a bulky black poly dubbed thorax, and white poly or organza wings tied spent. Add a touch of Flex cement at the base of the wings to stiffen the wings. Tiny Trico No-Hackle patterns are not easy to tie but can be extremely effective when the duns are emerging on slow, flat water. When the trout are on top gulping Tricos I have had great success with a size #18 Renegade pattern, it is much easier to see on the water.