The blue whale, the largest animal on the planet, can grow to 130 tons feeding on tiny shrimplike krill less than 1/2 inch long. On many tailwaters and spring creeks, midges are the staple food item for trout of all sizes, and 20-inch trout grow fat feeding on midges smaller than a grain of rice.
In many waters, midges from the family Chironomidae (order Diptera) represent over half the biomass available to trout and provide them with an almost year-round food source. While caddis might provide blizzard hatches around Mother’s Day, and mayflies, flying ants, and other insects provide epic events at other times, the midge hatch is bread and butter for trout. This is especially true on tailwaters, where the water temperature remains cold and stable regardless of the season.
Midge Life Cycle
The midge life cycle consists of egg, larva, pupa, emerger, and adult. Emerger is not a scientifically recognized stage, but it is extremely important to trout anglers.
Midges begin their lives as tiny eggs laid on rocks or on the water’s surface. These eggs hatch into larvae within one week, and the larvae burrow into the silt.
Larvae. In rivers with decomposing organic matter, midge larvae live in vegetation or moss that is attached to rocks or logs. Some species construct silken tubes to stabilize themselves in the current. These larvae feed primarily on organic debris.
Some midge larvae in fertile lakes are as large as 1/2 inch long, but most in moving water are best matched with #18-24 hooks. This stage can last anywhere from two to six weeks, depending on the species of midge.
Because they don’t swim, midge larvae are available to trout mostly because of fluctuating water flows that knock helpless larvae into the current, or because of wading anglers disturbing the bottom. Midge larvae look like tiny worms with little or no recognizable taper.
The best midge larvae patterns are simple and slender. They vary in color from pale olive/cream to brilliant red. Red midge larvae—or bloodworms as they are commonly called—derive their color from storing hemoglobin in their bodies. This allows them to survive in oxygen-deficient environments or during low water periods. These midges often have two colors of red inside their bodies. The outer, more translucent covering is a lighter red, and the inner core is usually dark red or maroon. These bloodworms are best imitated with a Common Sense Chironomid.
Pupae. Midge pupae are short-lived and take only about a week to metamorphose from larvae. They pupate on the river bottom among organic debris, or inside their silken cases. Midge pupae range from #10-32, but the majority are #18-24.
Midge larvae are streamlined, but pupae are cigar-shaped and have noticeably bulging thoraxes containing the growing midge adult. Midge pupae run the gamut of colors from cream to black, but most are two-toned with segmented bodies that can be imitated with a faint, light-colored rib running up the length of the body.
Midge pupae thoraxes are about 11/2 times the diameter of the abdomens, and usually darker than the body. The best flies to imitate this stage of the midge are noticeably tapered, with prominent ribbing. Patterns like the Dr. Midge, Zebra Midge, or Black Beauty match midge pupae perfectly.
Emergers. In my experience guiding on the San Juan River in New Mexico, the most important midge stage is the emerger. Mature pupae are fatter, juicier morsels that squirm and twist their way to the top of the water where they remain stuck in the film, or just under the surface, while the adult midge escapes the pupal shuck. This stage is the most vulnerable to trout.
Anglers often see the riseforms of trout in slack water, and immediately assume that the fish are feeding on adults. However, if you observe closely, you may realize that the riseforms are not caused by a trout’s mouth breaking the surface, but rather by the dorsal fin or tail of a large porpoising rainbow or brown. This type of feeding indicates trout are taking midge pupae or midge emergers just below the surface. [See “Fooling Small-Minded Trout” on page 44 for more information on catching trout near the surface. The Editor.]
Patterns tied to imitate midges as they struggle to transform into adults usually include some type of flash, foam, or CDC material that resembles the emerging wings and legs of the adult midge as it crawls out of its shuck. I prefer simple flash patterns like the Jujubee Midge, Envy Midge, Fluoro Fibre Midge, or the Shucka Khan.
Adults. Fish keying on single midge adults are often the toughest trout to catch. These fish stay sheltered on the bottom most of the day, eating one out of every hundred midge larvae that hit them in the head, until the midge pupae begin to rise through the water column and hatch into adults. Then, these trout hover just under the surface, using their pectoral fins like an osprey riding an updraft.
These fish are ultraselective, and getting them to eat your fly takes the correct size and color of the adult midge. Size is easy enough to recognize if you bend close enough to the water to see the naturals floating down the river as they dry their wings.
Color is almost as important as size, but often harder to determine unless you look closely. When you pluck a natural off the water or off your waders, turn it over and pay close attention to the underside. Adult midges float with their bodies resting on the surface and their wings folded over their backs, so the body color is both discernible and important.
The best patterns to match this tough hatch are olive, tan, dun, or black #26 Hi-Vis Parachute Adams, Brook’s Sprout, or 8N10 Midge.
Midging Trout Behavior
On waters like the Green, San Juan, and Colorado rivers, it is vital to observe which stage of midge is most prevalent, or which stage the trout are keying on. On the San Juan, the fish are usually hunkered down on the bottom eating midge larvae, and the occasional drifting pupa, until about 10 A.M.
Keep your flies on the bottom and absolutely dead-drifting during this nonhatch period. If your flies aren’t within 4 to 6 inches of a trout’s mouth, they will probably go unnoticed.
As the water begins to warm in the late morning, the hatch progresses to the pupal stage. The fish become more active, and matching the hatch often means drifting two midge pupa patterns with a tiny #9 split-shot in the middle of the water column, or fishing a greased leader with no weight just under the surface film.
Use emergers when the trout are breaking the surface or feeding just under the surface. Don’t switch to a midge adult pattern until you are positive that the fish’s mouth is breaking the surface, the fish is in a specific feeding lane, and the fish is rising consistently every 10 to 20 seconds.
Jude Duran is a guide on the San Juan River and a professional fly tier. His web site is customflys.com.