Montana Field Guide

This species is not as famous as the Giant Salmonfly, but is still one of the largest species of all mayflies, stoneflies or caddisflies hatching on any cold water river, up to 2.5 inches long. In late-May thru early July, adults are hatching and providing trout fisherman with exciting dry-fly fishing on the surface or with large artificial nymphs. Adults are fairly weak fliers and are very conspicuous by their size and erratic flying over the river making them easy pickings for feeding trout and insectivorous birds. The American Salmonfly range in Montana has been reported in the Smith and Missouri River systems potentially being confused with the Giant Salmonfly.

From late-May thru early July, adults are hatching and flying. June is the prime month for emergence
(Gaufin et al., 1972).

Salmonflies are easy to identify mostly by their tremendous size; often measuring nearly three inches in total length. As adults, they have a bright orange or red band behind the head and the underside of abdomen with broad wings containing dark veins

This species is not known to migrate, but does preferentially fly upstream as adults.

Salmonfly nymphs live on the bottom, crawling around on cobbles and feeding on large organic materials (leaves) in the spaces between rocks for three to sometimes four years. They require well-oxygenated water, so they thrive in swift, bouldery, riffly stretches of the river; narrow canyon reaches are ideal habitat.

Pteronarcys nymphs generally are considered shredders as a functional feeding group, and feed on large particulate organic and leaf materials that have collected in debris dams or behind boulders or logjams.

Pteronarcys nymphs grow through many instars (12 - 20). The length of the life cycle is 3 to 4 years depending on elevation and degree days (water temperature + air)(Baumann et al. 1977). Mature nymphs migrate to shore in the spring and climb out of the water (mostly at night) before the final molt. Adults live for a few days to few weeks among shoreline vegetation or rocks and they feed little, if at all.

From late-May thru early July, adults are hatching on adjacent riparian vegetation. Males attract mates by drumming rocks or wood, and mate while on the wing. Females lay the eggs directly on or above the water. The eggs separate and sink to the bottom to begin the next generation. Multiple cohorts may exist in any given river allowing a salmonfly hatch to occur every year, but some cohorts are stronger than others.

The biggest threats to the salmonfly are attributable to the loss of habitat through siltation, sediment filling interstitial spaces in the cobbles or increasing water temperatures beyond the species thermal limits.