Montana Field Guide

This species is the smaller cousin to the Giant Salmonfly, and is still a fairly large stonefly species, up to 2 inches long. In late-May to early July, adults are hatching after spending 2 years as nymphs on the bottom of the stream. The Least Salmonfly range in Montana occurs in most western drainages and in the mountain and foothill Missouri and Yellowstone Drainages downstream until the water temperatures become to warm. They can tolerate warmer water temperatures than other species in the Pteronarcys genus and overlap with the giant salmonflies in the upper reaches of the larger rivers like the Yellowstone, Clark Fork, Smith and Missouri.

In late-May to early July, adults are hatching after spending 2 years as nymphs on the bottom of the stream.

The diagnostic characters to seperate this large stonefly species from the others in the Pteronarcyidae family are smaller size and lack of orange coloration in adults and the 2 sets of gills on the underside of the abdomen in the nymphs, whereas the Pteronarcys nymphs have 3 sets of gills on the first abdomonal segments.

Pteronarcella nymphs are found in fast flowing streams and rivers below 8000 ft (Baumann et al. 1977). They prefer large cobbles and boulders as substrate, as well as logjams where leaf materials can accumulate.

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

Pteronarcella nymphs grow through many instars (8 - 14) during their 2 year life cycle depending on degree days (water temperature + air). They are found in fast flowing streams below 8000 ft (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. Males attract mates by drumming rocks or wood. After mating, females lay the eggs 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.