Montana Field Guide

This cold-stenothermic caddisfly larvae is free-living without a case and moves actively around cobbles and boulders searching for food, usually preferring smaller insects, especially chironomids (midge larvae) and simulids (blackfly larvae) (Merritt and Cummins 1996). There is very little ecological information available on this species. According to Anderson (1976) species of this genus typically inhabit clear, cool creeks, and the known locations in Montana fit this general description, including being steep gradient and forested.

See Denning 1950 for detailed adult description. Mature larval length is 10-11 mm. Head widest medially, as long as wide. Frontoclypeus with dark shading posteriorly, separate muscle scars distinct; maxillary palpus stout. Mandibles with a single apical tooth and abdominal segments are without gills like other members of this species group (Giersch 2002).

There is very little ecological information available on this species. According to Anderson (1976) species of this genus typically inhabit clear, cool creeks, and the known locations in Montana fit this general description, including being steep gradient and forested. These caddisfly larvae are free-living and move actively searching for food (predatory) with no case, until just before pupation. The trophic relationship of Rhyachophila is usually predatory on other insects, especially chironomids (midge larvae) and simulids (blackfly larvae) (Merritt and Cummins 1996).

Most Rhyacophila species are predators feeding mostly on aquatic insects, especially midges and blackflies.

A cold water stenotherm preferring year round water temperatures lower than 12 degrees Celcius.

In Montana, adults were collected from mid-July-August.

R. alexanderi has been described as a rare species due to few reported collections, habitat specificity and it is never abundant when collected (Wiggins 1996). It has no USFWS status at the present time, although it is currently a USFS Species of Concern (SOC); ranked globally rare/uncommon (G2) by Natureserve (2015), and ranked S2 in Montana.

Specific threats to Montana populations of R. alexanderi would include mismanagement of forested riparian areas, including sediment and temperature increases associated with road building and timber harvests not following BMPs. In general, cold-stenothermic (cold-water specialists) invertebrate populations can be affected by slight changes to the thermal characteristics of their aquatic habitats, such as alteration of flow patterns, increased sunlight exposure, streambed substrate and water quality. Alteration and degradation of riparian and aquatic habitat is the primary concern for these populations.