Alexander's Rhyacophilan Caddisfly - Rhyacophila alexanderi
State Rank Reason (see State Rank above)
Alexander's Rhyacophilan Caddisfly is currently ranked a "S2" Species of Concern in MT and at risk because of very limited and/or potentially declining population numbers, range and/or habitat, making it vulnerable to extirpation in the state. Limited sites with small populations, but also difficult to identify without adult specimens.
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).
Rangewide, this species is known only from the type locality in the Bitterroot Forest & Ravalli County, at 2 sites in Lake County, Montana (Yellow Bay Creek) (Newell and Potter 1973), and a new 2007 record in Specimen Creek basin of Yellowstone Park (Jeff Arnold personal communication 2009) as well as Manitoba (Denning 1950).
Observations in Montana Natural Heritage Program Database
Number of Observations:
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(Observations spanning multiple months or years are excluded from time charts)
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.
Threats or Limiting Factors
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.
- Literature Cited AboveLegend: View Online Publication
- Anderson, N.H. 1976. The distribution and biology of the Oregon Trichoptera. Oregon Agricultural Experiment Station Technical Bulletin 134:1-152.
- Denning, D.G. 1950. Records and descriptions of nearctic caddisflies. Part II. Journal of the Kansas Entomological Society 23:__.
- Giersch, J. J. 2002. Revision and phylogenetic analysis of the verrula and alberta species group of Rhyacophila pictet 1834 with description of a new species (Trichoptera: Rhyacophilidae). Master's of Science Thesis. Montana State University, Bozeman, MT. 206 pp.
- Merritt, R.W. and K.W. Cummins. 1996. An introduction to the aquatic insects of North America. 3rd Edition. Kendall/Hunt Publishing Company. Dubuque, Iowa. 862 pp.
- Newell, R.L. and D.S. Potter. 1973. Distribution of some Montana caddisflies. Proceedings of the Montana Academy of Sciences 33: 12-21.
- Wiggins, G.B. 1996. Larvae of the North American caddisfly genera (Trichoptera). University of Toronto Press, Toronto, Ontario. 2nd Edition. 457 pp.
- Additional ReferencesLegend: View Online Publication
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- Wold, J.L. 1974. Systematics of the genus Rhyacophila (Trichoptera: rhyacophilidae) in western North America with special reference to the immature stages. M.S. thesis. Oregon State Univ., Corvallis 229 pp.
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