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Montana Field Guide

Montana Field Guides

Alexander's Rhyacophilan Caddisfly - Rhyacophila alexanderi

Species of Concern

Global Rank: G2
State Rank: S2
* (see State Rank Reason below)

Agency Status
USFWS:
USFS:
BLM:


 

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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.
 
General Description
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.

Diagnostic Characteristics
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).

Species Range
Montana Range

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Range Comments
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).

Summary of Observations Submitted for Montana
Number of Observations: 4

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Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Habitat
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).

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

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

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

Management
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.

References
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Citation for data on this website:
Alexander's Rhyacophilan Caddisfly — Rhyacophila alexanderi.  Montana Field Guide.  .  Retrieved on , from