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Montana Field Guide

Montana Field Guides

A Rhyacophilan Caddisfly - Rhyacophila gemona

Species of Concern
Native Species

Global Rank: G2G3
State Rank: S2
(see State Rank Reason below)

Agency Status


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State Rank Reason (see State Rank above)
This 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, and the species is difficult to identify without adult specimens.
  • Details on Status Ranking and Review
    A Rhyacophilan Caddisfly (Rhyacophila gemona) Conservation Status Review
    Review Date = 09/18/2008
    View State Conservation Rank Criteria
    Population Size

    ScoreU - Unknown


    Range Extent

    ScoreE - 5,000-20,000 km squared (about 2,000-8,000 square miles)

    Comment5000-20,000km (2,000-8,000square miles)

    Area of Occupancy

    Comment40-200 km (25-125 miles) linear river

    Length of Occupancy

    ScoreLC - 40-200 km (about 25-125 miles)

    Long-term Trend

    ScoreE - Relatively Stable (±25% change)

    CommentSiltation and stream temperature increases with loss of riparian shading and lower snowpack probably contributed to some decline

    Short-term Trend

    ScoreE - Stable. Population, range, area occupied, and/or number or condition of occurrences unchanged or remaining within ±10% fluctuation


    ScoreG - Slightly threatened. Threats, while recognizable, are of low severity, or affecting only a small portion of the population or area.

    CommentClimate Change, increasing stream temperatures and lower snowpack could seriously impact the habitat that this speces exists in

    SeverityLow - Low but nontrivial reduction of species population or reversible degradation or reduction of habitat in area affected, with recovery expected in 10-50 years.

    ScopeLow - 5-20% of total population or area affected

    ImmediacyLow - Threat is likely to be operational within 5-20 years.

    CommentThreat is not fully operational now, but some areas have been lost.

    Intrinsic Vulnerability

    ScoreC - Not Intrinsically Vulnerable. Species matures quickly, reproduces frequently, and/or has high fecundity such that populations recover quickly (< 5 years or 2 generations) from decreases in abundance; or species has high dispersal capability such that extirpated populations soon become reestablished through natural recolonization (unaided by humans).

    Environmental Specificity

    ScoreC - Moderate. Generalist. Broad-scale or diverse (general) habitat(s) or other abiotic and/or biotic factors are used or required by the species but some key requirements are scarce in the generalized range of the species within the area of interest.

    CommentCold water stenotherm, cannot survive increases in water temperatures or will have to migrart to cooler temps

General Description
We do not yet have descriptive information on this species.  Please try the buttons above to search for information from other sources.

Diagnostic Characteristics
This species is a member of the R. vemna species group that includes three species reported in Montana that are inseparable as larvae: Rhyacophila vemna Milne, Rhyacophila robusta Schmid, and Rhyacophila gemona Ross.

Species Range
Montana Range


Range Comments
Known from Washington and Montana (Crane Creek in Lake County)

Observations in Montana Natural Heritage Program Database
Number of Observations: 1

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



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

This Rhyacophila species seems to prefer the small forested streams along the east side of Flathead Lake and potentially a few other similar streams in Lake and Sanders County. These steep-gradient, shaded, mossy cobble and woody rivulet streams fit the description of many habitats of our most narrowly restricted caddisflies.

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

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

Threats or Limiting Factors
Specific threats to Montana populations of R. gemona 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.

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Citation for data on this website:
A Rhyacophilan Caddisfly — Rhyacophila gemona.  Montana Field Guide.  .  Retrieved on , from