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Montana Field Guides

An Agapetus Caddisfly - Agapetus montanus

Potential Species of Concern
Native Species

Global Rank: G3
State Rank: S3
(see State Rank Reason below)

Agency Status


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State Rank Reason (see State Rank above)
This Glossosomatid Caddisfly is currently ranked a "S3" Potential Species of Concern in MT and is potentially at risk because of limited and/or potentially declining population numbers, range and/or habitat, making it vulnerable. This species was ranked a species of concern in 2006, but additional occurrence records and range expansions has moved it to the Potential Species of Concern list.
  • Details on Status Ranking and Review
    An Agapetus Caddisfly (Agapetus montanus) Conservation Status Review
    Review Date = 09/18/2008
    View State Conservation Rank Criteria
    Population Size

    ScoreU - Unknown


    Range Extent

    ScoreF - 20,000-200,000 km squared (about 8,000-80,000 square miles)

    Comment20,000-200,000 km2 (125,000-500,000 acres)

    Area of Occupancy

    Comment1000-5000 km (620-3000 miles) linear river

    Length of Occupancy

    ScoreLE - 1,000-5,000 km (about 620-3,000 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 species 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.

General Description
This small rock-cased larval caddisfly emerges to a small black caddisfly as an adult. The larvae occur on the upper surfaces and sides of cobbles and boulders in moderate gradient, fast flowing, foothills to mountain streams (Wiggins, 1996).

Diagnostic Characteristics
Adult morphology: Adult characteristics are covered in Denning (1949). They are commonly known as the Little Black Caddisflies

Larval morphology: Body length: up to 6mm. Larvae of Agapetus could be confused with Protoptila (another Glossosomatidae), but are readily distinguished by the presence of 2 mesonotal sclerites instead of 3. There have been few larval-adult associations of the Agapetus spp. (7 of 30), so in areas with multiple species, adults or mature pupae are needed for species level identification. The saddle-type rock cases for larval Agapetus usually have larger rocks along the edge of the case (see photo).

Species Range
Montana Range


Range Comments
Rangewide, Agapetus montanus occurs in Idaho, Montana, and Manitoba (Wiggins 1996, Morse 2016).
In Montana, A. montanus is our only known species based on collections (Gustafson 2006), thus any genus level identification to Agapetus should be our species, A. montanus. Since this is the case, A. montanus has been reported from ~30 streams in Missoula, Mineral, Gallatin, Granite, Powell, Meagher, Flathead, Deer Lodge, Lewis and Clark, Lincoln, Beaverhead and Sanders Counties.

In Idaho, A. montanus was collected from 2 locations at a small mountain stream near 1800 m elevation (Newell and Minshall 1976b). This represents the only documented Idaho distribution (Newell and Minshall 1977). Although due to a lack of larval species identifications and multiple Agapetus species reported for ID, there are probably far more streams containing this caddisfly species than have been reported.

Observations in Montana Natural Heritage Program Database
Number of Observations: 122

(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)

Adult caddisflies generally fly in an upstream direction, but do not usually migrate long distances.

The larvae of A. montanus occur on the upper surfaces and sides of cobbles and boulders in moderate gradient, fast flowing, foothills to mountain streams (Wiggins1996). This genus inhabits streams with more intermediate characteristics between the higher elevation, cold mountain streams (more likely to find Glossosoma & Anagapetus), and the large warmer transitional rivers downstream (more likely to find Prototila) (Wiggins 1996). Generally the riparian canopy of the occupied streams is mostly (>50%) open, and less shaded than mountain streams. In clear streams and rivers during low flows, it is typical to be able to locate & identify Agapetus larvae on the tops of rocks. In relation to trophic status, A. montanus larvae scrape, graze and digest algae and diatoms from the surfaces of rocks (Merritt and Cummins 1996).

Reproductive Characteristics
Adults of this species emerge from mid-June to mid-August (Wiggins 1996).

A. montanus has no USFWS status at the present time, nor is one warranted. It is currently a US Forest Service Species of Concern (SOC); listed as endangered in Idaho (S1) and vulnerable to extirpation (S2) in Montana, but these rankings were largely based on a paucity of locations that this species has been previously reported. With this new compilation work and additional targeted survey work, we feel confident that the rank of this species will be downgraded to at least a G3 and an S3 for the state of Montana due to the number of additional streams and river miles the species now appears to occupy.

Threats or Limiting Factors
Specific threats to Montana & Idaho populations of A. montanus have not been identified. In general, aquatic invertebrates that feed by grazing and scraping are intolerant of silt and sedimentation which tends to embed cobbles that contain their food source. Thus, improper management practices in the riparian zone (i.e. intensive livestock use) that would increase fine sediment in the streambed substrate and otherwise degrade aquatic habitat is the primary concern these populations.

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Citation for data on this website:
An Agapetus Caddisfly — Agapetus montanus.  Montana Field Guide.  .  Retrieved on , from