Ecology and coexistence of two species of Brachycentrus (Trichoptera) in a Rocky Mountain river

Life-cycle dynamics, growth rates, and relative abundance of Brachycentrus occidentalis and Brachycentrus americanus were studied over a 3-year period in the Flathead River, Montana. Brachycentrus occidentalis appeared as early instar larvae in mid to late summer, grew rapidly during autumn, and reached fourth and fifth instars prior to winter conditions. Most larvae remained active until mid-spring and the onset of spring runoff. Individuals pupated during late April through May and adults emerged after peak runoff in mid-June. Brachycentrus americanus larvae appeared as first instar larvae in autumn and overwintered as early instars. Larvae grew rapidly during spring as temperatures began to rise, but growth was discontinued during spring runoff. Larvae completed growth during summer after the runoff period and emerged in late August and early September. Growth patterns and emergence of both species occurred primarily during fall or spring and late summer, with little growth during winter or spring freshet. Thus growth was associated with quantitatively similar but temporally different periods of organic seston, temperature, and moderate flow.