Bionomics of Dicosmoecus gilvipes (Trichoptera: Limnephilidae) in a Large Western Montane River

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Abstract
D. gilvipes (Hagen) had a 2-yr life cycle in the upper Flathead River, Montana. First-instar larvae appeared in spring and grew to 4th instar by late summer. Larvae typically entered 5th instar during late summer or autumn and spent the winter months attached to the undersides of large rocks in apparent diapause. Fifth-instar larvae were again active as water temperatures warmed > 1.degree. C in March. In midsummer, 5th-instar larvae again attached their cases to the undersides of rocks and entered a 4- to 6-wk prepupal diapause prior to metamorphosis. Adults emerged from late Aug. through Oct. Early-instar larvae occurred primarily in depositional areas at the edge of riffles or in pools and backwater areas. Late-instar larvae were most frequently observed on and within the rubble substrate of lotic erosional zones, yet outside turbulent riffles. The change in habitat from depositional to erosional zones was coincident with decreased river discharge. The 2-yr life cycle is explained in terms of temperature and food quantity-quality relationships.