Life history and production of the predatory caddisfly Rhyacophila vao Milne in a spring-fed stream

1. The predatory caddisfly Rhyacophila vao Milne (Rhyacophilidae) displayed a 2-year, semivoltine life cycle in a small, spring-fed stream in southern Alberta, Canada. Three overlapping cohorts were identified throughout the 2-year sampling program, with five larval instars recognized. Larvae overwintered in instars I —ELI, developed to instar IV by summer and instar V by autumn. The second winter was spent in instar V, Pupation occurred from late May to August and did not commence until stream temperature exceeded 3°C. Adults were collected from mid-June to early September. 2. Larvae displayed a diphasic growth pattern: Phase I, a positive, non-linear growth rate for instars I-IV; Phase II, a constant growth rate during instar V. Phase I coincided with increasing stream temperatures in late winter and spring, where mean instantaneous growth rates (maximum of 2.78% dry wt day^?1) were significantly correlated with stream temperatures. Although the duration of Phase II spanned a temperature range similar to that in Phase I, the instantaneous growth rate remained temperature-independent at 0.87% dry wt day^?1. 3. Larvae exhibited a type III survivorship curve (i.e. an exponential decrease on an arithmetic scale), with the finite rate of mortality averaging 0.80% larvae day^?1. 4. Cohort 1 (later part of 1983 year class) displayed lower total production compared with the equivalent growth phase in the 1984 year class (Cohort 2). In contrast, production of the non-linear and linear growth phases of Cohort 2 was similar. Periods of similar growth characteristics for Cohorts 1 and 2 had comparable P/B ratios for both the unadjusted and time-adjusted annual estimates, although higher ratios were observed for the non-linear growth phase of Cohort 2. Total cohort production (linear + non-linear growth phases) could only be calculated for Cohort 2, and was 870.2 ± 1011.4 mg dry wt m^?2. The corresponding cohort P/B ratio was 5.01 and the adjusted annual P/B, 3.01. 5. Annual larval production (±SE) for the first and second years of the study was similar (Year 1, 480.0 ± 387.5mg dry wt m^?2; Year 2, 526.9 ± 967.5mg dry wt m^?2) as were unadjusted and annual P/B ratios.