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

Abstract

SUMMARY1. 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.