1. The life history and trophic basis of production of the caddisfly grazer Agapetus quadratus were studied in the torrent Gorg Blau, a spring-fed stream on the island of Majorca that dries annually during summer.
2. Quantitative random samples were taken every 2–3 weeks during an annual surficial flow period, from November 2000 to mid-July 2001. Instars of field-collected larvae were determined by measurements of head width and pronotum length, and the sex of all pupae was determined to study sexual dimorphism and sex ratio.
3. Stage-frequency histograms suggested a trivoltine population, with an average cohort time of 4 months. Larval development was asynchronous, with continuous growth and overlapping generations. Recruitment peaks were identified in mid-November, early March and late June, indicative of winter, spring and summer generations. On average, females were larger than males and the mean sex ratio was 2 : 3 (females : males). Population densities and biomasses derived from the field data were used to calculate production and turnover rate.
4. Annual production of A. quadratus in the torrent Gorg Blau (4.80 g dry mass m−2 year−1) was the highest ever reported for the genus, being comparable with that estimated for some insects with rapid development and multiple cohorts.
5. Estimates of production of A. quadratus were combined with foregut content analysis to estimate the fraction of total production derived from the principal food sources: algae and organic detritus. Algae supported a major proportion of the production of this grazer.
6. The low density of predators characteristic of many temporary streams, and the small amplitudes in discharge and temperature during most of the wet period that characterise the spring habitats might allow high levels of grazer production in this particular Mediterranean stream.