Quantitative field and laboratory data was gathered on the utilization of deciduous leaves as food by L. quercina Ross (Trichoptera: Lepidostomatidae) and the effect of this food processing on the stream ecosystem was estimated. Samples were taken monthly in a riffle-pool section of Berry Creek, Benton County, Oregon [USA]. Maximum larval density was 382/m2, instantaneous growth rate was 2.7%/day, instantaneous mortality rate was 1.4%/day and production was 0.19 g.cntdot.m-2.cntdot.yr-1. The life cycle of L. quercina and its period of maximum larval growth corresponded closely with the period of maximum availability of its preferred food (alder leaves) in the stream. Consumption and fecal production rates were measured gravimetrically. Rates (mg.cntdot.mg-1.cntdot.day-1) increased with temperature, food quantity and conditioning time of the leaves, and decreased with increased size of the larvae. Mean respiration rates of larvae were higher at 10.degree. C than at 5.degree., but there was no significant difference in mean rates at 10, 15 or 20.degree. C. Respiration rate decreased with increased size of the larvae. Size-specific respiration rates showed regulation of respiration with respect to temperature for small individuals (present in the field in Sept. and Oct. when temperatures are variable) and little or no regulation by large individuals (present in Dec. and Jan). Simulation modeling of larval growth based on laboratory data demonstrated that growth and production of L. quercina in the field may be limited by a lack of high-quality food (alder leaves) in late summer and early fall. Consumption of leaves by the simulated population was estimated as 3.1 g.cntdot.m-2.cntdot.yr-1. L. quercina comprised only a small part of the secondary production in Berry Creek (0.19 g.cntdot.m-2yr-1, vs. 2.2 g.cntdot.m-2 for simuliids) and processed only a small portion of the allochthonous input to the stream. Significant quantities of fecal material were produced and it was estimated that these fine particles would be sufficient to support 1/4-1/2 of the production of simuliids, the dominant riffle species in Berry Creek.
