Populations Dynamics and Role of Two Species of Lepidostoma (Trichoptera: Lepidostomatidae) In an Oregon Coniferous Forest Stream

The aim of this study was to gather quantitative field and laboratory data on the role of Lepidostoma cascadense (Milne) and Lepidostoma unicolor (Banks) in the processing of conifer needles in a stream ecosystem. Samples were collected monthly in a riffle—pool section of Mack Creek, Lane County, Oregon, USA. For L. cascadense, maximum larval density was 812/m 2 and instantaneous growth rate was 1.5%/d. The larvae grew slowly throughout the winter and pupation occurred in May and June. In contrast, L. unicolor increased very little in size during the winter and grew very rapidly during June and early July, reaching a maximum larval density of 320/m 2 and with an instantaneous growth rate (March through July) of 2.7%/d. Production of L. cascadense was estimated as 0.31 g°m — 2 °yr — 1 and that of L. unicolor was 0.23 g°m — 2 °yr — 1 . In addition to temporal separation in periods of maximum growth, the two species occurred in different microhabitats. L. cascadense larvae were found within the sediments and debris while L. unicolor occurred on the surface of the debris. Laboratory studies were conducted with L. unicolor larvae. Consumption and fecal production rates (measured gravimetrically) increased with higher temperature, greater food density, or longer conditioning time of the food, and decreases with increased size of the larvae. Production and biomass of L. cascadense and L. unicolor in Mack Creek are minor in relation to other insects in the system. However, because of high consumption rates and low assimilation efficiency, the processing of large particulate organic matter by these two species contributed significantly to the food available to collectors in the study area.