Ecology and Life Histories of Three Net-Spinning Caddisfly Species (Hydropsychidae:Hydropsyche) in the Flathead River, Montana

Abstract

Three species of the net spinning caddisfly genus Hydropsyche occur sympatrically in the Flathead River, Montana. Life histories, relative abundance, longitudinal distributions and microhabitats of H. cockerelli, H. oslari and H. occidentalis were investigated with particular reference to the role of temperature and organic and inorganic seston. Larvae of all three species occurred at all riverine (4th to 6th order) sampling sites except in the immediate tailwaters of Hungry Horse Dam. There were no apparent differences in longitudinal distribution between species and microhabitat differentiation was not observed. Frequently, retreats of different species were located beside that of another on the same rock or boulder. H. cockerelli and H. oslari were significantly (P < 0.05) more abundant than H. occidentalis at all sampling sites. H. cockerelli larvae grew primarily during late summer and early autumn while temperatures were > 7°C. Larvae overwintered in 5th instar and emerged as adults in mid-June. H. oslari larvae, although present in early instars during autumn, remained in early instars through the winter months and grew during spring. Adults emerged during late June and July. H. occidentalis larval appearance and growth occurred approximately six weeks behind H. cockerelli larvae. Thus, most growth occurred at temperatures < 7°C. We concluded that the delayed growth may have contributed directly, in terms of temperature response, to the infrequency of H. occidentalis larvae.