Responses of Caddisfly Larvae (Brachycentrus spp.) to Temperature, Food Availability and Current Velocity

Larvae of the stream caddisflies, Brachycentrus americanus and Brachycentrus occidentalis, were studied in eight simulated stream channels to determine their behavioral responses to temperature, food availability (brine shrimp) and current velocity. For both species, filtering, withdrawn and case-building were the primary behavior patterns of larvae that had attached their cases to the substrate. Most larvae not attached to the substrate were crawling or holding. As temperatures increased above 8 C, B. occidentalis larvae filtered more frequently; but above 20 C the percentage of larvae filtering steadily decreased and the percentage withdrawn increased dramatically with increasing temperature. Percentages of larvae case-building and unattached generally decreased over the range of 4 to 27 C. Despite this decrease in case-building, B. occidentalis larvae generally grew faster as temperature increased from 4-16 C. Behavior of B. americanus as a function of temperature was similar to behavior of B. occidentalis. Both species responded to decreased ration by increasing the percentage of time filtering. Although many larvae were unattached and probably grazing in Lawrence Creek, few larvae were unattached in the laboratory, even at the lowest ration (1.2% of the bodv weight per day). Growth and case-building activity of B. americanus larvae were directly related to ration. Over the range of current velocities of 7-26 cm/sec, behavior of B. occidentalis changed little. At 5 cm/sec fewer larvae filtered and more were unattached; this suggested a threshold response to current velocity. Increasing temperatures from 10-20 C caused the percentage withdrawn at low velocities to increase; however, this trend was hardly noticeable at velocities above 10 cm/sec. In these tests. Brachycentrus were more responsive to temperature and food availability than to current velocity. The interaction of temperature, food availability and current velocity largely controls the bioenergetic state of larvae and thus the behavior and ecology of Brachycentrus.