Factors Influencing Dispersion of Larval Black Flies (Diptera:Simuliidae): Effects of Current Velocity and Food Concentration

Abstract

Positioning and feeding of Simulium vittatum larvae were studied on a ceramic tile in a flume at three current velocities and two food concentrations. Locomotory activity and drift decreased with increasing current but were independent of food. Using nearest neighbor analysis, we detected significant aggregation at 15 cm∙s−1 that resulted from accumulation of individuals at upstream boundaries. Larvae also aggregated at 43 cm∙s−1, but independently of tile margins. Dispersion was random at 24 cm∙s−1. Gross pattern formation results from individual responses to prevailing flow conditions independently of food concentration. Relative ingestion rates decreased with increasing food concentration, larval size, velocity, and incidence of parasitism. Individual larvae avoided locations directly upstream or downstream of a nearest neighbor at 15 and 24 cm∙s−1 when food concentration was low (0.5 mg∙L−1), but not at higher food levels (3.5 mg∙L−1). These trends were reversed at high velocity. Larvae immediately downstream of neighbors had reduced ingestion rates. Larvae selected positions lateral to others under low food conditions. These animals had greater ingestion rates than did individuals in other locations. Results are consistent with hypotheses of flow disruption and/or food preemption by upstream larvae and of mutually induced beneficial flow between laterally adjacent individuals. Positioning within aggregations reflects complex behavior among conspecifics that varies with the interaction of flow and food concentration.