Abstract
In order to clarify distribution of blackfly larvae in relation to channel slope and stream discharge, studies were carried out in Guatemala in streams infested with Simulium ochraceum, S. metallicum, S. horacioi, S. callidum and others. A total of 485 larval collections were made during the dry season from 90 sampling units each consisting of 20m stretch. It was found that the incidence rate of each species was regulated by both channel slope and stream discharge. The higher-than-80% incidence rate was observed at the sites with high slope (20-40°) and medium discharge (0.3-3l/s) for S. ochraceum, with low slope (0-25°) and medium to large discharge (0.3-30l/s) for S. metallicum, and with high slope (20-40°) and small discharge (0.03-1l/s) for S. horacioi. The higher-than-50% rate for S. callidum was obtained at the sites with medium slope (5-25°) and large discharge (1-30l/s). S. ochraceum was positively associated with all other species. S. metallicum and S. callidum showed strong positive association with each other, but negative one with S. horacioi. Three types of dominant species pairs were most frequently observed, i.e., the horacioi-ochraceum pair at high slope and small discharge, the ochraceum-metallicum pair at low slope and small discharge, and the metallicum-callidum pair at low slope and large discharge. Species composition in tributaries was successfully explained by their mean slope and geometric mean discharge.
