PREIMAGINAL BLACKFLIES (DIPTERA, SIMULIIDAE) AND THEIR PREDATORS IN ACENTRAL SCANDINAVIAN LAKE OUTLET STREAM

In this study I describe the seasonal patterns of the occurrence of th e blackfly species inhabiting a boreal Swedish lake outlet. Ten specie s were present, and the numerically dominant species were Cnephia lapp onica, Simulium rostratum/truncatum, and S. ornatum. Densities were ve ry high reaching maximally >120 cm-2 in C. lapponica, and > 140 cm-2 i n mixed aggregations. Along with the description of the simuliid commu nity that of their predators was investigated. The larvae of eleven in sect species of four orders were found predating on simuliid larvae. T he predators with the highest average simuliid consumption were the tr ichopterans Halesus sp.and Hydropsyche siltalai, and the perlodid plec opterans Isoperla grammatica, and Diura nanseni. Due to differences be tween predator and prey phenologies, predators were variously importan t to different prey species. Thus, Diura nanseni primarily ingested S. ornatum, whereas Halesus sp. preyed heavily on larvae and pupae of Cn ephia lapponica and Simulium rostratum/truncatum. Since Hydropsyche si ltalai and Isoperla grammatica were common and present nearly at all t imes they had the greatest influence and affected all blackfly species . Presented data suggest that the impact from predators is greatest in the summer (July-August), after the main peak of blackfly larvae and pupae.