BACTERIA AS FOOD FOR BLACKFLY LARVAE (DIPTERA: SIMULIIDAE) IN LABORATORY CULTURES AND IN NATURAL STREAMS

Abstract

Planktonic bacteria as food for filter-feeding blackfly larvae were investigated in laboratory tests and in the field. Three of the commonest blackflies in Western Canada, Simulium venustum, S. verecundum, and S. vittatum, developed to adults from first-instar larvae and a fourth species, S. arcticum, developed to, or nearly to, last-instar larvae, when offered only washed suspensions of Bacillus subtilis, Aerobacter aerogenes, or Escherichia coli as food.Average concentrations of 1.3 × 10 6 to 34.9 × 10 6 cells per ml of the gram-negative A. aerogenes allowed production of adults, with best larval growth and survival in the higher concentrations. In local streams and rivers the bacterial content ranged from 0.1 × 10 6 to 25.0 × 10 6 (av. 2.2 × 10 6 ) cells per ml. Ultraviolet irradiation of the bacteria to reduce the proportion of live cells to as low as 0.1%, near the proportion in many streams, and the use of the gram-positive B. subtilis, adversely affected survival rates of the larvae.In certain streams bacteria and blackfly larvae apparently represented the basic trophic levels in the food web. Also, bacteria leached from soils and sewage may have been important in the development of outbreaks of S. arcticum.