Interaction of Factors Governing the Distribution of a Predatory Aquatic Insect

We investigated the interaction of factors regulating the larval distribution of the predatory chironomid Procladius culiciformis in Lake Thonotosassa, Florida. Hypotheses were tested experimentally to relate sediment grain size, organic content, prey density, diet and feeding selectivity to larval growth, feeding habits, and distribution. The distribution of P. culiciformis was determined by monthly sampling (June 1978 to May 1979) of the benthic macroinvertebrate community along a littoral zone transect. Experimental and field observations were compared for final analysis. The distribution of P. culiciformis, within 0.2—1.5 m depth, correlated positively with density of total organisms, density of total chironomids, and median grain size of the sediment. However, the distribution was correlated negatively with density of oligochaetes, water depth, and sediment organic content. The annual mean density of P. culiciformis across all transect stations was 371 larvae/m² and was highest (1024 larvae/m²) at the shoreline station (0.7 m depth). The influence of food on distribution was measured by larval preference for naturally occurring prey and by larval growth on four common diets. Feeding selectively by P. culiciformis was positive for chironomids, ostracods, and cladocerans, and negative for rotifers. Survival and growth of larvae were highest on a diet of oligochaetes, significantly lower on diets of chrionomids and zooplankton, and lowest on a diet of algae—detritus. Sediment grain size and organic content were tested for larval preference and their effect on predation rate. Larvae preferred sediment with high organic content, but no preference was found among natural sediments with different median grain sizes. Large—grain sand and high prey density promoted high predation rates in the laboratory. However, the presence of organic matter decreased the predation rate and nullified the effect of grain size. Multiple environmental factors, especially those influencing prey availability, interact to regulate the larval distribution of P. culiciformis.