EFFECT OF SUBLETHAL CONCENTRATIONS OF PESTICIDES ON THE FEEDING-BEHAVIOR OF DAPHNIA-MAGNA

Daphnia magna was exposed to sublethal levels(1/4, 1/2, and 2/3 LC(50)) of endosulfan and diazinon to determine the effect of these pesticides on filtration and ingestion rates. The experiments were performed with the unicellular algae Nannochloris oculata in a density of 5 x 10(5) cells/ml. Prior to these experiments, the acute toxicy of both pesticides was evaluated to calculate the LC(50)s., The 24-hr LC(50) values were 0.62 mg/liter and 0.9 mu g/liter for endosulfan and diazinon, respectively. Rates of filtration and ingestion declined with increasing toxicant concentrations after a short exposure of 5 hr. The effective concentration at which feeding rates were reduced to 50% of those in controls (EC(50)) was calculated for both pesticides. These values were 0.44 and 0.61 mg/liter for filtration and ingestion rates, respectively, in the case of endosulfan and 0.47 and 0.60 mu g/liter, respectively, in the case of diazinon. (C) 1994 Academic Press, Inc.