Comparative Toxicity of Chlordane, Chlorpyrifos, and Aldicarb to Four Aquatic Testing Organisms

Laboratory toxicity data contrasting responses of aquatic organisms to insecticides are important for focusing on sensitive species (steepest exposure-response slope) exposed to aqueous concentrations of these insecticides in field studies. These data also allow prediction of expected responses of aquatic species to a range of insecticide concentrations in situ. Aqueous 48-h toxicity tests were performed to contrast responses of Daphnia magna Straus, Hyalella azteca Saussure, Chironomus tentans Fabricius, and Pimephales promelas Rafinesque to acetylcholinesterase-inhibiting insecticides: chlorpyrifos, aldicarb, and chlordane. As expected, invertebrates tested (H. azteca, C. tentans, and D. magna) were >/= 200 times more sensitive than the vertebrate P. promelas to chlorpyrifos exposures. H. azteca was approximately 3.5 times more sensitive to chlorpyrifos (453% mortality/&mgr;g/L) than D. magna (128% mortality/&mgr;g/L). For both aldicarb and chlordane, C. tentans was the most sensitive species tested (2.44 and 2.54% mortality/&mgr;g/L, respectively). Differences in chlordane potency for test species varied only by a factor of approximately 2-3 (0.88% mortality/&mgr;g/L for H. azteca to 2.54% mortality/&mgr;g/L for C. tentans). Although point estimates of population responses such as LC50s, NOECs, and LOECs are of some utility for predicting effects of pesticides in aquatic systems, exposure-response slopes are also useful for extrapolation of laboratory data to diverse field situations, especially where sediment sorption may regulate insecticide exposure or bioavailability.