Relative susceptibility of stream macroinvertebrates to temephos and chlorpyrifos, determined in laboratory continuous-flow systems.

Laboratory techniques are described for evaluating the lethal and behavioral impact of pesticides on a range of stream macroinvertebrates under continuous through-flow and simulated stream conditions. The same basic test unit has been used, with slight modifications, to study the reactions of both Simulium larvae and non-target stream invertebrates. On the basis of a standard 1-hr exposure period to different concentrations followed by a 24-hr holding period in a continuous flow of clean water, different test organisms showed wide and consistent differences in tolerance to each of the two insecticides tested. The widest difference between two organisms occurred in the case of the Amphipod, Gammarus pulex (LC90-95, greater than 1 ppm) which was found to be about 5000 x more tolerant to temephos than are nymphs of the mayfly, Baetis rhodani. (LC 90-95, 0.001-0.002 ppm) The widest difference in the reactions of any one species is shown on the part of Gammarus which is about 100 times more susceptible to chlorpyrifos (LC 90-95, 0.05-0.1 ppm) than to temephos. The susceptibility levels of other indicator species such as Agrion, Hydropsyche, Brachycentrus, Ephemera, etc. are discussed in relation to susceptibility levels of Simulium larvae under the same test conditions, and also in relation to current field dosages of the two insecticides in practical and experimental Simulium control.