In rapid through-flow laboratory tests on the acute toxicity of the synthetic pyrethroid, permethrin, to select stream macroinvertebrates, estimated LC90-95 values based on mortalities 24 hr after a 1 hr exposure ranged from 0.001 ppm in the case of Baetis, Brachycentrus and Gammarus to 0.1 ppm for Hydropsyche. Simulium larvae- late instar- occupied an intermediate position with the LC90-95 approximately 0.005 ppm. Studies on behavioral reactions, with particular reference to insecticide-induced activation, detachment and downstream drift were carried out in a laboratory simulated-stream, using 30-min exposures to 2 critical concentrations, viz 0.0005 ppm and 0.005 ppm. These showed that on the one hand, Baetis, Gammarus and Simulium showed a high degree of activation, leading to drift, during the actual 30-min exposure period, and that with Gammarus at least this behavior pattern was consistent at both lethal and sublethal exposures. On the other hand Brachycentrus showed little sign of activation either during the exposure or subsequently up to 24 hr; at lethal concentrations over 90% of the Brachycentrus died in situ without drift. Hydropsyche showed an intermediate behavior pattern, activation leading to downstream drift only beginning to appear after the end of the 30-min exposure period, than increasing steadily to a peak of over 50% after 2-4 hr. This reaction was consistent even through the concentration of permethrin was one from which over 95% of the test animals survived. The implications of these findings are discussed in relation to the general problem of insecticide-induced downstream drift of stream benthos, and in relation to the findings, already reported, from similar laboratory studies on temephos and chlorpyrifos methyl.
