Stream microcosm toxicity tests: Predicting the effects of fenvalerate on riffle insect communities

Stream microcosms were used to predict toxicity of the pyrethroid insecticide fenvalerate to riffle insect assemblages. Over a 30-d test period, stream microcosms were exposed in triplicate to 0.0, 0.01, 0.1, 1.0, or 10.0 μg/L fenvalerate. Effects were determined by statistically comparing insect abundances over all concentrations. Initial exposure resulted in a significant increase in drift in the 1.0- and 10.0-μg/L treatments. After 30 d, several taxa exhibited density reductions at 0.01 μg/L, but this reduction was significant for only one taxon. Densities of most other taxa decreased significantly at 0.1 μ/L. Significant reductions in species richness and total density also were observed at 0.1 μg/L fenvalerate. Overall, the results suggest that at environmental concentrations of 0.1 μg/L, mayflies and stoneflies would be eliminated and riffle beetles, caddisflies, and some chironomids would be present in significantly reduced numbers.