Derivation of combined species sensitivity distributions for acute toxicity of pyrethroids to aquatic animals

The aquatic toxicity profiles of synthetic pyrethroid insecticides are remarkably similar, and results for a large number of species can be combined across compounds in Species Sensitivity Distributions (SSDs). Normalizing acute toxicity values (median lethal concentrations, LC50s) for each species and each pyrethroid to the LC50 of the same pyrethroid to the freshwater amphipod Hyalella azteca (the most sensitive species to all pyrethroids tested) enabled expression of LC50s as Hyalella equivalents that can be pooled across pyrethroids. The resulting normalized LC50s (geometric means for each species across pyrethroids) were analyzed using SSDs. Based on tests with measured exposure concentrations, the fifth percentiles (Hazard Concentrations, HC5s) of the SSDs were 4.8 Hyalella equivalents for arthropods (36 species) and 256 Hyalella equivalents for fish (24 species). HC5 values are useful as effects metrics for screening-level risk assessments, and the full SSDs can be integrated with estimated exposure distributions for higher-level risk characterization. The combined pyrethroid SSDs provide a more taxonomically representative and statistically robust basis for risk characterization than data for the most sensitive single species or SSDs based on data for a single pyrethroid alone, and are especially useful for pyrethroids that have been tested with smaller numbers of species.