Effects of river water and salinity on the toxicity of deltamethrin to freshwater shrimp, cladoceran, and fish

Deltamethrin is a pyrethroid insecticide used extensively to control invertebrate pests on cotton and other crops. It is acutely toxic to nontarget aquatic organisms, but existing toxicity data are mostly from toxicity tests using purified laboratory water that differs greatly from the turbid, high-conductivity rivers in the cotton-growing regions of Australia. The aim of this study was to determine whether the water quality variables conductivity, suspended particles, and dissolved organic matter alter the toxicity of deltamethrin to freshwater crustaceans and a fish. We tested three Australian native species: a cladoceran (Ceriodaphnia cf. dubia), a freshwater shrimp (Paratya australiensis), and larvae of the eastern rainbow fish (Melanotaenia duboulayi). Conductivity of the test solutions ranged from 200 to 750 μS/cm, but such changes did not modify the toxicity of deltamethrin to any of the test species. However, the toxicity of deltamethrin to C. cf. dubia and P. australiensis in river water was significantly decreased (1.8-fold to 6.3-fold reduction) compared to that in laboratory water. Variability in the toxicity data limited our ability to detect differences between laboratory and river water for M. duboulayi. Despite reductions in toxicity in natural waters, deltamethrin remained highly toxic [all L(E)C₅₀ values <0.26 μg/L] to all organisms tested; thus, further investigation of the hazard of deltamethrin is warranted.