River water and sediment reduce the toxicity of deltamethrin to paratya Australiensis

Deltamethrin is a pyrethroid insecticide used extensively to control cotton pests in Australia and worldwide. Deltamethrin readily binds to organic and particulate matter in the environment, thereby reducing its bioavailability and toxicity, yet most toxicity data come from studies using clean, organic matter-free water that were conducted under conditions that differ greatly from those in the turbid rivers of the cotton-growing regions of Australia. The aim of this study was to assess the toxicity of deltamethrin to the native glass shrimp, Paratya australiensis, and to consider the role of suspended and bottom sediment in the amelioration of deltamethrin toxicity. We conducted a series of acute single-species toxicity tests in the laboratory and in the field in the Namoi-Gwydir cotton region of northwest New South Wales, Australia. The toxicity of deltamethrin was significantly (p≤0.05) reduced in river water compared with that in laboratory water in laboratory but not field-based tests. The toxicity of deltamethrin in river water was further reduced with the addition of bottom sediment. Despite reductions in toxicity in natural waters, deltamethrin remained highly toxic (i.e. 60-h EC50 values <200 ng/L) to P. australiensis, and thus further investigation of the hazard of deltamethrin is warranted.