San Francisco Estuary and Watershed Science

Chemical and toxicological testing in the Cache Slough complex (the slough) of the North Delta indicated the aquatic biota are exposed to a variety of wastewater-derived food additives, pharmaceuticals, and personal care products in highest concentration during dry periods, and many insecticides, herbicides and fungicides with peak concentrations after winter rains. The insecticide groups currently known to be of greatest toxicological concern are the pyrethroids and the fiproles (i.e., fipronil and its degradation products). After stormwater runoff enters the system via Ulatis Creek, both pesticide groups attained concentrations that posed a threat to aquatic life. When the commonly used testing species, Hyalella azteca, was placed in Cache Slough, toxicity — and, at times, near total mortality — was seen over at least an 8-km reach of Cache Slough that extended from the uppermost end almost to the junction with the Deep Water Ship Channel. Previous work over many years has shown similar results after other winter storms. However, when H. azteca that carried a mutation providing resistance to pyrethroid pesticides were also deployed in the slough, no ill effects were observed, which provided strong evidence that pyrethroids were responsible for toxicity to the non-resistant strain. Abundant resident H. azteca in Cache Slough carry any of four mutations that provide resistance to pyrethroids. They also carry a mutation that provides resistance to organophosphate pesticides, and likely carbamate pesticides as well. After many years of exposure, sensitive genotypes have been nearly eliminated from the system, and replaced by a population unaffected by many insecticides now in common use. We offer a variety of reasons why this shift to a population with mutant genotypes is of considerable concern, but also note that society has yet to fully consider the ecological and regulatory ramifications of the evolutionary attainment of pollutant resistance.