Potential of 11 pesticides to initiate downstream drift of stream macroinvertebrates

The authors examined the potential of 11 pesticides to initiate macroinvertebrate drift in a stream microcosm containing Gammarus pulex, Simulium latigonium, and Baetis rhodani. Downstream drift is a common response of many lotic macroinvertebrates to disturbance, and can result in significant changes in the structure of the macroinvertebrate community at a particular location. The study included both acute toxicity tests to determine the LC50 for each species and stream microcosm experiments to evaluate drift at sublethal concentrations. The acute toxicity tests were 96 hours with mortality monitored daily. The 48 hour imidacloprid LC50 for B. rhodani was 8.49 µg/L. For S. latigonium and G. pulex, the 96 hour imidacloprid LC50s were 3.73 µg/L and 270 µg/L, respectively. The stream microcosms were designed to measure the downstream drift of the macroinvertebrates based on their distribution along the test chamber for 48 hours after contamination. Six of the 11 pesticides (thiacloprid, imidacloprid, acetamiprid, iprodione, fenvalerate, and indoxacarb) initiated drift at sublethal concentrations. Imidacloprid was tested with B. rhodani (1 µg/L) and G. pulex (30 µg/L), but not with S. latigonium because of a lack of black fly larvae. These values were chosen to be sublethal and do not represent a threshold test to determine the concentration at which drift would be initiated. Significant drift was seen in both species within four hours after imidacloprid contamination. Their results show that neurotoxic insecticides have the potential to initiate drift, and the rapid drift response (drift initiated within two hours and maximum drift reached within 4 hours of contamination for all contaminants that caused drift) suggest that these substances can be causing effects in the field from short-term pulse exposures. Drift could disrupt the community structure in a location, but also may serve to allow macroinvertebrates to avoid contamination by relocating, so it is difficult to discern its impact. At an organism level, it may be protective, but at a community level it can disrupt the structure and potentially ecological functions like leaf decomposition and nutrient cycling. The authors recommend that sublethal endpoints including macroinvertebrate drift should be considered during ecotoxicological risk assessments, especially for neurotoxic insecticides.