Comparison of press versus pulse exposure to insecticides: the role of sublethal effects

J.M. Culp^1,4, K. Liber², A. Cessna³, E. Luiker^1,4, A. Alexander⁴, K. Heard⁴, D. Baird^1,4, and P.J. Van Den Brink⁵.¹Environment Canada (NWRI), Fredericton, NB, Canada, ²Toxicology Centre, Univ. Saskatchewan, Saskatoon, SK, Canada, ³Environment Canada (NWRI), Saskatoon, SK, Canada, ⁴Canadian Rivers Institute @ Dept. Biology, Univ. New Brunswick, Fredericton, NB, Canada, ⁵Alterra Research, Wageningen Agricultural Univ., Wageningen, Netherlands

Maximum environmental exposure to soluble agricultural insecticides is more likely to occur during short-term pulses following heavy rains rather than through longer-term, continuous press exposures. This study used 25-d stream microcosm experiments to examine the effects on stream benthic communities of 24-h pulse versus continuous press exposure to a commonly applied insecticide (imidacloprid). Press and pulse concentrations of imidacloprid covered a broad range of experimental exposure (0, 0.1, 0.5, 1, 5, 10 ppb). Microcosms included natural stream substrate inoculated with periphyton and benthic macroinvertebrate communities from the Nashwaak River, NB, Canada. Experimental measurements included water chemistry, imidacloprid concentration, insect emergence, benthic invertebrate abundance and algal biomass. Imidacloprid had lethal effects on macroinvertebrates between 1-5 ppb press exposures, and sublethal effects between 1-10 ppb pulse exposures. Sensitive taxa were mayflies and chironomids, with both lethal abundance and sublethal emergence effects evident. Pulse exposures of imidacloprid resulted in lower levels of mortality compared to press exposures. However, pulse exposures at environmentally-relevant concentrations had sublethal effects on insect growth. Ongoing studies are determining further sublethal effects to mayflies and chironomids.