Severing Ties: Different Responses of Larval and Adult Aquatic Insects to Atrazine and Selenium

Aquatic insects link aquatic and terrestrial ecosystems through their metamorphosis and subsequent transition from water to land. Chemical stressors in freshwater, such as agricultural contaminants, can potentially disrupt insect life cycles and reduce the number of insects emerging as terrestrial adults, thereby damaging or severing this linkage. Atrazine and selenium, though frequently detected in waterways and often co-occurring, have not been previously studied together in controlled experiments. We conducted a six-week mesocosm experiment to measure the responses of larval and emerging aquatic insects to treatments of atrazine (15 μg/L), selenium (10 μg/L), and a direct combination of the two. Peak adult insect abundance was reduced in all treatments by 35% to 45% relative to the control. Further, cumulative adult emergence in the combined treatment was 33% lower than in the control. However, no reductions in primary production were observed with treatments, and consistent reductions in benthic insect abundance relative to the control were not observed until the end of the experiment, when overall abundance was low. Results suggest that adult insects are more sensitive than larval insects to atrazine and selenium and that the impacts of these contaminants are stronger on the terrestrial than the aquatic ecosystem.