Legacy effects of four decades of insecticide applications on contemporary riverine benthic macroinvertebrates

Insecticides have known effects on riverine benthic macroinvertebrate (BMI) assemblages. However, there is limited understanding of the legacy effects of insecticides, particularly in watersheds that received decades of historical applications. From 1952 to 1993, over 6.2 million ha in the province of New Brunswick (Canada) was treated with one to twelve different insecticides including dichlorodiphenyltrichloroethane (DDT), aminocarb, fenitrothion, and phosphamidon. Using a contemporary BMI dataset that included 274 sites within watersheds that cover 50 % of New Brunswick, we evaluated the relative importance of historical insecticide applications and contemporary environmental variables in explaining variability in BMI assemblages. We found that historical insecticides explained a significant, but small, amount of variation in contemporary assemblages. The number of insecticide applications showed a stronger association with BMI metrics than the total amount of insecticide(s) applied, though contemporary environmental measures such as urban land use, substrate size, geology, and climate all had more explanatory power than insecticide metrics. Our results suggested that while contemporary environmental variables and historical insecticide applications both affect BMI assemblage composition, the former often had a stronger role in structuring assemblages. Recognizing the influence of legacy stressors provides important context for understanding contemporary bioindicator responses to environmental change.