Insecticides in agricultural streams exert pressure for adaptation but impair performance in Gammarus pulex at regulatory acceptable concentrations.

Pesticide exposure in agricultural streams requires non-target species to adapt. However, pesticides may reduce performance in between exposure events due to long-term effects and physiological fitness costs of adaptation. Here, we investigated the long-term consequences of pesticide exposure to low concentrations in the widespread crustacean Gammarus pulex. We collected populations from six German streams covering no to moderate agricultural pesticide exposure. Peak concentrations ranged up to 1/400 of their acute median lethal concentration (Toxic Unit = -2.6), resulting in significant changes in the macroinvertebrate community composition (SPEAR = up to 0.12). Acute toxicity tests revealed up to 2.5-fold increased tolerance towards the most frequently found insecticide clothianidin compared to populations from non-contaminated streams. However, populations showing increased insecticide tolerance were characterized by reduced survival, per capita growth and mating when cultured under pesticide-free conditions in the laboratory for three months. We conclude that pesticide pollution triggers adaptation both at the species and the community level even at concentrations considered to be safe according to the European pesticide legislation. In G. pulex, exposure and adaptation are associated with impaired performance which potentially affects ecosystem functions such as leaf litter degradation. These long-term impairments need to be considered in deriving safe concentrations.