Adaptation of Gammarus pulex to agricultural insecticide contamination in streams

Highlights: • Non-target species in agricultural streams can acquire increased tolerance to pesticides. • Tolerance and species composition were affected even at contamination below regulatory thresholds. • The adaptation to pesticides significantly increased with local concentration. • Non-contaminated refuge areas hindered local adaptation to pesticides. Exposure to pesticides affects non-target aquatic communities, with substantial consequences on ecosystem services. Adaptation of exposed populations may reduce the effects of pesticides. However, it is not known under which conditions adaptation occurs when only a low toxic pressure from pesticides is present. Here, we show that Gammarus pulex, a dominant macroinvertebrate species in many agricultural streams, acquires increased tolerance to pesticides when recolonization from non-contaminated refuge areas is low. Populations in the field that were exposed to pesticides at concentrations several orders of magnitude below considerable acute effects showed almost 3-fold higher tolerance to the neonicotinoid insecticide clothianidin (mean EC₅₀ 218 μg L⁻¹) compared with non-exposed populations (mean EC₅₀ 81 μg L⁻¹). This tolerance of exposed populations increased from 2- to 4-fold with increasing distance to the next refuge area (0 to 10 km). We conclude that the development of tolerance for non-target species may occur at very low concentrations, much below those affecting sensitive test organisms and also lower than those predicted to be safe by governmental risk assessment frameworks.