Temperature- and latitude-specific individual growth rates shape the vulnerability of damselfly larvae to a widespread pesticide

1. Freshwater ecosystems are especially vulnerable to climate change and pollution. One key challenge for aquatic toxicology is to determine and manage the combined effects of temperature increase and contaminants across species' ranges. 2. We tested how thermal adaptation and life-history evolution along a natural temperature gradient influence the vulnerability of an aquatic insect to a pesticide under global warming. We applied a space-for-time substitution approach to study the effect of warming on the vulnerability of Ischnura elegans damselfly larvae to the pesticide chlorpyrifos in a common garden warming experiment (20 and 24 degrees C) with replicated populations from three latitudes spanning >1500 km in Europe. 3. Chlorpyrifos was more toxic to damselfly larvae at the higher temperature: mortality only occurred at 24 degrees C and the reductions in growth rate were stronger at 24 degrees C. This could partly be explained by parallel reductions in food intake but not by the activities of two widespread enzymatic biomarkers, glutathione S-transferase (GST) and acetylcholinesterase (AChE). 4. There was some evidence that the increased toxicity of the high chlorpyrifos concentration at 24 degrees C was stronger in terms of growth reduction in the faster-growing larvae from the low-latitude populations. This is consistent with energy allocation trade-offs between growth rate and pesticide tolerance, but suggests that local thermal adaptation does not play a role in coping with pesticide stress. 5. Synthesis and applications. Damselfly larvae from populations in lower latitudes were more vulnerable to a common pesticide at higher temperatures and pesticide concentrations, whereas evidence for the influence of local thermal adaptation on the vulnerability of larvae was weak. These results emphasize the need for spatially explicit bioassessment and conservation tools. Management practices aimed at mitigating pesticide run-off into aquatic ecosystems are particularly important in agricultural areas at low latitudes.