Prior pesticide exposure reduces drought tolerance in Arctic and temperate springtails–effects of sequential stressors

In the environment, organisms are exposed to a combination of fluctuating environmental conditions as well as anthropogenic toxicants such as pesticides. Fluctuations in temperature can affect the tolerance of terrestrial arthropods to toxicants, but far less is known of drought-influenced changes in the responses to toxicant exposure. Here, we compare mortality responses to drought under two optimal temperatures following prior pesticide exposure in two species of springtails (Collembola); the drought-susceptible Folsomia quadrioculata and the drought-tolerant Hypogastrura viatica. Two Arctic and two temperate populations from each species were exposed to three sub-lethal concentrations of the neonicotinoid imidacloprid (0.01-0.5-0.1 mg/kg dry soil) at 15 °C for 14 days, followed by different levels of drought stress (100-85% relative humidity [RH]) at 15 or 20 °C, and survival was assessed. Drought tolerance of all populations from both species at both temperatures was reduced by imidacloprid in a concentration-related manner. Temperate populations of H. viatica subjected to a drought stress of 98%RH showed an increase in mortality with increasing imidacloprid concentration of approximately 40% compared to a mortality increase of approximately 20% in the Arctic populations. This difference was less pronounced in F. quadrioculata, with an increase in mortality of approximately 50% in both temperate and Arctic populations at a drought stress of 98% RH. Drought stress at 20 °C further increased mortality with approximately 20% in the drought-tolerant H. viatica from both climatic regions. Thus, local adaptations and species identity should be considered when assessing the effects of combined stressors.