Sequential pesticide exposure: Concentration addition at high concentrations - Inhibition of hormesis at ultra-low concentrations.

Sequential pesticide exposure is a common scenario in both aquatic and terrestrial agricultural ecosystems. Predicting the effects of such exposures is therefore highly relevant for improving risk assessment. However, there is currently no information available for predicting the effects of sequential exposure to the same toxicant at both high and low concentrations. Here we exposed one-week-old individuals of Daphnia magna to the pyrethroid Esfenvalerate for 24 h and compared the effects with individuals treated twice with half the concentration after 7 and 14 days. We showed that at the concentrations close to the LC, both the survival and population growth rate from the two half-pulses were consistent with the concentration addition approach. At low (1/10th to 1/100th of the LC) and ultra-low concentrations (1/100th to 1/1000th of the LC), survival was around 100 %, while the population growth rate showed a hormetic increase following the one-pulse exposure but not for the two-pulse exposure. We hypothesize that this hormetic effect is due to lower systemic stress (SyS) after pesticide exposure in combination with only one rebound stress pulse. Our study suggests that while the lethal effects of sequential exposure are according to the concentration addition model, the sublethal effects at low and ultra-low concentrations need to consider hormetic effects.