Multiple exposure routes of a pesticide exacerbate effects on a grazingmayfly

Summary

Hydrophobic pesticides such as pyrethroid insecticides tend to occur in their soluble form mainly astransient pulses in streams. In addition, they are regularly detected in significant quantities adsorbed tostream sediments and other organic in-stream structures. Consequently, stream biota is likely subjectedto pesticide exposure via multiple routes. In this study we aimed at investigating the influence of exposureroutes for the pyrethroid insecticide lambda-cyhalothrin on the grazing mayfly Heptagenia sulphurea. Therefore, H. sulphurea was exposed to lambda-cyhalothrin via single- (water or biofilm) or biphasicexposure (water and biofilm) at environmentally realistic concentrations (0, 0.1, 1 ug L−1) and exposure duration (2 h) in a full factorial design (n = 5). Mortality, moulting frequency, and biofilm accrual (proxy for feeding rate) were recorded subsequent to a 7 d post exposure period. Mortality significantly increased and moulting frequency significantly decreased with increasing concentrations of lambda-cyhalothrinin the water phase whereas exposure via biofilm prompted no significant effects on these endpoints(alpha = 0.05). Effect predictions systematically underestimated and overestimated effects for mortality and moulting frequency, respectively. Similarly, mayfly feeding rate was significantly reduced by water phaseexposure whereas pre-exposed biofilm did not significantly affect this variable. However, we found a significant but non-systematic interaction between water phase and biofilm exposure on mayfly feeding rate. Our results show that exposure to the same pesticide via multiple exposure routes may increase the magnitude of effects beyond the level predicted from single phase exposures which has clear implicationsfor the aquatic risk assessment of hydrophobic pesticides. However, our results additionally reveal that interactions between pesticide exposure routes may vary between selected dependent variables. We emphasize that unravelling the underlying mechanisms causing these discrepancies in interactive effects between exposure routes is a major aspect that should receive further attention in future research.