Extremely low repeated pyrethroid pulses increase harmful effects on caddisfly larvae (Chaetopteryx villosa) and influence species interactions.

Abstract

Based on effect data, regulatory acceptable concentrations (RACs) are derived for surface waters to avoid unacceptable effects on the environment. RACs often relay on acute tests with single species, which may underestimate the effects under field conditions. Therefore, we applied a higher tier approach with artificial indoor streams (AIS). We exposed representatives of the benthic community in lotic surface waters to varying numbers (one to four times) of 12-hour deltamethrin pulses over a 35-day period with intervening recovery phases, to simulate multiple pesticide peak exposures caused by rain events or spray drift. The deltamethrin concentration of each pulse was equivalent to its RAC value of 0.64 ng/L and consequently should have no unacceptable effects on the tested species. In contrast, we observed that the mortality of caddisfly larvae increased significantly with the number of pulses at the RAC. In addition, larval development was significantly delayed after four deltamethrin pulses, while the gammarids apparently benefited from the toxicity-induced mortality of the larvae. This study underlines the importance of considering higher tier approaches that include species interactions and additional stressors in order to obtain more realistic effect data and optimise regulatory risk assessment. These are not considered in acute tests with single species, which usually leads to an underestimation of the effects. Based on the results of this study, we propose to lower the RAC value for aquatic environments due to the uncertainties mentioned above.