Neonicotinoids Thiamethoxam and Clothianidin Adversely Affect the 1 Development of Invertebrate Populations in Aquatic Microcosms 2

25 Surface waters are sometimes contaminated with neonicotinoids: a widespread, persistent, systemic 26 class of insecticide with leaching potential. Previous ecotoxicological investigations of this chemical 27 class in aquatic ecosystems have largely focused on the impacts of the neonicotinoid imidacloprid; few 28 empirical, manipulative studies have investigated the effect on invertebrate abundances of two other 29 neonicotinoids which are now more widely used: clothianidin and thiamethoxam. In this study, we 30 employ a simple microcosm semi-field design, incorporating a one-off contamination event, to 31 investigate the effect of these pesticides at field-realistic levels (ranging from 0 to 15 ppb) on 32 invertebrate abundance in small ephemeral ponds. In line with previous research on neonicotinoid 33 impacts on aquatic invertebrates, significant negative effects of both neonicotinoids were found. There 34 were clear differences between the two chemicals, with thiamethoxam generally producing stronger 35 negative effects than clothianidin. Populations of Chironomids (Diptera) and Ostracoda were negatively 36 affected by both chemicals, while Culicidae appeared to be unaffected by clothianidin at the doses used. 37 Our data demonstrate that field-realistic concentrations of neonicotinoids are likely to reduce 38 populations of invertebrates found in ephemeral ponds, which may have knock on effects up the food 39 chain. We highlight the importance of developing pesticide monitoring schemes for European surface 40 waters. 41 42