Nontarget impacts of neonicotinoids on nectar-inhabiting microbes

Plant-systemic neonicotinoid (NN) insecticides can exert non-target impacts on organisms like beneficial insects and soil microbes. NNs can affect plant microbiomes, but we know little about their effects on microbial communities that mediate plant-insect interactions, including nectar-inhabiting microbes (NIMs). Here we employed two approaches to assess impacts of NN exposure on several NIM taxa. First, we assayed in vitro effects of six NN compounds on NIM growth using plate assays. Second, we inoculated a standardized NIM community into nectar of NN-treated canola (Brassica napus) and assessed survival and growth after 24 hours. With few exceptions, in vitro NN exposure tended to decrease bacterial growth metrics. However, the magnitude of decrease and the NN concentrations at which effects were observed varied substantially across bacteria. Yeasts showed no consistent in vitro response to NNs. In nectar, we saw no effects of NN treatment on NIM community metrics. Rather, NIM abundance and diversity responded to inherent plant qualities like nectar volume. In conclusion, we found no evidence NIMs respond to field-relevant NN levels in nectar within 24 h, but our study suggests that context, specifically assay methods, time, and plant traits, is important in assaying effects of NN on microbial communities.