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.