19Neonicotinoid pesticides are currently implicated in the decline of wild bee populations. 20Bumble bees, Bombus spp., are important wild pollinators that are detrimentally affected by 21 ingestion of neonicotinoid residues. To date, imidacloprid has been the major focus of study 22 into the effects of neonicotinoids on bumble bee health, but wild populations are increasingly 23 exposed to alternative neonicotinoids such as thiamethoxam. To investigate whether 24 environmentally realistic levels of thiamethoxam affect bumble bee performance over a 25 realistic exposure period, we exposed queenless microcolonies of Bombus terrestris L. 26workers to a wide range of dosages up to 98 µg kg -1 in dietary syrup for 17 days. Results 27showed that bumble bee workers survived fewer days when presented with syrup dosed at 98 28 µg thiamethoxam kg -1 , while production of brood (eggs and larvae) and consumption of 29 syrup and pollen in microcolonies were significantly reduced by thiamethoxam only at the 30 two highest concentrations (39, 98 µg kg -1 ). In contrast, we found no detectable effect of 31 thiamethoxam at levels typically found in the nectars of treated crops (between 1 and 11 µg 32 kg -1 ). By comparison with published data, we demonstrate that during an exposure to field-33 realistic concentrations lasting approximately two weeks, brood production in worker bumble 34 bees is more sensitive to imidacloprid than thiamethoxam. We speculate that differential 35 sensitivity arises because imidacloprid produces a stronger repression of feeding in bumble 36 bees than thiamethoxam, which imposes a greater nutrient limitation on production of brood.