This dataset contains a colony performance indicator monitored from Bumblebee colonies of similar mass after 43-48 days in the field. The weight in milligrams (mg) is given of each surviving bee recorded as it was removed from the outside of the nest as we progressed towards the nest core. Bumblebee colonies of similar mass were provided with sugar syrup (UT, n=12), containing chlorpyrifos (CP, 150 nanomolar nM, n=6) imidacloprid (IMD, 10 nM, n= 6) or both (IMD + CP, n=12). No pollen was provided and bees were free to forage in a wilderness/grassland area in the west of Scotland. The data were collected as part of a project within the UK Insect Pollinators Initiative (2010-2015).