UNLABELLED: Commercially reared bees provide economically important pollination services for a diversity of crops. Improving their health is important both to maximise their pollination services and to avoid possible pathogen spillover or spillback with wild pollinators. Diet quality may directly or indirectly affect diverse aspects of bumblebee health, including colony development, individual size and immune health, but the impact of this remains unclear. Here we investigate experimentally the effect of diet quality during bumblebee colony development using three diets: (1) a standard pollen diet used in commercial rearing of colonies for sale, (2) an enhanced diet comprised of a wildflower pollen mix that was expected to be nutritionally superior (including by having an elevated protein content) and (3) a diet of pollen substitute mixed with the standard diet that was expected to be nutritionally poorer. The effect of diet quality on colony health was quantified using colony-level measures (colony weight, size [number of live workers] and number of dead individuals), and individual-level measures (body size, fat body size [proportion of body weight], total haemocyte count and phenoloxidase immune enzyme activity). Diet quality significantly affected colony growth, with colonies fed the enhanced diet growing larger and producing more reproductives than those fed either a standard or poor diet. The enhanced diet also resulted in bees that were significantly larger and had better immune health. The results show that diet can have important effects on the health of commercially reared bumblebees and suggest that the enhancement of standard-rearing diets may improve colony health. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13592-024-01132-1.