Social bees perform a highly valuable ecosystem service, and accomplish this through complex social organisation and behaviour, which allows efficiently coordination of tasks and division of labour. However, reports of colony losses suggest that social bees can suffer fundamental breakdowns when placed under mild environmental stress. I will present some work in which investigated the effect of neonicotinoid pesticides, which have been implicated in bee decline, on bumblebee colonies. When applied at low doses, neonicotinoid pesticides do not kill individual bees, it can have profound effects on the functioning of a colony, causing colonies to decline and whither. Here, we investigate how these results can be due to such sublethal stress through impairing colony function. We mathematically modelled stress on individual bees which impairs colony function and found how positive density dependence can cause multiple dynamic outcomes: some colonies fail while others thrive. The model accurately describes the dynamics of experimental bumblebee colonies, and we think our model can explain the enigmatic aspects of bee colony failures, highlighting an important role for colony functioning in bee decline.