Bumble Bees inquilinism. - In social insects, severe brood care costs have favoured the evolution of cheaters that exploit workers services of conspecifics or heterospecifics. In Bumblebees, a lot of species use hosts facultatively as an alternative to care for their own brood, while Psithyrus species have lost their worker caste and are completely dependant on hosts to produce their sexuals. Here is an overview of the adaptations and strategies used by bumblebees inquilines to successfully enter and exploit host social systems.