 Bumblebees of the subgenus Psithyrus ( known as ' cuckoo bumblebees ' , and formerly considered a separate genus ) are brood parasites , sometimes called kleptoparasites , in the colonies of other bumblebees , and have lost the ability to collect pollen . Before finding and invading a host colony , a Psithyrus female , such as that of the Psithyrus species of B. sylvestris , feeds directly from flowers . Once she has infiltrated a host colony , the Psithyrus female kills or subdues the queen of that colony , and uses pheromones and physical attacks to force the workers of that colony to feed her and her young . Usually , cuckoo bumblebees can be described as queen @-@ intolerant inquilines , since the host queen is often killed to enable the parasite to produce more offspring , though some species , such as B. bohemicus , actually enjoy increased success when they leave the host queen alive . 
