 The female Psithyrus has a number of morphological adaptations for combat , such as larger mandibles , a tough cuticle and a larger venom sac that increase her chances of taking over a nest . Upon emerging from their cocoons , the Psithyrus males and females disperse and mate . The males do not survive the winter but , like nonparasitic bumblebee queens , Psithyrus females find suitable locations to spend the winter and enter diapause after mating . They usually emerge from hibernation later than their host species . Each species of cuckoo bee has a specific host species , which it may physically resemble . In the case of the parasitism of B. terrestris by B. ( Psithyrus ) vestalis , genetic analysis of individuals captured in the wild showed that about 42 % of the host species ' nests at a single location had " [ lost ] their fight against their parasite " . 
