Specialist Bombus vestalis and generalist Bombus bohemicus use different odour cues to find their host Bombus terrestris

Cuckoo bumblebees (subgenus Psithyrus) are social parasites in colonies of their host bumblebee species (Bombus). In spring, parasitic females awaken from hibernation and start searching for established host nests. Some Psithyrus species are specialized on one host species, whereas generalists parasitize several species. In previous investigations, nest-marking signals of the bumblebee hosts have been shown to play a role in host recognition and discrimination between host and nonhost. We performed behavioural experiments and comparative chemical analyses with the specialist Bombus (Psithyrus) vestalis and the generalist Bombus (Psithyrus) bohemicus to identify semiochemicals that the females use to recognize the common host, Bombus terrestris. In chemical analyses of footprint samples, we mainly identified nonpolar hydrocarbons and polar wax-type esters. Bioassays with nonpolar and polar fractions obtained by solid phase extraction indicate that specialized parasites use a complex bouquet of compounds for host recognition, whereas generalists need only selected substances common to the signals of all of their hosts. The evolutionary significance of the results is discussed.