Yarrow (1970) suggests that Bombus inexspectatus may be a workerless obligate parasite.
B. inexspectatus is characterized by a number of peculiar biological and morphological traits, suggesting that this species might be an obligate social parasite in the nests of other bumblebees like the cuckoo bumblebees of the subgenus Psithyrus (Yarrow, 1970). These traits include: (i) absence of a worker caste (the two individuals originally considered as workers by Tkalců (1963) were later classified
as small overwintered females (Yarrow, 1970)); (ii) probable inability to produce wax; (iii) reduction of both the auricle and the corresponding hair brush at the apex of the hind basitarsus which probably makes the compression of collected pollen within the receptaculum and its transport to the corbiculae
impossible; and (iv) lack of pollen harvesting activities as indicated by the absence of pollen
loads in the corbiculae of all females known so far. Though these peculiarities clearly point to a parasitic habit of B. inexspectatus, there is still no direct evidence for such a behaviour (Williams, 1998).