Bumblebees of the World Blog Series… #7 Bombus inexspectatus

18/07/2019 0 By wildfeed

When Tkalcu first made his discovery of a new/overlooked European bumblebee species it prompted European bumblebee specialists to resample potential locations and look back in their own collections to reassess potential specimens. As a result, the original four records soon became eighty or so. Interestingly, the original records included two males and two supposed workers, which happen to be the only two workers ever found for this species. The status of these females as workers was called into question by Yarrow in 1970 who proposed that inexspectatus could potentially be a social parasite of the closely-related Red-shanked carder bee (B. ruderarius), which he always found in good numbers wherever inexspectatus was found. He argued that the workers were merely ‘runt’ females which were underdeveloped in comparison to the rest of the female specimens. Close examination of collected specimens revealed that unlike social bumblebees and similar to cuckoos, inexspectatus specimens do not appear to secrete wax from their abdominal segments, which is an essential part of colony building among the social bumblebees. Yarrow noted that none of the specimens he had seen had any evidence of pollen collecting, and that the structures on the legs associated with pollen collection seemed to have degenerated slightly in comparison with social bumblebees, although not to the same extent as the main cuckoo group (Psithyrus subgenus).

It was not until 2005, when Andreas Muller and colleagues stumbled upon a Bombus ruderarius nest in the Swiss Alps, that Yarrow’s suspicions were supported with the first and only evidence of parasitism by inexspectatus. The nest they found was close to completion with no stored nectar, eggs or larvae, a selection of empty cocoons from various generations of workers and four unhatched cocoons, which four ruderarius queens later emerged from. No founding queen/females of either species were found, however two ruderarius workers, two males and one new queen remained in the nest when it was found, as well as one fresh inexspectatus female. This indicated that both the social queen and the parasitic female were co-existing in the nest, which has been found when other cuckoo bumblebees invade nests, but often the social colony does not successfully produce new queens or produces fewer than normal. As this is the only nest that has been found to date – we do not know how common it is for the two species to co-exist or if the invading bees manage to completely take over nests in the same way that some other cuckoo bumblebees do. There are also larger questions about this species’ general ecology and life history still to be answered, for example, whether inexspectatus can invade the nests of other hosts, similar to the other known parasitic bumblebee species.

Links to videos of this species:

Bombus inexspectatus males feeding on Centaurea uniflora. Video credit: Maurizio Cornalba

Bombus inexspectatus female sheltering under a Phyteuma flower. Video credit: Maurizio Cornalba

Links to further information:

IUCN Redlist page

Natural History Museum species account

Natural History Museum Bombus – Bumblebees of the world homepage

IUCN Bumblebee Specialist Group

Hines, H. M., and Cameron, S. A. (2010). The phylogenetic position of the bumble bee inquiline Bombus inexspectatus and implications for the evolution of social parasitism. Insectes Soc. 57, 379–383. doi: 10.1007/s00040-010-0094-1

Müller, Andreas. (2006). A scientific note on Bombus inexspectatus (Tkalců, 1963): Evidence for a social parasitic mode of life. Apidologie. 37. 408-409. 10.1051/apido:2006005.

Yarrow I.H.H. (1970) Is Bombus inexspectatus (Tkalců) a workerless obligate parasite? Insectes Soc. 17, 95–112


Many thanks to Maurizio Cornalba of the University of Pavia, Italy, and Paul Williams of the Natural History Museum, London, for their help and advice.