Is the caste-ratio of the oligolectic bumblebee Bombus gerstaeckeri Morawitz (Hymenoptera: Apidae) biased to queens?

Three bumblebee species, foraging on Aconitum spp. have been commonly observed in Eyne (France, East Pyrenees): Bombus gerstaeckeri, B. hortorum and B. wurflenii. We estimated the population of these three species, For B. hortorum and B. wurflenii, the total workers populations foraging on Aconitum spp. ranged from 101 to 523 and 156 to 270, respectively. These two species also forage on other plants while B. gerstaeckeri visits only Aconitum spp. The population of B. gerstaeckeri observed was extremely small, founded by 33 queens only in 2001. With a total number of workers estimated from 40 to 102, the observed workers/queens ratio, 1 to 3 workers for each queen, is very unusual for a eusocial species. Also we observed queens foraging during the whole life of the colony. This kind of social organisation is similar to that of some high arctic species. It could be interpreted as the result of an insularity syndrome.