This mass of bumblebees landed heavily in front of us. A Queen Tree bumblebee, Bombus hypnorum, and two males, the one ar the rear mating with her, the other one trying too. Quite impressive she just managed to fly with the load!
In most bumblebee species, queens mate just once, but Tree Bumblebees are an exception, and queens often mate with more than one male. In experiments by Brown and colleagues, 72 queens that had mated the previous day were offered the chance of mating again. Sixteen of them (22%) mated again. The shorter the mating on the first instance the higher the chance the queens would choose to remate. The average copulation duration was about half an hour.
Paxton and coworkers (2001) analysed queens, workers and males of 14 Tree Bumblebee colonies using very variable genetic markers. They wanted to find out how many males had fathered the colonies workers, and therefore, the level of relatedness between workers. They found that workers of single colonies are the offspring of up to six males, although most of them (60%) were full siblings, os normally one male sires most of the queen's offspring. A further, very interesting finding was the presence of a few number of 'alien' workers in some nests, which although unrelated to the queen were related amongst them. Their explanation for this was that a nest usurpation, a phenomenon that occurs in bumblebees, had taken place. In the spring, after emergence from overwintering, there is high competition for nest sites amongst queens. In tree bumblebees nest boxes and under eaves are favoured, and dead queens are often found just outside occupied nests. Why would this nest usurpation take place and what are the consequences? Usurper queens take over a ready made, established nest with some larvae or pupae - so they are already ahead of the competition with no effort. From then on, the usurper queen rears the already present larvae or pupae, which, when workers will help her rear her own offspring (in the same way that cuckoo bumblebees take over colonies of other species). Without genetic analysis, the success of not of the usurpation events couldn't be assessed, who was the dead queen, the usurper of the initial queen that had established the nest? The fact that the 'alien' workers were small, suggests that they were produced early in the life of the colony, when the queen had to feed the larvae herself. The fact that they were related amongst them rules out that they are 'lost' workers that have entered the wrong nest. The alien workers were presumably early produced offspring of an usurped queen and had helped rear the offspring of the usurper, so they were effectively 'slaves'.
Paxton RJ, Thorén PA, Estoup A, & Tengö J (2001). Queen-worker conflict over male production and the sex ratio in a facultatively polyandrous bumblebee, Bombus hypnorum: the consequences of nest usurpation. Molecular ecology, 10 (10), 2489-98 PMID: 11742549
Brown, M. J. F., Baer, B., Schmid-Hempel, R., & Schmid-Hempel, P. (2002). Dynamics of multiple-mating in the bumble bee Bombus hypnorum. Insectes sociaux, 49(4), 315-319. DOI: 10.1007/PL00012654