Vestal cuckoo bumblebee
Bombus vestalis, the vestal cuckoo bumblebee, is a species of cuckoo bumblebee that lives in most of Europe, as well as North Africa and western Asia. It is a brood parasite that takes over the nests of other bee species. Its primary host is Bombus terrestris (the buff-tailed bumblebee). After its initial classification as Psithyrus vestalis, this bumblebee recently was reclassified into the genus Bombus, subgenus Psithyrus.
The queen is a large bumblebee with a length up to 21 mm (0.83 in) and a wingspan of 37 mm (1.5 in); the male is considerably smaller (16 mm (0.63 in)). The bumblebee is predominantly black, with an orange collar. The third tergite has a border of yellow hairs, and the hairs on the fifth tergite are mostly white. The males are similar to the females, but smaller and with longer antennae.
It is similar in appearance to another cuckoo bumblebee, Bombus bohemicus, but is distinguishable by either the length of the antennal segments or dissection and comparison of the genitalia. In Bombus vestalis, the fifth antennal segment will be the same length as the third and fourth together.
A key characteristic of Bombus vestalis is the lack of corbiculae or a pollen basket on the hind legs of the bee. This is evolutionarily relevant because Bombus vestalis does not make its own nests or collect its own resources. With its larger body and poor foraging skills, the female Bombus vestalis are adapted to taking over nests of other species which have already been made and using the workers in these colonies to collect pollen.
The bumblebee is common from North Africa in the south to southern Sweden in the north, and from Ireland in the west to northern Iran in the east. It is common throughout England and Wales, but has only been sighted in Scotland since 2009. Using data from the National Biodiversity Network gateway, Bombus vestalis is most prominent in south-east England.
In Ireland the bee was believed to be all but extinct in the 20th century with the last recorded sighting in 1926 in Carlow. However, in 2014 a population was rediscovered in the walled garden at Saint Enda's Park in Rathfarnham.
Bombus vestalis is a cuckoo bumblebee; it does not construct any nest of its own, but usurps the nest of Bombus terrestris, kills the host queen, and forces the host workers to raise its offspring. The male bumblebees often congregate in gardens in the suburbs.
The queen emerges early in the spring; the males emerge later, in late May to early June. Its food sources are flowering plants such as clover (males especially often visit white clover), tufted vetch, knapweed, and others. In the spring, the emerging queens frequently fly to flowers such as deadnettles, sallows, blackthorns, and dandelions.
The life cycle of Bombus vestalis is dependent on its host B. terrestris, who begin the growth of their colony in the early spring, slowly building up the nest. The female Bombus vestalis hibernates until April, after the host has raised a first generation of workers. At this point, the nest is an ideal size for the female B. vestalis to take over. In taking over the nest, the invaders assert their dominance among the workers bees for the first few days without killing the queen. It takes nearly a month for B. vestalis to fully take over the nest and raise her brood.
Male Bombus vestalis have been found to be attracted to floral odors, particularly the polar compounds of Ophrys flowers, which mimic the sex pheromones of virgin females. In short, olfactory cues play an essential role to attract males, for virgin female Bombus vestalis, and their sexually deceptive orchid mimics, O. chestermanii and O. normanii. For these orchid mimics, the males are attracted to their floral cues, which leads to the male attempting to copulate with the orchid labellum, during which the flower is pollinated.
