Bombus vestalis

A brood of bees successfully raised by Bombus vestalis is mainly female.[8] This is evolutionarily beneficial since the female is the sex which takes over host species and allows Bombus vestalis to reproduce. In fact, many of the male bees which are hatched in a nest that has been taken over by Bombus vestalis are a result of host workers who were able to successfully lay eggs despite the aggression of the invaders. Although they display haplodiploidy where males are haploid and females are diploid,[9] morphologically, male and female Bombus vestalis are very similar and can be classified with a mosaic gynander. They have similar mandible and body structures, and it is likely that the differences are only derived from chromosomal alterations during development of the embryo.[12]

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.[16]

Under the subgenus Psithyrus, there are two types of parasitic bees; one type of bee is non-specific when choosing its host and the other (e.g., B. vestalis) chooses an exclusive host. These two different behaviors are likely to have developed because of a chemical difference in pheromones. Socially parasitic bees have been shown to identify their hosts through pheromone trails left by host workers while they are collecting pollen.[7] By following these natural footprints, a parasitic bee can find the entrance to the hive of its desired host. Experiments show that B. vestalis is attracted to a specific combination of both nonpolar and polar pheromones, a combination that is specific to B. terrestris.[7] Though the latter can be parasitized by other bees, B. vestalis will not take over the nests of any other species. It can also distinguish the odors of differently aged host workers, thus determining which workers to attack when invading a nest.[7]

The success of Bombus vestalis is dependent on the size of the nest which it invades. As soon as a female enters a colony, she is investigated by the workers and often faces attacks from them.[8] Larger colonies will have more workers who are capable of defending the nest. Thus a female B. vestalis must weigh the costs of parasitizing a nest when the colony is very large.[11] Experiments have been done to reproduce this scenario. However, B. vestalis cannot take over a nest which is too small. During an invasion of a small nest, though successful, too many host workers will die and there will be a shortage of workers to take care of the invading queen and her eggs. Before attacking a colony, the female must accurately judge the size of the nest, through the frequency and size of workers moving in and out, and the pheromones of these workers before attempting an attack.[11]

In experiments with B. vestalis and its host B. terrestris, an interesting phenomenon has been observed. Any host workers born after the invasion of a female B. vestalis were unharmed despite the takeover of the queen. Similarly, newborn B. terrestris were not attacked during the invasion.[8] A female B. vestalis will only target older host worker bees who are a threat to her and her eggs.[11] She is able to identify these workers through the detection of pheromones. The older a terrestris worker is, the more pheromones it secretes.[11] Since B. vestalis is so perceptive of these odors, she knows which worker bees to kill so that the brood of eggs is safely and solely her own. The remaining younger Bombus terrestris and those born soon after takeover of the queen are available to help care for the new B. vestalis nest.

Many parasitic bees use mimicry to introduce themselves into the nests of their hosts.[11] They mimic the odor of the host and oftentimes live in cooperation with the host species when raising their eggs. This is more common in larger colonies. Bombus vestalis is unique in that it has a mauling behavior.[11] With a smaller colony to take over, the female B. vestalis can individually kill off the workers in the B. terrestris nest. When it comes to the queen, she may simply push her off the nest.[8] This behavior is evolutionarily possible because Bombus terrestris nests are not too large and because Bombus vestalis invades early in the season. It will then suppress ovarian development of the remaining workers.[10]