Polistes atrimandibularis

Polistes atrimandibularis is one of three obligate social parasites among the Polistes wasps found in Europe. Of the three social paper wasp parasites, it is the smallest. It parasitizes multiple species such as P. dominula, P. nimpha, P. associus, P. gallicus, and P. biglumis. Females of P. atrimandibularis are unable to build a nest or produce workers, and therefore rely entirely on the host colony. P. atrimandibularis is under the subfamily Polistinae (paper wasps). It is closely related to the other two obligate social parasites: P. sulcifer and P. semenowi. These three Polistes inquiline species are more closely related to P. nimphus and P. dominula rather than to P. gallicus and P. biglumis. P. atrimandibularis is the smallest of the three social Polistes parasites. The size of the host’s species brood cells determines P. atrimandibularis size, with the smaller P. atrimandibularis emerging from the nests of P. gallicus and P. associus. Likewise, the larger P. atrimandibularis emerge from the nests of P. dominula instead. This paper wasp has enlarged mandibles, used as weapons to injure hosts that resist parasite intrusion. They also have an enlarged first femur and a longer posterior tibia that is useful for when they must maintain a dominant position within the host colony. P. atrimandibularis is a rare European species found mainly around the Mediterranean and Caspian basin. They typically position themselves at higher elevations, although it is not unheard of to find colonies at lower elevations. Accordingly, one of the species that P. atrimandibularis parasitizes, P. biglumis, lives mainly in montane climates in Southern Europe. They do not possess the ability to build their own nests, so they must parasitize other wasps’ colonies. Similar to the two other Polistes social parasites, P. atrimandibularis usurps colonies in the late spring, which is roughly one month before the emergence of host workers. This allows the parasite sufficient time to reproduce and exploit the worker force. As soon as the usurper has successfully dominated the host’s queen, it begins to lay its own eggs, and will remain in the host colony until the end of its life cycle. It is not until late summer that the newly emerged parasites migrate to the mountains to mate and then overwinter for several months under a thick blanket of snow and ice. When P. atrimandibularis mate, males and females migrate to the tops of high mountains. At the top of the mountains, the males will occupy and aggressively defend their mating territories from male competitors. Once inseminated, females remain for overwintering for several months, under a thick blanket of snow and ice. To ensure that they arrive and usurp their host colonies at the correct time, P. atrimandibularis postpone the overwintering exit. These severe, high altitude climate conditions extend parasite diapause for about a month compared to the lowland host colonies, which allows the P. atrimandibularis to usurp colonies that are more developed. Since Polistes social parasites are very rare, these altitudinal migrations also give them an advantage. The migrations promote encounters between the two sexes and give more opportunities for matings between non-relatives. Each colony has its own unique hydrocarbon chemical signature that allows P. atrimandibularis to distinguish nestmates from non-nestmates. When a female Polistes wasp first emerges, she is unaware of the colony’s unique odor. However, after a few hours the female Polistes wasp will be able to recognize and identify the odor of the nest and create a template that it can refer to in the future. Being a social parasite, the queen has the ability to modify its own chemical signature along with the host’s nest signature. Studies have shown that the presence of P. atrimandibularis lowers the reproductive capacity of the host queen and also inhibits the fertility of host workers. Host foundresses in colonies that were parasitized by P. atrimandibularis were found to have repressed ovarian development, with the female parasite becoming the sole female laying eggs in the colony. P. atrimandibularis is interesting in that the usurper cohabits with the host foundress for a long period of time. This suggests that the host foundress is needed in order to exert reproductive control on the host workers.