Polistes semenowi is a species of paper wasp in the genus Polistes that is found in southeastern and southern central Europe, as well as central Asia, and was until 2017 erroneously known by the name Polistes sulcifer, while a different species was incorrectly believed to represent P. semenowi (this species has been named Polistes austroccidentalis since 2017). It is one of only four known Polistes obligate social parasites, sometimes referred to as "cuckoo paper wasps", and its host is the congeneric species Polistes dominula. As an obligate social parasite, this species has lost the ability to build nests, and relies on the host workers to raise its brood. P. semenowi females use brute force, followed by chemical mimicry in order to successfully usurp a host nest and take over as the queen.
Taxonomy and phylogeny Polistes is the only genus in the tribe Polistini of the subfamily Polistinae. Species in this subfamily are one of several lineages often known as paper wasps. Polistes semenowi, first named by Morawitz in 1889, is a member of the monophyletic species group once known as Sulcopolistes, which contains only four species (the others being Polistes atrimandibularis, Polistes austroccidentalis, and Polistes maroccanus), all of them obligate social parasites on other Polistes species. Sulcopolistes was mistakenly thought to be an entirely different genus but was incorporated into the subgenus Polistes by Carpenter in 1991. Additionally, this species group is phylogenetically close to Polistes dominula, the only host species of P. semenowi.
Description Polistes semenowi, like most other paper wasps, are typically 0.7–1.0 inches long. Because it is a social parasite, it is adapted for fighting off the host species, and has some morphological distinctions. P. semenowi is known to have larger mandibles and anterior legs than those of related species. It also has a more square-shaped head, which is hypothesized to have evolved from the need for robust muscles to power the larger mandibles. Additionally, there is often a black band in the lower half of the clypeus. This trait in particular distinguishes it from its host species P. dominula. P. semenowi males typically have a more complete coloration of the body than the females, with the colors ranging from yellow to almost white. A typical paper wasp nest resembles a papery material, and is made of saliva and fibers from wood and plants. P. semenowi, however, has lost the nest making ability. Instead, it relies on the nests of its host species Polistes dominula, whose nests are made of the typical paper material with many combs.
Distribution P. semenowi is found in southeastern and south-central Europe, as well as central Asia.
Colony cycle P. semenowi is a permanent workerless species, and consists only of reproductive males and females. Because P. semenowi is an obligate social parasite, its cycle depends heavily on that of its host species P. dominula, also known as the yellow paper wasp. As mentioned previously, P. semenowi queens (mated females) overwinter at high altitudes. Come May, they emerge. They tend to overwinter for a longer period of time than their host P. dominula, which emerges in April. This ensures that the parasites reach the host nest at the optimal time: just before worker emergence, when only the host foundresses are in the nest, leading to an easier invasion. Once a queen P. semenowi has successfully invaded a nest, the host nest is built up by the host workers after their emergence in early summer. P. semenowi has an annual cycle, and it has been shown that parasitic members only stay in the host nests for around 50 days. After emergence of the P. semenowi reproductive individuals, mating occurs, and fertilized females overwinter while males die off.
Brood parasite P. semenowi is an obligate social parasite. Even more specifically, it is a brood parasite. This means that they lay their eggs in the nests of other species and influence the host into providing the parental care for their young. The cuckoo (bird) is the most familiar example of a brood parasite, hence P. semenowi's common name, the cuckoo wasp. P. semenowi has lost the ability to take care of its own young, so if an overwintering female fails to find a host nest to usurp, she leaves no offspring. A successful female relies on the host workers to care for her brood. The parasite brood develop from eggs to adults more quickly than the host brood, 25 days vs. 33 days respectively. Based on behavioral observations, host workers tended to visit parasite young much more frequently than host young, and each parasitic larva was visited much more often by a host worker than by its own mother. Surprisingly, it is not the parasite queen who is controlling this phenomenon, but it is the parasite larva that are somehow able to attract unrelated host workers on their own. One hypothesis is that the larva have 'hungry signs', much like in cuckoo birds, that manipulate the host workers into thinking they are still hungry. Since food supply during the larval stage dictates which caste a wasp will become, this manipulation of host workers is essential to P. semenowi because all larvae need to emerge as reproductives.
Usurpation
In order for P. semenowi to have offspring, it must invade the nest of its host and exploit its colony. This invasion of a colony is called usurpation, and it is done in a very systematic way. After a long hibernation, the overwintering females, all fertilized queens, search for a host nest. The females emerge from overwintering relatively late, which allows them to look for a host colony before worker emergence, when there are only foundresses present in the nest. This allows for the simplest usurpation. When a suitable host nest is found, the female initially uses brute force to fight the host foundress and gain control of the nest. The outcome of these fights is often predicted by the relative body size of the two females. Most often, the host foundress is evicted or killed. Once inside the host nest, the P. semenowi female undergoes the process of changing her cuticular hydrocarbons to match that of the host colonies (more information on cuticular hydrocarbons can be found in the camouflage section). This helps the parasitic queen remain accepted by the host colony. The parasitic queen now controls the nest, and can lay her eggs. The host workers continue on as normal, raising both the host and parasite brood.
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