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Polistes

Polistes is a cosmopolitan genus of paper wasps and the only genus in the tribe Polistini. Vernacular names for the genus include umbrella wasps, coined by Walter Ebeling in 1975 to distinguish it from other types of paper wasp, in reference to the form of their nests, and umbrella paper wasps. Polistes is the single largest genus within the family Vespidae, with over 200 recognized species. Their innate preferences for nest-building sites leads them to commonly build nests on human habitation, where they can be very unwelcome; although generally not aggressive, they can be provoked into defending their nests. All species are predatory, and they may consume large numbers of caterpillars, in which respect they are generally considered beneficial.

Description

As part of subfamily Polistinae, Polistes wasps are covered in short and inconspicuous hair, have a clypeus with a pointed apex, have a gena that is wide throughout, tergum 1 of the metasoma is almost straight to gently arched in profile, the tibia of the mid leg has two spurs, and the legs end in simple tarsal claws. The genus can be distinguished from other Polistinae by a sessile metasoma (the first segment at most slightly longer than wide) and the fourth tarsomeres of the mid and hind legs being symmetrical.

Polistes show sexual dimorphism, with males having seven externally visible metasomal segments whereas females have six. This trait is shared with other vespid wasps.

Polistes species have single-layered nests which are shaped like an umbrella, with the cells exposed to the air from the bottom, and no layer wrapping around the nest. The nests are suspended from a surface by a petiole and are constructed from a paper-like substance made of a mix of saliva and wood fibres chewed off old and soft wood or dead twigs. Many Polistes species in general often have nests supported by a longer petiole than those of Vespula.

Biochemistry

Carlson et al 1998 finds the cuticular hydrocarbons of Polistes to be, similarly to many other insects, predominantly many-branched methyl branched alkanes. The reviews of Nelson 1978, Lockey 1988 and Nelson 1993 concur.

Life cycle

The general life cycle of Polistes can be divided into four phases:

  1. Founding (or pre-emergence) phase
  2. Worker phase
  3. Reproductive phase
  4. Intermediate phase

Founding (or pre-emergence) phase

The founding stage begins in the spring when a solitary female (the "foundress") (or a small group of related females) initiates the construction of a nest. The wasps begin by fashioning a petiole, a short stalk which will connect the new nest to a substrate (often the eave of a house or outbuilding), and building a single brood cell at the end of it. Further cells are added laterally in a hexagonal pattern, each cell surrounded by six others. Although nests can achieve impressive sizes, they almost always maintain a basic shape: petiolated (stellocyttarous), single-combed, unprotected, and open (gymnodomous).

Eggs are laid by the foundress directly into the brood cells and are guarded by the foundress and the assisting females (if present). After the first larvae hatch, the foundress feeds them via progressive provisioning, bringing softened caterpillar flesh to the larvae multiple times throughout their development (as opposed to the one-time provisioning seen in some other hymenopteran groups). Each of this first seasonal brood of new paper wasps is exclusively female and destined to a subordinate worker position inside the nest; they do not found their own nests and instead assist their mother in the care and maintenance of future sisters.

Some foundress wasps do not build their own nests, but rather attempt to usurp that of another female. These usurpation attempts may or may not be successful, but almost always result in impressive displays of aggression and violence. Females may also adopt a more peaceful alternative reproduction strategy by joining the nest of a close relative (usually a sister) and working as assisting females. In the latter case, such cofounding females are generally, but not exclusively, close relatives.

Worker phase

The worker phase usually begins in the early summer, roughly two months after colony initiation, with the emergence of the first workers. These new females take up most of the colony's work duties, foraging, caring for brood, and maintaining the structure of the nest. Around this time, those females which assisted in nest foundation (if present) are driven from the nest by aggressive behavior on the part of the foundress, and leave either to start their own late-season nests or usurp another's.

Reproductive phase

The reproductive phase of the colony begins when the first female reproductives (the gynes) emerge from their brood cells. These reproductives differ from their worker sisters by having increased levels of fat stores and cryoprotectant carbohydrate compounds (allowing them to survive the overwintering period). These reproductives contribute genes directly to the next generation, while their worker sisters normally pass along their genes indirectly.

Intermediate phase

Once male reproductives emerge and both males and females disperse from the natal nest for mating flights, the so-called intermediate phase begins. Brood care and foraging behavior decline and worker numbers drop as dying individuals are no longer replaced by new ones. Intracolonial aggression increases and the social cohesion of the nest declines. In temperate Polistes species, individuals (almost exclusively inseminated females) gather in groups of up to 50 individuals and seek a sheltered location (called a hibernaculum) in which to overwinter.

Behavior

Kin selection

The reproductive behavior of Polistes wasps provided some of the first evidence for the mathematical biologist W. D. Hamilton's 1964 theory of kin selection. Hamilton showed that animals such as workers could be expected to provide assistance to relatives such as their queens according to the costs and benefits involved (K) and their degree of genetic relatedness (r), and gave the rule that now carries his name, K > 1/r. Early caution existed among researchers as to whether social insects could really assess their relatedness. Hamilton himself suggested an alternative possibility, namely that kin could become associated simply by "population viscosity" —that offspring tend not to disperse far from their birthplaces— and West-Eberhard (1969) found some evidence for this in Polistes. However, Polistes species are now known to learn and remember chemical signals (hydrocarbons) picked up from the nest to distinguish nestmates accurately from non-relatives.

Dominance hierarchy system

Morphologically, the foundress and subordinate reproductive members of the colony differ little. However, behavioral differentiation occurs among females both between and within generations. For example, in the species Polistes humilis the queen displays a "tail-wagging" behavior to assert her dominance over the worker class. Similarly, Polistes canadensis also possesses behavioral differentiation between the queen and her nestmates, with the queen often suppressing the aggressive behavior of subordinates through lateral abdominal vibrations and stroking. In contrast, unmated females are not aggressive. In Polistes exclamans queens have different amounts of glucose, fructose, and trehalose which lead to different cryoprotectant levels. This alters their survivability in different temperatures, increasing their odds of reproduction. Females in P. bellicosus are also morphologically similar between caste separations. For example, a P. bellicosus worker could become queen, and egg-layer, if all of the original foundresses die or leave the nest. This is also true for Polistes dorsalis, which also displays dominant behavior. Despite having no distinct morphological caste, roles of P. dorsalis tend to be fixed in a system with division of labor.

Nestmate recognition

Polistes spp. discriminate colony mates using an acquired (i.e. learned) cue, absorbing hydrocarbons from the natal nest at eclosion. This cuticular hydrocarbon "signature" is derived both from the plant material and the foundress-applied substances from which the nest is made. Studies of Polistes fuscatus have researched the molecular basis of the recognition "pheromone" used by the wasps, and indicate at least some of the recognizable labels have the same chemical constituents as the adult cuticular hydrocarbons. Similar recognition is found in Polistes metricus. Dominant individuals of P. dominula have differing cuticular profiles from workers, and the frequent observations of the dominant female stroking its gaster across the nest surface, combined with its staying on the nest for longer times than subordinates, suggests the dominant individual may contribute more to the nest odor.

P. carolina females do not preferentially feed their own progeny (as larvae).

Further to this recognition of nestmates, Polistes biglumis foundresses discriminate between 'alien' eggs and their own via differential oophagy.

Gallery

Species

203 species were described in the genus by 1996, and new species continue to be described. There are nine species in Europe.

229 species and 125 subspecies are as follows:

Pest status

Along with the German and common wasps, the Asian and Australian paper wasps (P. chinensis and P. humilis) are considered pests in New Zealand. Arriving in 1979, the Asian paper wasp has established itself in both the North Island and the northern parts of the South Island. Because it competes with native species (such as the kaka) for insects, nectar, and honeydew, it is a hindrance to conservation efforts.

In North America, the introduced European species Polistes dominula has rapidly colonized a significant area, and is considered an invasive pest. It is a concern for cherry and grape growers in British Columbia, as it injures the fruit by biting off the skin. It also spreads yeast and fungi that harm fruit and can be a nuisance to workers and pickers at harvest. There is evidence it has also displaced native paper wasp species by outcompeting them.

Parasites

Various other insects are parasites or parasitoids of Polistes, including flies (e.g., Sarcophagidae), mantispids, and wasps in the families Torymidae, Mutillidae (rarely), Braconidae, and Ichneumonidae (e.g. Latibulus argiolus). Some more specialized groups are more intimately associated with Polistes; this includes strepsipterans in the family Stylopidae (genus Xenos), wasps of the genus Elasmus (formerly placed in their own family, "Elasmidae"), and wasps in the family Trigonalidae.

The nests of many species of this wasp genus are invaded by the parasitoid caterpillars of the moth Chalcoela iphitalis which feed on the wasp larvae and pupas at night, spinning their cocoons in empty cells.

Within the subgenus Polistes are four known social obligate parasites: P. atrimandibularis, P. austroccidentalis, P. maroccanus, and P. semenowi, which parasitize other Polistes wasps. Known host species of these parasites are P. dominulus, P. gallicus, P. nimphus, P. associus, and P. biglumis. Although these parasites differ in their host invasion strategies, their end goal is to successfully infiltrate the host nest and reproduce at the host's expense.

References

Further reading

External links