| Polistes | |
|---|---|
| Polistes dominula | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hymenoptera |
| Family: | Vespidae |
| Subfamily: | Polistinae |
| Tribe: | Polistini Lepeletier, 1836 |
| Genus: | Polistes Latreille, 1802 |
| Type species | |
| Polistes gallicus Linnaeus, 1767 [1] |
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Wasps of the cosmopolitan genus Polistes (the only genus in the tribe Polistini) are the most familiar of the polistine wasps, and are the most common type of paper wasp. It is also the single largest genus within the family Vespidae, with over 300 recognized species and subspecies. Their innate preferences for nest-building sites leads them to commonly build nests on human habitation, where they can be very unwelcome; although generally non-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. The European paper wasp, Polistes dominula, was introduced into the US about 1981 and has quickly spread throughout most of the country, in most cases replacing native species within a couple of years. This species is very commonly mistaken for a yellowjacket as it is black, strongly marked with yellow, and quite different from the native North American species of Polistes. Polistes wasps can be identified by their characteristic flight; their long legs dangle below their body, which is also more slender than a yellowjacket.[2]
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The general life cycle of Polistes can be divided into four phases[3]:
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). 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 (see above). In the latter case, evidence shows that such co-founding females are generally, but not exclusively, close relatives.[3]
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 who 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 nest or usurp another's.
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 over-wintering period). These reproductives will contribute genes directly to the next generation, while their worker sisters will normally pass along their genes indirectly.
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 fifty individuals and seek a sheltered location (called a hibernaculum) in which to over-winter.
Morphologically, there is little difference between the foundress and subordinate reproductive members of the colony. However, several studies have shown that behavioral differentiation occurs among females both between and within generations. This has been best-studied in P. dominula.
Polistes discriminate colony mates using an acquired (i.e. learned) cue, absorbing hydrocarbons from the natal nest at eclosion.[4] 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 looked into the molecular basis of the recognition "pheromone" used by the wasps, and indicate that at least some of the recognizable labels have the same chemical constituents as the adult cuticular hydrocarbons.
Dominant individuals of P. dominulus have differing cuticular profile to workers,[5] 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 that the dominant individual may contribute more to the nest odor.
A study of P. carolina showed that females do not preferentially feed their own progeny (as larvae),[6] so it may be the case that nest odour only serves as a likely indicator of relatedness, rather than a specific label of kinship.
Further to this recognition of nest-mates, a study on Polistes biglumis illustrated how foundresses discriminate between 'alien' eggs and their own, via differential oophagy.[7] Interestingly, the discrimination focused upon eggs destined to be reproductives, with 'alien' worker destined eggs allowed to remain on the nest. The authors speculated that the benefits of allowing worker destined eggs to remain (and so hatch to become workers which will then aid the colony) outweigh the costs of initially provisioning the resultant larvae.
The mechanism of differentiation was not elucidated, but was thought to be based upon differences in cuticular hydrocarbon odor. Whether the discriminatory oophagy was a result of decreased tolerance of alien odors during the later, reproductive phase of the colony cycle, or an actual discrimination between worker and reproductive destined eggs, remains to be supported with good evidence.
Along with the German and common wasps, the Asian and Australian paper wasps (Polistes chinensis and P. humilis) are considered pests in New Zealand. Arriving in 1979,[8] the Asian paper wasp has established itself on both North Island and northern parts of South Island. Because it competes with native species (such as the kākā) for insects, nectar and honeydew,[9][10] it is a hindrance to conservation efforts.
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). There are also some more specialized groups that 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.