Paper Wasps are social wasps and make up the genus Polistes. They form small colonies with umbrella-shaped nests in sheltered spots, and are quite mild-mannered. They feed on caterpillars and other soft-bodied insects, most of them pests.
Despite the use of the proper name, most social wasps make nests from paper, although some tropical wasp species such as Listenogaster Flavolineata use mud, a far more easy resource for the wasp to collect. The larger colonial species, Yellowjackets, Hornets, can be very defensive and should not be approached unless one is experienced. All are beneficial and should be protected if possible. In Europe, hornets are a threatened species and are protected by law.
Paper wasps gather fibers from old wood and dead dry plant stems, chew them, and mix with saliva to make a water-resistant gray paper. The nests are characterized by having open cells and a petiole attaching the nest to the substrate. The wasps produce a chemical which repels ants and secretes it around the base of this petiole in order to avoid letting their brood become ant food. Humans may have learned to make paper from watching wasps.
The European Polistes wasp, Polistes dominulus 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 years. It nests earlier in the spring, in a wider variety of nest sites, is more aggressive, and feeds on a larger variety of insects. Native species feed almost entirely on caterpillars. Some entomologists are considering this to be an invasive species, while others note that it may become an important control of Japanese beetle a serious invasive pest.
The general life cycle of Polistes follows a cycle that can be divided into four phases (after Reeve 1991):
- Founding (or pre-emergence) phase
- Worker phase
- Reproductive phase
- “Intermediate” phase
The founding stage involves a solitary female initiating a nest, building 20-30 cells before initially ovipositing. This phase begins in spring, depending on climatic conditions. The foundress or foundresses begin by fashioning a petiole and produce a single cell at the end of it. Further cells are then added around this, 6 cells surrounding it to produce the characteristic hexagonal shape of the cells.
After the hatching of the first larvae, the foundress progressively provisions (brings food multiple times throughout development) them with malaxated caterpillar flesh, halting further oviposition until some of the larvae have pupated. These larvae will eventually hatch to become 1st generation workers.
As well as founding a nest, a female can also choose to attempt to usurp a nest of a conspecific, or join conspecifics upon another nest. In the case of the latter, evidence shows that such co-founding females are generally, but not exclusively, closely related (reviewed in Reeve 1991).
The worker phase usually begins in early summer, roughly 2 months after colony initiation, with the emergence of the first workers. On emergence, the workers take up most of the colony’s foraging, brood care and nest maintenance. Typically at this stage, the auxillary (subordinate) foundresses are driven from the nest, leaving the alpha female and newly emerged workers.
Emergence of the first female reproductives (Gynes) is taken to signal the start of the reproductive phase of the colony. The reproductives differ from workers produced at this latter phase of the colony by having increased levels of fats stores to allow it to survive the over-wintering period, as well as having increased levels of cryoprotectant carbohydrate compounds for the same purpose.
Finally, the “intermediate” phase is really just the period in which the gynes and males mate and then disperse from the natal colony, before over wintering (hibernating) until the start of the next colony cycle. Typically this period is characterized by a decline in brood care and foraging, as well as fewer workers (due to mortality without the worker being replaced by newly hatched brood). In P. dominulus the colony disperses in the late summer.
Dominance hierarchy system
Morphologically, there is little difference between the foundress and subordinate reproductive members of the colony. However, several studies have shown that behavioral differentiation occurs (Theraulaz 1992, Pardi 1942), the role the individual female taking determined by social interaction within the colony. Typically, the alpha female dominates all other individuals of a colony, and this female lays the majority of eggs, and partakes in differential oophagy. The alpha female devotes much of her time to social interaction, in comparison to subordinates that are much more involved in foraging and brood care (Theraulaz 1989).
These behavioral divisions are not permanent; if an alpha female is removed from a nest then another female (usually the second-most dominant, beta female) assumes the role and behavioral profile of the removed dominant. Indeed, individuals alternate between different profiles of behavior within their own dominance rank position.
Some studies (Pardi 1946) seem to indicate that the dominant female, through its behavior, suppresses the ovarian development of subordinates. Abdominal wagging is thought to serve as a dominance signal between dominant foundresses and subordinates, but studies by Roseler and Roseler (1989) showed that ovariectomiszed dominants failed to restrict subordinate reproduction whilst still retaining dominance.
Polistes discriminate colony mates using an acquired (i.e. learnt) cue, absorbing hydrocarbons from the natal nest at eclosion (Gamboa 1996). This cuticular hydrocarbon “signature” is derived both from the plant material and the foundress-applied substances that the nest is made of. Studies of Polistes fucatus have looked into the molecular basis of the recognition “pheromone” used by the wasps, and indicate that at least some of the recognisable labels have the same chemical constituents as the adult cuticular hydrocarbons.
Dominant individuals of P. dominulus have differing cuticular profile to workers (Bonavita-Cougourdan 1991), and the frequent observations of the dominant female stroking its gasters 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 odour.
A study of P. Carolina showed that females do not preferentially feed their own progeny (Strassman 2000) (as larvae), so it may be the case that nest odor 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 (Lorenzi 2000). 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.