Polistes annularis

Polistes annularis
Nest of Polistes annularis
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Vespidae
Genus:	Polistes
Subgenus:	Aphanilopterus
Species:	P. annularis
Binomial name
Polistes annularis (Linnaeus, 1763)
Synonyms
Vespa annularis Linnaeus, 1763

Polistes annularis (P. annularis) is a species of paper wasp which lives in North America. A eusocial wasp, P. annularis demonstrates behavior typical of other Polistine wasps, such as a dominance hierarchy, relatively small colony size, and a female-biased sex ratio. Unlike other wasps, P. annularis is relatively robust in winter conditions, and has also been observed to store honey in advance of hibernation. It is closely related to P. major, P. buysonni, and others in the subgenus Aphanilopterus, and slightly less related to the more common P. bellicosus, P. carolina, P. metricus, and P. fuscatus.

Description

Unlike other North American Polistes species, P. annularis shows little sexual dimorphism in coloration. This species differs from Polistes metricus in terms of the coloration of the antennae and thorax. There is geographical variation in coloration between northern and southern populations. In the north, the thorax of P. annularis has ferruginous (rust-red) markings on a predominantly black background, while in the south, the thorax is mostly ferruginous, with black markings. The legs also vary from black to ferruginous. In terms of size, the forewings are 18.5–23.5 mm (0.73–0.93 in) long in females, and 17.5–19.5 mm (0.69–0.77 in) long in males.

Taxonomy and classification

The first description of P. annularis was published by Carl Linnaeus in his 1763 Centuria Insectorum, where he named the species Vespa annularis. It was moved to the genus Polistes by Johan Christian Fabricius in 1804, two years after Pierre André Latreille had erected the new genus.

P. annularis is within the order Hymenoptera, which includes most of the social insects. It is further placed within the subfamily Polistnae (paper wasps), which is the second largest of the subfamilies within Vespidae. Polistinae contains two main behavioral groups: wasps which form nests with large number of workers, and those wasps which found nests with few workers and foundresses (the latter including P. annularis). This subfamily likely arose in the mid to late Jurassic period, around 145 to 175 million years ago. P. annularis diverged from P. bellicosus between 10 and 80 million years before the present day. It is currently placed in the New World subgenus Aphanilopterus.

Ecology and Distribution

P. annularis is found across the eastern United States from New York to Florida, and west to a line from South Dakota to Texas. This range is similar to that of Polistes exclamans.

P. annularis forms its nests on the branches of trees and shrubs as well as in sheltered parts of some buildings. The nests differ markedly from those of other species in the genus Polistes. They are much larger, with around 500 cells, and are wide, rather than the slender, elongate nests seen in some other species.

P. annularis preys on caterpillars from a large number of lepidopteran families, including Arctiidae, Saturniidae, Geometridae, Limacodidae, Lymantriidae, Notodontidae, Nymphalidae, Sphingidae, Erebidae, Noctuidae, Amphisbatidae and Elachistidae.

Behavior

Dominance hierarchy

P. annularis, a eusocial animal, demonstrates a dominance hierarchy, due to group competition over scarce resources. Within a colony, certain wasps will chew on or attack others to demonstrate power. Individuals that are ranked differently demonstrate different behaviors; higher ranked wasps have a propensity for “tailwagging” and “checking cells” as compared to lower ranked workers, similar to Polistes dominula. Workers have been observed to forage for caterpillars more often than queens. Queens, the highest ranked individual in a nest, will usually lay the highest percentage of eggs, though subordinates are sometimes allowed to lay eggs as well. Should a foreign organism attempt to land, a female on the nest will engage the arriving wasp with her antennae. This clash may last for several minutes and may escalate into grappling. Queens are more active on larger and newer nests. Many of these behaviors are the result of high reproductive competition between females. Despite the hierarchy and its dependence on queen size, larger queens do not necessarily inhibit egg laying by subordinates better than smaller queens. However, queens do develop far larger and more developed ovaries than their subordinates, and often have more mature eggs within.

Much of P. annularis’s dominance hierarchy is dependent on dry weight, residue weight, wing length, and fat content. Females from different nests exhibit considerable variation in these factors while females originating from the same nest are quite similar to each other in these factors. In some cases, the queen may be smaller than some of the worker wasps on the nest, weighing less and having a lower fat content, though this is atypical. Heavier wasps, as well as those with more fat content, typically have greater ovarian development; dry weight is typically more correlated with ovarian development than any other parameter. Ovarian development has been correlated with dominance hierarchy in other polistine wasps, Polistes gallicus and Polistes metricus.

Foundress grouping

Group nesting is especially prevalent and varied in P. annularis, with reports of foundresses per nest ranging from one to 28, with an arithmetic mean of 3.82 to 4.93, depending on the year. However, variation by year only explains two percent of the variance in the grouping size of foundresses. Only a small number of queens run a nest without a partner (about five percent), while about three quarters of foundresses become subordinate to a queen on the nest. This species will only become foundresses with other wasps if they were born in the same nest. Large aggregations of foundresses are seen when females reuse the nest in which they were born, either by reusing the cells themselves (uncommon) or by building a new nest on top of the old one. Where females reuse the natal nest, an average of between seven and eight foundresses may be seen on the nest, a significantly greater number than those seen on new nests (4.34). Foundress number plays a major role in determining the success of a colony. Colonies are on average 60 to 65% successful at producing workers and reproductive, respectively. However, nests with one foundress are only 20% successful at surviving until workers emerge. Nests with four or more foundresses have an 80% chance of success. Strangely, the inclusive fitness of subordinate foundresses is lower than their fitness in the case where they established a nest alone; it remains unclear why such subordinates do not leave the nest to establish their own. They may be “making the best of a bad job,” though there is scant evidence to indicate this.

Foundress eviction and mortality due to worker emergence

Multiple foundresses may create a nest; some studies have shown that an average of four to five foundresses may inhabit a P. annularis nest. However, selective pressure tends to eliminate any additional foundresses once workers emerge. This is due to multiple factors. First, given the scarcity of space and resources, the foundresses will compete with each other to raise the best offspring. In addition, workers can perform all the same functions as foundresses with minimal competition. Lastly, the relatedness between workers and their eggs versus those of the foundress; workers would be more related to their sisters than their own offspring. These factors may combine to result in the expulsion of the subordinate foundresses. Such behavior was first seen in another polistine wasp, Polistes gallicus. Subordinate foundress mortality has been found to significantly increase following worker emergence when compared to mortality rates prior to worker emergence. This pattern is also observed in P. exclamans and P. carolinus, but not P. bellicosus. In some of these species, including P. annularis, subordinates decrease foraging rate following worker emergence. Unlike other polistine wasps, P. annularis tends to more severe consequences after worker emergence, despite this change in behavior. This puzzle is currently unsolved, though it may deal with senescence. Decreased foraging also accompanies decreased aggression. Comparing across polistine wasps, foundress eviction is generally independent of rate of usurpation; P. annularis has a usurpation rate of about nine percent. When all foundresses are evicted, colony failure rate is high: in nests where multiple foundresses are evicted, around 19% of nests fail, but when only one foundress was originally present, almost 80% of nests will fail. When pupae appear, queens are less aggressive towards their subordinates, as compared to the times at which only larvae are present. Some other social insects, such as ants, evict queens, while others, such as termites, seem to permit multiple queens.

Colony cycle

P. annularis has been found to exhibits slight variations in its colony cycle from year to year based on environmental conditions, in part due to the ability of females to switch castes. Typically, the largest and top ranking female founds a nest and lays the largest percentage of eggs, while subordinates forage. After workers emerge from the eggs, many of the subordinate foundresses disappear; the nest will usually lose its original queen before more eggs can be laid at the end of the season. A worker with fully developed ovaries will take over the nest and become a gyne. It has been demonstrated that earlier queen death corresponds to an earlier arrest in rearing brood, which may be due to a decrease in relatedness between the new queen and the nascent females, or due to internal conflict on the nest. Additionally, if resources decrease, such as during a drought, brood rearing will end sooner than in more prosperous years, and females will choose to become gynes as opposed to workers. Much of these variations can be rationalized via relatedness and Hamilton’s Rule: females are more related to sisters than to brothers, and will try to skew the queen's sex ratio to be three females to one male.

Queen determination, behavior, and succession

In the absence of a queen, dominance conflicts will arise between foundresses on the nest. Wasps will routinely attack each other over a several week period, and certain wasps will eventually remit, leaving room for a new queen. Subordinates may then be chased from the nest. During the competitions, cell construction has been observed to stop, as well as oviposition. The new queen will lay more eggs than her co-foundresses, and dominate them as explained above. In so doing, she will cause the ovaries of other workers to decrease in size, to the point that they will be eliminated following the hatching of new workers. The queen will lay eggs, while the other females are constrained to laying eggs solely at the beginning of the association of the group. The queen may lay up to 55% of eggs, while her direct subordinates, named beta subordinates, will lay most of the remaining eggs. Should the queen disappear, the previous beta subordinate will take over as the new queen, leading to increased aggression. This aggression may result from the competition to lay eggs. When a foundress succeeds as the new queen, no additional aggression is observed, but if a worker becomes queen, aggression rises, likely because the dominance hierarchy is not as well established on these nests. Often when the queen dies, the oldest foundress will succeed her, due to her size and dominance in the nest. Despite aggression after the removal of a queen, brood care does not decrease.

Sex ratios

Other species of Polistes exhibit a female biased sex ratio due to the 3:1 relatedness between sisters, while still other members of the genus have a virtually unbiased sex ratio. Research has demonstrated that northern wasps tent to have biased ratios, while southern wasps are less biased, in large part because southern wasp sisters have lower relatedness. Since the relatedness between sisters in P. exclamans is 0.39, and since P. exclamans and P. annularis have similar geographic distributions, one may presume that the sex allocation patterns of P. annularis follow that of P. exclamans.

Recent studies have noted that members of the brood are highly related to the queen, but are less related to the subordinates and their mates, aligning more with a 1:1 sex ratio. As such, it is to the benefit of the wasp to be a reproductive queen as opposed to a worker, if feasible.

Honey caching

P. annularis was the first wasp which did not form large colonies to be observed storing honey in the autumn. The wasps store the honey over the winter in order to facilitate their survival in colder months and assist in the recognition of sisters, who will help construct new nests near their original one. The caching begins in early to mid-autumn, after which time the wasps retreat from the nest to hibernacula when the temperature drops below approximately 5°C. They return to the nests when the weather permits, such as on warmer days. This behavior exposes P. annularis to attack from other insects of the same species, wasps of other species, such as those of the genus Vespula, and from mammals. However, Polistes have evolved to hide their nests from most mammals or put them in an area as to make them inaccessible. Invading insects are attacked. Despite the fact that birds are common enemies of polistine wasps, none have been observed attacking nests for honey. The wasps may either gorge on the honey, or ration it, given that they are social insects. Additionally, it has been found that wasps with nests containing honey are significantly more likely to survive the cold winter than those wasps whose nests do not contain honey.

Altruism

As with many other species in Hymenoptera, as well as other polistine wasps, P. annularis has been noted to engage in altruistic behavior. For example, despite the lack of drastic morphological differentiation between workers and foundresses, and the benefits procured by a worker becoming a foundress on a new nest, a worker may lay less than ten percent as many eggs as her queen, independent of the number of females on the nest. This results in vastly decreased inclusive fitness for the worker and greatly increased inclusive fitness of the foundress, even if the sisters are related by a factor of 0.75, the maximum possible relatedness for outbred sisters. Should the works be acting in such a manner, it may create a direct cost upon which selection can act. However, evidence for this has been scant, and researchers are currently investigating why and how such levels of altruism are sustained in P. annularis.

Pre-nesting aggregations

As new nests are established by P. annularis, many behavioral characteristics from the prior nest are taken to the new one. Notably, related siblings tend to aggregate together and go to the same nest, where possible. However, unlike in other Hymenoptera (eusocial insects), the wasps tend to act as independent agents; while the queen is at the top of the dominance hierarchy, the other wasps may lay eggs and forage for themselves as they see fit. As queens or foundresses evict other females, the aggregations will begin to dissolve.

Nest construction and site selection

Characteristic design of a paper wasp nest

P. annularis females select a new nest site every year; they never use the same nest twice. The site is often within several meters of the previous one. Nests have been seen on cliffs and buildings, and less commonly in trees and on shrubs. Occasionally, wasps will construct a new nest on the face of the old one, but will not use the old one to store larvae. Researchers have speculated that the old nest is not reused because it is often damaged, dirty, and parasitized in the late summer or winter. Other animals, such as moths, may use the empty nest during the winter.

Being a paper wasp, P. annularis will gather wood fibers to construct the new nest. The queen will typically remain on the nest while subordinates search for wood. Fibers are often acquired by scraping the surface with the mandible, and mixing the resulting fibers with saliva to create a pulp. The pulp is added to the nest in a consistent manner.

Collection of food

Due to the dominance hierarchy, the queen leaves the nest as little as possible, and has the other wasps forage on her behalf. In small groups of wasps (two to three foundresses), the queen may be forced to forage. As workers emerge, they take over the role of foraging. P. annularis preys on moths, butterflies, flies, and other insects. Unlike other organisms in the genus, P. annularis generally takes its food from wooded areas, and not from fields. However, prey represents only 20% of food resources; the other 80% is liquid.

Winter behavior and cold hardiness

Despite morphological similarities, queens and workers exhibit different behavior over the winter months. Queens will overwinter, whereas workers will not. Researchers have theorized that this difference in behavior is a direct consequence of the dominance hierarchy. Experimental treatment of wasps to cold conditions resulted in increased trehalose in both sexes, though females also increased levels of glucose and fructose, while males maintained or decreased these levels. Wasps at normal foraging temperature showed significantly lower carbohydrate levels than wasps treated at colder temperatures. In addition, exposing P. annularis to low temperatures over a two week period caused many workers, but no foundresses and few males, to die. Foundresses appear to have greater glycogen storage capability than workers, helping them survive low temperatures. Males often have higher glucose, fructose, and trehalose levels higher than those of queens, indicating that mating may continue into autumn or winter. P. annularis cannot tolerate frost, though it has been shown to be able to survive at temperatures below the minimum temperature in the area in which it resides.