||This article's lead section may not adequately summarize key points of its contents. (November 2013)|
|Metricus paper wasp|
|A pinned metricus paper wasp|
|Species group:||Polistes fuscatus|
The Metricus paper wasp (Polistes metricus) is a wasp native to North America. It ranges throughout the southern Midwest, the South, and as far northeast as New York, but has recently been spotted in southwest Ontario. A single female specimen has also been reported from Dryden, Maine. Polistes metricus is dark colored, with yellow tarsi and black tibia.
Nests of Polistes metricus can be found attached to the sides of buildings, trees, and shrubbery.
Polistes metricus, like other Polistes species, has evolved eusociality and demonstrates behaviors including nestmate discrimination and local mate competition. Polistes metricus' populations also demonstrate a deviation from Fisher's 1:1 sex ratio. Like the other members of the order Hymenoptera, Polistes metricus has a haplodiploid genetic system. Nests of Polistes metricus have distinct characteristics like the ability to share nests with other Polistes species and reutilize nests multiple seasons.
- 1 Description
- 2 Behavior
- 3 Effects of Resources
- 4 Nest Characteristics
- 5 Parasites and Parasitoids
- 6 References
Polistes metricus has a dark ferruginous (rusty) color with black markings on its thorax and a mostly black abdomen. Its tibia is black, and the tarsi are yellow. The three ocelli are contained within a black spot, separate from the antennae. Females bear six abdominal segments, while males bear seven. Swelled parts of the abdomen of this species is a key defining element.
Polistes metricus is found to be most closely related to Polistes carolina. The most recent phylogenetic analysis shows that both Polistes carolina and Polistes metricus share a common ancestor with Polistes bellicosus.
Polistes wasps including Polistes metricus prey on Lepidopteran larvae (i.e. caterpillars) most frequently. They are considered generalist predators. Polistes also eat other insects. Manipulation by parental feedings has been hypothesized to pay a role in the development of larvae. The proponents of the hypothesis argue that the well fed larvae grow to into be reproductive adults. However, it was found that parent manipulation of the brood is not done is Polistes metricus.
In order to differentiate female workers from female reproductive wasps in Polistes metricus, behavior must be studied because they are morphologically similar. Female reproductives are referred to as queens or foundresses. The reproductive queens have been shown to have more developed ovaries compared to the workers. Consequently, female Polistes have the ability to be reproductive depending on the social and physical environment they occupy. Polistes metricus is considered to be a eusocial organism, characterized by sterile castes, overlapping generations, and cooperative care for offspring. Polistes metricus, like Polistes dominula and Polistes annularis, differs from many other advanced eusocial insects in that the distinction between reproductive females and non-reproductive females is subtle.
Deviation from 1:1 Sex Ratio
Polistes metricus has been shown to demonstrate a deviation from Fisher’s prediction of a 1:1 male to female sex ratio. Despite the equal parental investment in the sexes, the average frequency for females in Polistes metricus is approximately .55. In most species that deviate from a 1:1 sex ratio, there is a degree of sexual dimorphism, with the more common sex being smaller. Polistes metricus follows this trend, with females being smaller than males on average.
Like other insects of the Hymenoptera order, Polistes metricus has a haplodiploid genetic system. When a queen’s eggs are fertilized, they produce diploid daughters; unfertilized eggs produce haploid male offspring. Consequently, in colonies with a monogamous queen, females are more closely related to their sisters than to their brothers, mothers, or future offspring. It has been argued that, due to the 3/4 relatedness between haplodiploid sisters, there exists a conflict between the queen and her female offspring. Robert A. Metcalf argues that data on the skewed sex ratios does not prove or disprove the hypothesis of parent-offspring conflict over male production and parental investment. Metcalf’s study shows that the foundresses control the production of males by restricting sperm availability to workers.
The importance of haplodiploidy in terms of the evolution of eusocialty was brought into question using a phylogenetic study of Polistes by Pickett et al. The paper argues that through a phylogenetic test they found that there is little support for the idea the haplodiploidy lead to early social evolution in the Polistes genus.
Local Mate Competition
In a population that has a female biased sex ratio, according to W. D. Hamilton, there could be Local Mate Competition, or LMC. Local Mate Competition occurs in female biased populations because brothers must compete with each other for mates. These mates might be their sisters; therefore, inbreeding should be higher in these populations. Robert A. Metcalf's study of inbreeding in both Polistes metricus and P. variatus using phenotypic differences found that there were no signs of inbreeding in either population. According to Metcalf’s research, the lack of inbreeding in Polistes metricus makes it unlikely that LMC occurs. Additionally, there has been research that shows that some populations produce only male reproductives. This occurs when the queen dies and the workers lay eggs. The female workers are not fertilized so they produce males who are haploid.
Effects of Resources
Local Resource Enhancement, Size, and Reproductive Ability
Food supplementation and access to food does not affect the number or the sizes of worker offspring. This could suggest that there is not the adaptive advantage to make larger workers. Stabilizing selection is present to make a fixed number of workers regardless of nest size or food availability. The selection shifts toward the production of more sexual females than larger males. The colonies that are supplemented with food also produced more reproductive females than larger reproductive females which suggests that female sexual production in resource limited conditions is very expensive. Males however, are slightly larger on average which could mean that the number of males produced is not as critical as male size. It has been proposed that larger males helps the colony by attacking foreign males, and defending nest territory.
Nutrient level of certain macro and micronutrients varies throughout the life history of the organism. There are few large differences in nutrient levels between workers and gynes however, gynes process lipids differently by retaining it. On the other hand the differences in nutrient levels in reproductive larvae and non-reproductive larvae are strong. Additionally, nutrient levels does not change throughout the lifetime of both male and females Polistes metricus. Based on these observations, the nutrient level at the larval stage tends to correlate with the reproductive ability of the larvae as it develops.
Increased food resources have also been shown to lead to an increase in the production of gynes at the expense of worker production.
Nest Sharing and Nest Reutilization
In certain locations across the United States, such as eastern Kansas, individual foundresses of Polistes metricus can maintain more than one nest or even share a nest with another foundress. In cases where multiple nests are maintained by a single foundress, males are not produced. This phenomenon is due in part to the extensive parasitism found in eastern Kansas. Chalcoela iphitalis is a parasitic moth that is commonly found in the nests of Polistes metricus. It is not known why Polistes metricus has evolved this behavior. It is has been proposed that the behaviors are able to limit the chance of reproductive failure for any one foundress.
In some situations, it has been observed that a nest is shared between two species of Polistes. One case is the sharing of a nest by Polistes metricus and Polistes fuscatus. The different species of wasps were seen to coexist without conflict. The evolutionary significance of joint nesting is unclear. Proposed hypotheses include the fact that it might represent the beginning stages of interspecific mutualism or exploitation.
Nest reuse has been observed in Polistes metricus species. It is common for a foundress to use the nest multiple times within a reproductive season. However, it is very unlikely for foundresses of any species to return to a nest in a subsequent season.
Adult female Polistes use their salivary enzymes to soften wood or paper products to form a nest pedicel. The adult female foundress or foundresses will then begin to build the nest cells. They will also lay eggs. In colonies with multiple foundresses, one will become dominant and lay the most eggs. Once the first worker develops, the nest moves from the founding phase into the worker phase. Following the worker phase is the reproductive stage, where the dominant foundress loosens her control over the other colony members and the workers lay sterile male eggs. The period in between the reproductive phase and the founding phase of the next season is the intermediate phase. Adult female gynes and adult males leave the nest to mate.
It has been suggested that Polistes metricus females discriminate between nestmates and non-nestmates using genetically determined cues regarding food, nesting materials, or nest site. These genetically predetermined cues have been found in Lasioglossum zephrym, a species of bee at a similar social level. Polistes metricus gynes can detect and work with former nestmates without being inside the nest in which they were born. In situations where there is no nest, nestmates are more tolerant of each other than are nestmates in Polistes metricus. Former nestmates are often assumed to be sisters or other close relatives.
Nestmate discrimination does not occur in workers or non-reproductives, which could suggest a relationship between nestmate discrimination, nutrient availability, and the environment.
Multiple Foundress Colonies
In Polistes metricus, colonies with more than one queen are aptly named multiple foundress colonies. It is beneficial for a colony to have many foundresses because they can protect the nest from usurpers, parasites, and help rebuild the nest if it is destroyed. It has been shown that individuals in multiple foundress colonies spend less time at the nest compared to individuals in single foundress colonies. The dominance hierarchy in multiple foundress colonies consists of the dominant foundress at the top, and subordinate foundresses beneath her. Gynes and worker wasps make up the base of the hierarchy. Dominant foundresses in multiple foundress colonies forage for wood, pulp, and nectar, while subordinate foundresses take long foraging trips and return with the majority of the sustenance for the colony. Foundress mortality increases after the workers emerge. Also, less dominant foundresses are often kicked out of the nest prior to the laying of eggs that will become reproductive. It is improbable that subordinate foundresses will become dominant. Lastly, if a subordinate foundress lays eggs, the dominant foundress will often consume them. Based on the subordinate foundresses status in a multiple foundress colony, it may appear to be better for her to create a new colony of her own. However, creating a new colony is more difficult to do than waiting to displace the queen. It has also been proposed that natural selection favors foundresses that have offspring that are a mixture of dominant foundresses and workers.
Parasites and Parasitoids
Parasitoids and parasites of Polistes metricus tend to affect the colony at different stages in their development. It has been shown that C. pegasalis is able to infect a colony at any stage except the egg stage. C. pegasalis prefers to infect colonies that have larvae and pupae. X. peckii is similar because it prefers to parasitize larvae and pupae. In contrast, X. peckii can infect any stage of development. Infestations of Xenos parasites happen frequently in Georgia. 
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