Species of wasp
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Protopolybia exigua is a species of vespid wasp found in South America and Southern Brazil. These neotropical wasps, of the tribe Epiponini, form large colonies with multiple queens per colony. "P. exigua" are small wasps that find nourishment from nectar and prey on arthropods. Their nests are disc-shaped and hang from the undersides of leaves and tree branches. This particular species of wasp can be hard to study because they frequently abandon their nests. "P. exigua" continuously seek refuge from phorid fly attacks and thus often flee infested nests to build new ones. The wasps' most common predators are ants and the parasitoid phorid flies from the Phoridae family.
Taxonomy and phylogeny.
The Epiponini belong to the swarm-founding wasp subfamily Polistinae. Epiponini are neotropical swarm-founding wasps characterized by large, multiple-queen colonies. The "Protopolybia "genus consists of small paper wasps. Ducke (1905) morphologically separated "Protopolybia" from "Pseudochartergus" and established their close relation due to physical similarity of a medial posterior process on the metanotum.
Description and identification.
"P. exigua" are small, swarm founding wasps. One can determine the wasps' age by evaluating the darkness of the cuticle on the apical edge of the fifth gastral sternite. Older wasps typically exhibit darker shades relative to others in its colony. The front wing length is generally 4.15mm, and their wings are more rounded than other wasps. This round wing shape may enhance the wasp species' flying abilities and increase fitness. The muscle mass on P. exigua's mesosoma directly contributes to flight power; the larger the muscle, the stronger the output. Instead of flapping their wings, "P. exigua" row them due to increased wing drag. Mature "P. exigua" eggs have a complete chorion and are tinted blue with a shiny coat. Immature oocytes that are equal in size to mature eggs can be distinguished due to their cream and less glossy appearance.
Nest composition.
"P. exigua" nests are white or light brown and exhibit domed, paper envelopes. These fibrous envelopes cover the entire comb. The comb is suspended from a leaf or branch by one or more peduncles, with an exit-hole on the side or bottom of the envelope. The nests are also observed to provide many entrance holes, which seem to be a unique feature to the species. On average, there are 1.7 combs and 566 cells per colony. While "P. exigua" nests typically have one comb, additional combs are sometimes found on certain nests. Cells with brown wafers of meconium towards the bottom indicate that there has been an emergence of new larvae; this discoloration is the result of the larva shedding its waste products during pupation. "P. exigua" tend to build their nests in locations facing west; it is suggested that this specific placement leads to higher environmental temperatures and proportionally greater foraging activity.
Distribution and habitat.
"P. exigua" are found from Bolivia to southern Brazil, with particular sightings in Venezuela, Guatemala, and Colombia. The wasps can inhabit orange orchards and eucalyptus plantations. They settle in green areas surrounded by small-sized trees and bushes. Unlike the mud nests of "Polybia emaciata" and "Parachartergus colobopterus" that are strongly attached to stable foundations, "P. exigua" nests are more frequently destroyed. Their nests generally hang from the underside of plant leaves and tree branches. The nests rarely exist for more than 6 months since they are attached to leaves that eventually fall; in addition, the nests are constantly vulnerable to parasitoid infestations. Parasitoid nest destruction can uncouple the wasps' colony cycle from their nesting cycle and ultimately disunite the colony.
Colony cycle.
In the course of a colony's life, epiponine wasps flux from polygyny (many queens) to oligogyny (few queens) to monogyny (no queen). This cycle was first proposed by West-Eberhard (1978,1981) and is referred to as cyclical oligogyny. It has been suggested that this cycle is crucial for maintaining high genetic relatedness found among epiponini colonies. The series begins when a group of workers and queens found a nest. As time passes, the colony's total egg layers diminish due either to death or from wasp dominance interactions. The number of egg layers decrease until a couple of queens are left in the colony. Once the colony's queen population is significantly reduced, then a new generation of reproductive females are conceived. Since only a couple of mothers produce this new generation, the female offspring are closely related. Sometimes new queens will leave the old nest and found their own colony, thus repeating the colony cycle.
The different stages in the wasp's colony cycle are labeled and separated by emergence. The pre-emergence stage refers to when there are not many wasp larvae present in the nest. The post-emergence stage is when a colony grows from an increase in larvae population. There is a higher rate of egg-laying and greater number of pupae developing at this phase. According to Noll (1995), the post-swarming stage is when immature wasps are in the mother nest, after the production of sexed individuals and swarming. A small number of old queens and males may also be present at this time. Colonies produce swarms when they have many laying queens; they produce new queens when there is only one queen left in the colony. The number of males produced by queens is lower compared to the number of males produced during swarms. However, colonies that produce swarms also produce males. Swarm queens and swarm workers are produced at different points in the queen cycle, with workers being produced after new queens become reproductive on the parental colony.
In a report comparing 37 "P. exigua" colonies from Venezuela, the colony population averaged 171 workers, 20 queens, and 24 males. When there was at least one male per colony, the average population of males in colonies was 69.
Kin selection.
Caste differentiation.
The morphological differences in" P. exigua" queens and workers become more distinct as the colony cycle progresses. There are fewer differences within early stage colonies, and correspondingly greater discriminations among later stage colonies. In addition, physical discrepancies between wasp castes are more prominent in colonies with a smaller number of queens. "P. exigua" queens are generally easier to identify, while it is harder to distinguish between workers and intermediates. The thoracic labial gland is better developed in workers than in queens, but wasp brain size does not differ between castes. Both inseminated and non-inseminated "P. exigua" females exhibit a greater presence among colonies that frequently experience nest destruction. This might be because nest destruction uncouples the colony cycle from the nesting cycle and can also cause colonies to separate when this might not otherwise have occurred. Males are identified by the presence of testes. Queen body size and relative fecundity seem to be important determinants of queen success within a given colony.
Genetic relatedness.
Since "P. exigua" colonies consist of many queens, wasps within a given colony should be expected to have low relatedness among one another. However, cyclical oligogyny seems to help colonies maintain a higher level of relatedness. In cyclical oligogyny, new queens are only conceived after most of the colony's queens have died. This late production of new queens guarantees that only a few queens from the previous generation participate. The relatedness among workers in "P. exigua" is 0.39, which is sufficient for wasps to exhibit worker behavior. Among queens from the same colony, 0.82 are genetically related. Therefore, queens demonstrate a significantly greater genetic relatedness than do workers from their same colony. Thus far, every epinonine species studied exhibits high relatedness among queens. This outcome of relation seems to play a crucial role in the wasp's social behavior and in the colony's success.
Queen selection and worker preference.
Although queen selection is not fully understood, workers clearly have an important role in the queen elimination process. Workers often test queens, and depending on the queens' response, the workers may remove them from the colony. It has been suggested that workers discriminate between queens based on their relative fertility. Workers seem to identify the more fecund queens through various behavioral tests and dismiss any queens with inhibited ovaries. Queen size could be an additional determinant in queen selection. Since size differences between queens and workers increase as the colony ages, smaller queens could be eliminated to create this morphological gap. This selection process also correlates with the fact that a colony's queen population decreases as the colony ages. Colonies begin to gain more queens once they have reached a minimum queen population. The new queens usually reproduce in the old colony, which lowers relatedness. Swarming occurs afterwards, frequently during the queen-reduction cycle. The dissociation between queen cycle and the colony cycle allows worker interests to be satisfied with respect to the timing of queen and male production, without sacrificing the ability to initiate new colonies at appropriate times. Workers prefer this selective pattern of queen emergence because they are three times more closely related to their sisters than to other brothers on single-queen colonies. Workers are equally related to the nieces and nephews that make up most of the brood on multiple-queen colonies.
Intermediates.
Intermediates are workers with fully developed ovaries. These wasps are non-inseminated laying females, and they are present throughout the entire colony cycle. Intermediates lay eggs, but the eggs are eaten either by themselves or by other colony members. It is possible that non-inseminated laying females are always present in order to provide extra energy in the colony. Their eggs may be used for trophic purposes, since male production is controlled entirely by queens. This oophagy (egg eating behavior) can provide additional sustenance for the colony. Inseminated workers are typically older and are likely to have been potential queens at colony initiation. Therefore, non-inseminated egg layers are young and seem to be about to swarm. In a study on "P. exigua "foraging activity, a higher proportion of workers than intermediates were observed in all the colonies. Their study observed 12 colonies in Bom Jesus da Lapa, Bahia, Brazil.
Ovarian distinction.
Three kinds of ovary development have been recognized in "P. exigua". Queen wasps develop ovaries that bear a range from two to several mature oocytes. Intermediates exhibit ovarioles with some oocytes at the beginning of development. Oocytes were sometimes found in the final phase of vitellogenesis as well. Vitellogenesis is the process of yolk formation through deposition of nutrients in the oocyte. Workers have filamentous ovarioles with either no visible oocytes or slightly developed oocytes. Only queen females' spermatheca contained sperm. On average, inseminated queens contain 3.6 mature eggs in their ovaries. Egg laying workers appear to be common in the species, though they frequently eat their own eggs. This oophagy suggests that worker wasps do not compete reproductively with the queens. Most young workers of "P. exigua" apparently oviposit before their ovaries regress and they begin foraging. Egg eating must occur within this species; otherwise the large numbers of worker-laid eggs would yield many more males than were found in collected colonies.
Behavior.
Foraging.
Foraging wasps gather nectar and prey for nourishment and water, resin, and wood pulp for the nest. Nectar is a crucial resource for any colony since it is used for feeding both immatures and adults. Foragers also acquire nectar at lower energy costs compared to that needed to capture prey. It seems to be more advantageous for "P. exigua" to take the prey in the crop, since their flight becomes less difficult. The number of prey captured by foraging wasps tends to increase as the colony grows. In addition, "P. exigua" have been reported performing trophallaxis with adults and between adults and larvae. Foraging for animal proteins is the most complex task for worker wasps. In order to successfully hunt prey, wasps require prey recognition, predation ability and the capacity to return to the nest. Foragers must make recognition flights to get an image of the surrounding landmarks that compose its environment at different distances. These flights ultimately enable wasps to use visual markers to direct them back to their colony. It is extremely important that workers carry water to the nest; water collection occurs more often in the warmer periods of the day. Water is a crucial source for nest building, and in its absence, this activity would not happen efficiently. Wood pulp is also collected and taken to the nest for building and nest infrastructure.
Environmental influences.
The earliest "P. exigua" foragers leave the nest around 6am and foraging stops around 7pm. Foraging activity has been recorded from