Polistes carnifex

Polistes carnifex is a neotropical vespid wasp in the cosmopolitan genus Polistes and is native to Central and South America. It is a small, yellow and brown social insect that establishes small colonies which build papery nests under the eaves of buildings or suspended from branches.^[3] The colonies are often founded by solitary queens, many times in the form of winter aggregations to wait until environmental conditions are more suitable for building a colony.^[4] Not all nests have a female with developed ovaries.^[3] Foraging adults bring nectar and small caterpillars back to the nest to feed to the developing larvae which are individually housed in separate cells in the nest.^[5] Evolutionary adaptations include mandibles and teeth. Worker-queen conflict, worker policing, and dominance interactions all come into play for the species. Polistes carnifex is territorial and is parasitized.

Taxonomy and phylogeny

Polistes carnifex belongs to the Polistes genus, which is the largest genus in the Vespidae family and the only genus in the Polistini tribe.^[6] Polistes carnifex is a species of paper wasp and, as like other members of the subfamily Polistinae, is a eusocial wasp.^[6] One of the hypothesized phylogenetic trees puts Polistes carnifex most closely related to Polistes major and more distantly related to the following species: Polistes apachus, Polistes aurifer, Polistes bellicosus, Polistes carolina, Polistes metricus, Polistes poeyi haitiensis, and Polistes perplexus.^[7] However, there has not been a consensus with regard to the phylogeny of Polistes carnifex so no one phylogenetic tree can be termed correct.^[7]

Description and identification

Polistes carnifex is the largest neotropical wasp in the Polistes genus.^[8] Despite its size, it is a relatively non-aggressive insect.^[3] It can be distinguished by its brown and yellow stripes.^[9]

The maximum size for a nest of Polistes carnifex was approximately 9 cm in diameter.^[3] One nest in which one emergence at minimum occurred had an average length of 27.8 mm.^[3] Nests range in the adult population from 4 to 13 individuals.^[3] In the nest sampled in a study, there were 28 cells and this number remained constant for the duration of the observation(17 days).^[3] There was only one cell observed as being enlarged, which was on the periphery.^[3] Nests are sometimes built under the eaves of buildings and in other areas where they can hang.^[3] Polistes carnifex is a social species and the nests consist of a number of horizontal papery cells in which the young are reared.^[3] The nests are hanging and open-faced, supported by a single pedicel in the centre which is strengthened by a tough gelatinous material^[3]

Distribution and habitat

Polistes carnifex is native to Central and South America; its range extends from Mexico to northern Argentina.^[3] The species is found in coastal, humid, and open areas, such as in evergreen tropical forests.^[10] In a study in Costa Rica, nests were found hanging from branches of various species of low thorny trees near an ephemeral swamp. Nests have been seen under the eaves of buildings. The nests were sometimes found within about a meter of nests of a Polybia sp. wasp and occasionally in similar close proximity to a Mischocyttarus sp. wasp nest.^[3] Some trees bore several nests of different wasp species while many other similar trees bore none, which suggests that there is a nonrandom distribution.^[3]

Colony cycle

"Little is known of its [Polistes carnifex's] biology or behavior."^[3] As a member of the Polistes genus and the Vespidae family, it is possible to predict the species' colony cycle.

The nest is originally founded by a solitary queen which builds first one and then further cells from macerated pulpy material.^[5] To create a colony, the queen enlarges a cell by introducing a ball of recently macerated pulp on her own. Using her mandibles, the queen loads the ball while holding the sides of the wall being constructed with her foretarsi. While completing this task, the queen moved her antennae in circles about her head and she touched the opposite wall that lied parallel. The antennae-wall contact allows the queen to construct straight sides on the inner wall.^[3] She lays eggs and feeds the larvae, bringing them nectar and caterpillars as they grow. The female workers that emerge from the first cells then assist with the further building and development of the colony, and can themselves mate and lay eggs.^[5]

Given that Polistes carnifex inhabits temperate climates, it is expected to have a particular annual colony cycle and for young queens to hibernate in the winter. Females in the subtropics undergo “hibernation” in the form of winter aggregations under harsh climate conditions. In the species studied thus far in temperate climates, including paper wasps, body sizes increase as an unfavorable season approaches. Studies show that larger females arise as a result of winter aggregations, and they become new queens. Scientists predict that winter aggregations in tropical species allow certain females to wait for improved environmental conditions before starting a new nest, instead of starting a new nest immediately upon emerging.^[4]

Nest Morphology

The maximum size for a nest of Polistes carnifex was approximately 9 cm in diameter. One nest in which one emergence at minimum occurred had an average length of 27.8 mm. Nests range in the adult population from 4 to 13 individuals.^[3] While there was not information on the growth rate of the colony in Polistes carnifex, another species in the Polistes genus, Polistes metricus grows slowly as the solitary founder may cannibalize eggs.^[11]

Reproductive Behavior

One point worth noting is that not all nests contained a female with "well-developed ovaries." Another interesting point is as follows: in a study in Colombia, twenty-nine foraging wasps were observed returning to a particular nest with twenty-five loads of nectar, three loads of macerated prey and one of nest-building pulp. When a foraging wasp arrived, the highest ranking wasp present demanded food and then both fed the larvae. Each wasp pushed its head into a cell, drummed on the cell walls with its antennae and then deposited the food. The drumming noise could be heard a meter away and may have alerted the larvae to the presence of food.^[3]

Feeding

The English naturalist Thomas Belt observed how a Polistes carnifex wasp which had found a large caterpillar, chewed it up and made half of it into a macerated ball. Picking this up, it hovered for a few seconds and then circled several times round the place among the dense foliage where the other half of the caterpillar lay. It then flew off but returned a couple of minutes later and quickly located the correct hole among the leaves. Making its way in among the foliage, it could not at first find the exact leaf on which the caterpillar lay. After several fruitless hunts interspersed with short circling flights, it finally located the corpse and flew off with its trophy. Belt marvelled that the insect could use a mental process so similar to that a human might have used to remember the specific location of its prey.^[9]

Evolutionary adaptations

As a member of the order Hymenoptera, Polistes carnifex has mandibles, which may be used to cut vegetation, obtain wood fibers, dig nests, or capture and destroy prey.^[12] The mandibles of Polistes carnifex are short. Yet, they are markedly wide at their base, with a length to basal width ratio of approximately 2.0. An external basal area stretches "from the basal margin ... to a point situated about half-way the mandible's length." Polistes carnifex also have teeth. "A convex distal posterior area ... is continuous with the posteriormost apical tooth and stays adjacent to a distal media area." This area is convex in Polistes carnifex. In Polistes carnifex, the third tooth's anterior edge is elongated, compared to in other species.^[13]

Interactions within the species

Queen-worker conflicts

Given that the queen is equally related to her daughters and sons, she should produce male and female offspring in the same proportions according to Fisher's theory of equal investment. Workers, in contrast, are more closely related to their sisters than to their brothers. Thus, workers would prefer to have more female siblings. If workers rear too many females than the drones will gain a reproductive advantage over the queen. The stable sex ratio from the worker's point of view is 3 females to 1 male.^[14]

Worker policing

As a member of the order hymenoptera, Polistes carnifex is subject to worker policing. There is a conflict between workers and queens over who should produce the males. While workers do not mate, they are able to lay unfertilized eggs that will develop into males (since they are haploid). Since the relatedness of a worker to its offspring is r=0.5, while its relatedness to the queen's sons is r=0.25, workers prefer to produce males. However, the queen's relatedness to its own offspring is r=0.5, where as the queen's relatedness to her workers' sons is r=0.25, thus the queen prefers to bear her own sons. The other workers are more closely related to the queen's offspring than to their brothers' offspring. The consequences of these differences in incentives is as follows: queens try to suppress the production of eggs by workers and workers act to suppress other workers from producing eggs. This phenomenon is known as policing.^[14]

Dominance interactions

Dominance interactions amongst females were not overly aggressive. The wasps with the highest status would be the first to receive food upon the forager's return. The higher ranking wasp remained a farther distance away from the substrate than those with lower statuses.^[3]

The dominance interactions are almost only related to feeding. One of the few times in which such an interaction was seen was when an unprovoked queen grasped a third-tier individual by the legs. The queen then bit the wings, eyes, neck, thorax, and head of the lower ranking wasp. After the attack, the subordinate returned to the top of the nest and the queen went to the face of the nest.^[3]

Interactions with other species

Nests (hanging from low branches on thorny trees near a swamp) were sometimes found within about a meter of nests of a Polybia sp. wasp and occasionally in similar close proximity to a Mischocyttarus sp. wasp nest. Polybia and Mischocyttarus are often associated in the same province; however, Polistes carnifex only occasionally was found in it. (There is a change that Polybia and Mischocyttarus are in fact the same species.) “The association of carnifex with other species of social wasps has not been reported outside of Costa Rica.”^[3]

Territorial behavior

In Costa Rica, male Polistes carnifex congregate on the top of ridges where they maintain territories. The males chase away other conspecific males from these territories which consist of groups of trees and shrubs with no nests. It is suggested that females only mate with males in occupation of these territories.^[15]

Parasites

In a study of the nests of Polistes carnifex, there was no conclusive evidence in any of the nests that suggested parasitism. Although in three cells there was an additional egg that may appear to come from another species, these eggs were of the Polistes carnifex species. Moreover, when 10 larvae were dissected at random, they did not suggest parasitism.^[3]

Polistes carnifex wasps are parasitized by strepsipteran insects in the genus Xenos.^[16] These obligate parasites infect the developing wasp larvae in the nest and are present within the abdomens of female wasps when they hatch out. Here they remain until they thrust through the cuticle and pupate (males) or release infective first-instar larvae onto flowers (females). These larvae are transported back to their nests by foraging wasps.^[17]

References