Polybia sericea

Taxonomy and phylogeny

Polybia sericea is a species within the genus Polybia of subfamily Polistinae. Polybia is the most diversified genus within tribe Epiponini, which is characterized by colony founding via swarming. Placement of Polybia among related genera has fluctuated, and the genus may be paraphyletic.

Description and identification

P. sericea individuals are medium sized wasps, about 17 mm long. They are dark in color, and tend to be slow-flying and docile, unless they are disturbed. Queens typically have larger abdomens, and smaller heads and wings than workers.

Workers build enclosed, multi-tiered roughly spherical nests of carton paper maché-like pulp of plant fibres chewed with saliva. Nests are reddish-brown in color, and are not very large. They are typically built between two and five meters off the ground, in the branches of trees or shrubs.

Distribution and habitat

Individuals of P. sericea live in South America, and are commonly found throughout Brazil. A large presence of Polybia sericea inhabits the lower tropical Amazon region of Brazil. They tend to live in semi-arid climates, in open areas, around forest edges, and near rivers. This species prefers to live in areas with grasses, trees, and shrubs. This is likely because these areas make good places to build nests and to forage for food.

Colony cycle

P. sericea colonies can range from a few individuals, to a few thousand, but are typically made up of several hundred workers. The number of queens in a nest can range from one to several queens. New colonies are founded by swarming emigration, where the entire colony travels to a new nest site. This may happen when a nest becomes unsuitable for living or when a predator attacks the nest and forces the colony to leave.

Swarm emigration

Swarm emigration occurs when a colony moves from the location of one nest to a new nesting site. This may occur for several reasons. One is if the colony sends a reproductive swarm from the original nest. This may happen if the colony has grown too big, and must split into two new colonies. In this situation, part of the colony will remain in the original nest, with a subgroup leaving to colonize a new nesting site. Other reasons swarm emigration occurs is if an old nest becomes unsuitable, or if the nest is attacked by a predator. At first, when the colony is driven from the nest, individuals assemble in groups of 2 or 3 around the original nest. Wasps will exchange between groups until the entire colony is assembled in one cluster. The colony may remain in this cluster for up to 2 days before migrating to the new site. Once the colony begins to travel, leading individuals will drag their gaster over prominent objects such as branches or tall grass. This releases a pheromone that other workers can follow to the new nesting site.

Pheromone communication

Pheromone communication can be used to recruit workers to a food source, a new nesting site, or a place where work is required. The pheromone is released from the abdomen of the wasp, at the base of the fifth sternite. It is excreted by Richard's glands. The pheromone has a strong leather-like odor, and looks like brownish wax. Because of the strong odor, workers can easily follow a pheromone trail left by other colony members. The pheromone is produced slowly and continuously throughout life, so it can be used up if used too often or too quickly. Another species that exhibits this behavior is Synoeca surinama.

Foraging

P. sericea individuals forage for flower nectar, nest building materials, and for prey. Workers tend to search for building materials such as wood pulp in the mid-morning, and for food in the later morning and early afternoon. Workers generally forage close to the nest because they cannot fly over long distances. These wasps can fly for about 75 meters, which is low compared to other species in the genus Polybia. When searching over longer distances, return rate back to the nest decreases. When availability of food resources is low, colony development becomes limited. Polybia sericea can store nectar as honey to use in times of scarcity.

Hunting

When workers hunt for arthropods, they fly from side to side about half a meter off of the ground. Visual cues are used to determine where to search for prey, whereas olfactory cues are used to determine that prey is near, and elicit landing. Green and brown caterpillars are the most common prey for P. sericea. Once a caterpillar or other insect has been found, workers bite the insect behind the head. They will then drag the prey up onto a twig or a piece of grass, or will sometimes fly the insect to a different location; they do this in order to keep ants or other insects from stealing their prey. Once the insect is in a safe location, the wasp will malaxate the prey, or soften it by mixing it with a thinner substance, presumably saliva. The worker will then bring what it can back to the nest to feed other individuals. If prey is too large to bring back to the nest, foragers will leave the prey, and fly in side to side arcs back to the nest to recruit other workers to help carry the load.

Genetic relatedness

Colonies of Polybia sericea are polygynous, and generally have more than one queen. Typically, the number of queens in a colony is three or four. This results in a lower degree of relatedness among workers, and it has been questioned why workers should care for offspring that are related to them by less than half. Although this is not fully understood, it has been suggested that the advantages of group living compensate for the low degree of relatedness among workers, so workers will still care for offspring.

Caste differentiation

There are some morphological differences between queens and workers of P. sericea. Generally, queens are larger than workers in the abdominal region, but have smaller head width and wing length. Queens are the only ones with well-developed ovarioles, mature oocytes and sperm-filled spermathecae. Workers' ovaries are either underdeveloped or not developed at all, and so they are not reproductive. Queens perform the reproductive tasks of the colony, while workers forage for resources such as food and nest-building materials. Thus, it makes sense that workers would have larger brains and wings because they must hunt and forage for food. The morphological differences seen in this caste differentiation are also seen with the species Protopolybia chartergoides and Polybia rejecta.

Nest defense

Polybia sericea are generally docile wasps, and will not attack other organisms unless they are stimulated. When they are attacked, however, they may chase predators up to 15 meters away from the nest. In addition they may sting and bite the predator. The magnitude of a defense response is proportional to the energetic investment by the colony in rearing individuals, and also to the size of the colony. More individuals are likely to attack a predator if there are more adults, eggs, larvae, and pupae in the nest, because the colony has already made a large investment in developing itself. Polybia sericea relies upon active defense to defend their nest from ants, whereas other species produce glandular secretions that act as ant repellant to deter ants from entering the nest.

Diet

The diet of P. sericea consists of arthropods such as caterpillars and flies, as well as energetic carbohydrates such as nectar and fruits. Larvae feed only on animal proteins that must be supplied by adult workers. The workers must hunt for insects and feed them to larvae by chewing up the prey and transferring it to the larvae. The most common natural prey of this species is green or brown caterpillars. They also may prey on cashew fruits which can be found in Brazil, as well as the white fly, which parasitizes the cashew fruit.

Cultural significance

P. sericea is a species that is historically and culturally significant to the Pankararé Indians who live in northwest Brazil. They have developed a folk taxonomy of many social bees and wasps that inhabit the area. Polybia sericea is classified as a fierce wasp that is likely to bite more than once. This classification contrasts with other findings that describe this species as docile, and not aggressive unless provoked. The Pankararé often roast the combs of Polybia sericea larvae, and then extract them to be eaten alone or mixed with manioc flour. In addition, they use this species in their healing practices. A bath of smoke from burning the nests of Polybia sericea is used to treat evil eye and stroke.