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Scaptotrigona postica
Scaptotrigona postica
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Apidae
Genus:	Scaptotrigona
Species:	Scaptotrigona postica
Binomial name
Scaptotrigona (Latreille, 1807)
S. postica distribution
Synonyms
Nannotrigona postica or timbra amarella

Scaptotrigona postica (also called the Nannotrigona postica or, locally, timba amarella) is a species of stingless bees that lives mainly in Brazil. These bees are eusocial bees in the Meliponini tribe. S. postica is one of 25 species in the Scaptotrigona genus and are critical pollinators of the tropical rain forests of Brazil. They construct their nests in hollowed sections of tree trunks, allowing for effective guarding at the smaller entrance of the nest. The S. postica is a very important pollinator of the Brazilian tropical rain forests, with a focus on the Eucalyptus genus that is common in this environment, and is widely appreciated for their honey. These bees, along with other stingless bees, account for approximately 30% of all pollination of the Brazilian Caatinga and Pantanal ecosystems and up to 90% of the pollination for many species of the Brazilian Atlantic Forest and the Amazon.

Appearance

S. postica are medium-sized, averaging 1.2 centimeters in length and cross-sectional area of 5.3 mm. They appear dark gray in color with some sections of dark yellow with black eyes. . The workers and drones appear the same in color as they are black and close to identical in size. The drones tend to weigh slightly more than the female workers. However, queen S. postica are much larger than the other colony members and have more of the yellow coloring than the workers and drones.

Distribution and habitat

S. postica is found in the southern, central-western and northern regions of Brazil as the dense tropical rain forests provide ideal nesting locations.This habitat provides the ideal environment for S. postica. These bees maintain an average nest temperature of 32˚C, which is a few degrees above the average temperature of the Brazilian rain forests (27˚C). With the addition of an insulating layer, the nest temperature can be easily maintained in this environment. S. postica have also been found in Peru but are significantly more common in Brazil.

Taxonomy and Phylogeny

S. postica is one of the 25 species in the Scaptotrigona genus, a eusocial genus of bees. This bee is also a member of many taxonomic sub groups including the subclass pterygota (winged insects), suborder apocrita, and subfamily apinae (stingless bees). These groups of bees are winged, have a social hierarchy, and do not have defensive stingers.

Nest

Location

S. postica nest in partially hollow sections of trees in the tropical rain forests of Brazil. The nests range from 3–7 meters above ground with a canopy overhead, approximately 15–20 meters above the nest.One of the common trees nested by S. postica is the Caryocar brasiliense, also known as the cerrado-tree.

Shape

The nest consists of inner brood cells and short entrance tube. The brood cells are sometimes compacted into combs and large wax pots for storage of honey/pollen. The cells are composed of cerumen, are vertical in shape, and open at the top of the cell. The entrance to the nest has an average cross-sectional area of 143.5 mm and is guarded by about 8 guards. These bees stand at attention near the entrance, occasionally flying back and forth in front of the entrance.

Size

Nests contain one colony of S. postica. They carry between 2,000-50,000 individuals, averaging to 10,000 bees per colony.

Colony members

The roles for S. postica are workers (female), queens (female), and drones (male).

Worker

Worker S. postica are the smallest bees in the nest, weighing between 15–22 mg and are black in color. They assume different roles in the nest based on their age.

0 days old = produce wax

16-20 days old = provision cell broods

21-35 days old = colony cleaning

21-45 days old = nectar reception and dehydration

31-40 days old = colony defense

26-60 days old = foraging

Queen

Queens are the largest bees in the nest, averaging 38–50 mg, and are identifiable by their swollen abdomens. There is one queen per colony, but once virgin queens are born, they are tolerated for 15 days on average until they are killed. Queen S. postica cannot independently colonize. While virgin queen bees must leave the colony, a queen holds its position for several years due to its long lifespan.

Drone

Drones weigh on average 17–30 mg and are black in color. The drones are the male bees hatched from unfertilized eggs. Their role is to mate with the queen to produce female bees. They do not participate in many other activities.

Colony cycle

Initiation

Worker bee swarming initiates new colonies. These bees occasionally invade the nests of nearby bee species. New nests house up to three virgin queens during initiation. The queen arrives at the new colony within 5 days of the beginning of swarming. While there can be three virgin queens to begin a nest, the workers will kill two of them to leave a single, reproductive queen for the nest.

Growth

Colony growth depends on nest productivity. During colonization, productivity is low, restricting population growth. As productivity increases, colonies begin male production, increasing overall population growth until reaching an average size of 10,000 individuals. This growth is season dependent. During the rainy period, colony growth rate raises due to increased resource availability.

Lifespan

The average lifespan of S. postica workers ranges from 30–40 days. They have a low mortality rate during the initial stages of life. Death of individuals becomes significant after approximately 15 days and continues increasing until about 40 days. Unlike the worker, the queen bees can live for multiple years, averaging a longer lifespan.

Male production

Male production depends on the season. Production of males is restricted during food shortages, creating short periods of male production with longer periods of female production. Males are produced by unfertilized eggs laid by the workers.

Behavior

Communication

S. postica communicate to recognize nest-mates, identify the caste of any individual bee, locate food, and signal danger.

Cuticular hydrocarbons

Cuticular hydrocarbons are waxy coatings on S. postica bodies that signal the hierarchy and original colony of any bee. Workers have cuticular hydrocarbons lacking oxygens while the drones have more oxygenated compounds. Hydrocarbons also provide social dominance and fertility cues that are important when determining the queen during colonization.

Scent markers

S. postica workers leave scent paths between the nest and food sources, facilitating the successful foraging of others. The threshold distance from food to nest for the scent trail is 11–12 meters, although the flight radius from the nest is larger (600 meters). These marks last about 15 minutes before dissipating. Workers follow the tracks of other populations of the same species, increasing foraging success of both colonies. These scent substances are composed of two different ketones: 2-heptanone and 2-nonanone.

Communication for Reproduction

The specific scents that attract drones to virgin queens prior to reproduction are 2-alcohols and 2-ketones. The 2-alcohols attract the males from long distances to the virgin queen, while 2-ketones induce copulatory attempts when the male is closer to the queen. These compounds have been found in the queens’ mandibular glands but are absent in those of the worker female S. postica

Reproduction and kin selection

Queen bees are the colony reproductive heads. While there is only one queen per colony, workers produce males without mating. However, these hemizygous individuals will always be male. So, eggs can be laid by the unfertilized females (workers) or the fertilized females (queens).

Mating

Virgin queens are most attractive to workers. Males are attracted to the queens via olfactory signals composed of hexyl hexanoate. These scents indicate the queen's reproductive status. S. postica males respond to the pheromones of virgin queens and congregate in groups of variable size at the netrance of the nest. These groups can be from just a few individuals up to thousands of S. postica males. The individuals of the group become a compact swarm as some drones sit on the backs of others. While the drones compete to mate with the virgin queen, they do not act aggressively towards each other. These individuals will quickly disperse upon any intrusion, extra commotion, or disturbance due to an alarm pheromone that spread from individual to individual. Queens make one mating flight. The males lose their genitalia during mating, only inseminating one female. Flights generally occur during the afternoon, with low flight activity in the morning hours. When mating with the queen, the S. postica drones assume an upright posture, antennae stretched as far and outward as possible. The queen will store the spermatozoa, returning to the nest to lay her now fertilized eggs.

Genetic relatedness

The genetic relatedness of individuals depends on the egg. The worker eggs are hemizygous, only containing genetic information from the worker. Thus, all offspring from one worker are genetically identical, also known as "full sisters". The fertilized eggs of the queen contain genetic information from the queen and from one of the up to 6 mates of the female. This increases the genetic variance in the offspring. Furthermore, the drones mating with the queen have low genetic relatedness due to drifting of drones between colonies. This increases the genetic relatedness between fertilized offspring.The overall genetic relatedness within a colony for S. postica is between approximately .7 and .85, averaging to a relatedness of around .8. This suggests that while mainly workers produce males, queens can also produce male offspring.

The costs of hemizygous male production include inbreeding due to the decreased genetic variance and producing nonviable or sterile males. To prevent this, drones drift between colonies. However, there are few populations within the flight radius, so drone genotypic diversity is low.

Sex determination

Sex is determined through controlled fertilization of eggs. Drones are the fertilizers of the nest and will only fertilize the eggs laid by the queen of the colony. These fertilized eggs are heterozygous with complementary sex-determining loci and will always become female. Since drones do not fertilize the worker bees, the worker eggs are haploid, and they produce male offspring. However, workers are not always laying eggs. Thus, male production is limited to certain periods of time (see above). While the workers of S. postica produce approximately 95% of all males, the queen still produces that remaining 5% of males. The queen will release some haploid eggs among her main diploid eggs. However, after the queen has produced a certain small amount of male offspring, she will revert back to producing females only.

Worker-queen conflict

Conflict exists between the egg-laying females in the colony. Workers only produce males with identical genetic information while the queen will produce females. Workers lay their eggs in cells after the queen has laid hers and in the same cells where the queen has already laid hers. Their eggs are identified as larger, more rounded, and laid on the upper inside wall of cells. If the queen does not re-check the cells, the eggs of the workers will produce the male. However, if other workers or the queen checks on a cell and sees the unfertilized egg, they will eat it. This shows a competition between the egg-laying females of the colony.Furthermore, there is conflict between the larvae of both the worker and the queen. If there are two eggs in a cell, the larvae from the worker egg will develop more rapidly and devour that of the queen egg. The reciprocal has not been observed.

Thermoregulation

Nest location helps regulate nest temperature. However, the S. postica can help warm or cool the nest. At low temperatures, the bees mass incubate the brood chamber, increasing temperature. Cerumen coverings, produced by workers, insulate cold spots. During overheating, bees evacuate the nest and reduce the temperature by fanning. There is direct cooling via evaporation due to nest location. The temperature homeostasis is important because of the hot climate of the tropical Brazilian rain forests.

Diet

The main plant used by the S. postica for both pollen and nectar is a member of the eucalyptus species, Eucalyptus spp., a very popular species in the Brazilian rain forests. The pollen grains from Eucaplytus spp. can be found in the midguts of all members of the S. postica colony. These pollen grains are digested fully between 6 and 28 hours. The protein requirements for the S. postica differ based on the role and age of the individual. Worker and queen bees have similar protein requirements (between 20 and 200 pollen grains). Larvae ingest a significantly higher amount of pollen (between 1000 and 2300 pollen grains), as the protein is essential during development. Drones require a high-protein, pollen diet in order to function properly. However, older drones have more responsibilities outside of the nest (see above section on roles of the bees), thus they prefer nectar and honey to pollen. Their change in diet from a pollen-base to a nectar/honey-base is most likely an adaptation to their new roles, which require more flight activity and increased energy expenditure.

Predators

A main predator of the S. postica is the sphecid wasp Trachypus boharti. This wasp is also found in Brazil and exclusively prey on the males of S. postica. The wasps hover near the entrance of the nest in groups averaging to 11 wasps in a group. These wasps capture up to 50 S. postica drones per day close to the entrance of the nest. It captures the individual and preserves it using a secretion from the wasp's post-pharyngeal glands.Fortunately for the S. postica, these predation attempts are generally unsuccessful as the T. boharti only captures a male drone 7% of the time. Most of these failed attempts are due to conflict with other wasps (interfering with the flight path, collisions, and competition). Furthermore, if a wasp captures a different individual of the colony (mainly all female bees), it immediately releases the individual with no harm done. While the T. boharti only predate the drones of S. postica, they do not seek specifically those individuals, rather they are attracted to all S. postica and select their prey after capture.

Defense

The defense for S. postica nest is to have on average 8 workers guarding the front of the nest at all times. Sometimes the guards are sitting nearby and others they fly back and forth across the entrance. These bees are part of the Meliponini tribe, which do not have stingers. However, S. postica guard bees have been observed to bite nest intruders as they alert the other colony members through pheromone signaling. These alarm pheromones contain 2-heptonal and other ketones as active substances. S, postica workers also will display a specific leg posture when other members of the nest or other organisms attempt to take cerumen from the cubiculae (structures located on the hind legs functioning as pollen baskets).

Importance

Pollination

S. postica are pollinators for plants in the Brazilian rain forests. S. postica exploit a smaller number of the many plant species. The main flower visited by this species is the Eucalyptus spp., making up over 45% of the pollen collected by the workers. Some secondary sources for pollen and honey are Mimosa daleoides, Lithraea molleoides, Leucaena leucocephala, and Piptadenia gonacantha.

As pollination is vital for the plants in the Brazilian rain forests, some plant species have developed pheromone mimicry to attract the drones of S. postica. Virgin S. postica queens have a mixture of 2-alkanols in the pheromones that attract the drones for mating. Many Orchidacae species common in Brazilian rainforests, such as Mormolyca ringens, have a similar mixture of alkanes/alkenes that will attract those same drones to the flower. Following the attempted "copulation" of the drone with the flower, the chemical composition of the flower’s mimicked pheromone changes so that it does not attract any more males.

Pesticides

One insecticide in Brazil is Fipronil which inhibits GABA receptors of the nervous system, resulting in seizures, paralysis, and death. S. postica is in contact with Fipronil which is extremely toxic to bees. Compared to other bees, S. postica tolerate higher doses of Fipronil, but the toxicity is still high. Another toxic insecticide widely used in Brazil is imidacloprid. Similar to Fipronil, S. postica are relatively more tolerant of this insecticide. The route of intoxication is very important in determining how lethal imidacloprid is to the S. postica bees. Imidacloprid is significantly more toxic to S. postica through topical exposure compared to consumption of the insecticide . The increase in use of pesticides could cause a decline in pollinators like S. postica hurting the environments of Brazil.

Antiviral properties

S. postica creates propolis with plant secretions, saliva, and wax. It seals the nest but also has significant antiviral properties. The propolis from S. postica reduces production of pocornavirus 64-fold, production of influenza virus 32-fold, and production of measles virus 8-fold. It can decrease the replication of rubella virus at a rate of 10.

References

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19. Engels, W (24 March 1988). "Age-Dependent Queen Attractiveness for Drones and Mating in the Stingless Bee, Scaptotrigona Postica". Journal of Apicultural Research. 27 (1): 3–8. doi:10.1080/00218839.1988.111000773. Retrieved 22 September 2015.
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