Tetragonula carbonaria

Tetragonula carbonaria
File:Sugarbag bee.jpg
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Superfamily:	Apoidea
Family:	Apidae
Subfamily:	Apinae
Tribe:	Meliponini
Genus:	Tetragonula
Species:	T. carbonaria
Binomial name
Tetragonula carbonaria Smith, 1854
File:LocationAU.svg
The range of Tetragonula carbonaria.
Synonyms
Trigona angophorae Cockerell, T.D.A. 1912

Tetragonula carbonaria (previously known as Trigona carbonaria) is a stingless bee, endemic to the north-east coast of Australia. Its common name is Sugarbag bee. The bee is known to pollinate orchid species, such as Dendrobium lichenastrum, D. toressae, and D. speciosum. It has been identified as an insect that collects pollen from the cycad, Cycas media. They are also known for their small body size, reduced wing venation, as well as a highly developed social structure compared to honey bees.

T. carbonaria forms honeycombs in their nests. The bee produces an edible honey; the whole nest is sometimes eaten by Indigenous Australians. The bees "mummify" invasive small hive beetles (Aethina tumida) that enter the nest by coating and immobilizing the invaders in wax, resin and mud or dirt from the nest.

Stingless Bees

There are 21 genera of stingless bees (Apidae family) worldwide. As the name would suggest, the stings of these bees are vestigial and useless in defence. There are about 14 species in Australia, mostly found in the tropical north.T. carbonaria is one of the few exceptions, in which they are found as far south as Bega in southern New South Wales . Stingless bees and Honey bees are expected to have evolved from a common ancestor, like bumblebees, which would explain their similarities in social behavior. Some of these similarities are cooperative brood care, and having different castes of queens, workers and drones. The workers are infertile females, while the drones are males.

Taxonomy and phylogeny

The eusocial stingless bees (Apidae, Apinae, Meliponini) comprise about 374 species. Two genera occur in Australia, with Tetragonula being one of them. Tetragonula is the largest genus of stingless bees, with T. carbonaria being one of approximately 150 species in this genus. There are only minor structural differences at the species level of the genus Tetragonula. T. carbonaria are nearly identical to T. hockingsi, besides a few differences in their nest architecture.

Description

Bees

File:Tetragonula carbonaria f.jpg

Tetragonula carbonaria

Compared to other stingless bees, T. carbonaria tend to be "medium-sized." However, their size can vary based on location. For instance, T. carbonaria from Queensland can be as small as T. mellipes, but in New South Wales, they can get as large as T. hockingsi. They are all predominantly black bodied, covered in microscopic hairs. The adult workers and males are all black, with some brownish tint in certain areas such as the legs. The worker’s body length is 3.9-4.3 mm, and the wing length (including tegula) is 4.1-4.6 mm. The male drones have very similar bodies and wing lengths. T. carbonaria are very strong fliers, being able to fly up to one kilometer. However, they will not fly any further than they have to and so close resources are preferred over those farther away. T. carbonaria and its closed related species have high levels of morphological similarities. There is also very low genetic variation existing within T. carbonaria.

Nest structure

The entrance and surrounding areas of their nest are coated with a smooth, thick layer of black, red, or yellow cerumen. Cerumen is a material formed by mixing beeswax (a glandular secretion of worker bees) with propolis. They lack external entrance tunnels. T. carbonaria build brood cells arranged in combs or semicombs. The cells are a single layer of hexagonal combs that are built in a spiral. They are vertically elongated and in a regular vertical orientation. They are built out of brown cerumen, and house the eggs and larvae. New cells are added to the brood by the advancing front. The nest cavity may be sealed off from adjoining cavities by a hard batumen layer of cerumen or field-collected material. The brood chamber is centered in the hive, which makes T. carbonaria suitable for hive propagation. The brood chamber is also made up of multiple horizontal layers which allows for easy division of the brood comb.

Distribution and habitat

The nests are found in open forests and woodlands. They are usually built in tree cavities and have small cryptic entrances, with no external entrance tube. Four or five workers are usually visible at the entrance and are expected to be guards. They tend to choose larger trees and wider cavities in order to produce insulation valuable for their survival in the cool regions. Some features that would favor survival in a cooler climate are a high tree height and large feeding pots. The nesting sites of T. carbonaria are located near the top tree trunks that are 1.5 m in diameter, and are predominantly found in trees that are well insulated. Members of T. carbonaria also create the largest honey and pollen pots compared to the other species of the Tetragonula genus, which may help with efficient food storage.

Colony cycle

Colony Activity

Colonies of T. carbonaria tend to be active all year round. The daily activity period, however, is longer in the warmer months (October to March inclusive) compared to the cooler months (April to September inclusive). The intensity of these daily flights are greatest in September, and least intense in May. There is a temperature threshold on all of this activity. Flight can only occur at temperatures greater than 18°C. This year-long period of activity is beneficial for the pollination of crops flowering at any time of the year by T. carbonaria.

Colony Initiation

Each brood cell is stocked almost to the brim with honey and pollen. An egg is laid in the cell by the queen and then the cell is closed. Complete larval and pupal development occurs in the closed cell . Once the adult emerges, the cell is destroyed. Most stingless bee species are monogynous, meaning that when the colony divides, one of the daughter colonies will be queenless. T. carbonaria colonies are frequently divided by bee keepers to increase the number of colonies. They build emergency queen cells by fusing two worker-sized cells that contain eggs or young larvae.

The queens cannot live alone and they are not transferred to a new nest until it has been fully prepared by workers. There is a lot of back and forth action until the new nest is finally ready. The new queen is the bee that makes the flight to the new nest, with the old queen remaining in the parent nest . When the old queen has died, mating swarms can occur at the established nest to replace the old queen with a young, unmated one.

Colony Growth

A study performed by Tim Heard in 1988 looked at the propagation of hives in T. carbonaria. He successfully transferred colonies to boxes, and then once the available space was occupied, he would split the box by prying apart the two halves of the box. He recorded that colony weight increased much more in spring and summer compared to autumn and winter. After about 17 months, the final weight was established. The rate at which colony weight increases is dependent on the availability of nectar and pollen, not age. A heavy hive suggests that there was plenty of filled storage pots, and a large population of workers and brood, meaning that the hive is ready to be split.

Behavior

Division of labor

There is a division of labor among the workers. The young bees perform tasks within the nests, such as brood care. As they mature, they become foragers and their tasks are performed outside of the nest.

Foraging behavior

T. carbonaria depend on nectar and pollen for survival. Workers tend to exhibit characteristics of group-foraging behavior called "opportunism." In short, opportunism is when many foragers search for resources independently, and once they find a highly resourceful flower, they will rapidly recruit nest mates. In other words, they are optimizing the feeding intake of the colony. The success in this kind of practice is dependent on chance. If a forager encounters an area full of rich resources, then recruitment and harvesting will be extremely heavy in this area until the resources are depleted. Workers will look for areas with the highest sugar concentration in the nectar, as they have the ability to physiologically identify the richest sugar solutions. As more nest mates arrive to the area with rich resources, the availability of this high concentration sugar will decrease to a point where it would be best to move onto another area that might be lower in concentration, but hasn’t been visited yet. In T. carbonaria colonies, only some of the bees do the foraging. Workers will spread out in all directions surrounding the colony, and will quickly locate the best option nearest the nest. Once this area is found, they will mark the food sources with a pheromone. Marking is used as a guide to make the location easier to find for their nest mates.

Reproduction

A study performed by Gloag et al. used microsatellites to determine the origin of males. Their results showed that the resident queen was the sole mother of the males. This meant that the workers did not contribute to the production of males. Ovaries were sometimes present in the workers, but not activated. This is unusual because most stingless bee workers can produce unfertilized eggs that develop into haploid males, therefore having both the queen and the workers with potential to be the mother of the males in the colony. This is also unusual because there is usually some sort of kin-selected benefits towards worker reproduction. One possibility could be that the queens have power over their workers. The aggressive oviposition can sometimes be seen as the queen "bullying" the other party into refraining from reproduction. However, there is very little queen-worker agonism during oviposition in T. carbonaria. Another possibility could be some sort of "evolutionary arms race" between workers and queens over which caste will have power to produce males. This could depend on some extrinsic factors such as the size of the colonies, the number of brood cells available for oviposition, and size dimorphism of queens and workers. A final possibility could be that workers have evolved to "self-restrain" from egg-laying because worker reproduction creates a significant cost to the colony. Some of these costs could be low reproductive success of worker-laid males or reduced colony productivity since the workers now have to focus on reproduction instead of colony maintenance.

Kin selection

Genetic relatedness within colonies

The workers tend to be the progeny of a singly-mated queen. The colonies are predominantly haploid males who arose from queen-laid eggs. Mating frequency is a central factor in kin selection arguments. There are some cases of diploid males, who are generally sterile and are considered to have a very low fitness. Diploid males tend to have a cost to the colony because diploidy can result in a reduced proportion of workers able to perform their tasks, which is pivotal to the colony’s survival. In some extreme cases, workers have been reported to kill a queen producing diploid males, to help the future success of the colony.

Worker-queen conflict

When workers do lay eggs, there tends to be a direct conflict within the colony between the queen and the workers over the source of male eggs. Queen-worker conflict is found in cell provisioning and the oviposition process of most stingless bee species. This conflict is usually very elaborate, and very apparent, but tends to not involve acts of aggression, which other species of stingless bees have been known to perform. Although worker oviposition is known to be controlled by worker policing, it can sometimes be controlled through queen dominance/policing. This is where the queen patrols the area where new brood cells are being produced, being able to have a hands-on policing which tends to be quite effective.

Interaction with other species

Defense

When the colonies are attacked, nest defense relies on the ability to recognize intruders. T. carbonaria sometimes display a behavior known as a "fighting swarm" when a non-nest mate is encountered. Thousands of workers will pair together through their wings and form a cloud. The signal to form this cloud is most likely mediated by alarm pheromones, which workers release from their mandibular glands. As one entity, they will drop to the ground and wrestle the intruders, which often leads to death of both parties. This behavior is also a common defense mechanism against large predators such as humans and bears. T. carbonaria are highly sensitive to intruders, since they will even attack invaders that are carrying pollen or nectar. Even if an intruder found a way to make it past the swarm, they still wouldn’t make it through the congested entrance tunnel.

Predators

One predator known to the T. carbonaria is an Australian crab spider, Diaea evanida. This organism, along with others, is able to exploit the interaction between plants and their pollinators. These nectar robbers pierce a hole in the corolla of the flower and drink the nectar without touching the pollen or stigma. There seems to be a correlation between nectar production and corolla tube length. And so the crab spiders will preferentially exploit the flowers with longer corolla tubes for higher nectar content. These crab spiders attract and ambush pollinators on flowers. They produce UV-reflective body colors that attract pretty to the flowers they are occupying. However, Australian native bees are able to detect and avoid flowers harboring crab spiders despite the fact that they are initially attracted to them. D. evanida spiders can generate color contrasts for bees’ individual preferences, however T. carbonaria did not show any preference for any of the contrasts.

Parasites

The braconid subfamily Euphorinae has several genera, including Syntretus, known to be parasitoids of the adult stage of insects. They are a highly diverse group and tend to be very successful parasitoids worldwide. A new species of Syntretus, trigonaphagus, has recently been discovered as parasitizing workers of T. carbonaria. Females of S. trigonaphagus are frequently found at the entrances of T. carbonaria hives near Queensland, Australia. They will approach workers that land nearby and oviposit on the host by curling the abdomen. The workers will repeatedly brush their abdomens afterwards, suggesting that they were aware that an attack occurred. The overall effect of this parasitism is usually fatal. Older workers are more likely to be parasitized. Because of this, as long as the number of parasites in minimal, there isn’t too large of an overall cost to the colony, since these workers have already contributed substantially to the colony’s welfare.

Importance to humans

Bee keeping

File:Meliponikultur di Malaysia.jpg

Meliponiculture in Malaysia

Meliponiculture is the practice of stingless bee keeping. This is where bee keepers maintain, reproduce, and utilize stingless bee colonies for their own profit. These colonies tend to be managed through artificial hives so that the bee keepers have the ability to propagate the colonies and produce hive products such as honey and pollen. These products are then sold to various buyers from health food stores to gift shops. The honey of T. carbonaria possess a peculiar smell which makes it quite the appealing product. When the first set of work began in 1984 on this stingless bee, the human industry was practically non-existent. Since then, the interest in stingless bees, more specifically T. carbonaria has greatly increased. This has allowed for the establishment of conservation groups along the eastern regions of Australia. T. carbonaria is the most popular species that bee keepers tend to, followed by the A. australis and then T. hockingsi. The main reason for most people to keep T. carbonaria is for enjoyment and conservation. With this, the amount of honey produced is constantly increasing at a fast pace. T. carbonaria is the main species that bee keepers produce honey with out of the stingless bees in Australia. Bee keepers report that one of the major limiting factors in propagating colonies is the availability of queens. More research is needed on queen rearing to fix this limiting factor.

Agriculture (as pollinators)

Individual T. carbonaria demonstrate a consistency with floral choice. In other words, individuals restrict their foraging activity to one kind of flower during a particular trip. These consistency in a single pollen type enhances the pollinator efficacy by increasing the chances of pollen being transferred to stigmas of the same plant species. This increases their importance ecologically as crop pollinators. At the level of the colony, however, the species can utilize many different flowering species. So although the species is polylectic, individual bees will remain consistent with their flower choice.

Honey

Meliponines store their honey in pots, not in combs like the honeybees. Compared to A. mellifera, a honeybee species that is well known for their honey, T. carbonaria honey had higher values in moisture, water activity, and electrical activity. The two different honeys can also be distinguished by flavor and aroma. Also, the antioxidant activity of T. carbonaria honey has such a high value that it has potential to serve medicinal needs both nutritionally and pharmaceutically.