Tetragonula carbonaria (previously known as Trigona carbonariaBHL) is a stingless bee, endemic to the north-east coast of Australia. Its common name is sugarbag bee. They are also occasionally referred to as bush bees. 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, and highly developed social structure comparable to honey bees.
Tetragonula carbonaria produce edible honey, which they store in pots in the nest made from propolis. These pots are referred to as sugarbags, giving the bees their common name of sugarbag bees. These pots and the whole nest are eaten by some Indigenous Australian communities as bush tucker. The bees themselves are also important cultural symbols for many Indigenous Australians.
Stingless bees Twenty-one genera of stingless bees (family Apidae) are described worldwide. As the name would suggest, the stings of these bees are vestigial and useless in defence. About 14 species are found in Australia, mostly in the tropical north. T. carbonaria is one of the few exceptions, being found as far south as Bega in southern New South Wales. Stingless bees and honey bees are thought to have evolved from a common ancestor, like bumblebees, which would explain their similarities in social behaviour. Some of these similarities are co-operative 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. The Tetragonula species of Australia were once in the larger genus Trigona, but were moved into a new genus in 2013. Only minor structural differences are seen at the species level of the genus Tetragonula. T. carbonaria is nearly identical to T. hockingsi, besides a few differences in their nest architecture.
Description
Bees
Compared to other stingless bees, T. carbonaria tends 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 and the wing length (including tegula) is . The male drones have very similar bodies and wing lengths, but can be identified by different antennae. T. carbonaria is a very strong flier, being able to fly up to . Female bees generally only forage within of their colony, while male bees can fly much farther, some dispersing as far as from their original colony. T. carbonaria and its closely related species have high levels of morphological similarities. Also, very low genetic variation exists within T. carbonaria. T. carbonaria can be distinguished from other species of Australian Native Stingless Bees by their brood (in a distinctive spiral unique to the species), and by the entrance (these bees tend to daub resins around their entrances, where as other species, such as T. hockingsi generally keep their entrances clean).
Nest structure
The entrance and surrounding areas of their nest are coated with a smooth, thick layer of black, red, or yellow cerumen or propolis, a material formed by mixing beeswax (a glandular secretion of worker bees) with resin (collected from plants). They lack external entrance tunnels, but do build internal entrance tunnels where guard bees patrol, looking out for any intruders (including Small Hive Beetle, Phorid Fly, and other bees). T. carbonaria builds brood cells arranged in combs or semicombs. The cells are a single layer of hexagonal combs that are built in a distinctive 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 bitumen layer of cerumen or field-collected material. The brood chamber is centred in the hive, which makes T. carbonaria suitable for hive propagation. The brood chamber is also made up of multiple horizontal layers which allow 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 to produce insulation valuable for their survival in the cool regions. Some features that would favour 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 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 genus Tetragonula, which may help with efficient food storage. In urban and suburban areas, T. carbonaria have been found to nest in telecommunications pits as well as water meters when other habitat is not available.
Colony cycle
Activity Colonies of T. carbonaria tend to be active all year round. The daily activity period, however, is longer in the southern hemisphere's warmer months, October to March. The intensity of these daily flights is greatest in September, and least intense in May. A temperature threshold exists on all of this activity. Flight can only occur at temperatures greater than . This year-long period of activity is beneficial for the pollination of crops flowering at any time of the year.
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 beekeepers 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. 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.
Growth A study performed by Tim Heard in 1988 observed 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 (however, it usually takes 12 to 18 months for a hive to be ready to be propagated). A heavy hive suggests filled storage pots and a large population of workers and brood, meaning the hive is ready to be split.
Behaviour
Division of labour Division of labour exists among the workers. There is a progression from safer to riskier jobs as the bees mature. The young bees perform tasks within the nests, such as cleaning and brood care. As they mature, they move on to jobs involving building nest structures, like pollen and honey pots, while the oldest bees become foragers and their tasks are performed outside of the nest.
Foraging behaviour T. carbonaria depends on nectar and pollen for survival. They also collect resin for use in the nest structure. Workers tend to exhibit characteristics of group-foraging behaviour called "opportunism". In short, opportunism is when many foragers search for resources independently, and once they find a highly resourceful flower, they rapidly recruit nest mates. In other words, they are optimising the feeding intake of the colony. The success in this practice is dependent on chance. If a forager encounters an area full of rich resources, then recruitment and harvesting are extremely heavy in this area until the resources are depleted. Workers 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 decreases to a point where moving onto another area that might be lower in concentration is best. In T. carbonaria colonies, only some of the bees do the foraging. Workers spread out in all directions surrounding the colony, and quickly locate the best option nearest the nest. Once this area is found, they 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
T. carbonaria queens mate a single time. When the time comes for a virgin queen (sometimes referred to as a "princess") to mate, a "male swarm" (also referred to as a "drone swarm" or "mating swarm") will form. This entails hundreds to thousands of male stingless bees aggregating around the entrance to the hive of the virgin queen, usually for multiple days and sometimes multiple weeks. The virgin queen will then leave the hive to mate with a single male. When mating, the male will insert a mating plug into the virgin queens genitalia, after which the male will die. The newly mated queen will then return to the hive and begin laying eggs.
Queens are the only caste in a T. carbonaria colony that can produce eggs. In many stingless bee species, 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. Microsatellite analysis has shown that T. carbonaria workers do not contribute to the production of males. Workers have also been shown to be irreversibly sterile, having ovaries that are reduced in size compared to virgin queens with no developing oocytes.
Kin selection
Genetic relatedness within colonies The workers tend to be the progeny of a singly mated queen. The drones are predominantly haploid males which arose from queen-laid eggs. Mating frequency is a central factor in kin selection arguments. Some cases are seen of diploid males, which 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 Very little queen-worker agonism occurs during oviposition in T. carbonaria. In many stingless bee species, when workers do lay eggs, direct conflict tends to occur within the colony between the queen and the workers over the source of male egg during the cell provisioning and the oviposition process. Notably, T. carbonaria workers do not lay eggs, so it is unlikely this system occurs for this species, and has yet to be observed.
Interaction with other species
Defence