Bombus terrestris, the buff-tailed bumblebee or large earth bumblebee, is one of the most numerous bumblebee species in Europe. It is one of the main species used in greenhouse pollination, and so can be found in many countries and areas where it is not native, such as Tasmania. Moreover, it is a eusocial insect with an overlap of generations, a division of labour, and cooperative brood care. The queen is monogamous, which means she mates with only one male. B. terrestris workers learn flower colours and forage efficiently.
Taxonomy and phylogenetics B. terrestris is part of the order Hymenoptera, which is composed of ants, bees, and wasps. The family Apidae specifically consists of bees. It is also part of the subfamily Apinae. There are 14 tribe lineages within Apinae, and B. terrestris is in the bumblebee tribe, Bombini. It is in the genus Bombus, which consists entirely of bumblebees, and the subgenus Bombus sensu stricto. This subgenus contains closely related species such as Bombus affinis, Bombus cryptarum, Bombus franklini, Bombus ignitus, Bombus lucorum, Bombus magnus, Bombus occidentalis, and Bombus terricola. There are nine recognized subspecies: B. terrestris africanus, B. terrestris audax, B. terrestris calabricus, B. terrestris canariensis, B. terrestris dalmatinus, B. terrestris lusitanicus, B. terrestris sassaricus, B. terrestris terrestris and B. terrestris xanthopus, each with a distinctive coloration scheme.
Description and identification
B. terrestris are pollen-storing bees that generally feed and forage on nectar and pollen. The queen is between 20 and 22 mm long, males range from 14 to 16 mm, and workers from 11 to 17 mm. Workers have white-ended abdomens, and look just like workers of the white-tailed bumblebee, B. lucorum, a close relative, apart from the yellowish bands of B. terrestris being darker in direct comparison. The queens of B. terrestris have the namesake buff-white abdomen tip ("tail"); this area is white as in the workers in B. lucorum. B. terrestris is unique compared to other bees in that their caste of workers exhibit a wide variation in worker size, with thorax sizes ranging from 2.3 to 6.9 mm in length and masses ranging from 68 to 754 mg.
Distribution and habitat B. terrestris is most commonly found throughout Europe and generally occupies temperate climates. Because it can survive in a wide variety of habitats, there are populations in the Near East, the Mediterranean Islands, and Northern Africa as well. Additionally, it has escaped captivity after being introduced as a greenhouse pollinator in countries where it is not native, so this bee is now considered an invasive species in many of these places, including Japan, Chile, Argentina, and Tasmania. Nests are usually found underground, such as in abandoned rodent dens. Colonies form comb-like nest structures with egg cells each containing several eggs. The queen will lay egg cells on top of one another. Colonies produce between 300 and 400 bees on average, with a large variation in the number of workers.
Colony structure
Social castes Like in most social bees, there are three main social caste divisions in B. terrestris. This ensures a division of labor and efficient colony functioning. Queens become the main female individual to reproduce in a future colony. There is only one per colony. Her sole responsibility is to lay eggs after she founds a nest. This fate is determined for larvae that receive more food, have longer instar stages, and higher levels of juvenile hormone biosynthesis. Workers, an entirely female caste, mainly forage for food, defend the colony, and tend to the growing larvae. They are usually sterile for most of the colony cycle and do not raise their own young. Unlike queens and workers, which develop from fertilized diploid eggs, drones, or male bees, are born from unfertilized, haploid eggs. Drones leave the colony shortly after reaching adulthood to find a mate outside the nest. Mating is their sole role in the colony.
Life cycle A solitary queen hatched from her abandoned colony initiates the colony cycle when she mates with a male and finds a nest. She will stay in this nest over winter and then will lay a small batch of diploid (female) eggs in the spring. Once these hatch, she tends the larvae, feeding them with nectar and pollen. When the larvae are grown, they pupate, and about two weeks later, the first workers emerge. This is known as the initiation phase of the colony. Workers forage for nectar and pollen for the colony and tend later generations of larvae. The workers are smaller than the queen, and usually die while foraging in the jaws of predators like birds or robberflies. The foraging range and frequency of workers depends on the quality and distribution of available food, but most workers forage within a few hundred meters of their nest.
This first phase can last a variable amount of time in B. terrestris, after which a switch point is reached, and the queen begins to lay some unfertilized eggs, which develop into males. When the male drones emerge from the nest, they do not return, foraging only for themselves. They seek out emerging queens and mate with them. The remaining diploid eggs hatch into larvae that receive extra food and pupate to become new queens. The queen can use pheromones to discourage the workers' inclination to invest more in these larvae, thereby ensuring that not too many become queens. The resolution of this worker/queen conflict can be complex and is discussed below. The colony persists until fall in temperate zones and then workers begin to lay unfertilized eggs that if they mature will become males. At this point, outright aggression among workers and between the queen and workers begins. This is a predictable time point that occurs about 30 days into the colony cycle in very temperate climates.
Usually, the worker–queen conflict will force the queen out and the new workers will become queenless. A "false queen" might take control of the colony for a short period. The newly emerged queens sometimes act as workers and help to raise another brood of queens. During this time they daily leave the nest looking for food, during which time they may mate. Eventually they find a site to dig a "hibernaculum" where they will hibernate until the next spring, when they emerge, seek food — primarily to build up their ovaries — and soon seek a site to found a new nest. (In warmer climates they may skip the hibernation stage.) Almost always the old colony will have died out, and if the site is free of parasites one of the new queens will return and reuse that site.
Reproductive behavior
B. terrestris is a singly mating species. Mating with multiple males might provide benefits of genetic variability among the brood, but it does not happen in this or any but the most highly derived social bees. The lack of multiple matings by B. terrestris queens may be partly due to male interference. B. terrestris males plug the female's sexual tract with a sticky secretion during mating, which appears to reduce the female's ability to successfully mate with other males for several days. However honeybee males also plug the female's reproductive tract to no avail; honeybees mate tens of times on one mating flight. While there may be genetic fitness benefits in colony heterogeneity from a polyandrous mating system, bumblebees are also likely to be monandrous due to social constraints, risks associated with multiple matings, and phylogenetic inertia since the ancestral bees are singly mated. Finding multiple mates might be energetically costly and expose the queen to higher predation risks. Additionally, while queens may prefer multiple matings to ensure more genetic variability and viable offspring, workers are more closely related to full sisters than to paternal half sisters. This is due to haplodiploidy in Hymenopteran social insects in which males (drones) are haploid and females (workers and queens) are diploid. This confers greater genetic similarity between sister workers (relatedness of 0.75) than between mother and offspring (relatedness of 0.5), making the relatedness component of kin selection higher between sisters.
In addition to the queen, the workers can lay eggs. Since workers do not mate, all of their eggs are haploid and will develop into drones. There are multiple factors that determine whether a worker bee will become reproductively active. Workers born early in the first brood are more likely to become egg layers due to their increased size and age, which allows more time for ovarian development. Workers usually have to be at least 30 days old to become an egg layer. Individuals that spend less time foraging and more time near the queen are also more likely to become reproductive. Lastly, due to intense competition for the opportunity to reproduce, older workers often harass the queen by attacking her and buzzing loudly. Once this point is reached the colony is usually abandoned.
Due to the variability in the switch point to male production from worker production of B. terrestris colonies, there are varying levels of sex ratios among nests. Early-switching colonies have a much smaller number of future queens compared to males (1:17.4), which may give them a competitive advantage in mating with later emerging queens. Late-switching colonies have fewer males and a more even sex ratio of 1:1:3, thus indicating the queen's control over her colony (she prefers a 1:1 ratio, since she is equally related to both sons and daughters). On the other hand, workers prefer a 3:1 ratio, as they are more related to each other than to their mother. Although early and late switching colonies are usually balanced equally in numbers in the population, the overall demographic in one study was found to be male biased, resulting in an overall sex ratio of 1:4 (female to males). However, most studies show that this balance of bimodal sex determination between early and late-switching colonies creates the queen's preferred 1:1 sex ratio in B. terrestris populations. This is unusual for monogamous social insects, which usually have a 3:1 sex ratio indicative of worker colony control. B. terrestris often does not conform to standard predictions of sex ratios based on evolutionary theory and haplodiploid theory.
Reproductive suppression
Queen bees can control oogenesis in worker bees by suppressing juvenile hormone (JH) in the workers, which regulates egg development. Among queenless B. terrestris workers, the corpora allata, which secrete JH, was noticeably enlarged compared to queenright workers. JH concentrations were also higher in the hemolymph of queenless workers. This suggests that the presence of a queen is enough to prevent workers from laying eggs, which helps her maintain genetic control over her colony's brood. The mechanism through which the queen induces this behavior is likely through pheromones.
While the queen controls much of the egg laying and larval development in the colony, it is likely that workers play a much bigger role in controlling egg laying than previously thought. Dominant workers will often inhibit younger workers from laying eggs. Workers have low levels of JH and ovarian development during the early stages of the colony cycle and also after the competition point. Workers introduced into queenright and queenless colonies experience similar levels of inhibition from fellow workers during the competition point, indicating the key role of worker policing of fellow nest mates later in the colony cycle. This suggests that worker reproductive development will be highest between early development and the competition point in the colony.
Kin selection
Worker–queen conflict