Bombus terrestris, the buff-tailed bumblebee or large earth bumblebee is one of the most numerous bumblebee species in Europe. Bombus terrestris is the largest of the bumblebee species. It is one of the main species used in greenhouse pollination, and consequently, can be found in many countries and areas where it is not native; Tasmania for example. It is a eusocial insect that is characterized by unique Hymenopteran sex ratios, where male drones dominate most colonies. The queen of B. terrestris is often highly dominant over her colony and exhibits behaviors such as altering the sex ratio in her favor over the workers and controlling queen larval development with pheromones. However, after aggression breaks out in the nest, the workers can usually gain control of the nest and restart the colony cycle. The queen is monandrous and only mates with one male after leaving the nest, despite the potential genetic benefits from polyandrous mating. B. terrestris demonstrates noteworthy learning tactics with flower color and alloethism in foraging behaviors. They have also been implicated in a number of bee pathology studies.
- 1 Taxonomy and Phylogenetics
- 2 Description and Identification
- 3 Distribution and Habitat
- 4 Colony Cycle
- 5 Reproductive Behavior
- 6 Social and Foraging Behavior
- 7 Kin Selection
- 8 Parasites and Disease
- 9 Environmental Concerns
- 10 Human Importance
- 11 References
- 12 External links
Taxonomy and Phylogenetics
B. terrestris is part of the order Hymenoptera, which is composed of ants, bees, wasps, and sawflies and the family Apidae, which specifically consists of bees. It is also part of the subfamily Apinae, which includes most species of bees within the family most of which are solitary. 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. There are two documented subspecies: Bombus terrestris terrestris and Bombus terrestris sassaricus.  Two closely related species, B. canariensis and B. maderensis, are thought to have evolved from European B. terrestris strains and then diverged on the Canary Islands and Maderia respectively.
Description and Identification
B. terrestris are pollen-storing bees that generally feed and forage on nectar and pollen. These bees can navigate their way back to the nest from a distance as far away as 13 km (8.1 mi), although most forage within 5 km of their nest, with one mark and recapture study finding the average foraging distance to be approximately 663 m. The queen is between 20–22 mm long, males range from 14–16 mm, and workers from 11–17 mm. The latter are characterized by their white-ended abdomens and look (apart from their yellowish bands being darker in direct comparison) just like those of the white-tailed bumblebee, B. lucorum, a close relative. The queens of B. terrestris have the namesake buff-white abdomen ("tail") tip; this area is white like in the workers in B. lucorum. B. terrestris are unique compared to other bees in that their caste of workers exhibits a wide variation in worker size with thorax sizes ranging from 2.3 to 6.9 mm 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. Nests are usually found underground, such as in abandoned rodent dens. Colonies form comb-like nest structures with eggcells, each containing several eggs. The queen will layer these eggcells on top of one another. Colonies produce between 300-400 bees on average with a large variation in the number of workers.
A solitary queen hatched from her abandoned colony will initiate the colony cycle when she mates with a male and finds a nest, after which she will lay a small batch of diploid eggs. 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 worker bumblebees emerge. This is known as the initiation phase of the colony. Workers will forage for nectar and pollen for the colony and tend later generations of larvae. The workers are smaller than the queen and only live for a few weeks. 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 the switch point is reached and the queen begins to lay some unfertilized eggs, which develop into male bees. When the male drones emerge from the nest, they do not return, foraging only for themselves. They seek out new queens and mate with them. Remaining diploid eggs receive extra food and pupate to become new queens, though the queen can use pheromones to discourage to some extent the workers' inclination to invest more in larvae around this time, thereby ensuring that not too many become queens. The colony persists until the competition point is reached, when workers begin egg laying. At this point, outright aggression amongst workers and between workers and the queen begins. This is a very predictable point that occurs at about 30 days into the colony cycle.
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 colony cycle starts again when the newly hatched queens leave the nest in search of a mate and a nest for themselves to start a new colony.
B. terrestris is thought mainly to be a singly mating species. This is unusual for social insect queens where mating with several males (polyandry) has potential genetic benefits. The lack of multiple matings by B. terrestris queens may be partly caused by 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. While there may be genetic fitness benefits in colony heterogeneity from a polyandrous mating system, bumblebees are also likely monandrous due to social constraints and risks associated with multiple matings. 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, the queen-worker conflict dictates that workers will be more apt to raise larvae from a single male. 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 (75%) than between mother and offspring (50%), making kin selection stronger between sisters. This selective force would be reduced if workers were the offspring of multiple males, which might lead to increased conflict in the nest.
Due to the variability in the switch point of B. terrestris colonies, there are varying levels of sex ratios among nests. Early-switching colonies have a much larger number of males (17.4:1), 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 1.3:1, thus indicating the queen's control over her colony as she would prefer a 1:1 ratio since she is equally related to both sons and daughters. On the other hand, workers would prefer a 1:3 ratio as they are more related to each other than to their mother. Although early and late switching colonies are usually balanced in the population, the overall demographic in this study was found to be male biased, resulting in an overall sex ratio of 4:1, males to females. 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 social insects, which usually have a 1:3 sex ratio indicative of worker colony control.
Worker Egg Laying
In addition to the queen, the workers can lay eggs for the colony brood. 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 obtain a reproductively active fate. Workers born early in the first batch of eggs 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.
Queen bees can control oogenesis in worker bees via juvenile hormone (JH), which regulates egg development. Among queenless B. terrestris workers, the corpus allata, which secretes 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 worker sisters 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.
Social and Foraging Behavior
Workers start out at the bottom of the dominance hierarchy in the social colony. As they age, they move up closer to the position of the queen. Queen-side workers are often egg layers and interact more frequently with the queen. This social position may pay later, after the competition point is reached. When the queen is overthrown by the aggression of the workers, the most dominant worker will have the best likelihood of contributing more eggs to the colony brood and perhaps climb to the position of “false queen.” The queen appears to maintain a constant distance of social dominance from her workers at all points in the cycle, suggesting that she is displaced by the sheer number of workers later in the cycle.
B. terrestris bees exhibit alloethism in foraging behavior in which larger bees are more often found foraging outside the nest and will return to the nest with larger amounts of nectar and pollen. It is possible that larger bees might be able to withstand greater temperature variation, avoid predation, and travel larger distances making them selectively advantageous. Distinct social roles based on morphology might also be beneficial for individuals of the colonies, by making the colony operate more efficiently. Small bees can be reared more cheaply and kept for in-nest tasks, while only some larvae will be fed enough to become large foraging bees.
Individuals who return from the nest after a foraging run often recruit other bees in the colony to leave the nest and search for food. In B. terrestris, successful foragers will return to the nest and run around frantically and without a measurable pattern, unlike the ritualized dance of the honeybee. Although the mechanism by which this is an effective recruitment strategy is unclear, it is hypothesized that running around likely spreads a pheromone that encourages other bees to exit and forage by indicating the location and odor of food nearby.
B. terrestris has an impressive homing range, where bees displaced from their nests can relocate the colony from up to 9.8 km away. However, the return often takes several days indicating B. terrestirs might be utilizing familiar foliage and natural landmarks to find the nest, which may be a tedious process if an individual is outside the conventional foraging range of the nest.
Bumblebees and honeybees are extremely influenced by innate preferences for blue and yellow color. When they have no training, they will often just visit flowers that naturally attract them. However, it is generally thought that bees will learn to visit more nectar rewarding flowers after experience associates the reward with the color of the petals. This has been demonstrated in B. terrestris where bees trained on artificially colored flowers will pick a similar color to the one they were trained with when tested with an array of flower choices. Though if individuals were tested with flower colors significantly different than from what they were trained with, they just visited flowers most closely aligned with their innate color preferences. In addition to identifying specific colors for foraging purposes, it has also been shown that young worker bees have to learn complex motor skills in order to efficiently collect nectar and pollen from flowers. These skills might take several days to develop, as memory does not always hold perfectly on a day-to-day basis, sometimes deteriorating overnight.
Limitations on Foraging Precision
While bees are highly adept at discrimination tasks, they are still limited by the magnitude of difference needed in hue to properly carry out these tests. Error rates of color recognition go down in B. terrestris when flower pigments are closer together on the color spectrum. This might have damaging effects on pollination efficiency if bees visit different flower species with similar, but distinct colors, which can only be mediated if the flowers have unique shapes.
While bees often forage alone, experiments demonstrate that young foragers might learn what flowers provide the most nectar more quickly when foraging with older workers. B. terrestris individuals have a faster learning curve for visiting unfamiliar, yet rewarding flowers when they can see a conspecific foraging on the same species. The discovery of this type of associative learning is a novel insight into bee behavior and may supplement learning via color reward association.
Conflict is expected between queen and workers over the sex ratio and reproduction of males in the colony, especially in monandrous colonies where workers are more related to their own sons and nephews than to their brothers. In early-switching colonies, workers might start laying eggs when they know it will be in their own genetic interests, perhaps from a cue that indicates the switch point has been reached and the queen is now laying haploid eggs. This might be delayed, because sex can only be differentiated in mature larvae by workers. In late-switching colonies (where the competition point still occurs at the same time in the cycle), workers may start laying eggs when they detect a change in the queen’s pheromone that indicate larvae are developing into new queens. Thus, the outcome of this conflict is mediated through the dominance of the queen and the information available to the workers. While it is assumed that queens usually win this conflict, it is still unclear because some studies have indicated that up to 80% of males are produced by workers. These asymmetries in the timing of egg lying and dominance in B. terrestris might explain why it often does not conform to predicted sex ratios and kin-selection hypotheses.
Although B. terrestris workers are most directly in competition with the queen for egg laying opportunities, they will still inhibit their sisters from laying eggs in order to have their own sons, which they will share the most genes with than their nephews.
Parasites and Disease
Effects of Foraging on Resistance
Foraging is considered energetically costly and it is possible that individuals that spend more time foraging suffer costs to their overall fitness. For example, B. terrestris is often vulnerable to parasitism by conopid flies in Central Europe and it has been hypothesized that foragers might suffer higher incidences of parasites due to the increased metabolic costs of flying. This was demonstrated in a population in which foraging workers had significantly lower levels of encapsulation of an experimental parasitic egg when compared to non-foraging workers. This suggests that foragers have compromised immune systems due increased energetic expenses and might be predisposed to fly parasites.
Effects of Polyandry on Resistance
While B. terrestris is a singly mating species, it might be beneficial to mate with multiple males in a polyandrous system to attain greater genetic variability for resistance against disease. Accordingly, artificially increasing the number of mates a B. terrestris queen obtains through artificial insemination has shown that the increased genetic variability in her offspring confers greater resistance to the most common bumblebee parasite, Crithidia bombi.  However, the average reproductive success between one and multiple matings is not linear. Queens that mated once and mated four times had a higher fitness than those that mated twice. This suggests that there might be a fitness barrier to increased matings, which might be why colonies are usually monandrous.
Surprisingly, the immunocompetence, as measured by the ability to encapsulate a novel antigen, does not vary based on the local environment. Experimental studies demonstrate that B. terrestris have equal levels of encapsulation in poor and stable environments. This is unexpected, because immunity should be compromised in conditions where food supply is low to save energy, but perhaps encapsulation represents an invariable trait of bumblebees or immunity is far too complex to characterize solely based on measurements of encapsulation.
Threats from Disease
Deformed wing virus (DMV) is normally a honeybee pathogen that results in reduced and crumpled wings, making those individuals inviable. This virus is thought to have spread to B. terrestris and in 2004 as many as 10% of queen bees bred commercially in Europe were found dead with deformed wings. This was confirmed as DMV when B. terrestris colonies tested positive for the presence of DMV RNA. This could indicate that DMV is a broad range pathogen among bees or perhaps it has recently been infecting new hosts after transmission from honeybees.
While native to Europe, B. terrestris has been introduced as a greenhouse pollinator into many foreign ecosystems. The presence of B. terrestris is becoming an ecological concern in many communities in which it is not native. It is classified as an "invasive alien species" in Japan. For example, B. terrestris has a large niche overlap with local Japanese bee species in flower resources and nest sites. B. terrestris queens competing for local underground nest sites are displacing B. hypocrita sapporoensis. However, B. pseudobaicalensis, which visits similar flowers, but only forms nests above ground has not seen a rapid decline in population numbers.
In 2008, the Australian government banned the live import of B. terrestris into Australia on the grounds that it would present a significant risk of becoming a feral species and thereby present a threat to native fauna and flora. In 2004, this bumblebee was classified as a 'Key Threatening Process' by the Scientific Committee of the New South Wales Department of Environment.
This species was introduced to Chile in 1998. It has since crossed into Argentina, and is spreading at about 275 km per year. Where it spreads, the only bumblebee native to southern South America, Bombus dahlbomii, disappears within weeks. Bombus ruderatus, a bee previously introduced in 1982, is also seriously affected. The cause is thought to be the parasite Apicystis bombi, an organism carried by the buff-tails, but which has no adverse effect on that species.
Colony Development in Changing Environments
In temperate areas, variable climates and environmental conditions occur during changing seasons. Lack of available food due to these unpredictable circumstances can often negatively affect colony growth, reproduction, and resistance to parasites. In poor environments with limited food, the few workers born are smaller than average. However, it appears that B. terrestris is well adapted to a changing environment considering colony growth is higher under variable feeding conditions. Workers and reproductives are also heavier with a variable food supply when compared to stable food availability. This might indicate an adaptive strategy of increased provisioning to save for days it is hard to find food.
In their 2014 study published in Functional Ecology researchers using Radio-Frequency Identification (RFID) tagging technology on the bees, found that a sublethal exposure to either a neonicotinoid (imidacloprid) and/or a pyrethroid (?-cyhalothrin) over a four-week period caused an impairment of the bumblebee's ability to forage. Research published in 2015 showed that bees prefer solutions containing neonicotinoids, even though the consumption of these pesticides caused them to eat less food overall. This work implies that treating flowering crops with such pesticides presents a sizeable hazard to foraging bees.
Since 1987, B. terrestris has been bred commercially for use as a pollinator for European greenhouse crops, particularly tomatoes—a task which was previously carried out by human hand. B. terrestris has been commercially reared in New Zealand since the early 1990s and is now used in at least North Africa, Japan, Korea, and Russia, with the global trade in bumblebee colonies probably exceeding 1 million nests per year.
B. terrestris are key commercial pollinators in Europe, which has driven researchers to investigate the influence of agricultural land on the foraging and survival of this species. Monoculture reduces biodiversity in farmland areas and likely decreases the number of flowering species bees can forage on. B. terrestris bees consequently exhibit greater nest growth in suburban areas than in farmland, because local suburban gardens promote more plant diversity for bees to feed from. Agriculture has a profound impact on many bumblebees and is causing widespread decline in several species. However, B. terrestris is still widespread, likely because it can forage at very long distances, making it less sensitive to changes in biodiversity and the environment.
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- What Harm Could Exotic Bumblebees Do in Australia? - a report by Australian Native Bee Research Centre