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.
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.
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.
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.
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