Earth bumblebee

Earth bumblebee / Bombus terrestris / نحل الأرض الطنان

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Status: Common

Mostly known as earth bumblebee, Bombus terrestris is a social bee, with a large body and is the most common bumblebee of the three species found in Palestine. However, it is not a native, but invasive species in some Mediterranean areas. The earth bumblebee is a Palearctic species, native of temperate Eurasia and has been worldwide moved since the 1800s. This species is most commonly found in Europe and generally occupies temperate climates. Additionally, it has escaped captivity after being introduced as a greenhouse pollinator in countries where it is not native, being now considered an invasive species in many places, including Northern Africa, Japan, Chile, Argentina, and Tasmania.

Bombus terrestris is a big bee species and its size varies among queen, workers and drones. The queen is between 20–22 mm long, while males are 14–16 mm and workers are 11–17 mm long. The general basic colours characteristics of this species are black on the head, thorax, legs, first and third segments of the abdomen. They have, in addition, an intense yellow stripe on the thorax and on the second segment of the abdomen and white abdomen tip. This colouration pattern does not vary so much among the B. terrestris castes and among individuals, but the intensity of the yellow parts. A further and interesting characteristic of these short-tongued bees (4.5-6.5 mm tongue), is their capacity to robber nectar of long tube flowers, by piercing them from the outer part of the flower.

As a social bee species, the earth bumblebee nest is a colony and the social and working divisions are in castes (queen, workers and drones). The queen is the only reproductive member of the colony and her function is to lay eggs, when she finds a proper local for a nest. Workers are all sterile daughters of the queen, with functions of colony defence, foraging for larvae and queen food and colony cleaning. Queen and workers develop from fertilized eggs (diploid), whilst drones (male bees) develop from unfertilized eggs (haploid). Directly after hatching, drones leave the colony and mate a new queen. When drones emerge from the nest, they do not return, foraging only for themselves. Therefore, mating is their only role in the colony.

A colony of B. terrestris starts when a queen hatches, leaves the colony and search for food (nectar) to gain enough energy for maturate its ovaries, then she mates a drone and finds a new nesting place. The new queen will then lay the first diploid eggs (female) and will feed them with pollen and nectar until they hatch as adult in the next spring. This is the beginning of the colony, which can reach from 100 up to 600 individuals till the end of the entire cycle. The newly hatched workers will forage for the colony and will take care of the next generations. In autumn in temperate zones, workers start laying unfertilized eggs that will become drones. At this point, begins aggression among workers and between the queen and workers. The queen will then be forced to leave the colony and the remaining workers will raise new queens. The newly emerged queens may forage for helping to raise another brood of queens and, during this time, they can mate. In temperate climates the new queen may find a place for hibernate until next spring. However, in warmer climates they do not hibernate.

Commonly, the old colony will die and one of the new queens may return to clean up and reuse the nest.

Curiously, a B. terrestris queen mates just a single drone (monandrous mating system), in opposite to what happens to honey bees, where a single queen mates around 15 drones (polyandrous) in one swarming. Mating with multiple drones provide benefits of genetic variability for the offspring. The reasons of mating with a single male may, however, be due to risks associated with multiple mattings, to high energetic costs and to the risk of exposing the queen to predation. Further, this mating system may also be explained by its phylogenetic position, since the social organization level of B. terrestris colonies is not as complex developed as in the honeybees (Apis mellifera) colonies.

Bombus terrestris started to be worldwide spread in the 1980s when bees were reared artificially and commercially in Europe for pollination in greenhouses. Interestingly, this species was very fast recognized as invasive and would, therefore, disturb local ecosystems. Thus, the invasive characteristics of B. terrestris are: high migration ability, early seasonal emergence, high adaptability to adverse climatic conditions in several habitats and to be a generalist or polylectic species, enabling it to forage on a wide variety of flowers. Further invasive features of these bees are their capacity of foraging over wide distances; a metabolism that enables them to support low temperatures; no natural enemies to control population growth in areas outside its natural range and the fact that it may reproduce twice in a year (bivoltine). In addition, commercial bees produce better competitors than the local populations and may replace these if those escape. The biological impacts of invasive B. terrestris are, for example, bad interactions with local bee fauna; increasing competition for nest sites and for nectar and pollen resources; genetic contamination of local Bombus spp. and spread of parasites and pathogens.

The earth bumblebees are commonly parasitized by B. bohemicus and B. vestalis. B. bohemicus is a more generalist parasite and attacks B. terrestris by invading its colonies and taking over reproductive dominance from the host queen. The parasite lays its own eggs that will be cared for by the host workers. In contrast, B. vestalis is highly specialized in parasitizing only B. terrestris. Moreover, a common honeybee pathogen called deformed wing virus (DMV) has spread to B. terrestris. This virus results in reduced and crumpled wings, making the ill individuals impossible to fly. In 2004, as many as 10% of queen bees bred commercially in Europe were found dead with deformed wings.