Biology and ecology
Bumblebees are large, colourful and furry bees. The most common species are characterised by black, yellow and white body hairs, often in bands; however, some species have orange or red body hair, or may be entirely black.
Bumblebees are described as primitively eusocial, since their social organisation is considered to be simpler than that of the honey bees. A bumblebee colony is composed by a single queen, many female workers and males only for a short period of colony life. Among workers there is not a division of labour as in the honey bees. The size a colony reaches depends on the bumblebee species and the food supply: some can have as few as 30 bees, others can reach as many as 300.
Bumblebees collect nectar and pollen from many botanic species and are therefore considered polilectic pollinators. There is a high variability in the length of the ligula among different bumblebee specie (long-tongued bumblebees and short-tongued bumblebees), which determine a specialisation of the species in the pollination of flowers with perianth of different size and shape. Thanks to their ability to be active even at low temperatures, unlike honey bees, they are reliable pollinators even in very cold and unpredictable climates.
The life cycle
Almost all bumblebees species have an annual cycle, which is divided in a solitary phase and a social phase. The solitary phase starts in spring, when the queen exits the hibernation and searches for a proper place where starting a new colony (tipically an abandoned mouse nest). During this phase only the queen is present, she collects nectar and pollen and lays fertilised eggs, which develop into females (workers). When the first workers emerge, the social phase begins, where the queen stops her foraging activity and devotes herself exclusively to egg laying for the rest of the summer. Workers take her place in all the other duties, like foraging, nest cleaning and larval nutrition.
In a late phase of the colony (around August in North Italy) the queen starts to lay unfertilised eggs, which develop into males, whereas the last group of fertilised eggs develop into queens. In the last phase of colony development usually the queen dies, adult males and young queens leave the colony for mating and the colony itself goes into decline. After the mating, males die and the young fertilised queens search for a proper place to hibernate alone (they usually dig a short tunnel and rest in a cavity barely as large as her body). The new queens will leave hibernation the next spring to start a new colony each.
The majority of bumblebees has very similar colony cycles, the main differences can be found in the timing of the different phases, in the final size of the colony and in the way of larval nutrition “Pollen storer” bumblebees store pollen in separate wax pots and workers feed it to the larvae in the same fashion as nectar. “Pocket makers” bumblebees create pockets of pollen at the base of the brood cell clump from which the larvae can feed themselves.
Taxonomy and distribution
Bumblebees are social insects belonging to the superfamily Apoidea and family Apidae.
About 250 bumblebee species have been described worldwide, while in Europe 72 species have been reported so far (81 including those from Anatolia). The most common bumblebee species in Europe are Bombus lucorum, B. terrestris, B. hortorum, B. lapidarius, B. campestris, B. pratorum, B. humilis, B. pascuorum, and B. ruderarius.
In Italy there are about 43 species, the 6 most common being B. terrestris, B. lucorum, B. pratorum, B. lapidarius, B. hortorum, B. pascuorum; the less common species in Italy are B. alpinus, B. confusus, B. argillaceus, B. gerstaeckeri, B. sichelii, B. mendax, B. flavidus, B. norvegicus, B. hypnorum, B. jonellus, B. monticola, B. pyrenaeus, B. wurflenii and B. mucidus, all species restricted to the high mountains of central and northern Italy.
Bumblebees are found mainly in northern temperate regions, though there are a few native South American species and New Zealand has four species that were introduced around 100 years ago to pollinate red clover and become established.
Bumblebees distribution ranges much further than that of honbey bees, and their colonies can be found also in very northern region, near the north pole. They are capable of thermoregulation and therefore they are well adapted also in very cold climate, like alpine and arctic zones.
Due to its wide commercialisation (see following chapter), the specie B. terrestris is now spread as alien specie in countries where it is not native (Mexico, Japan, China, Korea and Taiwan) and in others where it was present only in some regions or was present with different sub-speciesr (Israel, Jordan, Spain, Italy and Turkey). Among the four bumblebee species which were introduced and become established to New Zealand, recently one spread also to Tasmania.
Since 1980 bumblebees are reared and commercialised extensively for the pollination of crops, especially in greenhouses. Bumblebees are capable of buzz pollination, which consists of a strong vibration of anthers to make the pollen come out. This makes them excellent pollinators of Solanaceae, whose flowers have anthers with a tubular shape and an opening at the end, which require buzz pollination. The main crop pollinated by bumblebees is by far tomato, but they are also used to pollinate green pepper, aubergine and musk melon in greenhouses and, to a lesser extent, almonds, plums and cherries in open orchards.
Currently, five species of bumblebees are reared commercially. The main species is the Eurasian B. terrestris which distribution is all over Europe, in coastal North Africa, and in West and Central Asia. It is commercialised in Europe, North Africa, Asia, Australasia and South America. The other reared species are B. ignitus in West Asia (China, Japan and South Korea), B. lucorum (China), B. impatiens in North America (Canada, U.S.A. and Mexico) and B. occidentalis in western North America and Canada.
Recently the commercial use of bumblebees has rise many environmental concerns, for the massive collection of queen from nature and the risk of genetic pollution due to the introduction of non-native species and subspecies.