Garden bumblebee, Small garden bumblebee
Bombus hortorum, the garden bumblebee or small garden bumblebee, is a species of bumblebee found in most of Europe north to 70°N, as well as parts of Asia and New Zealand. It is distinguished from most other bumblebees by its long tongue used for feeding on pollen in deep-flowered plants. Accordingly, this bumblebee mainly visits flowers with deep corollae, such as deadnettles, ground ivy, vetches, clovers, comfrey, foxglove, and thistles. They have a good visual memory, which aids them in navigating the territory close to their habitat and seeking out food sources.
This bumblebee has an oblong head and a very long tongue, about 15 mm (0.59 in), and in some cases even 20 mm (0.79 in). The tongue is so long that the bee often flies with it extended when collecting nectar. The queen is variable in size, with body lengths between 19 and 22 mm (0.75 and 0.87 in), and wing spans from 35 to 38 mm (1.4 to 1.5 in). The workers are almost as large, the larger ones overlapping the smaller queens. Their color is black with a yellow collar, a narrow yellow band on the scutellum, and a third yellow band on terga (abdominal segments) 1 and 2. The tail is white. Darker forms, with little yellow in their fur, are common.
This species is found in Europe as far north as 70ºN (in Scandinavia, south of the tundra). In the west, its distribution reaches Iceland, where it probably has been introduced. In the south, it extends to the middle of the Iberian Peninsula, to southern Italy (Calabria), northern Turkey, and to the Mediterranean islands except Corsica, Sicily, and (probably) Sardinia. It continues in northern and central Asia through Siberia to the Altai Mountains, and, in the southeast, to northern Iran. In 1885, it was introduced in New Zealand, where it still exists, but without being particularly common.: 219–220 It is also found in America, particularly Florida. It is found throughout the British Isles, including Orkney and Shetland. Bombus hortorum are commonly found in grassland environments with abundant flowers that they can feed on.
In a study analyzing spatial patterns of Bombus in different habitats, it was discovered that B. hortorum are most abundant in recently cattle-grazed grasslands, as compared to arable, sheep-grazed, unmanaged, and disturbed land plots. Cattle play an important role in the habitat of Bombus species because their feeding actions in grasslands cause a more diverse floral environment, which is preferred by the bumblebees for their own foraging behavior.
Due to their localized lifestyle and the destruction and loss of grassland habitats, Bombus hortorum populations are expected to decline in the future. In order to address the conservation of Bombus hortorum, it is important that large areas of foraging plant diversity and nesting sites either receive no intervention as to foster a natural habitat, or are extensively grazed by cattle during the summer in order to promote plant diversity and create an environment rich in favored flowers for Bombus.
Bombus hortorum do not readily communicate with others when foraging. Instead, they collect food independently using a method called ‘trap-lining', in which individuals follow a regular route and visit the same flowers each time they forage.
Bombus hortorum have been found to forage on plants in the genus Aquilegia; specifically the species A. vulgaris in Belgium and A. chrysantha in North America and Belgium. In regards to Aquilegia foraging, Bombus hortorum do not show any preference in color of the flowers.
When Bombus workers set out to forage, they typically restrict themselves to either pollen collection or nectar foraging on one specific plant species. On the other hand, queens combine both pollen and nectar foraging in one expedition and visit multiple plant species. The queens use their tongue and jaws to grasp the stamen and petals of the flowers and collect food. While several other Bombus species perforate the flowers when foraging, Bombus hortorum do not perforate the flowers they feed on. This lack of perforation is thought to be attributed to the species' extremely long tongue length, which sufficiently reaches the nectar without any trouble.
Bombus individuals forage in small areas of several square meters for many days. Small areas they have been found to feed in are commonly separated by brush from neighboring spaces. In a study examining diet preferences of Bombus hortorum, it was determined that foraging behavior of individual bumblebees affects isolation and hybridization of flowering plants. In comparing choices of Rhinanthus plant species, B. hortorum queens and workers showed a strong preference for R. serotinus which has a long corolla length, but it easily reached with B. hortorum’s long tongue length. Observations of B. hortorum foraging patterns reveal that they are nototribic, or upright pollinators. Thus, when feeding, pollen is deposited on their head and thorax rather than on their legs and the underside of their abdomen as seen in sternotribic, or inverted foragers. When feeding on plants in quick continuation, bumblebees have their proboscis extended as they approach the flower. Data from this study reveals that when pollinating, Bombus hortorum visit eighteen flowers per minute, which is more than other species. This large quantity is most likely due to the greater efficiency B. hortorum experience by having longer tongues. B. hortorum play a prominent role in causing cross-pollination in Rhinanthus flowers and thus, make hybridization possible.
Bombus hortorum queens searching for locations to build a nest are most frequently found along forest and field boundaries and in open uncultivated fields. B. hortorum usually build their nests under the ground. They need moss and dried grass to be present in their habitats in order to successfully build their homes. Thus, they prefer grassland habitats with ample sunlight reaching the land in order to ensure secure and warm nests beneath the ground. Both this species and Bombus pratorum are known to build their nests in unusual places.
Bombus hortorum females mate once only; this increases the overall relatedness of individuals in their colony. At the end of the season in late summer, females mate and then store the sperm in spermatheca during their hibernation period. Then, at the start of the season when the queen emerges to start her colony, she uses the stored sperm to fertilize her eggs and produce workers. The average ejaculate size in B. hortorum is 6,800 sperm, which is relatively small compared to other Bombus species.As with most bumblebees, the males of this species patrol a fixed circuit, marking objects along the route, about a meter above ground, with a pheromone to attract queens. This behaviour was noted by Darwin 1886 in his own garden.: 47
