Bombus hortorum

Bombus hortorum
Queen on blackberry
Garden bumblebee on foxglove
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Apidae
Genus:	Bombus
Subgenus:	Megabombus
Species:	B. hortorum
Binomial name
Bombus hortorum (Linnaeus, 1761)

The garden bumblebee or small garden bumblebee, Bombus hortorum, is a species of bumblebee found in most of Europe north to 70ºN, as well as parts of Asia and New Zealand. It is distinguished from other bumblebees by its long tongue used for feeding on pollen in deep-flowered plants. They have a remarkable visual memory capacity, which aids them in navigating the territory close to their habitat and seeking out food sources. Due to its long tongue, this bumblebee mainly visits flowers with deep corollae, such as deadnettles, ground ivy, vetches, clovers, comfrey, foxglove, and thistles.

Taxonomy and phylogeny

Bombus hortorum belongs to the Bombus, or bumblebee, genus. It holds the position of being one of the six most common bumblebees throughout Europe. Of the six species, only two are long-tongued: B. hortorum and B. pascuorum. It is closely related to the species Bombus ruderatus. The two species are nearly identical in terms of morphology, but a DNA sequencing test serves to distinguish them. The test uses electrophoresis-banding patterns to assign identities to the two physically similar species, and taxonomists have discovered an 89-100 percent success rate of accurately recognizing individuals from the two species based on their genetic information.

Queen

Description and Identification

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.

Eye anatomy and vision

The compound eyes of Bombus hortorum are two dark, oval structures situated on the dorsal-ventral axis of their head. They lack interfacetal hairs and feature a relatively thick cornea and large retinal cells. Three spectrally sensitive cells were identified: UV with peak sensitivity to light of 353 nm wavelength, blue and green sensitive receptors with peak sensitivities to wavelengths of 430 nm and 548 nm, respectively. The ratios of these three cell types were 1:1:6. Polarization sensitivity occurred mostly in the UV cell types. Intracellular recordings to brief flashes of lights of different durations, intensities and spectral properties were carried out and acceptance angles (ca. 4 degrees) and flicker fusion sensitivities (approx 130 Hz) were determined. The median ocellus was found to possess a much wider acceptance angle and a higher UV : green receptor cell ratio than the compound eye.

Distribution and habitat

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. It is also found in America, particularly Florida. In Britain, it is found through the entire region, 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 sedentary 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 cattle-grazed during the summer in order to promote plant diversity and create an environment rich in favored flowers for Bombus.

Nests

Bombus hortorum queens searching for locations to build a nest are most frequently found along forest and field boundaries and in open uncultivated fields. Bombus hortorum usually build their nests under the ground in their ecosystems. 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.

Colony cycle

Colony growth

A single queen starts a new colony in the springtime. They carry out all necessary duties by themselves for over a month until the first batch of workers develops from larvae. In late summer and early fall, new breeding females and males emerge. The males do not return to the nest after they have developed. Instead, they care for themselves independently. The colony size of Bombus hortorum is small compared to other bumblebee colonies. A large bumblebee colony is considered to be several hundred individuals. The typical season of bumblebees lasts from mid-March to October, with summer showing the largest population size.

Colony decline

At the end of the species’ season in the winter, mated queens search for hibernation sites underground while the rest of the colony dies off. In comparison to the six common Bombus species in Europe, B. hortorum is one of the last species to emerge after hibernation. The modernization of agriculture and demand for crops to feed the human population has brought about a significant increase in pesticide use. This has an adverse effect on the Bombus genus. Pesticides can impact the B. hortorum colonies by reducing brood development and also negatively impacting their memory, preventing them from remembering the locations of their foraging sites and nests. When a single queen first establishes her colony at the beginning of the season, pesticide risks could pose consequences for colony development and size, therefore resulting in colony decline. However, this species overall is not in danger of extinction, despite this use of pesticides.

Behavior

Communication

Bombus hortorum do not readily communicate with others when foraging; instead, they collect food independently using a method called ‘trap-lining.' Trap-lining refers to a feeding behavior in which individuals follow a regular route and visit the same flowers each time they forage.

Mating behavior

Bombus hortorum females showcase single mating behavior; 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.

Foraging behavior

Bombus hortorum have been found to forage on plants in the Aquilegia genus; specifically the species of 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 separated by brush from neighboring spaces, displaying ecological isolation. 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. Bombus hortorum play a prominent role in causing cross-pollination in Rhinanthus flowers and thus, make hybridization possible.

Traplining

Bombus hortorum exhibit traplining, a foraging behavior in which they visit the same feeding areas using regular routes that they repeat over several days. The routes are unique to individual bees. Using their preferred routes, Bombus individuals move between plant groups and forage patches as they search for pollen and nectar to consume. The flight direction among these routes is often irreversible, unless environmental factors like wind interfere.

Movement behavior

Bombus hortorum have been found to fly distances up to 2.5 kilometers in a relatively short period of time, approximately one to four days. Queens in particular travel long distances in dispersal flights in order to positively influence gene flow in the species. When traveling long distance, bumblebees occasionally stop to rest, often taking their breaks on prominent structures in their landscape, such as trees.

Unusual behavior due to transmitter attachment

In a study assessing the movement behavior and flight distances of Bombus hortorum, radio tracking was used as a method to gather data on the bee’s routes. To carry out this research, transmitters were attached to the bumblebee’s bodies, directly on their abdomen. After being released with transmitters attached to them, B. hortorum exhibited unique behaviors not commonly observed in the species. These uncommon behaviors included long periods of rest and cleaning in the middle of their flight, resulting from the handling and attachment of the device. One B. hortorum individual in particular showed to take breaks longer than forty-five minutes, exhibiting how transmitters contribute additional weight to the bee, causing them further energy expenditure and the need for more rest.

Interaction with other species

Diet

Bombus hortorum lack much variety in their diet, as the majority of the pollen they collect comes from one source, Fabaceae. They are especially fond of one specific species of Fabaceae, Trifolium pratense. Bumblebees tend to favor perennial flowers rather than annual species because perennials produce more nectar, providing the bees with more sustenance. Because Bombus hortorum has better visual sensitivity compared to other bees, they can start foraging earlier in the morning and return to their nests later in the day. B. hortorum exhibit buzz pollination, a foraging behavior in which they generate vibrations that are transmitted onto flower’s anthers, thus ejecting the pollen that they gather and then consume. In a study comparing other Bombus species, B hortorum was found to create higher buzz amplitudes, thus exhibiting them as one of the more efficient species that was able to collect more pollen.

This preference can be explained by Bombus hortorum tongue length, which is the longest compared other long-tongued species and to the majority of bumblebees, which are short-tongued species. Because T. pratense is a flower species with a long corolla tube depth, B. hortorum can easily reach the nectar and pollen within the tube with their unique tongues.

Defense

Bombus hortorum exhibit defensive behavior through the use defensive buzzes. Defensive buzzes can be differentiated from other buzzes because they tend to have much greater amplitude values. Since defensive buzzes are used as warning signs and to show aggression, they are produced using much more power than other buzzes.

Competition

Because they forage at the same sites, B. hortorum compete largely with honeybees. To resolve the issue and maximize their consumption during foraging, bumblebees forage early in the morning and evening while honeybees forage during the afternoon.

Parasites

Neogregarine Parasite

In 1988, a new pathogen was discovered to be affecting Bombus hortorum workers and queens, causing the appearance of unusual spores on the bumblebees. Research confirmed that a new parasitic protozoan that belongs to the order Neogregarinida caused the infection; this was attributed to the type of spores and the life cycle associated with the protozoan. It is presumed that the parasite is widely distributed throughout Europe.

Crithidia bombi

Bombus hortorum serve as hosts for Crithidia bombi, a widespread gut parasite that is present in many bumblebee species. The pathogens negatively impact reproductive fitness of Bombus queens, as they affect their ovarian development as well as early colony establishment after the queens emerge from hibernation. Queens hibernating underground during the winter are not directly affected by the parasite. C. bombi’s dominant route of infection into Bombus individuals is by ingestion of the infectious agents. This process is known as vertical transmission and is greatly influenced by the number of available individuals who can serve as potential vectors. Another infection pathway of C. bombi into its’ host is through horizontal transmission in which foraging worker bees catch the infection from flowers they are feeding on. While inside their host bodies, Crithidia bombi have been discovered to reproduce clonally as well as sexually. After being ingested, the genotype of the host can play a prominent role in the development of the parasite. Bumblebee colonies exhibiting little variety in their gene pool, typically as a result from inbreeding, tend to have a higher occurrence of C. bombi as compared to Bombus populations with high levels of heterozygosity. Furthermore, the host genotype may affect the response to the parasite by triggering the bee’s innate immune system as well as up-regulated effector genes that defend the host.

Human importance

Agriculture

Bombus hortorum play a very active role in agriculture, as they are frequent pollinators of many crops, including sunflowers, strawberries, apples, and tomatoes. The modernization of agriculture and demand for crops to feed the human population has brought about a significant increase in pesticide use. This has an adverse effect on the Bombus genus. The bees are directly exposed to the chemicals in two ways: by consuming nectar that has been directly treated with pesticide, or through physical contact with treated plants and flowers. Pesticides can impact the B. hortorum colonies by reducing brood development and also negatively impacting their memory, preventing them from remembering the locations of their foraging sites and nests. Furthermore, when a single queen is first establishing her colony at the beginning of the season, pesticide risks could pose consequences for colony development and size.