Garden bumblebee (Bombus hortorum)

The garden bumble bee (Bombus hortorum) is a relatively large bumble bee common in Europe. Queens, drones, and workers look similar featuring the same color pattern. They are identified by their white tail, three yellow bands, and long, thin face.

The garden bumble bee looks very similar to thee other types of bumblebee. They are the Barbut´s cuckoo bumblebee (B. barbutellus), the Heath bumble bee (B. jonellus), and the Ruderal bumble bee (B. ruderatus). Cuckoo bumble bees have no pollen baskets because they are predators and live of the brood and food of other bumble bees. Both, the Barbut´s cuckoo bumble bee and the Heath bumblebee have round faces compared with the garden bumble bee.

The Ruderal bumblebee, also called the ´large garden bumblebee´ is almost identical to the Garden bumblebee. It is often makes it impossible to tell them apart.

Garden bumblebees belong to the Apidae family. They are widespread and abundant in northern Europe including Scandinavia and Iceland and in the south up to the Mediterranean. There have been no sightings in Corsica, Sicily and Sardinia, however, their distribution spreads to northern and central Asia up to the Altai Mountains in Siberia and northern Iran. They can also be found in Florida and New Zealand and are most active in warm, sunny weather.

Habitat of the Garden bumblebee

Garden bumble bees are most abundant in grassland areas that were used for cattle according to a study. This is because the feeding of cattle leads to a larger diversity of blooming flowers that these bees forage on. These species typically make their habitat in gardens, clearings and edges of woodland, grassland, and brownfield sites.

They like to feed on Red Clover, Cowslip, Foxglove, Vetches, Lavender, Honeysuckle, and Nettles. Garden Bumble bees typically prefer perennial flowers (aside from Rhinanthus) that tend to produce more nectar than annual ones.

Nesting Habits

Garden bumble bees prefer to nest underground and typically construct their dwellings facing south in cooler regions to keep warm. These resourceful insects commonly repurpose deserted rodent burrows, however, they also occasionally nest in unusual places above ground like bird boxes, buckets, or even abandoned machinery.

Because they need moss and dried grass to build their nests, they prefer grassland habitats with ample sunlight.

Unique Traits of the Garden Bumblebee

Only two of the six most common species of bumble bees are long-tongued (proboscis). One of these is the garden bumble bee and and the Bombus pascuorum. Did you know that the garden bumblebee´s tongue can stretch up to 2 cm? This is as long as their own body!

Garden bumblebee Foraging Method

Also, unlike most other bees, the B hortorum do not typically communicate with others regarding foraging grounds, instead they use a trap-lining method. This is a feeding strategy whereby individuals repeatedly visit the same food sources on a regular basis similar to hunters who check their traps regularly.

Workers of the garden bumble bee species typically collect either pollen or nectar during a foraging flight on one specific plant species. In contrast, the B hortorum queen collects both pollen and nectar in one expedition by using her tongue and jaws to grasp the stamen and petals of the flowers. In addition, the queen also visits multiple plant species.

Pollination Method

Garden bumble bees are upright pollinators. This means the pollen is deposited on their head and thorax instead of on their legs and the underside of their abdomen when feeding. This may be the reason for premature baldness that is  observed in some individuals due to the constant rubbing their heads and bodies against the petals during foraging. According to research, garden bumblebees visits eighteen flowers per minute. This is more than other species. They also play a key role in generating cross-pollination in Rhinanthus flowers and therefore enable the creation of hybrids.

B. hortorum are excellent buzz pollinators that create higher amplitudes of vibration than other bumble bee species. According to research, their buzz is also louder and produces much more power than other bumble bees. This makes them much more efficient at collecting pollen. When they forage for food, the buzzing vibration is transmitted onto the anthers of flowers. This results in the pollen being ejected by the vibration which the bees gather and consume.

Visual Sensitivity

Garden bumble bees have better vision than other bees, allowing them to forage for longer periods by starting earlier in the morning and returning home later than other bees. Because they often compete for nectar with honey bees, this allows them to avoid direct competition as honeybees generally forage during the afternoon.

Movement

Scientists attached transmitters to the bumblebee’s bodies and found that Garden bumblebees, especially queens can fly up to 2.5 kilometers in approximately 1 to 4 days. The study discovered that bumblebees take unusually long periods of rest and cleaned themselves in the middle of their flight.

Colony Structure and Life Cycle of Garden Bumblebees

During springtime, a single queen initiates the establishment of a new Bombus hortorum colony. In Europe, this species is typically the last of the commonly observed Bombus species to emerge from hibernation (late April to early October).

The queen locates a suitable spot for her nest and proceeds to construct a circular chamber. Using wax that she secretes, she builds an egg cell and lays her first batch of eggs. Garden bumblebees do not produce honey. In front of the cell, she arranges a pot-shaped structure made of wax to hold the nectar she collects. This provides her with sufficient energy to incubate the eggs for several days. To keep the eggs warm and comfortable, the queen shivers her muscles, actively contracting and relaxing them to generate heat. When the larvae emerges, resembling small grubs, the queen tirelessly scavenges for nectar and pollen to nourish her brood.

The queen undertakes all vital tasks alone for a span of over a month, until the initial batch of workers take over foraging and other duties, leaving the queen to lay more eggs. Their population is at its peak during summers. The average lifespan of the Bombus hortorum workers and drones is approximately 14 weeks. Queens live around one year.

Later in the season, during the early fall or late summer, new queens and males appear. Once the males are fully developed, they do not return to the nest. They fend for themselves independently pursuing only one goal – to mate. Males typically patrol for new queens less than one meter above the ground.

At the same time the new queens also leave the nest pursuing their goal to mate and start a new colony after hibernating until the following spring.

Compared to other bumblebee colonies, Bombus hortorum colony sizes are small, consisting of 50 to 120 individuals. A large bumblebee colony can comprise of several hundred individuals.

Parasites and Predators

To defend themselves against predators, garden bumblebees use defensive buzzes as warning signs. The garden bumblebee is prey to badgers and birds. Wasps and flies often lay their eggs inside live bumblebees or bumblebee nests. Another insect that preys on bumble bees is the crab spider.

Garden bumblebee workers and queens can become affected by a recently discovered single-celled microscopic parasite (protozoan) that causes the appearance of unusual spores on their bodies. According to research, this parasitic protozoan belongs to the order Neogregarinida. Researchers believe the parasite is widely distributed throughout Europe.

Another parasite that affects the Bombus hortorum is the widespread gut parasite Crithidia bombi. The parasite affects the ovarian development and therefore the reproductive fertility of the queens. The parasite is distributed by ingestion of the infectious agents by larvae from the workers that feed them (vertical transmission). Another source is the horizontal transmission. This is when workers become infected by foraging on flowers that were previously visited by other infected bees.

Threats to Survival

Bombus hortorum play a crucial role in the pollination of various crops such as sunflowers, strawberries, apples, and tomatoes. However, the modernization of agriculture and its high demand for crop production has led to an increase in pesticide usage, which adversely affects this bee species.

B. hortorum are directly exposed to pesticides through nectar consumption from treated plants or by coming into physical contact. This pesticide exposure can disturb their colonies, reducing brood development and impairing their memory, making them forget the location of their foraging sites and nests.

Additionally, pesticide exposure can have severe consequences for the colony during the early days when a single queen is establishing the colony, leading to poor development and smaller colony size. It is, therefore, essential to take necessary measures to protect Bombus hortorum and other pollinators from pesticide exposure to ensure the sustainability of our crops and the ecosystem.