Tree Bumble Bee (Bombus hypnorum)

The Tree bumblebee (Bombus hypnorum), is a species native to the European continent and parts of Asia. Some also refer to it as the also known as the new garden bumblebee. It was first seen in 2001 in the United Kingdom and in 2008 in Iceland. These little bumblebees are around 0.4-0.6 in (1-1.6 cm) long and weigh between 0.0014-0.03 oz (0.04-0.85 g).

The Tree bumble bee can be identified by its predominant ginger-brown thorax, black abdomen, and white tail. It is sometimes mistaken for the Common carder bumblebee but the carder bee lacks the white tail. Although Tree bumblebees are not really aggressive, they will defend their nest if they feel threatened. Only the female Tree bumble bees can sting, males have no stinger.

These little fellows have a rather broad diet, collecting nectar and pollen from an array of flowers. However, they appear to have an especially strong attraction to raspberries and blackberry blossoms. Such fruits have made gardens and allotments great places to spot these bees.

Habitat of the Tree Bumble Bee (Bombus hypnorum)

Tree bumblebee species often prefer the proximity of human settlements and urban areas, which provide them with both a safe habitat and an abundance of food. As the name suggests, they nest above ground (about 1,50 m), often in bird boxes, wall cavities, roof spaces and hollow tree stumps. They sometimes also build their nests in trees and hedgerows but usually close to human habitats. Other nesting sites can include compost heaps and even tumble drier vent pipes, although this is uncommon. Another frequent location for Tree bumblebee colonies are rabbit hutches and horse stables where they take up residence in the piles of straw. Nests made in lofts, as long as they are clean and free of parasites, are often re-used by subsequent queens.

Signs of a Tree Bumble bee Nest

Yellow smudges (Bee poop) on the front of a bird box are often a sure sign that Tree bumblebees have taken over. There may also be some pieces of bird-nest material protruding from the entrance hole, which is another tell-tale sign of Tree bumblebees nesting inside.

A word of Caution! Tree bumblebees, often found in bird boxes attached to sheds or insecure fences, are extremely sensitive to vibrations. These vibrations can be triggered by activities as mundane as opening and closing the shed door. When disturbed, tree bumblebees may rush to the entrance of their nest all at once and may fly at, or even sting people nearby. In some cases, the bees can “boil out” of their nest, which can be intimidating – especially if someone wasn’t aware of the colony. Although they do not cause structural damage, in some cases it may be necessary to move and relocate these bumble bees.

Building structures like wall cavities and roof or loft spaces are far superior than the open environment in terms of their ability to absorb vibration. This significantly reduces the likelihood that colonies will experience this type of disruption.

Like honey bees, Tree bumblebees also fan their nest to cool down when the temperature gets too hot in the summer.

Unique Traits

• Unlike other species of bumblebees, the Tree Bumble bee prefers habitats that others do not, bringing much-needed pollination to wildflower habitats that many other species do not typically inhabit.
• Interestingly it appears to avoid and does not stay in areas with a high concentration of rapeseed cover.
• Tree Bumble bee queens sometimes evict blue tits from their bird boxes to construct their nest inside.
• These little bumble bees are extremely sensitive to vibrations.

Colony Structure

Tree bumblebees are social insects, meaning that they live together in groups with a single queen at the top of the hierarchy, with workers and drones making up the rest of the colony. Their colony size ranges between 150 to 400 individuals.

The caste system of Tree bumble bee colonies is significantly weaker than that of its close relative, Bombus terrestris. This may be because Tree bumble bee also have smaller overall colonies. A significant factor in determining the caste hierarchy is the amount of food the larvae receive during their development. When they are older, they arrange  themselves into groups based on olfactory cues they receive in form of pheromones.

Life Cycle of the Tree Bumble bee

The life cycle of Tree Bumble bees is relatively short. In late February or early March, a single fertilized queen forages for food to replenish herself before she looks for a suitable nesting place. She then lays her first batch of eggs, which develop into workers.  The queen will forage for food and take care of the young bees until they are ready to take over her duties. This cycle usually takes 6 weeks. Once there are sufficient workers, the second, smaller breeding cycle starts, with the queen producing new queens and drones. By the time the new queens and drones have hatched, most of the workers have died. The colony can remain active for 3 to 4 months, sometimes up to 5 months.

Once the new queens drones reach early adulthood, they will leave the colony in search of a mating opportunity with a new queen. Bumblebees typically do not display multiple mating behaviors, although some queens do mate with more than one male. When this happens, one male generally fathers most of the offspring.

Surveillance Flights

In early summer (usually between May and June) large groups of tree bumblebees can often be seen gathering around a particular location. This phenomenon, known as ‘Nest Surveillance’ or ‘Lekking’, is often confused with ‘swarming‘ but has nothing to do with it. It is the result of male bees congregating around a nest to mate with new queens when they emerge. Although this behavior may appear alarming, it is entirely harmless since male bumblebees do not possess stingers.

When male bees come across a potential mate, they will often dart towards one another in a brief but energetic display. During this moment of excitement, the bees may fall out of the air with an audible bang. This phenomenon is known as “mistaken-identity mating activity” and may occur when both bees believe they have found a compatible match.

Mating Process

Mating between two Tree bumble bees consists of two distinct parts: the approach and the copulation. When a male bee attempts to mate with a virgin queen, it is not uncommon for people to misconstrue the scene as a fight between bees. In reality, it is a courtship behavior with the male attempting to mate with the queen. The male bee will initiate the process by approaching the female. He hovers above her and inspects her with his antennae. If the queen is of the same species, he will land and continue his inspection using his antennae. She will decide whether to take off or remain with him. If she agrees, the pair may fly off together to a private area like foliage or vegetation. If not, the male may get knocked off immediately. It is the queen that ultimately decides whether or not to mate with the male bee.

When the queen is receptive she releases a pheromone from her mandibular gland. This signals her readiness to the drone, who then mounts her from behind by using his front legs to grasp her abdomen. B. hypnorum drones will tap the female’s abdomen with his legs in ten-second intervals for a few seconds before copulating. The copulation process typically lasts between 20 and 40 minutes. After mating, both male and female bees will fly off in search of nectar and pollen. The female will lay her eggs in a nest, while the male will continue to find other potential mates.

Workers

Workers tend to mature quicker than queens since their development time is shorter than the queens. They also have a relatively short lifespan, typically living for just four weeks. Queens live significantly longer with an average lifespan of up to one year.

The difference in development period is caused because queens have a larger amount of juvenile hormone than workers when they are larvae. This higher level of juvenile hormone allows them to remain in the larval stage for a longer period of time than workers. This extended larval phase is what allows queens to grow into larger and more heavily armored adults than workers. The additional growth time also allows queens to develop reproductive organs, giving them the ability to lay eggs.

Tree bumble bee colonies have three distinct worker groups – the dominant, subordinate, and foraging workers. The dominants are especially aggressive and are known to attack and sometimes even bite their fellow workers. They also secrete a special “queen-like” pheromone to demonstrate their dominance without resorting to physical violence. Egg-laying workers in a Tree bumble bee nest appear to have a strict hierarchy among themselves. When the queen was alive, the strongest egg-laying workers would eat any other workers that were laying eggs in the nest. If this worker dies, the next in the hierarchy would start up this act as well and at the same time defend her own eggs.

Subordinate workers help build and maintain the colony while forager bees search for food, return with it, and feed the larvae.

Predators and Diseases

Tree Bumblebees are frequently killed by a variety of predators, including avian species like the European Honey Buzzard, the Great Gray Shrike, and Great Tits. They may also fall prey to spiders, flies, bee-eaters, and other insect species. Human destruction of their habitat, the use of pesticides and herbicides as well as climate change can also reduce the abundance of their preferred food plants and result in decreased numbers of tree bumblebees.