Buff-tailed Bumble Bee (Bombus terrestris)

The buff-tailed bumblebee, also known as Bombus terrestis, belongs to the Apidae family. These fluffy little creatures thrive in temperate climates but can survive in all climates. They are common across Europe and Asia, parts of North Africa and the Middle East, as well as some parts of North America. This species is particularly common in all regions of the United Kingdom, except for Northern Ireland. It is one of the most numerous bumblebee species in Europe but considered an invasive species in Japan, Chile, Argentina, and Tasmania.

Habitat of the Buff-tailed Bumblebee

The Buff-tailed Bumblebee has distinctive yellow markings on its body and white buffy tail. These bees prefer open habitats like grasslands, meadows, and heaths. Being polylectic, they feed on any flowers or trees that produce fruit or sap, and often appear in urban areas like parks and gardens. The best chance to spot these bumble bees is in the morning when their activity is at its peak. Workers are active during the winter and are often seen feeding on Mahonia bushes. They typically nest underground, like in abandoned rodent dens.

Unique Traits

The B. terrestis is a bumble bee with some unique traits:

• Unlike many other bee species, the queen is monandrous, meaning she mates with only one male.
• These bees live longer than most other bee species, typically surviving for six months up to two years.
• Workers can be of different size. Their thorax sizes can range from 2.3 to 6.9 mm in length, while their mass can be between 68 to 754 mg.
• It is one of the few bees to actively forage in cold weather, thanks to their thick furry coats that help insulate them from the cold temperatures. They are also capable of producing larger nests than other types of bumblebees that can reach up to 700 individuals at a time.
• The remarkable homing capabilities of Bombus terrestris bees is well-documented. Studies show they can return to their nests from as far away as 13 km. Furthermore, the ability of males to make longer flights could potentially increase the range of pollen flow and improve pollination rates.

Colony Structure

Like most social bees, B. terrestris have three main social castes, i.e. one queen, various drones (males) and many workers. The labor is divided to ensure that the colony works efficiently. Similar to honeybees, the Buff-tailed bumble bee queen is the main reproductive female. Her only responsibility after she finds a nest is to lay eggs.

Workers (females) are usually sterile for most of the colony cycle, and forage for food, defend the colony, and care for the brood. Like honey bees, queens and workers develop from fertilized (diploid) eggs, whereas drones develop from unfertilized (haploid) eggs. Once drones reach adulthood, they leave the colony to find a mate outside the nest. Their sole responsibility is to mate.

Worker B. terrestris display a phenomenon known as alloethism, whereby different sizes of bees fulfill distinct roles. This behavior is most obvious in the context of foraging, with larger bees going away from the nest and returning with high amounts of nectar and pollen. It is possible that large bees may be capable of withstanding higher temperature variations, protecting themselves from predators, and going farther distances. This makes them more competitive in the environment. Meanwhile, small bees are cheaper to breed and use for in-nest tasks, as only some larvae will be provided with enough nutrition to become big.

Life Cycle of the B. terrestris

A single female queen bee is responsible for kick-starting the colony cycle. She finds a suitable nest, mates with a male and then spends the winter in the nest. In the spring, she starts laying small batches of diploid (female) eggs which hatch into larvae. The queen feeds these larvae with nectar and pollen until they pupate, and about two weeks later, the first adult workers emerge. This first phase is called the initiation phase.

Once the workers hatch, they are responsible for collecting the food necessary to feed the colony and care for subsequent generations of larvae. They fly in ever-expanding circles away from their nest to forage for nectar and pollen from flowers. Many workers die whilst foraging, falling prey to birds, bats, and robber-flies.

In some species of B. terrestris, the queen’s initial phase of reproduction can last for a varying length of time. After that, the queen switches to lay some unfertilized eggs that develop into drones. The drones stay in the nest until early adulthood. When they emerge from the nest they do not return. Instead they begin to search for virgin queens to mate with. The remaining diploid eggs hatch into larvae, which receive more food than the other workers and will eventually pupate to become future queens. The queen uses her own pheromones to prevent too many of these larvae from becoming queens.

In temperate climates, the colony continues to thrive until autumn. At this point, the workers become aggressive and  start to lay unfertilized eggs that may become males. This occurs at around the 30-day mark of the colony life cycle.

Conflict in the Colony

The development of worker bees into egg layers is a phenomenon that is determined by a multitude of factors. Generally, workers born early in the first brood are bigger and older with more developed ovaries. Also, workers that spend less time foraging and more time near the queen are also more likely to become egg layers. The result of worker bees becoming reproductive often creates an unsustainable environment for the queen.

When a worker-queen conflict takes place in a colony, the queen is usually forced out and the workers become “queenless”. A “false queen” may take over the colony for a short period. The newly hatched queens sometimes take over the role of a worker and mate during their daily foraging trips. Eventually, they find a place to hibernate (hibernaculum) until the following spring. When they emerge, they first seek food to replenish, which is primarily to prepare their ovaries. Once replenished, they seek a new nesting site and cycle starts again. In almost all cases the old colony will have died. If the site is free of parasites, it is re-used by one of the new queens.

Conflicts can also occur among workers. Workers of B. terrestris have evolved a behavior in which they actively inhibit the egg-laying opportunities of their sisters, in order to maximize the number of their own sons in the colony. In order to achieve this, workers actively patrol nest entrances and reject potential egg-laying attempts by their sisters. Workers may also engage in direct aggression, such as biting and stinging, towards their sisters when they attempt to lay eggs.

Mating System

Bombus terrestris, are a monandrous species, meaning they only mate with one male.

During mating, the male B. terrestris will secrete a sticky substance that plugs the female’s sexual tract, which has been shown to inhibit the female’s mating ability for several days afterwards. This may be an attempt by males to ensure that their paternity is secure.

Unlike honey bees, Bumblebees tend to exhibit monandry due to a variety of reasons, one of which is social constraints. The risks associated with multiple matings can be high. It costs a lot of energy and exposes the queen to a higher threat of predators. Even though queens may prefer to mate with multiple drones because it ensures a bigger gene pool and therefore higher chances of a healthy brood, mating with one male means that workers are more closely related. They are full sisters, rather than paternal half sisters.

Predators and Diseases

Brood Parasites

The Bombus terrestris has two different brood parasites. Both species of Cuckoo bee, the Gypsy Bumble Bee (Bombus bohemicus) and the Southern Cuckoo Bumble bee (Bombus vestalis) invade B. terrestris hives. They take over reproductive dominance from the host queen. When this is done, they lay their own eggs and force the Buff-tailed bumble bee workers to take care of them as if they were the queen.

Fungal parasites

Nosema bombi is a microsporidian parasite that infects the gut of various bumblebee species. As the parasite multiplies and spreads, spores are released into the environment through feces or the decaying host. It is believed that the disease is mainly transmitted through infected workers contaminating shared food sources, but transmission during mating is also possible.

The effects of N. bombi on bumble bee health are not always clear. Some field studies have found little to no negative impact on colony size or success, while others have found that infected queens produce smaller colony sizes and reduced sexual offspring. Under laboratory conditions, the fungus has been linked with lower adult survival rates and reduced sperm counts of male offspring.

Insects

Workers that spend more time foraging could compromise their fitness as foraging costs a lot of energy. Weaker Buff-tailed bumble bees are more prone to parasitic conopid flies in Central Europe.

Another enemy of the Buff-tailed bumble bee is the female Bee Moth (Aphomia sociella), who is notorious for her destructive behavior. Bee moths lay their eggs in the nests of bumblebees. The hatched larvae then feed on the bumble bee brood and sometimes destroy large parts of the nest on their search for food.

Disease

Deformed wing virus (DWV) is a honey bee pathogen that can have deadly implications for the bee species. However, Buff-tailed bumble bee colonies tested positive for the presence of DWV RNA. This indicates that DWV is either a broad range pathogen among bees, or that honey bees have recently transmitted the disease to new hosts.

The disease is characterized by reduced and crumpled wings, which make the individual inviable. It manifests in other symptoms like a shortened life span, decreased foraging ability and a weakened immune system.