Ruderal Bumblebee (Bombus ruderatus)

The Ruderal bumblebee (Bombus ruderatus) is also known as the Large garden bumble bee. It is geographically widespread but faces decline due to various contributing factors. Of the 23 bumble bee species in Britain, it is the most rare species and is listed on the Biodiversity Action Plan.

The Ruderal bumblebee is native to Europe, North Africa and North America. Since the 1800´s, the Ruderal bumblebee was used commercially to pollinate crops. It was first introduced to New Zealand (1885), and later to Chile (1982), to pollinate red clover. In the early 1990´s it was also spotted in Argentina (Patagonia).

The large B. jonellus has a very long tongue (proboscis), which is often held outstretched as it approaches a flower. All three castes look similar. They feature a yellow-black-yellow thorax, a yellow band at the base of the abdomen, and a pure white tail. The extent and brightness of the color bands and tail color can vary considerably. Some individuals are all-black including the tail. This variety of the Ruderal bumblebee is named Bombus harrisellus.

Drones of this large bumble bee typically measure between 15 mm (0.59 in), and queens up to 22 mm (0.87 in). Workers are slightly larger than drones, measuring about 16 mm (0.63 in).

Although slightly larger, the Ruderal bumble bees is often mistaken for the Garden bumble bee (Bombus hortorum), the Heath Bumblebee (Bombus Jonellus), or the parasitic Barbut´s cuckoo bumblebee.

Habitat of the Ruderal Bumblebee

The majority of Ruderal bumble bees tend to nest underground, with some queens even returning to their maternal nest site. However, according to research, a small percentage (around 8-32%) also occupied above ground locations, such as hives of bricks or pumice concrete.

The rare long-tonged Ruderal bumble bee favors meadows that are rich with a variety of long-tubed flowers. These include plants, such as foxglove, honeysuckle, comfrey, yellow iris, and marsh woundwort among others. These flowers provide them with nectar and pollen when they emerge from April to Sept./October within a short distance of their nests.

Colony Structure and Life Cycle

The size of a Ruderal bumble bee colony was reported to be large in 1912 (Sladen) with more than 150 workers. However in 1994 Hagens reported a smaller colony size of between 50 to 100 workers. The colony cycle of the Ruderal bumblebee is slightly later than that of the Garden bumblebee (B. hortorum).

From April until June, a single fertilized queen emerges from hibernation (diapause) to start a new Bombus jonellus colony.

Once she found a suitable nesting location, she replenishes her emaciated body with nectar and pollen to start reproducing. With the wax she secretes, she builds an egg cell to lay her first batch of eggs. To keep them warm, she shivers her muscles to generate heat. When the larvae emerge, the queen forages to nourish her brood with nectar and pollen. The larvae turns into pupae until the first batch of workers emerges around from May/June to August.

Once the first workers emerge, they take over the foraging and other duties the queen carried out until the point, leaving her to focus on laying more eggs. During the summer, the population is at its peak, with males and new queens emerging later in the season (July to Sept./October).

The males emerge slightly earlier than the new queens. Both leave the nest in search of a mate once fully developed. During this time they fend for themselves. After mating, the drones perish. The new queens forage to build up energy for their hibernation until the following spring. This is when a new cycle begins. 

Unique Breeding Behavior

New Zealand researchers observed Bombus ruderatus workers laying eggs while engaging in oophagy. This entails the queen consuming eggs deposited by the workers. Interestingly, both workers and queens participated in the common practice of ejecting live larva from the nest.

More frequently, female workers were observed to conduct ejection as opposed to their male counterparts. However, there were no observable differences in morphology between the ejected and surviving larva, thereby making the exact purpose of ejection unclear. Through comprehensive analysis, it was concluded that worker oviposition is heavily influenced by their engagement in oophagy, as well as certain factors unique to the nest. Additionally, the practice of larva ejection, while not fully understood, is a common trait seen within this species.

Parasites

Argentinian scientists recently discovered the presence of the pathogenic protozoan Apicystis bombi in Bombus ruderatus species. This highly virulent parasite poses a serious threat to bee populations, not only due to the danger of infect many different bumblebee species, but also because it impacts both commercially produced and native colonies.

The parasite can severely inhibit colony foundation. This is because individuals can suffer extreme physical and behavioral effects that leads to an increased mortality rate. The transmission of this parasite to B. ruderatus is believed to have occurred through interaction with another invasive species, the Bombus terrestris, which was introduced in Argentina in 2006.

Before this, no native or invasive species of bee had succumbed to Apicystis bombi. Unfortunately, the transfer of diseases from one species to another is common, especially when trading invasive species like the B. terrestris. This spillover may have contributed to the decline of bumblebee populations, underscoring the importance of regulating invasive species trading to safeguard ecosystem health.

Human Importance

Bumblebees are vital for pollinating crops and flowers, with their pollination activity remaining strong even in lower temperatures. This makes them more effective than honeybees, whose activity tends to decline as temperatures drop. In Chile, the introduction of the Bombus ruderatus bee species in 1982 aimed to aid in the pollination of red clover, a major crop for seeds.

Native bees were not as effective as honeybees in pollinating red clover, and when honeybee activity decreased during late summer to early fall, seed production suffered. The introduction of B. ruderatus bee species increased seed production, leading to positive agricultural outcomes. However, it also resulted in a decrease in the native bee population, which is a negative outcome. Chile’s economic success is closely tied to the effectiveness of bee pollination for red clover crops.

The decline of bee populations, particularly bumblebees, in numerous countries is attributed to various factors, including habitat fragmentation due to human development, climate change, disease, and the use of agrochemicals. The population of B. ruderatus in Britain has significantly declined, endangering many flowering plant species that rely on their pollination. This issue poses a considerable ecological threat and calls for immediate action to address the problem.