Bumble Bee Drones

How to Identify a Bumble Bee Drone

Bumble Bee Drones or male bumble bees have no stinger in contrast to female worker bees. They have longer antennae and a narrower abdomen than their female counterparts. In addition, they also have hairy hind legs without pollen baskets and are smaller than the queen but slightly larger than workers. Another feature to identify drones are their enormous eyes that almost touch at the top of their head. These are required to spot a potential mate. Workers and queens have much smaller eyes.

Antenna

By examining a bee’s antennae, it is possible to discern its gender. The antennae of bees consist of a scape (base) and a flagellum, with the scape serving as an upper arm-like appendage. Furthest from the head is the the flagellum. This is the segmented portion furthest from the head that is connected to the scape by the pedicel (stem).

The pedicel acts as an elbow joint, providing the antennae with a large degree of rotation. Female bees have a flagellum with 10 segments, while males have 11.  The antennae are used to sense the environment, which helps the bumblebees to navigate and communicate.

Like honey bees, they do not participate in gathering nectar or pollen. Instead, they rely entirely on worker bees to feed them. Although depending on the species, drones do carry some jobs within the colony, their main purpose is to mate with a new queen. This crucial task ensures the survival of the hive by promoting genetic diversity. Without the drone’s contribution, the queen would not be able to lay fertilized eggs and expand the colony. While lacking certain abilities and responsibilities compared to worker bees, drones play a vital role in the overall success of the hive.

Development of Bumble bee Drones

The development of drones, from egg to adult bee, varies among different species. It takes an average of approximately 24-28 days. Factors like brood nest temperature, worker attendance, and the availability of nutritional resources can cause developmental variations even within the same species.

The proximity of workers within the brood area triggers queen bees to lay haploid (male) eggs. Researchers believe that the emergence of male pupae stimulates the rearing of queen larvae. Although drones emerge sexually immature, they develop full sexual maturity within 6-20 days. During this time they must transfer sperm to their accessory testes.

Once sexually mature, drones leave their native colonies to find and mate with queen bees from other colonies. They never return to the nest. The lifespan of bumble bee drones differs based on seasonal fluctuations and the specific species of bee, depending on the climate region.

Nutritional Requirements to rear Bumble bee Drones

There is still little knowledge about the nutritional requirements of drone larvae and adults.

It is an area of great interest that needs to be further explored. Although drones play an important role in the life cycle of bees, no studies have yet been conducted to explore the quantity and quality of food required to successfully rear drones.

Role within the Nest

Male bumble bees have an important role in the nest by contributing to its function and dynamics. As soon as the drones hatch from the brood mass, they actively participate in caring for the immature brood. Recent studies have shown that Brown-belted bumblebee (B. griseocollis) males participate in pupal incubation during their first few days after the pupae hatch (post-eclosion), a task similar to that of the worker bees.

By incubating the pupae, the male bees increase their temperature by 4-6 °C. This is crucial for maintaining optimal brood growth rates. Although they cannot warm up the pupae to the extent of workers and queens, their participation provides an additional warmth. This ensures the optimal temperature is maintained for brood growth, as suboptimal temperatures can significantly impact pupal development in bumble bee nests.

While cooler temperatures may not lead to immediate pupal mortality, it ultimately alters their development time, even if it’s minimal. It is therefore imperative that male bees participate in brood incubation to ensure that the brood, including gyne brood, are maintained at an optimal temperature.

Mating Behavior

The behavior of drones during mating is crucial for successful queen fertilization and the establishment of new colonies. Depending on the species, drones may patrol an area around their nest, depositing scent markings along the way, or congregate at the nest entrance to mate with gynes as they enter or leave the nest.

Mating success of male bumblebees is influenced by several factors, including ambient temperature, age, weight, and virginity. Studies have shown that the most successful mating occurs at a temperature of 23°C, while the lowest success is observed at 29°C. Male bumblebees possess accessory testes that enable them to store large amounts of sperm, upwards of 600,000, to mate with multiple queens.

Once-mated queens retain between 40,000 and 60,000 sperm within their spermathecae. Interestingly, researchers have discovered that non-virgin drones produce more successful queens, indicating that multiple mating can enhance queen colony foundation and fitness. Inbreeding can reduce genetic diversity in bumblebee populations, but certain species have evolved mechanisms to avoid this.

For example, some species such as the Carder Bumblebee (B. muscorum), Frigid bumblebee (B. frigidus), and Two-form bumblebee (B. bifarius) can detect nest-mates during mating, so as to avoid inbreeding. In contrast, species like the California bumble bee (B. californicus) and Red-belted bumblebee (B. rufocinctus) may mate with nest-mates, placing these populations at risk of genetic homogeneity. However, the frequency of such events in natural settings remains unclear.

Factors that influence Mating Behavior

The ability of drones to mate successfully is influenced by both physical and chemical attributes. A study conducted with Buff-tailed bumblebees (B. terrestris) has shown that younger and heavier males tend to copulate more quickly and for shorter periods than their older and lighter counterparts. However, under situations of increased competition, age is no longer a factor for success. Instead, drones with longer leg lengths, specifically fore and hind tibiae, are more likely to achieve reproductive success. This finding suggests that such physical attributes may confer an advantage in competitive mating environments.

Nutrition

Interestingly, the quality and quantity of food resources within the drone’s natal nest play an important role in determining body size and other morphological characteristics. Nests with limited access to pollen, a crucial source of nutrition for larval development, tend to produce smaller adults.

This suggests that the nutritional quality of a drone’s development environment may have lasting effects on its reproductive success. With these insights, researchers can gain a better understanding of the complex factors that influence drone mating success, from physical traits to environmental conditions.

Pheromones

Aside from physical characteristics, male bumble bees also rely on the production of pheromones to attract potential mates. Recent studies have shown that the amount of pheromones produced by male bumble bees is dependent on their age. Both, the Buff-tailed bumblebee (B. terrestris) and the White-tailed bumblebee (B. lucorum) were found to reach peak levels of pheromones at around seven days after emerging from their pupal stage. However, B. terrestris showed a significant drop in pheromone levels after day seven, while B. lucorum’s levels remained consistent.

This suggests that B. terrestris may have a shorter mating window than B. lucorum. This makes it more challenging for them to find a mate. Interestingly, differences in the chemical composition of pheromones were also discovered between the two species. This indicates a potential mechanism by which female bumble bees can locate their preferred mates.

Further studies revealed that the age-related changes in pheromone production were controlled by variations in de novo synthesis of pheromones in the labial glands of these species. These are influenced by common hormones in immature insects. Consequently, any factors that disrupt or alter drone hormone production could have severe consequences for their mating behavior. This impacts not only the drones´ success but also the success of their queen.

Sperm Quality

Males have been observed to exhibit varying levels of success when it comes to mating. This is one factor that could explain the length of their sperm.

Sperm length tends to largely remain consistent within individual males. However, significant variations in sperm length can occur between siblings of the same nest and even among males from different nests and species. These differences suggest the possible influence of genetic and environmental factors in sperm production. Such variations are important in understanding the evolution and reproductive strategies of different species.

In the case of B. terrestris, there is a positive correlation between sperm length and body size. This suggests that queens could potentially select for drones with longer sperm by choosing larger mates. This phenomenon indicates that certain species exhibit sexual selection when it comes to sperm length. This could have significant implications for reproductive success.

What happens after Bumble Bee Drones Mate?

After mating, male bumble bees produce a gelatinous substance known as a mating plug, which is deposited into the female queen’s genital tract. The purpose of this plug is to prevent other males from inseminating the queen and thus, reduce sperm competition.

While the mating plug of monandrous species like the Buff-tailed bumblebee (B. terrestris) is highly effective, polyandrous species like the Tree bumblebee (B. hypnorum) show only partial effectiveness. Research has revealed that the primary components of the mating plug are fatty acids and a cyclic peptide called cycloprolylproline.

Interestingly, cycloprolylproline is not necessary for the physical formation of the plug. However, it influences the queen’s post-mating behavior and reduces her receptivity to further mating attempts. This phenomenon of male accessory gland products inhibiting female receptivity is not exclusive to bumble bees. It has also been demonstrated in other insects like the common fruit fly (Drosophila melanogaster).

Recent studies have identified linoleic acid (a polyunsaturated omega-6 fatty acid) as one of the compounds responsible for inhibiting further mating attempts. Thus, the mating plug plays a crucial role in dictating colony paternity. It also manipulates queen mating behavior in certain bumble bee species.