The Bumble Bee Lifestyle

Introducing the Bumble Bee

Of over 250 known bumble bee species, 18 are found in Pennsylvania (Kilpatrick & López-Uribe 2020). Just like honey bees, bumble bees store pollen within specialized branched hairs (or scopae), that are located in grooves on their hind legs (called corbiculae). Together, the scopae and the corbiculae make a pollen basket, which can be easily seen with the naked eye. Pollen stored on the body of bees is transmitted to the female reproductive organ (the stigma) of the flower as they fill their pollen baskets, resulting in fertilization and, ultimately, the development of fruits, seeds, and/or nuts.

Bumble bees are excellent pollinators due to the way that they attract and disperse pollen from their bodies with each floral visit, and because of a unique foraging behavior they engage in called buzz pollination. All bees possess branched hairs that enable pollen transfer between flowers, but buzz pollination is a specialized behavior performed by many large-bodied bees, including bumble bees. During buzz pollination, bees vibrate their flight muscles at just the right frequency while holding on to a blossom to dislodge pollen grains that are tightly packed away in the flower. This practice allows them to collect pollen from flowers that are more difficult for honey bees to reach. Two important crops grown in Pennsylvania, blueberries and tomatoes, have pollen reserves hidden in the flowers and must be buzzed or sonicated to be released.

Bumble Bees Versus Honey Bees: How Do They Compare?

Like honey bees, bumble bees work together as a colony through the late spring, summer, and fall months. And, as with honey bees, individuals within the colony are divided into three distinct castes. Each colony includes a single queen, non-reproductive female workers, and male drones. The queen is the only one in the colony to lay eggs, while the workers maintain the hive and bring in local pollen and nectar resources. Drones have fewer responsibilities to the colony and are less abundant than the workers. Their purpose is to mate with virgin queens, which are typically reared during late summer. Task allocation is also a feature shared by both honey bees and bumble bees. In honey bees, the division of labor is dependent on age; younger workers feed and care for the developing brood, while older workers are foragers. As new workers emerge, the more senior workers will adopt new colony tasks according to age (Johnson 2010). In contrast, bumble bee division of labor is correlated with body size, where larger bees can carry more pollen, and smaller bees perform in-colony tasks, such as caring for brood.

However similar bumble bee and honey bee colonies may seem, the appearance and life histories of the two species vary a lot. Bumble bees are an annual species that have one queen per year, whereas honey bees are perennial species, with one queen that can live for several years. The annual bumble bee life cycle is a fascinating process where the queen transitions from a seasonal solitary phase to a social phase later in the cycle.

The Bumble Bee Colony Life Cycle

Spring

During the spring and early summer months, bumble bee queens emerge from their hibernacula; underground alcoves where they shelter over the winter. Queens terminate diapause using cues related to warming temperatures and increasing day lengths. Before a queen can lay eggs, she must forage for pollen and nectar as a source of proteins and carbohydrates. These macromolecules play an important role in the activation of her ovaries and initiation of egg-laying. Queens can be seen flying close to the ground in search of pre-existing cavities, like rodent holes or other burrows, where they will start their colony by laying the first eggs. A queen has found a suitable habitat for her colony once she is seen with pollen on her hind legs. She then uses this pollen and nectar to feed her first brood as well as to form wax pots for the storage of nectar. The queen will continue to forage for resources that allow the growth and development of the brood. Bumble bee development includes four life stages: egg, larvae, pupae, and adult. After about 3-4 weeks of development, the first female workers will emerge. This is the onset of the social phase of the colony.

Summer

By summer, the queen transitions to solely in-colony tasks, such as laying eggs. While she is caring for her brood, she is also producing pheromones that inhibit reproduction by workers. This is called the "pre-competition phase," where the queen is the sole egg layer for the colony. Along with the queen, nurse workers assist in feeding the colony's brood while foraging workers search for floral resources throughout the day to supply the colony with sufficient pollen and nectar.

Fall

As late summer and fall arrive, bumble bee colonies grow to contain several hundred individuals. At this time, queens cease worker production and begin laying unfertilized eggs, which develop into drones, or male bumble bees. Nurse workers help rear the remaining fertilized eggs into new queens. Once queens stop laying reproductives (gynes and males), workers may start to lay their unfertilized eggs, initiating the "competition phase" between the more dominant workers and the queen and among other workers for male offspring. The new, virgin queens, called gynes, and the males will go out to forage and mate with members of other colonies. The original queen, or the founder, of the colony, will die naturally with the remaining workers.

Newly emerged, virgin queens leave the colony to mate and fill up on pollen and nectar resources in preparation for diapause. Diapause is an arrested state of development and behavior, which is like hibernation in mammals. Many bees and most insect species in temperate climates undergo diapause throughout cooler months, only to reemerge once the right temperature and day length is reached.

Winter

During late fall and winter, the mated queen is completely dormant inside a cavity under the soil. This diapause can last 6 to 9 months, with the queen surviving for months in cold temperatures without feeding. The physiological stress on the queen is why it is imperative for her to 'fatten up' prior to entering diapause. Floral resources contain pollen, which is the main source of protein, lipids, and other micronutrients, and nectar, the main source of carbohydrates (Treanore & Amsalem 2020) which are stored in the fat body and used throughout diapause. Having a sufficient amount of nutrients stored in the fat body during diapause plays an important role in the survival of the queen. After 6 to 9 months in the solitary phase of diapause, the surviving females emerge and continue the life cycle as the new founder queen.

The life cycle of bumble bee colonies is an ongoing area of research, especially the factors involved in the transition from the pre-competition to the competition phase as well as the transition into diapause for newly-emerged queens. The research on these fuzzy insects is critical for understanding the environmental and physiological mechanisms directing bumble bee behavior. Bumble bees contribute to ecosystem services, where humans can benefit so much from these pollinators. The next time you see these busy bees buzzing around the yard, appreciate their hard work!

References and Resources

Penn State Center for Pollinator Research. 2020. Resources and Outreach.

Johnson, B. R. (2010). Division of labor in honeybees: form, function, and proximate mechanisms. Behavioral ecology and sociobiology, 64(3), 305-316.

Kilpatrick SK, López-Uribe MM (2020) Apidae.

Thorp, R.W. (1979) Structural, Behavioral, and Physiological Adaptations of Bees (Apoidea) for Collecting Pollen. Annals of the Missouri Botanical Garden, 66, 788–812.

Treanore, E., & Amsalem, E. (2020). The effect of intrinsic physiological traits on diapause survival and their underlying mechanisms in an annual bee species Bombus impatiens. Conservation Physiology, 8(1), coaa103.