Bumble bees, essential contributors to ecosystem services, are herbivorous insects with specific dietary and foraging habits crucial for their survival and their role as pollinators. Humans benefit significantly from these pollinators. Unlike honey bees, bumble bees are annual species. Understanding their diet and foraging behavior is crucial for comprehending their life cycle and the factors influencing their behavior.
Dietary Components of Bumble Bees
Bumble bees primarily feed on nectar and pollen, with water and occasional honey consumption playing supporting roles.
Nectar
Nectar, a sugary liquid produced by flowers, serves as the primary energy source for bumble bees. They use their long proboscis to access nectar within flowers and prefer flowers with higher sugar content. As they collect nectar, they inadvertently transfer pollen, facilitating plant reproduction. This mutually beneficial relationship is vital for the sustenance of bumble bees and the reproductive success of plants.
Pollen
Pollen is a protein-rich substance crucial for bumble bees, serving as a critical source of protein and other nutrients. Bumble bees collect and store pollen in pollen baskets on their hind legs and mix it with nectar to form small pellets. This pollen is then fed to their developing larvae, providing the protein necessary for their growth and development. All bees possess branched hairs that enable pollen transfer between flowers.
Water
Water is essential for various functions in a bumble bee colony, including temperature regulation, dilution of stored honey, and hydration. Bumble bees collect water from sources like puddles or damp soil and distribute it within the hive to cool it during hot weather or maintain humidity levels. They may also use water to dilute stored honey, regulating the sugar concentration.
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Honey
While not a primary part of their diet, bumble bees have been observed occasionally feeding on honey produced by honeybees or other sources. Honey serves as a concentrated source of sugars. Unlike honeybees, bumblebees have relatively small colonies with no long-term honey storage.
Foraging Behavior and Adaptations
Bumble bees are excellent pollinators due to their unique foraging behavior and physical adaptations. They possess branched hairs that enable pollen transfer between flowers.
Buzz Pollination
Many large-bodied bees, including bumble bees, perform buzz pollination. During buzz pollination, bees vibrate their flight muscles at a specific frequency while holding onto a blossom to dislodge pollen grains tightly packed within the flower. This practice allows them to collect pollen from flowers that are more difficult for honey bees to access.
Pollen Baskets
Just like honey bees, bumble bees store pollen within specialized branched hairs (or scopae) 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.
Foraging Range and Learning
Foraging range varies among bumble bee species; some forage closer to the nest, while others travel further away. There may be a correlation between foraging range and nest size, with species from smaller colonies foraging closer to home (less than 1500m) than those with larger colonies. Bumble bees may learn about rewarding flower species with abundant nectar and pollen by observing other foragers.
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Avoiding Competition
Bumble bees tend to avoid flowers already visited by conspecifics or other bumble bee species.
The Bumble Bee Life Cycle and Diet
The life cycle of bumble bees significantly influences their diet and foraging habits.
Spring and Early Summer
During the spring and early summer months, bumble bee queens emerge from their hibernacula. They forage for pollen and nectar to activate their ovaries and initiate egg-laying. A queen with pollen on her hind legs indicates she has found a suitable habitat for her colony. She uses pollen and nectar to feed her first brood and form wax pots for nectar storage.
Late Summer and Fall
As late summer and fall arrive, bumble bee colonies grow to contain several hundred individuals. Queens cease worker production and begin laying unfertilized eggs, which develop into drones. Nurse workers rear the remaining fertilized eggs into new queens. New, virgin queens and males forage and mate with members of other colonies. Newly emerged queens fill up on pollen and nectar resources in preparation for diapause.
Winter
During the winter months, bumble bees enter diapause, an arrested state of development and behavior similar to hibernation. Mated queens are dormant inside a cavity under the soil, surviving for months in cold temperatures without feeding. They rely on floral resources, particularly pollen (for protein, lipids, and micronutrients) and nectar (for carbohydrates), stored in their fat body before diapause.
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Nutritional Needs and Diet Selection
Pollen provides bees with their primary source of protein and lipids, but the nutritional quality varies among host-plant species. Bumble bees appear to have adapted to assess resource quality and adjust their foraging behavior to balance nutrition from multiple food sources. Studies have shown that bumble bees can regulate their protein and lipid intake, selecting foods that provide them with a specific ratio of protein to lipids (P:L). For example, Bombus terrestris and Bombus impatiens have been shown to target a P:L ratio of approximately 14:1 and 12:1, respectively.
Colony Structure and Task Allocation
Like honey bees, bumble bees work together as a colony, divided into three distinct castes: a single queen, non-reproductive female workers, and male drones. The queen lays eggs, while the workers maintain the hive and bring in local pollen and nectar resources. Drones mate with virgin queens. Task allocation is also a feature shared by both honey bees and bumble bees.
Conservation and Research
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 and the transition into diapause for newly-emerged queens. Research on these insects is critical for understanding the environmental and physiological mechanisms directing bumble bee behavior. Bee population declines are linked to the reduction of nutritional resources due to land-use intensification.