Bumble bees are busy pollinators that build annual colonies with a single queen, workers, and male drones during warmer months. Unlike perennial honey bee colonies, which survive winter by clustering and eating stored honey, the entire social structure of a bumble bee colony collapses each autumn. Only the newly produced, mated females, known as gynes or new queens, survive the cold season to found the next generation.
The Annual Cycle’s Conclusion
The established bumble bee colony is designed to be temporary, reaching its peak size in late summer or early autumn before dissolving completely. At this point, the founding queen shifts her egg-laying from producing sterile female workers to producing reproductive individuals: new queens and male drones. Once this reproductive phase begins, the colony’s purpose for the season is essentially complete.
As temperatures drop and floral resources become scarce, the colony’s energy reserves dwindle. The non-reproductive members, including the workers and male drones, quickly perish. The original, founding queen also reaches the end of her natural lifespan and dies, leaving her nest abandoned. The nest cavity will not be reused by the new queens.
The New Queen’s Preparation for Winter
The newly emerged queens are the sole link to the next year, and their survival is entirely dependent on their actions in the late season. These queens leave their natal nest and immediately begin an intensive period of foraging, known as hyperphagia. This gorging behavior is necessary to build up a substantial reserve of fat in a specialized organ called the fat body.
The fat body serves as the primary storage site for lipids and proteins gathered from pollen and nectar. The reserves built up during this intense feeding period will be the queen’s only source of energy throughout her hibernation. Queens also mate during this time, storing sperm internally to fertilize eggs the following spring. Successfully mating and attaining sufficient body weight are the two major prerequisites for a queen to successfully enter the winter.
The State of Diapause
Once adequate fat reserves are accumulated, the mated queen seeks a safe place to enter a state of metabolic suppression known as diapause. Diapause is a hormonally-controlled arrest of development and activity. During this period, the queen’s resting metabolic rate can drop to less than five percent of her normal resting values, allowing her to conserve her limited energy stores.
The queen locates a hibernaculum, her chosen hibernation site, typically by digging a shallow hole into loose soil, often under leaf litter or rotten wood. The location must provide stable, cool temperatures while offering protection from freezing, flooding, and excessive moisture. Diapause can last anywhere from six to nine months, depending on the species and the regional climate.
Spring Emergence and Colony Foundation
The long period of diapause ends when rising soil temperatures and increasing day lengths in the spring provide the cues for the queen to emerge. Upon waking, the queen is solitary and must immediately begin foraging to replenish her depleted energy reserves. She requires nectar for carbohydrates and pollen for protein, which are necessary to activate her ovaries for egg-laying.
The queen then begins searching for a suitable nesting site, often inspecting cavities such as abandoned rodent burrows or dense clumps of grass. Once a dry, protected site is selected, she constructs a small wax pot to store nectar and a clump of pollen upon which she lays her first batch of eggs. For the next four to five weeks, the queen acts as a solitary mother, foraging for all resources, incubating the brood by shivering her flight muscles to generate heat, and defending the nascent nest. The first offspring to emerge are the sterile female workers, who then take over the duties of foraging and nest maintenance, allowing the new queen to focus solely on laying eggs.