Bumblebees are familiar, fuzzy pollinators, often seen buzzing around flowers throughout the warmer months. Unlike their close relatives, the honeybees, bumblebees do not maintain a perennial colony that survives the winter together by feeding on stored honey. Instead, their entire social structure is built around an annual cycle, meaning their winter survival strategy is solitary and relies on a profound physiological shift. This unique approach to surviving the cold ensures that the next generation of pollinators is ready to emerge with the first spring blooms.
The Annual Cycle’s End: The Fate of the Summer Colony
The bumblebee colony, which starts with a single queen in the spring, is an ephemeral structure that lasts only one season. By late summer, the colony reaches its peak size, consisting of hundreds of workers, the original founding queen, and finally, the production of reproductive individuals. The colony’s ultimate goal is to produce new queens and males, who leave the nest to mate with individuals from other colonies.
Once mating is complete, the original queen, her sterile worker daughters, and all the male drones begin to decline and perish as the weather cools and floral resources dwindle. The founding queen, having completed her life’s work of starting and provisioning the colony, typically dies of old age and exhaustion before the first hard frost. This leaves the newly mated, next-generation queens as the sole survivors of the summer’s population. The old nest is abandoned, and the new queens must now prepare to carry the colony’s genetic future forward alone.
Survival Mode: The Queen’s Diapause
The survival mechanism employed by the newly mated queen is a highly controlled physiological state called diapause, which is often mistakenly described as simple hibernation. Diapause is an arrested state of development and growth, triggered by environmental cues like decreasing temperature and shorter daylight hours, allowing the insect to survive harsh conditions without feeding.
Before entering this dormant phase, the new queen spends considerable time foraging to build up substantial fat reserves, primarily in her fat body. These reserves, composed of lipids and glycogen, serve as the only energy source the queen will utilize for the next six to nine months. The amount of fat she sequesters is directly linked to her survival rate through the winter.
During diapause, the queen’s metabolism drops drastically, allowing her to conserve the stored energy. To prevent her cells from freezing, she synthesizes cryoprotectants, which act as a kind of biological antifreeze. This controlled shutdown allows her to withstand prolonged periods of cold temperatures and lack of food until the warmer weather returns in the spring.
Finding Shelter: The Hibernaculum
Once the queen is physiologically ready for diapause, she must locate a suitable overwintering site, known as a hibernaculum. The selection of this site is crucial for her survival, as it must provide stable conditions away from extreme temperature fluctuations and excessive moisture.
Queens typically seek out soft, well-drained soil, where they can burrow to a depth of several centimeters. Many queens select shaded areas, such as under leaf litter or logs, to prevent the winter sun from prematurely warming the ground and triggering an early emergence. A premature awakening could deplete her limited fat reserves, leading to death. The queen remains dormant in her solitary chamber until rising spring temperatures signal that it is time to emerge and begin the annual colony cycle anew.