As temperatures drop and food sources become scarce, bees employ various methods to survive winter. Different bee species, from social honey bees to solitary dwellers, exhibit distinct adaptations to endure this period.
Honey Bee Winter Survival Strategies
Honey bee colonies exhibit collective behavior to survive winter. When outside temperatures fall to around 14°C (57°F), honey bees form a tight cluster inside their hive, acting as a warm entity. The bees on the outer layer of this cluster pack densely, forming an insulating shell, while bees in the core remain more loosely packed and active. This organized structure allows the colony to maintain a stable internal temperature, typically between 15°C and 30°C (59°F and 86°F), even as external temperatures plunge.
To generate heat within this cluster, honey bees rapidly vibrate their flight muscles without moving their wings, a process akin to shivering. This muscular activity consumes energy, which is primarily fueled by their stored honey. As winter progresses, the cluster slowly moves across the honeycombs, consuming their reserves to sustain warmth and energy. The colony needs a substantial amount of honey, often 60 to 90 pounds, to fuel this heat generation throughout the winter.
The queen bee plays a role in the colony’s winter survival by significantly reducing or even ceasing her egg-laying activity during the coldest months. This conserves the colony’s resources and minimizes the need to maintain a high brood-rearing temperature, which is typically around 33°C to 36°C (91°F to 97°F). As winter approaches, specialized “winter bees” are born; these bees have larger fat bodies and longer lifespans than summer bees, enabling them to endure confinement and contribute to the colony’s warmth and survival until spring.
How Other Bees Endure Winter
Unlike honey bees, many other bee species do not maintain a communal warm hive throughout winter. Bumblebees have a different life cycle where only the new, fertilized queen survives the cold season. The existing worker bees, male bees, and the old queen all perish.
The new queen bumblebee, after mating and building up fat reserves, seeks out a secluded spot, often underground in loose soil or leaf litter, to hibernate alone. She can burrow several centimeters deep to create a small chamber called a hibernaculum. During hibernation, her metabolism slows, and she produces a natural antifreeze, glycerol, in her body to prevent freezing. In the spring, she emerges to establish a new colony.
Solitary bees, such as mason bees and leafcutter bees, have unique winter survival strategies. These bees do not form colonies; their offspring typically overwinter within individual nesting chambers. Mason bees overwinter as fully formed adults inside their cocoons, having developed from larvae and pupae within their mud-capped nests. Leafcutter bees usually spend winter as dormant larvae or pre-pupae encased in their protective cocoons, often within hollow stems or tunnels. These cocoons protect the developing bees until they emerge as adults the following spring.
Winter Threats to Bee Colonies
Despite their adaptations, bee colonies face several threats during winter. Starvation is a danger if a honey bee colony does not have sufficient honey stores to last through the cold months. Bees consume honey to generate heat, and if their fuel runs out, they can freeze, even if temperatures are not extremely low. Starvation can also happen if the bee cluster is unable to move to new honey reserves within the hive, a phenomenon sometimes called “isolation starvation.”
Extreme cold can overwhelm a colony, particularly if the cluster is too small or unhealthy to generate and maintain adequate heat. Prolonged periods of severe cold can lead to bees on the outer edges of the cluster becoming too cold to rotate inward, potentially causing colony collapse. Chilled brood can also die if the temperature within the brood nest drops below the required range.
Disease and parasitic infestations pose risks during winter. Colonies, stressed by cold and confinement, can become more susceptible to pathogens like Nosema, a fungal disease, or to Varroa mites. Varroa mites weaken bees by feeding on their fat bodies and transmitting viruses, which can shorten the lifespan of individual bees and compromise the colony’s overall health and ability to survive winter.
Excessive moisture inside the hive is another threat. As bees generate heat and consume honey, they produce water vapor. Without proper ventilation, this vapor can condense on cold hive surfaces and drip onto the bees, leading to chilling, hypothermia, and the growth of mold or mildew. Damp conditions can weaken the colony and contribute to winter losses.