In nature bees have two general methods for maintaining colony temperatures in winter: 1) selecting a protected and well-suited cavity (Tab. 1) and 2) clustering. Clusters have a two-part structure (Fig. 1): 1) a dense outer mantle in which bees jam together, orienting their heads towards the center of the cluster and 2) a loose inner core where bees are free to move. The mantle insulates and, at its tightest, approaches the insulation of bird feathers or mammal fur (0.1 W/kg/ºC). Clusters move slowly from empty combs to ones full of honey. This movement is typically upwards and sideways, never downwards. know: 1) brood nest = 32-36ºC, 2) minimum thorax temperature needed for flight = 27ºC, 3) minimum temperature needed to pump flight muscles and warm up (analogous to mammal “shivering”) = 18ºC and 4) below which bees go into a “chill coma” = 6ºC. broodnest area Bees begin clustering when temperatures fall below 18ºC. Cluster size shrinks until -10ºC at which the cluster is tightest. The cluster shrinks 5-fold between 18ºC and -10ºC. Below -10ºC hive temperatures can only be maintained by increasing core heat production (Fig. 2). The core bees create this heat by “pumping” their flight muscles. This process is ultimately fueled by honey which prompted WF Cheshire to write in 1888: “Each bee is a tiny furnace carrying on a process in its tissues and fluids which is the exact chemical equivalent of oxidizing honey ”. y WORDS OF WISDOM - A hive’s metabolic rate is lowest when temperatures are 5-10ºC. This is why beekeepers who winter bees indoors maintain their buildings at 5ºC: the bees use the least amount of honey at this temperature. It is estimated that bees produce 0.68 kg of water per kg of honey they consume. This water is important to the bees and they use it to dilute honey, feed brood and flush metabolic wastes from their bodies. Nonetheless, some of this water escapes as a vapour, which in itself is important as brood develops best at 40% relative humidity. A problem occurs, however, when outside temperatures drop. Cold air does not hold as much water as warm air, so as warm moist air leaves the cluster, it condenses on the comb. Bees can cope with some moisture outside the cluster, but if the moisture accumulates and begins dripping back on the bees it saps them of heat. This issue is significant when we consider the role of the upper entrance in your colonies. Bees reduce their energy consumption through the winter by “turning their thermostat down”. They accomplish this by shutting down brood production in late fall and early winter (Fig. 3), which is thought to be triggered by shortening day length. Free of maintaining incubation temperatures (remember: 32-36ºC), the core bees lower their heat production and, thus, minimize their consumption of honey. Brood rearing begins slowly again after the winter solstice. The high metabolic cost of late-winter brood rearing translates into higher honey consumption rates at the end of winter. In one study, colonies ate honey at twice the rate in March (0.84 kg per week) compared to December (0.42 kg per week).

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