Overwintering Honeybee Colonies in Canada

The winter cluster

As daylight shortens and temperatures drop in late September and October, the colony stops raising brood and begins consolidating into a winter cluster. Worker bees press together between frames, forming a roughly spherical mass. The bees on the outer shell of the cluster act as insulation; the inner bees generate heat by vibrating their flight muscles. The queen is at the centre of the cluster, protected and kept warm.

The cluster moves slowly upward through the winter as it consumes stored honey. A colony that runs out of food above the cluster — even if honey remains in frames the cluster has not yet reached — will starve. This is why the cluster position in late October matters: it should be at the bottom of the honey stores, not already at the top of the upper brood box.

August and September: the critical preparation window

The bees that will carry the colony through winter are not the summer foragers. They are a physiologically distinct population of workers — often called "winter bees" — that develop in August and September when day length shortens and forage becomes less abundant. These bees have higher fat body reserves and can survive 4–6 months; summer workers typically live 6 weeks.

To produce a large population of winter bees, the queen needs to be laying vigorously through August. Any condition that reduces brood production in August — a failing queen, a heavy Varroa load, disease, or a nectar dearth — will result in a numerically weak cluster heading into October.

Varroa management before winter

Varroa destructor mites reproduce inside capped brood. Late summer treatment timing is critical: treatments applied when there is still open brood present knock down mite populations before the winter bee population is raised in broodless or low-brood conditions. In Ontario and most provinces, the recommended treatment window is late August to early September, after the honey supers have been removed and before the brood nest contracts significantly.

Oxalic acid dribble or vapour applied during a broodless period in October or November can reduce residual mite loads, but it does not substitute for timely August treatment when brood is still present and mites are reproducing.

Food stores

A two-deep Langstroth colony in Ontario or Quebec should enter winter with approximately 25–30 kg of capped honey. In colder, longer winters (Manitoba, Saskatchewan, northern Ontario), estimates run higher. The exact amount depends on colony size, winter length, and whether supplemental feeding is possible mid-winter — which it generally is not below −10 °C because bees cannot move to consume liquid syrup.

Stores can be estimated by hefting: a full 10-frame deep box with capped honey weighs roughly 27–30 kg. An experienced beekeeper can estimate remaining food by lifting the back of the hive slightly off its stand. Light hives in November are a warning sign that requires action — either a candy board (solid sugar placed above the cluster as an emergency food) or fondant.

Syrup timing

Autumn feeding should be completed early enough for bees to process syrup into capped honey before cold sets in. In most of southern Ontario and British Columbia, this means finishing syrup feeding by early October. Further north, late September is a more reliable cutoff. Syrup fed too late may not be fully capped, raising the moisture content of stores and increasing the risk of fermentation inside the hive during winter.

High-concentration syrup (2:1 sugar-to-water by weight) encourages faster evaporation and capping than 1:1 syrup. Adding a respiration stimulant or thymol is not necessary for overwintering preparation; the bees' ability to process the syrup is the limiting factor.

Ventilation and moisture

Moisture is the primary cause of winter colony loss that is not attributable to starvation or disease. A cluster of 20,000 bees exhales significant moisture. If this moisture cannot escape the hive, it condenses on the cold inner cover and drips back down onto the cluster, chilling it. Cold and wet is lethal; cold and dry is survivable.

Upper entrance

An upper entrance — a small notch in the inner cover or a dedicated winter entrance disc — serves two purposes: it provides ventilation for moisture escape, and it gives bees an exit if the lower entrance becomes blocked by snow or dead bees. Many Canadian beekeepers drill a 25 mm (1 in) hole in the upper brood box as a permanent upper entrance.

Moisture quilts

A moisture quilt is a shallow box placed above the inner cover, filled with wood shavings or another absorbent material. As warm humid air rises from the cluster, the moisture is absorbed before it can condense and drip. Moisture quilts are common in British Columbia and maritime provinces where winters are wetter than cold. In the drier prairies, good upper ventilation alone is often sufficient.

Insulation wraps

Black insulating wraps are widely used in Canada, particularly for apiaries with more than a few hives where individual management is impractical. The standard wrap is a black corrugated plastic or foam sheet that goes around the outside of the hive bodies, reducing heat loss from the walls while the black exterior absorbs solar heat on clear winter days — a non-trivial benefit when February sun can warm a hive surface even when air temperatures are below −15 °C.

Wraps should leave the upper entrance uncovered and should not trap moisture against the wood. Removing wraps too late in spring can overheat colonies on warm March days; most keepers remove them when daily highs consistently reach 10 °C.

Mouse guards

Mice seek warmth and food in autumn. A mouse that enters a hive in October will nest in the brood frames, consume honey, and contaminate comb with urine and nesting material. Metal mouse guards — strips of metal mesh with holes sized to let bees pass but not mice — should be installed over the bottom entrance before temperatures drop below 10 °C, at which point bees become too slow to evict mice effectively.

Monitoring through winter

A healthy colony in deep winter makes a soft sustained hum when the hive is tapped. No sound at all may indicate the cluster has moved out of range, the colony has died, or the colony is clustered very tightly. On warm days above 10 °C, bees will take cleansing flights — brief exits to void waste accumulated since the last warm spell. The presence of cleansing flights in February or March is a positive indicator.

Avoid opening the hive below 10 °C. Brief exposure to cold disrupts the cluster and can chill the brood if any is present. Winter checks should be limited to external observation and heft assessments unless there is strong evidence of a problem requiring immediate action.

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