Cold Storage Overwintering
Honey bees in temperate climates have a yearly cycle where colonies rear brood and populations grow vigorously throughout the spring and summer. By late summer and into the fall, progressively less brood is reared. As winter sets in and temperatures drop, there is little or no brood in the colony and the bees remain in their hives in a tight thermoregulated cluster surrounding the queen.
Many colonies used to pollinate early season crops like almonds spend the winter in southern states or California. In these areas, bees forage and can rear brood throughout the winter. There are challenges with placing colonies in areas where bees can rear brood and forage during the winter. Often floral resources are insufficient to keep colonies supplied with nectar and pollen. To prevent colony loss from starvation, beekeepers feed protein supplements and sugar syrups. Though protein supplements can meet some of the nutritional requirements of honey bees, if pollen is unavailable, colonies will show signs of malnutrition. Varroa mites can exacerbate effects of nutritional stress. Varroa parasitizes developing bees by crawling into brood cells just before they are sealed. The mites feed on larvae and pupae and can transmit virus. Adult bees parasitized during development have reduced lifespan particularly if they are infected with viruses. In addition, Varroa can migrate into colonies on foragers particularly in the fall and significantly increase mite populations even in colonies that were previously treated with miticides. Colonies that are infested with mites in the fall have little chance of survival overwinter.
An alternative to overwintering colonies in warmer climates is to put colonies into cold storage (CS) facilities in the fall. There are advantages to this management strategy. Colonies put into CS after a fall miticide treatment avoid reinfestation with Varroa that can migrate into colonies on foragers. Bees clustered inside the hive rather than foraging have greater longevity and require fewer resources. The cost of overwintering bees in CS also might be lower than in areas with warm winters if resources are limited and bees need supplemental feeding and miticide treatments.
This website provides the latest research findings and information on overwintering colonies in cold storage, including when to put colonies into cold storage, and which colonies to put into cold storage. We also provide access to a program (Cold Storage Overwintering Tool) that helps beekeepers choose which colonies to put into cold storage based on bee and Varroa populations in September. Our estimate of the cost per colony overwintered in cold storage and then moved to California for almond pollination is about $40 per hive. Choosing colonies to overwinter that have the greatest chances of being large enough to rent for almond pollination can reduce expenditures on colonies having little chance of being rented.