Survival of the honey bee is critical for spring pollination

Mary O’Riordan explains the complex processes involved in overwintering of honey bee colonies.

Bees and colonies form a warm cluster in winter.

GETTING through the winter isn’t simple. However, the honey bee is one of the few insect species that is adapted to survive winter without becoming completely dormant.

The bee and colony change not only their behaviours, in terms of forming a nice warm cluster which has been written about before, but they also change their physiology.

Before the destructive Varroa mite came to our shores, overwintering colony losses were about 10%. However, an average 33% winter loss was reported by Irish beekeepers in surveys since 2010.

Honey bees provide critical pollination services for the natural environment and for agriculture. Successful overwintering of honey bee colonies is critical to meet the pollination needs of early spring-blooming crops such as cherries, apples, and oil seed rape.

Besides the formation of the remarkable cluster, there is a complex interplay of other characteristic changes that protect honey bees in winter include becoming less active, changes in their various hormones levels, increasing their nutrient stores, increasing the length of their life, as well as changes at the colony level such as stopping brood rearing.

This ability to live longer is intriguing. Throughout most of the year, from late spring through summer and early autumn worker bees live for about 30 days.

The youngest bees, less than 10 days old, perform nursing tasks, middle age bees are busy comb building, food storing, cleaning and guarding, while the oldest bees — older than 20 days — serve as foragers flying out and bringing back nectar and pollen.

In late autumn as egg laying and brood rearing decline, long lived bees, living up to eight months, are produced and they will survive the winter — forming the insulating cluster when the temperatures drop.

Once the brood rearing restarts in early spring the division of labour resumes among overwintered worker bees. 

There seems to be several environmental cues associated with the seasonal changes such as length of sunlight, temperature, and nutrition, which all interact to bring about the cyclic physiological changes in the colony.

During the autumn, there is a decrease in available pollen and nectar — which in combination with days getting shorter and colder — results in a decrease in the foraging effort by bees.

With more foragers staying in the hive there is an increase in exposure to their pheromones which slows their maturation. Then the reduction in pollen foraging decreases levels of brood production and brood pheromones which also slows the maturation process.

With a reduction in brood pheromone, foraging is again reduced, further amplifying the effects of the lack of foragers’ pheromones. In late winter day length increases sufficiently to trigger the production of a small amount of brood.

The presence of brood pheromone stimulates behavioural maturation in some of the winter bees which results in the generation of foraging bees reversing the process which has happened in the autumn.

Brood pheromone also stimulates the collection of pollen by these foragers and the influx of pollen in turn stimulates brood production and facilitates colony growth in spring.

The genetic background of the colony also influences the overwintering success. In a study conducted during 2009 and 2014, spanning 20 apiaries across Europe, led by the Bee Institute in Hessen, Germany, it was found that winter survival was impacted upon by the genetic background of bees.

Colonies headed by local queens survived an average of 83 days longer than colonies headed by non-local queens. 

There are also other substantial queen effects on winter survival of colonies as requeening in mid-summer has more positive outcomes than late summer, or early autumn re-queening.

So if beekeepers need to requeen their colonies they should consider requeening during summer using queens from high quality local stocks.

Colony size and food stores also impact on colony survival. Larger colonies use their stored food more efficiently since per capita food consumption is lower in large colonies compared to small ones and colonies which are larger entering the winter are more likely to be successful in exiting the winter — ultimately producing greater annual yields of honey.

Overwintering in honey bees is a hugely complex process which integrates multiple triggers such as environmental, social, physiological, and genetic cues.

The beekeeper tirelessly tries to reduce winter losses by focussing on the areas he or she can control, such as enhancing colony strength and food stores in the autumn, improving our queen quality by queen rearing, and protecting our bees from mites, mice, and diseases.

An almost full-time job.

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