The inner workings of a colony

Honey bee colonies can be considered super organisms this is because they consist of a vast number of individuals that work together for the benefit of the colony as a whole. The social order of bees is termed eusociality. Eusociality is defined by the presence of a generational overlap, cooperative rearing of young and non-reproducing worker castes.

image from: bee-safe.eu

This non-reproducing worker caste is the most populous, with up to 50,000 individuals and is responsible for the upkeep of the colony. The two other castes in a colony are the queen and the drones who vary in number up to a maximum of 200 in summertime. The role of the drones is to mate with unfertilised queens which they do at drone congregation sites away from the colony. The queen lays eggs to replenish her workers and drones, at her peak a healthy queen can lay 1000-2000 eggs per day.

Queens frequent drone congregation sites to reproduce, they reproduce on only one occasion but mate with many drones at that time. This is known as polyandry, typically a queen will mate with around 12 drones and she stores the sperm in her spermatheca to fertilise her eggs later. Interestingly, honey bee colonies have a haplodiploid sex determination in which drones are produced from unfertilised eggs and are haploid. This means that all the genetic information within the drones comes from the mother, that is they have a single set of chromosomes. This also means that all the sperm cells produced by the drones will be identical.

On the other hand, the female workers are diploid and so have a set of chromosomes from the father and a set from the mother. And because a queen mates with around 12 drones this means that there will be roughly 12 half-sister groups or patrilines within the colony. On occasion the diploid eggs will be required to become queens to allow the colony to replace the old queen or to split and make a new colony. However, they more often become workers who are non-reproductive except under specific circumstances, their primary function is to take care of the colony by caring for brood, collecting food, and maintaining homeostasis of the nest. The nest needs to be kept at temperature of about 33 °C with 80% relative humidity. Workers can cool the nest using water and by fanning and conversely, they can heat it up by clustering and vibrating their flight muscles to produce heat.

The tasks of individuals within the worker caste are determined by age. As workers age, they go through tasks in a sequential manner the shift from one task to another is accompanied by behavioural and physiological changes. The average worker will begin as a nurse who will then begin to perform in hive tasks such as hygienic behaviour which is normally seen in bees that are 15-20 days old. As I mentioned in my post about deformed wing virus the final task of all workers is foraging where they leave the hive to collect supplies. Foraging is a major predictor of life span of a worker honey bee because whilst it can vary when bees start foraging, the foraging life stage because it is energy consuming and risky consistently lasts about 7 to 10 days.

The season also affects the partitioning of tasks within workers because the needs of the colony change between seasons. This also means that in different seasons worker bees have different lifespans. Summer worker bees who are reared between late winter to summer have a shorter mean lifespan of 30-40 days. They typically begin as nurses then switch to foraging approximately 2-3 weeks later. However, the workers who are active during the peak of the summer season have the shortest lifespan of 25-30 days because of their heightened activity. On the other end of the spectrum are the diutinus or “winter” bees which can live anywhere from 6 to 10 months. Why can they live so long even though they are produced from the same eggs and sperm as summer workers? Well, the answer to that may be the increased presence of a glycolipoprotein (a complex of fat, protein and carbohydrate) called vitellogenin in the hemolymph of winter bees. Vitellogenin is believed to act as an anti-aging agent in a number of ways. It carries zinc within the bee which is important for immune function and it acts as an antioxidant protecting cells from oxidative stress. Finally, it acts a signalling molecule which suppresses juvenile hormone, a hormone that normally promotes aging and the change to foraging behaviour. Indeed, winter bees have low levels of juvenile hormone whereas foragers have high levels of the hormone and low levels of vitellogenin.

This long lifespan of winter workers is crucial for the colony to survive overwinter. This is because they stop laying brood during winter and cluster within the hive for warmth. They do not leave the hive during winter but instead live off the resources stored within the hive and their fat bodies (which are enlarged in winter bees). Clustering produces a surprising amount of heat; at the core of a cluster the temperature can reach 33°C! The bees must take it in turns to be in the centre as if they remain on the outside of the cluster too long, they can become too cold and die. The winter bees are also needed to begin brood rearing mid-winter, this brood is important in helping to rebuild the colony. Thus, if the winter workers die prematurely it has a serious impact on the colony size and structure as it means reduced brood rearing and a smaller spring population with a skewed division of labour. The lifespan of winter workers can be cut short by many different stressors including diseases, bad weather, pesticides and parasites.

However, one of the most detrimental stressors to over winter bees is deformed wing virus. This virus shortens the lifespan of bees infected as pupae by around two thirds. This is problematic any time of the year, but it is especially significant overwinter as the winter bees will die much younger causing the colony to collapse. Because of this, beekeepers are encouraged to reduce their colonies Varroa mite population before autumn (when the winter bees are produced). The Varroa mite is what spreads the virus to the pupae and so reducing its population size means fewer infected winter bee pupae and thus fewer premature deaths during winter.