A matter of life and death

Knowledge has the fascinating attribute to evolve. Theories and explanations that we have believed in are replaced with new and improved ones. With time this evolution leads to an ever better understanding of the world around us. A certain explanation about the honey bee colony persists today although we know better at least since the 18th century.
It is time to replace it.

Author: Dr. Hannes Bonhoff
Email: hannes.bonhoff@gmail.com
When dies a honey bee colony? Photo by Åke H Nilsson
Our understanding of the honey bee colony has undergone many twists. In the Middle Ages, for example, the honey bee colony was considered to be ruled by a king. Later it was discovered that the king was actually laying eggs and in accordance with biology should therefore be considered as female. In an almost feministic act, the king was henceforth called the queen.

This article is about another twist, or rather what should have been another twist a long time ago.

The swarming of a honey bee colony shows many similarities with the birth of a child, where the swarm represents the offspring and the remaining colony the parent. Traditionally, therefore, a colony is defined by its nesting site and considered to be alive as long as there are enough living bees present. Since the queen can be replaced, the honey bee colony is potentially immortal. This traditional view suggests the beekeeper to assist the honey bee colony to live as long as possible by regular replacement of the queen and the combs and by treating against parasites and diseases.
Is a swarm a mother or daughter colony? Photo by Åke H Nilsson
At least since the 18th century, however, the mother queen is known to fly with the prime swarm, leaving the hive for one of her daughters to inherit. Just like the king was called the queen when he was found to lay eggs, the prime swarm should have been referred to as the parent colony after the discovery that it includes the mother queen. The same conclusion can be drawn from looking into the genetics of the honey bee superorganism during reproductive swarming.

Genetic consistency

One of the foundations of modern biology is genetics which describes inheritance during reproduction. Every animal has a specific genetic fingerprint with which it is possible to distinguish the animal from others and also from its parents and children. The genetic fingerprint of a honey bee colony consists of the genes of both the queen and the drones the queen mated with.1 When the colony swarms, it is the prime swarm with the old queen that maintains this genetic fingerprint. The prime swarm therefore represents the parent colony. The bees left behind in the original nesting site support the emerging daughter queen to establish her colony. After a few weeks the old bees have all been replaced by workers from the new queen. The colony at the original nesting site has then changed its genetic fingerprint and now represents a daughter colony.

Defining a colony of honey bees by its unique genetic fingerprint yields the following unambiguous picture of its life cycle.
The genetic setup of a honey bee colony is defined by the queen and the drones she mated with. Photo by Åke H Nilsson

Death and birth

The fact that the genetic fingerprint of a honey bee colony stems from the queen and the drones she mated with, ties the colony to the queen. When the queen is lost or loses her ability to lay fertilized eggs, the worker bees cannot be replaced by her anymore. With the passing of the worker bees, the genetic fingerprint of the colony ceases to exist. Simply put, the colony dies if the queen dies.1 For both beekeepers and bees, it is therefore impossible to save a colony that has lost her queen. Fortunately, however, a new colony can be created from the old one by replacing the old queen.

If there are fertilized eggs or young enough larva left in a queenless colony, the worker bees may create an emergency cell to raise a new queen. The genetic fingerprint of the colony begins to change when new worker bees start to emerge from the fertilized eggs of the new queen. Since the new queen represents a daughter of the old queen, the colony will become a daughter colony. This form of reproduction can be interpreted as an involuntary pregnancy with a 100 % death rate of the parent colony. As bad as this may sound, the successful passing-on of genes from parent to offspring secures a continued local adaptation and evolution.

If the current queen is showing signs of aging or has been injured, the worker bees may raise new queens in supersedure cells. One of these daughter queens will replace the old queen and the colony becomes a daughter colony. Although this form of reproduction can be interpreted as voluntary, it still results in a 100 % death rate of the parent colony.

Better survival chances for the parent colony are found in reproductive swarming. Here, the old queen leaves the hive with the prime swarm to continue her colony at a new nesting place. For the emerging daughter queens in the old nest there are two options. They can fly out with another swarm to start their colonies from scratch or they can battle to the death with the winner inheriting the old but well-equipped nest. Secondary swarms and the colony at the old nesting site have become daughter colonies when the worker bees from the old queen have all been replaced.
The prime swarm represents the mother colony. Photo by Åke H Nilsson
The fact that the parent colony does not necessarily have to die during reproductive swarming does not imply that her survival chances are any good. Studies of wild colonies in the forests around Ithaca, New York, show that the chances to survive the coming winter when establishing a new nest only are about 20 %.2,3 It is not surprising that for swarming the mother queen literally has to be shaken out of the hive.4

The honey bee colony does not age because it is replaced with its offspring as soon as the queen lacks in performance. The absence of aging does not mean that the colony is potentially immortal but on the contrary that it has a very short life span.

Life span

As the life cycle of the honey bee colony is directly linked to the queen, the highest possible age a colony can reach is the same as that of queen bees. A queen can live over five years but is replaced long before she reaches her maximum age. The average life span of honey bee colonies can be calculated from the probabilities of surviving the various stages of its life cycle. The following example is roughly based on the studies on wild colonies in Ithaca, New York.2,3

When the life cycle of a honey bee colony begins by replacing the mother queen with one of her daughters, the chances to survive the coming winter under natural conditions are 80 %. This comparably high rate of survival is due to the fact that the young colony is equipped with a complete nest filled with honey, pollen and even worker bees. With the exception of the year in which the colony is born, the vast majority of colonies swarm annually. After having survived the first winter, it is therefore already time to reproduce by leaving the nest as the prime swarm. Since the colony now has to establish a new nest and before the winter build up a sufficiently large honey storage and number of winter bees, the chances to see another summer drop to 20 %. With increasing age, the survival chances are further reduced due to the increasing likelihood that the queen will be replaced by one of her daughters. The low chances to survive the second and following winters have the effect that colonies primarily die during winter and only rarely during queen supersedure in summer.

Based on the survival chances above, the average life span of honey bee colonies under natural conditions is about two years. Due to the low survival rates when establishing a new nest, the average life span of a colony born as a secondary swarm is only about one year.

In an apiary, the life of honey bee colonies follows a different cycle. Swarming is effectively controlled so that new colonies are born by queen replacements and splits. The chances to survive the winter in Switzerland are about 90%.5 Regular queen replacements by the beekeeper result in an early death of the colonies during the summer. The life span of honey bee colonies in apiaries averages around two years as well, depending primarily on the frequency of queen replacements. With an annual requeening, for instance, the life span is just one year.

Winter losses

Under natural conditions, a stable population of honey bee colonies has almost 50 % winter losses.3 The population is kept constant over the years by virtue of the fact that almost every colony swarms each spring. Simply put, the number of colonies doubles in summer and halves in winter.
Under natural conditions, honey bee colonies die primarily in winter. Photo by Åke H Nilsson
With only 20 % winter losses in an apiary, the number of colonies can be kept stable by making a few splits or buying a few new colonies to replace those that died during the winter. But with an average life span of about two years, 50 % of the colonies are dying each year. The remaining 30 % are dying during the summer in connection with the replacement of the queens.

Birth and death of honey bee colonies by queen replacement during the summer differs significantly from birth through swarming in combination with death in winter. When a colony dies in winter, the mites in that colony die as well. The same goes for residues in the old combs if the beekeeper or wax moths remove these before a new colony can move in. During queen replacement, on the other hand, the fully equipped nest inherited by the young colony comes with strings attached as all mites and diseases follow with it.

Another important difference is that death during winter and birth through swarming leads to a natural selection of mites and diseases. A colony infected by an aggressive virus may be too weak to swarm so that the spreading of that virus is reduced. The lethal variations quickly die out, leaving only the less harmful variations of the diseases and parasites.6 In contrast, requeening a hive removes that selection pressure from the mites and viruses as they are readily transferred from the old colony to the new.

The birth of honey bee colonies through swarming also implies natural selection of the bees as only fit colonies manage to swarm. This selection results in an adaptation of the colonies to the local conditions as well as an increased resistance to viruses and parasites.6 When a new colony instead is created by requeening, any selection of the bees depends on how and from what stock the new queen has been reared. Moreover, if the new queen is not related to the old queen then the collected local adaptation of the old colony will go extinct.

Discussion

A good understanding of the biology of honey bee colonies forms the basis for a successful beekeeping. The traditional view of a potentially immortal honey bee colony does not suffice to describe its biology. In order to understand the life history strategy of honey bee colonies for instance it is necessary to abandon the potential immortality and to study the queen instead as representative of the colony.2 Allowing a colony of honey bees to have a unique genetic fingerprint, just like every other animal out there, yields a genetically consistent picture of its life cycle as a basis for ones beekeeping.

Under natural conditions, as based on the example of the forests around Ithaca, New York,2,3 a colony of honey bees produces offspring every year by means of swarming. This annual swarming implies that honey bee colonies are not sessile but move to a new nesting site every year. The prevention or suppression of swarming by adding supers or removing queen cells has the consequence that the colony cannot move in accordance with its nature. It therefore lives in captivity.

A colony lives only about two years, leading to a high replacement rate of old colonies with new ones. With such a short life span, they die primarily in winter instead of during queen supersedure in summer. The dying in winter can be viewed as a control mechanism against viruses and mites since these are not transferred to offspring as would be the case during supersedure. This life cycle demonstrates that the survival strategy of honey bee populations relies heavily on natural selection to force a rapid evolution of both the colonies and their diseases and parasites.

When a young colony inherits the parental nest, it begins its life on at least one year old comb. This colony will only live there for a year as it will swarm to a new site in the following year. Whether a swarm settles on old comb or has to build anew depends on how good other animals such as the wax moth are at cleaning out vacated nests.

In an apiary, the life cycle described above has been turned upside down. Instead of swarming, new colonies are primarily created by adding new queens to existing hives or splits. Due to swarm control measures, the colonies live in one and the same hive throughout their lifetime instead of the annual change of residence. And instead of dying in the winter, the majority of the colonies die during the summer by means of requeening. Natural selection is effectively removed from both the honey bee colonies and its parasites and diseases. The adaptation and evolution of the colonies, mites and viruses has therefore become the responsibility of the beekeeper.

In nature as well as in apiaries, honey bee colonies live only about two years which means that about half the colonies die each year. Under natural conditions, such 50 % losses occur during winter effectively halving the number of colonies. This raises the question why considerably lower winter losses in apiaries make headlines. The death of honey bee colonies is as much a part of their life cycle as is their birth. Under natural conditions, colony death is put to good use by imposing a selection pressure on colonies, mites and viruses.

The regular administration of treatments against mites is also put to question by the short life span of honey bee colonies. If a colony would be potentially immortal it would make sense to assist individual colonies to live as long as possible by reducing their mite loads. With a 50 % turnover of colonies each year on the other hand it is more sensible to assist the local population of colonies in its adaptation and evolution to survive.

The biology of the honey bee poses a problem for the monitoring of wild colonies. The length of time a nest is inhabited without interruption does not correspond to the life span of a colony. Instead, there are successive generations of colonies that inhabit the nest. Whether a population of colonies survives in the wild does not primarily depend on how long individual nests are inhabited. Since a colony only lives one to two years, the reproductive rate is the decisive factor instead. In principle, it would even be sufficient for the survival of the population if each colony survived only a single winter – provided that each colony swarmed once or twice. The colonies that are best adapted to the wild could therefore have an extremely short lifespan. In order to understand whether and how a population of honey bee colonies survives, it is essential to monitor swarming and population dynamics.

References

1. Moritz, R.F.A. & Southwick, E.E., Bees as superorganisms, Springer Verlag, pp. 62–67, 1992
2. Seeley, T.D., Life history strategy of the honey bee, Apis mellifera, Oecologia 32, pp. 109–118, 1978
3. Seeley, T.D., Life-history traits of wild honey bee colonies living in forests around Ithaca, NY, USA, Apidologie, 2017
4. Pierce, A.L., Lewis, L.A. & Schneider, S.S., The Use of the vibration signal and worker piping to influence queen behavior during swarming in honey bees, Apis mellifera, Ethology 113, pp. 267–275, 2007
5. Brodschneider, R., et al., Preliminary analysis of loss rates of honey bee colonies during winter 2015/16 from the COLOSS survey, Journal of Apicultural Research 55(5), pp. 375–378, 2016
6. I. Fries, I. & Camazine, S. Implications of horizontal and vertical pathogen transmission for honey bee epidemiology, Apidologie 32(3), pp. 199–214, 2001