Honey Bee Communication: Trophallaxis.

It’s really interesting studying honey bee behaviour and trying to work out what’s going on when watching bees at work. I wrote this original article for the “Communications Within The Hive” booklet published recently by the BBKA. You can buy a copy here: https://www.bbka.org.uk/shop/communications-within-the-hive-new

Food Sharing.

In our human world, food sharing is also important.  While it expresses the religious symbolism of holy communion through the giving and receiving of wine and wafers, it also demonstrates the cherishment of children, family and friends through creating and presenting special meals. Do you remember the last birthday cake you shared with your childhood friends? Mine was a multicoloured marble cake with raspberry jam and butter icing in the middle and on the top. Reflect also on how we show compassion for others less fortunate by providing soup kitchens and donating to food banks. Indigenous tribes of South America, Africa, Arctic regions and elsewhere still rely upon hunting and food sharing for survival and social cohesion. Therefore, it remains a crucial part of our social history and culture.

Honey bee colonies, like humans and other animals, including birds and bats, rely on trophallaxis and the associated exchange of information for social cohesion and survival. However, it is a more complex behaviour in honey bees and this paper aims to share what we know today and explain the processes and unravel some of the mysteries.

Trophallaxis.

Trophallaxis is rooted in the Greek language. Trophos refers to one who nourishes or is nourished, and allaxis means an exchange. It is the term used to describe the direct transfer of food or fluids from one individual to another and this activity is common among the social insects such as wasps, bees, ants and termites. Food may be transferred orally or anally depending on the species and circumstances. For example, during the early brood raising part of the wasp life cycle adult wasps utilise the sugary excretions of their larvae for energy in an anus to mouth exchange called proctodeal trophallaxis. The more aesthetically pleasing stomodeal exchange involves mouth to mouth contact and this is how honey bees feed their nestmates and it is the method of all trophallaxis in this species.

Trophallaxis is an evolutionarily advanced trait associated with sociality.  Solitary bees do not share their crop contents with other bees, so trophallaxis is a behaviour that evolved when honey bees became social. Probably, the drinking of food from another bee is derived from the behaviour of drinking nectar from a flower (a behaviour present in the solitary ancestors of honey bees). And it is also likely that regurgitation of food to another bee is derived from the behaviour of solitary bees of regurgitating nectar to form a ball of nectar and pollen inside a solitary bee’s brood cell. Trophallaxis is therefore a remarkable behaviour that is a special feature of the social lives of honey bees.

Much More Than Nutrition.

Ribbands (1) explains that food sharing among honey bees is a much larger practice than is necessary to prevent starvation, rather it serves mainly as a mode of communication. Later researchers, Korst & Velthius (2) stated that less than 5% of trophallaxis results in actual food exchange. For this reason, Ribbands called the activity food transmission rather than exchange.  He also believed that food transmission delivers information that forms the basis of effective division of labour within a colony. Later, Winston (3) would acknowledge that trophallaxis does indeed play a role in age polyethism, or age-related activities which effectively create a division of labour and influence the onset of age specific tasks such as wax making, guarding, foraging etc. We know that hormones such as juvenile hormone play a major role in colony flexibility. For example, if there is a shortage of nurse bees for some reason, perhaps disease, foraging bees can revert back to house bee status and brood feeding through the lowering of Juvenile hormone levels and raising of vitellogin to enlarge their hypopharyngeal glands thus enabling them to produce larval food again. Colonies of honey bees have the amazing ability to respond to changes in the internal and external environment, and we also believe that food transmission is involved in some way yet it requires further investigating and explaining.

Worker to worker transmission .

Imagine a fluffy grey coated new worker bee emerging from her cell at 21 days; she doesn’t go off immediately to uncap a honey-filled cell and drink. Rather she seeks out an older bee and begs food by palpating its antennae with her own and thrusting the tip her outstretched tongue towards the mouth of the other bee. She might have to intercept a few bees till she finds one loaded with stores and able to meet her needs. When she does, the willing donor will open her mandibles wide and, with her proboscis remaining retracted and tucked under her head, she will push the base of her proboscis slightly forwards and regurgitate a drop of fluid onto it.  The new bee receives the drop by thrusting forwards her tongue towards the drop and sucking up the liquid. If you were watching you would notice that the bees are constantly touching each other by feeling each other’s heads with their antennae. This action serves to keep their mouthparts correctly aligned during the transfer.

What does the honey bee transfer? It is mainly nectar or honey but there are also likely to be some glandular secretions, brood food and gut flora which may confer some immunity to the new bee. New-born human babies swallow maternal fluids during the birthing process that kick- start their immune systems through the colonisation of the gut with normal protective bacterial flora transferred from their mothers. The latter is essential for honey bee health too.  Importantly, during this first exchange the new bee will receive queen substance which is the key to colony cohesion.

This transaction does also ensure that the young house bee has the energy to carry on with cell cleaning duties because she will not actively feed herself with honey sipped from a cell till around her fourth day. However, this is a multifunctional strategy ensuring that colony odour and protective immunity factors are transferred to the new bee. That colony cohesion is bolstered is of great importance.

Queen Substance.

Dr Butler (3) and his colleagues at Rothamstead Experimental Station discovered queen substance in 1950’s when they were investigating swarming.  After the last war, there was much need for the UK to produce more food and agricultural practices changed markedly as a result of research carried out then, and farming on a large scale was born of that era. Beekeeping received a lot of attention, and if swarming could be better understood and controlled, then honey production should increase. Queen substance was isolated from the queen mandibular glands, and the main component 9-oxodec-2- enoic acid (9-ODA) was named and described as a pheromone and chemical messenger produced in large quantities in young well- mated queen honey bees. The distribution of 9-ODA in a colony inhibits queen raising in the presence of a vigorous queen, except when a colony prepares to swarm. As we know, swarming is a complex behaviour and we still do not know what actually triggers it though we know that there are several important contributing factors involved.

Queen substance was once thought to be key in preventing worker ovaries from developing thus giving the queen reproductive supremacy, but it is now known that it is the it is the presence of brood and their pheromones that mainly inhibits worker ovary development. However, it is not uncommon for some colonies to have a few egg-laying workers under normal circumstances. The situation is normally benign because workers closely monitor this activity and eat the eggs.  Usually, the beekeeper is totally unaware of it.

Interestingly, virgin queens receive little attention from workers who do not form a retinue around her until she is mated, but they are aware of her and if she is removed, they are immediately alerted and show signs of being without a queen.

 Butler didn’t discover the perfect solution to swarming with a synthetic pheromone because queen substance is so complicated and comprises many chemicals. However, Butler and colleagues opened up for us a new world of understanding about how honey bee colonies function. During their research, they discovered that swarm lures could be produced by mimicking the Nasonov gland pheromone.  Now we can buy a synthetic swarm lure based on the Nasonov pheromone.

Brood Feeding.

Let us return to our young house bee. She has been feeding up on proteinaceous pollen since her emergence and her hypopharyngeal glands are plump and ready for her next task of feeding the larvae and queen. If you dissect the head of a young house bee you will be amazed to find that most of the head space around her brain is full of strands of tiny white bead-like glandular cells. These are called acinus cells because they are clusters of glandular cells resembling a many-lobed berry. These glands secrete clear liquid whilst the mandibular glands, above the mandibles in both cheeks, secrete a thicker milky secretion full of protein and fats. Together these secretions with added pollen comprise brood food for the worker and drone brood. The queen larva, and her mature self, receive a special diet high in mandibular gland secretions and royal jelly containing an amazing array of nutrition compared with brood food. There are vitamins biopterin and pantothenic acid which are important growth factors as well as vitellogenin which is a crucial female-specific phospholipoglycoprotein which must be present before egg yolk can develop. Vitellogenin is a multifunctional component that acts as a storage protein too and is also found in fat body tissue.

Although our young bee will not directly feed larvae mouth to mouth, she will lean into their cells and let drops of food fall from her tongue down the sides of the cell. The larva, who is mobile within the cell until capping, will rotate its body until it reaches the food. Our bee will feed hundreds of larvae and not necessarily return to the same ones which means that many different nurses share their secretions and pheromones amongst the brood.

Our bee will feed the queen larvae and the mature queen. She will top up her own levels of queen substance every time she feeds the queen and this she will share with her sisters. She will feed the drone larvae and emerged drones. Although drones emerge around day 24, they become mature around 36 days from newly laid egg stage. They are larger than their sisters and have more complicated reproductive systems which require large amounts protein for this development. So, drones are fed by workers of the correct age, for proteinaceous glandular production, until their seminal vesicles are full at around 12 days post-emergence. After that drones help themselves to stores.

In preparation for foraging around 21 days, our worker bee moves on to different tasks as her glands develop. She moves from the brood nest and up into the honey supers to work in the honey factory where she receives nectar by trophallaxis from the foragers before storing and ripening it. She packs down pollen and helps unload resin for propolis manufacture. The latter is a time-consuming job and can take as long as 20 minutes a load.

Fuelling Heater Bees.

Our bee might be called into action as a “filling station bee”. Tautz (4) describes how heater bees are employed in empty cells in the middle of brood combs to generate heat for the brood. At first glance you might think that they were cell cleaning, or having a snooze, but they are working flat out head first in the cell generating heat by isometrically contracting their flight muscles with folded uncoupled wings. It is a thirsty energy-sapping job and honey stores are usually farther away from these cells so filling station bees are employed to continuously trek backwards and forwards with stores thus sparing them the long journey and interruption of heating duties. These bees locate the hot heater bees in the darkness by means of the efficient temperature sensitive receptors on their antennae. The heater bees are fed by trophallaxis.

Cooling the Colony.

If a colony is starting to overheat in summer the middle-aged worker bees are soon aware and they start to ventilate the nest. Those that are involved in spreading dilute and water to cool the combs will do so, and when they need more fluid, they will walk around begging water from inactive water collectors, many of whom will still have water in their crops, retained from the last bout of water collecting. Being begged for their water stimulates the water collector to return to the task of water collection, and they will do so until the need for water diminishes. These water collectors sense this drop in the demand for water when they return to their nest and search for bees that will receive their water loads and in doing so experience long search times. Thus, water collectors, like nectar foragers, get information about their colony’s need to raise or lower their colony’s intake of water/nectar from the delay experienced in initiating trophallaxis when they get back to the hive. These bees don’t get their information during the act of trophallaxis per se but from the ease or difficulty they experience in initiating trophallaxis with water receivers (5).

Research carried out by Ostwald et al (2016) (6) demonstrates that high nest temperatures alone do not activate water collectors; these bees are often stimulated to resume collecting water without heat stress, for example, in the spring when little nectar is being collected but the nurse bees need plentiful water to produce copious brood food. When their colony’s need for water is high, the water collectors are begged by the receiver bees unloading bees with water, and so, these water collectors are stimulated to resume their water collection. Previous water collectors who may be hanging about resting are also repeatedly begged for water which stimulates them to go back out and collect water once more.

Guarding and Defence.

Although in temperate climates honey bees do not usually store water in cells, this study showed how bees under extreme heat conditions will store water in comb cells. What is even more remarkable is that some bees became “reservoir” bees and store water in their crops overnight, passing on water, as required and requested, to house bees.

Before the final foraging stage, our bee will probably become a guard at the entrance tasked with apprehending any bee not from her colony. If there is a heavy nectar flow on and the newcomer offers nectar she may be allowed to enter. On the other hand, the guard bee is able to discriminate between bees by odour and deny entrance if threatened thereby demonstrating that trophallaxis also plays a key role in colony defence.

Disease.

What about trophallaxis and disease? The negative aspect of food sharing may be the orofaecal transfer and spread of diseases such as Nosema apis through a colony. Bees fed with Israeli Acute Paralysis Virus (IAPV) at the University of Illinois (7) were less likely to feed or touch nest mates indicating that they too use social-distancing to reduce the spread of a virus. However, in this study, infected and sick bees easily got past the guard bees in other colonies and gained entry to other hives to spread infection. It is thought that the virus alters the cuticular hydrocarbons of the sick bees and makes them less easy to recognise as non-nest mates, and it also makes then more submissive. Certain levels of a substance called octasane are associated with a greater acceptance of bees from another colony and it is higher in sick bees. Guard bees use their antennae to detect a collection of chemical signals on the outsides of other bees not from their colony. In the experiment, guard bees let in 30% sick bees from another colony compared with 15% healthy ones. This highlights the dangers of beekeepers placing hive close together, and it clearly demonstrates how viruses adapt and take advantage of a situation like this to increase their own survival chances.

Summary.

In summary, trophallaxis is a complex behaviour involving skilful coordination between the donor and the receiver which usually starts with the receiver begging food from the donor as described earlier in worker to worker transmission. However, there are times when the initiation goes the other way, such as when a nectar forager enters the nest and seeks a nectar receiver to quaff her nectar load. In this situation the nectar forager walks around holding a droplet of nectar between her mandible and offering it to other bees until she encounters a bee who puts out her tongue, after “tasting” what is being offered using sweetness receptors on her antennae, and drinks the offering.

Trophallactic transfers are made through:

• worker to worker adults transferring nectar, water, honey and queen substance
• Worker to worker larvae transferring brood food and queen substance
• Worker to queen larvae and adult queens transferring royal jelly and queen substance
• Worker to drone larvae and adult drones transferring brood food and honey and queen substance

The contexts are:

• Nutrition
• “Filling station” bees refuelling heater bees
• Recruitment
• “Reservoir” bees supplying water
• Colony cohesion
• Colony defence

If any hive commodity is in short supply, then the demand for it increases based on intelligence received through trophallaxis. Colonies can respond to external and internal environmental changes and rejig the ratio of workers engaged in various tasks which is a powerful means of collective colony control.

Trophallaxis is an important part of the social fabric of a colony for it is a critical part of the processes whereby nectar and water are distributed among the members of a colony. Some of this distribution is from nectar (or water) collectors, and to hive bees (and vice versa), and some is from one hive bee to another.  For example, this is how the queen gets all of her food, something that highlights the critical role of trophallaxis in the functioning of a honey bee colony.  However, there is still much that we do not yet know about the trophallactic behaviour of foragers but, with abundant available research material, the opportunities for future investigations into this fascinating subject look promising.

References:

1. Ribbands, C.R., 1953. The Behaviour and Social Life of Honeybees. Bee Research Association Limited, London.
2. Korst, P.J.A., Velthius, H.H.W., 1982. The Nature of Trophallaxis in Honey Bees, Apis, mellifera mellifera. Insectes Sociaux 29 (2): 209-221 doi:10.1007/BF 02228 753.
3. Butler, C. G., 1954. The World of the Honeybee, Collins, London.
4. Tautz, J., 2008. The Buzz About Bees, Springer.
5. Seeley, T.D.S., 1995. The Wisdom of The Hive, Harvard University Press, Cambridge, Massachusetts, London, England.

6. Ostwald, M.M., Smith, M.L., Seeley, T. D., 2016. The Behavioural Regulation of Thirst, Water Collection and Water Storage in Honey Bee Colonies, Journal of Experimental Biology (2016) 219, 2156-2165 doi: 10. 1242/jeb.139824.

7. https://www.sciencemag.org/news/2020/04/deadly-virus-turns-honey-bees-trojan-horses