Coping with Pests & Pathogens: Honey Bee Immunity.

Welcome to all the new subscribers who joined Beelistener this month, and to the new readers from Hong Kong, Sri Lanka, Qatar, Kenya, Monaco, Egypt, and Isle of Man bringing the blog to 61 countries. A big thank you to those of you who supported the running of the website this month.

We’re having ferocious gales this week and empty stored bee hives have been crashing all over the patio but the colonies are safely anchored down in place. Yesterday I heard a crash in the nearby woods and my son who was closely watching the storm from the doorstep saw an enormous trees fall to the floor. Dried leaves were whipped up into a swirling mass like a snowdome scene and the air was full of dust.

It wasn’t safe to go to the woods so Connie and I spent the afternoon playing the animal game and reviewing Miss Johnstone’s B Class virtual classroom. It’s not live yet and I can’t share it but Connie’s fascinated to learn how some Scottish secondary school pupils are studying beekeeping and learning how to diagnose Nosema spp. at the Roslin Institute using sophisticated equipment. See the Beekeeping in Scottish Schools blog for links to the project.

Connie is moving to Wales in a few months with her family so we’ve been talking about all the good things that might happen then like living close to a beach. One of the schools she may attend keeps bees which is exciting. Once we know where they will be living, I shall soon be on the lookout for a beekeeper to help Connie and her mother get started with bees. I’ll miss Connie lots but we’ve had a great time beekeeping and exploring nature over the last 3 years and she’s keen to keep on learning and look after her own bees in a year or so.

How Honey Bees Deal with Pests and Pathogens.

Like all insects, honey bees have an immune system that is similar to ours in many ways but lacking the unique innate adaptive response present in mammals. More about that later. Honey bees are under constant barrage from pests and pathogens ranging from varroa mites and small hive beetle to bacterium, fungi, toxins and viruses. They respond to such attacks in various ways using physical, chemical and behavioural strategies. They fight back at individual level and at colony level.

Cell Mediated Immunity.

An individual bee responds to wounding, and the introduction of bacteria at cellular level, through cell mediated immunity. Special cells called haemocytes are present in the bee body and are rather like our mammalian neutrophils that fight infection in a similar way. If the bacterial particles are small enough the honey bee haemocytes engulf them up and swallow them in a process called phagocytosis. Where there are many particles, they are dealt with slightly differently in a process called nodulation. What happens is, somehow, the haemocytes stick the invaders together to form rafts of inactive nodules incapable of causing damage. Another strategy for dealing with larger particles is encapsulation whereby haemocytes surround them in large numbers forming layers and basically suffocating them.

Humoral Immunity.

At individual level, there is also humoral immunity which is found in the haemolymph and other body fluids and works on pathogens outside the cell using something like mammalian antibody mediated immunity. In mammals this works on the basis that when there is a foreign protein in the body (antigen) the body produces antibodies to fight the alien protein. This is what happens in our bodies when we get an infection like shingles which will cause an increase in antibodies with a memory. So, if we are exposed to shingles again our antibodies will immediately recognise the invader and start fighting the virus and we may not get sick the second time. The ability to train the immune system to recognise and attack a foreign protein is the basis of vaccination which is currently an everyday word.

Antimicrobial Peptides.

It is not quite the same for honey bees because they do not have antibodies or the adaptive response system that we have, but we know that they produce protein substances called antimicrobial peptides (AMPS) in the fat body tissue in response to infection. There are many kinds of AMPS specific to particular invaders that are involved in attacking them. The presence of them shows that honey bees have an active humoral immune system. There is still much research required before we fully understand honey bee immunity but an exciting discovery has been made recognising something called transgenerational immune priming. What this means is that when a colony is sick and fights infection the next generation somehow benefits and is able to mount stronger humoral and cellular responses and they cope better with the infection becoming less sick than the previous generation of bees. 

Recent research from Sweden shows how this works and that vitellogenin (Vg) plays a key role in pathogen recognition and preparing the next generation to get ready to deal with the attack. I discussed Vg in a previous blog in connection with fat body tissue and winter bee production, and you can see that it is a very important glycolipoprotein. Vg is a specific female protein and required in egg production and also in process of immunity. Inside the egg, the researchers found Vg with attached pieces of pathogen that stimulated the immune system allowing the new bee’s system to recognise the invader and attack it. So, transgenerational immunity passes from the queen via her eggs to the next generation of colony bees and this will be the basis for any future honey bee vaccines that might come on the market. The queen will be fed a vaccine and she will transfer immunity vertically to offspring via the Vg in her eggs.

Social Immunity.

Immunity at colony level involves; keeping things clean, isolating from the sick, polyandry and self-medication. We refer to this as social immunity in the colony and it helps protects a normally functioning colony from disease. It is quite fascinating to examine all the behaviours and strategies that are involved in social immunity.

Starting with hygiene, we immediately think of propolis produced from plant resins and collected by foragers. These resins are actually part of plant immune systems so bees are visiting “pharmacies” for their own health needs. Propolis, which means before the city in ancient Greek, is collected by foragers and used to block draughty gaps in the nest cavity and entrances. But that is not all and is it used to line the nest cavities of wild colonies and it plays a large role in immunity as research by Dr Marla Spivak of Minnesota demonstrates. Bees self-medicate using propolis and Spivak’s laboratory studies show that propolis can kill Nosema ceranae spores. However, be warned that we do not know what a safe or effective dose would be yet, and it is not appropriate to feed our bees with any propolis concoctions. The presence of propolis reduces the cost of investing in the immune system in other ways.

Honey bees embalm hive intruders such as mice with propolis which prevents infection as they cannot always remove a mouse from a nest. They coat frames with propolis, and how many times have you heard beekeepers complain about it and advise you to rear bees from colonies that don’t make lots of propolis. We now know that it is a very good thing to make lots of propolis and we must embrace it for the colony benefit. Having said that, it makes my finger and hands itch so I must always wear thin disposable gloves when handling frames.

Did you know that bees have glucose oxidase in their salivary glands and they use it to clean things up in the hive because it creates a strong disinfectant called hydrogen peroxide that some of us might use around our own homes to keep things clean?

Then there is grooming and removing of varroa mites, and grooming a nest mate which is called allo grooming. Often sick bees remove themselves from the hive in a situation called altruistic self-removal. They come out of the hive and crawl off to die thus reducing the risk for nestmates left at home.  Dead and sick bees are usually removed by the middle-aged house bees that are no longer involved with feeding brood. They are aptly named undertaker bees.

What is pretty amazing is that these undertaker bees can detect a sick larva by the odour they give off and they can be removed before the infective stage is reached in some cases. This situation can lead to a spotty brood pattern on the frame so keep a look out on colony inspection.  Sometimes you can see dark areas on dead pupae and these are caused by haemocytes killing themselves and becoming coated with melanin polymers which may prevent infection spreading.

Where does polyandry come into it? Well, this refers to the queen mating with many drones and research shows that this promiscuous behaviour confers increased immunity and a decrease in disease due to genetic diversity making these colonies much stronger and more resilient than otherwise if the queen mates poorly with fewer drones. This might have implications for instrumentally inseminated queens if semen was used from only a small number of drones.

Pathogens are sensitive to temperature which is why we get a fever when we are sick with flu sometimes because our bodies use increased temperature to fight infection.  If we let the fever do its job, we can speed up our recovery process but, because we don’t like being uncomfortable, we tend to take paracetamol or aspirin to reduce the temperature but this actually prolongs pathogen life. Of course, human babies and young children are unable to easily regulate body temperature, due to an immature hypothalamus, so must be treated with appropriate medicine to reduce high fevers which might cause convulsions in severe cases. Honey bees can thermoregulate and raise the temperature inside the brood nest to kill pathogens.

Trophallaxis.

Have you heard of the social stomach of a honey bee colony? This relates to shared ingested good bacteria and fungi and other substances acquired through 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.  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. Ribbands 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

What does the honey bee transfer? It is mainly nectar, or ready to consume diluted 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- starts 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. Trophallaxis is a multifunctional strategy ensuring that colony odour and protective immunity factors are transferred among the colony.

Most diseases affect the bee and larval guts which are the weakest links in the armoury of defences against disease. However, the mid gut or ventriculus is lined with a peritrophic membrane which offers some protection from infection and damage from spiky pollen grains.

Lastly, pollen comes in from the field carrying beneficial moulds and bacteria that help boost honey bee immune systems. Pollen is stored as bee bread which is produced by fermentation following the additions of acids such as lactic acid. Fresh pollen is pretty indigestible for honey bees because of its hard outer shell but when fermented the shell is broken down and pollen is of greater food value to bees. It can also be stored for quite a few months in this form, though not much longer than around 9 months. By the way, fungicides may seem benign but they can damage the good yeasts and other fungi in a honey bee colony which will affect the immune system negatively. 

Keeping Health Colonies.

A strong healthy colony has the capacity to fight infection but can easily be compromised and overwhelmed. The best we can do as beekeepers is understand what is going on in and around our colonies and try to prevent stress by keeping varroa levels down and ensuring that the colony has enough  stored pollen and honey. We can allow colonies to raise plenty drones to ensure genetic diversity and not kill off too many drones during our biotechnical varroa control methods. Good apiary hygiene is the key beekeeping strategy in maintaining healthy colonies.

Coming Soon.

Look out for a guest blog from Professor Tom Seeley on Darwinian beekeeping.