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Honeybee Nutrition

By Chris Strudwick
Australian Bee Journal (December 2012)

“Bees eat pollen and drink nectar, and nectar is made into honey to keep as a food store.” This is an oversimplification but likely sums up most people's understanding of bee nutrition.

Beekeepers know that availability of pollen and nectar at the right times are critical to colony development and honey production and also that bees will sometimes voluntarily take other foods, like protein patties and sugar syrup, even rye and soy flour. Possibly they have participated in the discussion around the ethics of feeding sugar – a hot topic for 'natural beekeepers'. Bee nutrition is a much more interesting subject than that narrow scope suggests. First, a lot of what is commonly believed is not exactly true; second, nutrition is intimately associated with colony health.

Do you like chestnuts? If you have ever harvested your own chestnuts you will know the tasty nut is enclosed in an unpleasantly tough and spikey case. Few people would enjoy chestnuts if they had to swallow them whole in their cases. Pollen is much the same. Browse through the Australian pollen and spore atlas http://apsa.anu.edu.au and you find images of the tough, rough surfaced, outer walls of pollen which can survive for millennia in the soil and are pretty much indestructible. Only fine pores give access to the nutritious contents. The brightly-coloured bee poop that sometimes ends up on your laundry is mostly undigested pollen walls. If you use pollen supplements yourself you might want to consider that piece of information. It is unlikely your system can extract much goodness directly from pollen.

Bees clearly do collect and consume pollen and even have a special valve in the honey crop to separate pollen from nectar and ingest it. Honey also has some pollen in it, although it could be considered an accidental contaminant as it usually makes up less than 1%, often much less, by weight. Honey is low in protein and amino acids which both come from pollen rather than nectar, which is mostly carbohydrate. So the pollen bees collect usually does not end up in honey, it is hard to digest directly, so what are they doing with it? Well, we can see they are stuffing it into cells around the brood but this is not just a handy store. Bees get around the indigestibility of pollen and tap into its rich stores of protein by mixing it with sugars, in the form of nectar or honey, and processing it into 'bee bread', allowing fungi and lactic acid bacteria to ferment it and break it down it to release its nutrients.

Suddenly we have a more complex picture. Just like us, bees rely on a specialised ecosystem of other organisms and some sophisticated processing operations to make their primary food available to them. Protein and amino acids are essential for bee development both for growing larvae and for newly emerged bees. Availability of the right sort of protein is important to young bees to ensure their hypopharangeal and mandibular glands develop properly and can secrete protein-rich royal jelly and amino acids to feed the queen and growing queen larvae, also to supplement the diet of drone and worker larvae. These start off on the same diet but then get a mixture of bee-bread and honey along with the food secretions of nurse bees. Consumption of high-quality protein allows young bees to lay down stores of vitellogenin, another protein, in their bodies. The richness of these stores affects later longevity and ability to forage. In particular, long-lived 'winter bees' need to accumulate good vitellogenin stores, which is why good pollen supplies are very important late in the season and why beekeepers might choose to provide supplements at this time, especially if their colonies are struggling with Varroa, Nosema or other problems. Well-fed nurse bees may have surplus glandular secretions to supplement the diet of foragers and help extend their working lives. Foragers otherwise subsist on carbohydrates and the vitellogenin stored in their bodies, consuming these until they work themselves to a halt.

But it goes further than just a question of nutrition. Bee-bread is a living culture, containing fungi that can suppress brood diseases and others that can cause them. Fungal brood diseases include Chalkbrood (Ascosphaera apis) and Stonebrood (various species of Aspergillus, mostly A. flavus) and both will grow in bee-bread. When spores are transferred to a larva during feeding, fungal hyphae rapidly invade its body and kill it. Mostly these diseases are relatively minor in their effect on the colony and the causal fungi are out-competed and directly suppressed in the bee-bread by other beneficial fungi. There are however times when Chalkbrood is more prevalent, especially earlier in the year, or during cool damp periods, when the hive's biological processes may not be fully up-to-speed. Bee-bread in higher-intensity agricultural areas has been shown to support a less diverse range of fungi, most likely due to fungicides being brought in on pollen. The incidence of Chalkbrood seems to be increasing under these conditions and perhaps this is not a total coincidence.

If the image of microbial wars going on in bee-bread seems alarming, it is just a warm-up for what follows. We are familiar with the idea of probiotics. Bees have a less sophisticated immune system than we do and a simpler digestive system, however they share with us, and other warm-blooded creatures (another hint that bee colonies are surrogate mammals), a number of Lactobacillus and Bifidobacterium species that can act against disease organisms. These lactic acid bacteria are also amongst those that help break down pollen in bee-bread. A bee is not born with a gut microflora, it is acquired from the bee-bread it receives as food and the house-bee that delivers it, but the microflora seems to play an important protective role. The outbreak of foulbrood for example is the outcome of a battle between 'good' and 'bad' bacteria. If enough foulbrood spores – of either species – are present and the disease gets a head start, an infected bee larva starves to death in competition with the invading bacteria. If enough good bacteria are around in the larval gut to outcompete and suppress the foulbrood, the larva might fight off the infection. As we know from annual testing, low foulbrood spore counts do not usually lead to an outbreak of the disease. The downside of surviving the disease is a bee that might still be infectious and further spread disease to the larvae it feeds once it becomes a nurse. So we have a bit of an arms race. If foulbrood always won the battle it would quickly wipe out its host and possibly itself. Bee larvae with a rich and healthy microflora are more likely to fight off infection and live, but in doing so also may allow the disease to survive and be passed on in a form of competitive symbiosis.

One recent discovery is that adult bees are strongly dependent on their relationship with lactic acid bacteria, not only for fermenting pollen and fighting infection but also for stabilising nectar to prevent fermentation before it gets transformed into honey. And that brings us around nicely to the other main food source, nectar. If bees get all their protein, amino acids and a range of micronutrients from pollen, they get their carbohydrate fuel from nectar and its manufactured derivative, honey. We know that honey contains many constituents that give each batch a unique flavour and scent and some of these may have value to bees. Mostly it is concentrated energy – fuel for foragers and fuel for central heating – and that energy source is sugar, which makes up over 80% of the honey. Most of the remainder is water and the bees reduce this as far as they can to inhibit fermentation. Bees have to add water back to the honey taken out of store to bring the level to about 50% before they can use it themselves. Honey is the reverse of nectar, which mostly consists of water and a little sugar. Again it looks simple: nectar is dilute sugar which bees transform to simpler sugars using enzymes and the lactic acid bacteria in their crops, then concentrate by removing the water until they are left with honey.

There are many forms of sugar. Common sugar, or sucrose, is often the main one in nectar although frequently it is fructose or glucose (or their isomers, laevulose and dextrose) which are simple sugars derived from sucrose. There are a handful of other sugars bees can use but there are many more they cannot, and most of these are positively toxic to them along with many other common carbohydrates, such as pectins. Not only that, but nectars can contain harmful alkaloids and essential oils, also HMF (HydroxyMethylFurfuraldehyde, also produced by over-heating honey) and PA (Pyrrolizidine Alkaloids) which are toxic substances considered as honey contaminants. All these get concentrated when making honey, a reason to be grateful they are usually there in trace quantities.

Bees are sensitive to many of these substances. It turns out that bees – specifically foragers – seem to do best and live longer on a sucrose diet, or a mixture of fructose and glucose, which is what bees convert sucrose into. Other sugars, often also present in honey, can shorten their lives. In the laboratory at least, bees have been shown to live longer feeding on sucrose syrup than on honey. One might make the controversial speculation that sucrose syrup is likely what bees prefer, but honey, with its mixtures of good and bad sugars and other constituents, is what they are capable of manufacturing. My own colonies are always overwintered on their own honey but I have no hang-ups about feeding sugar syrup to weak colonies or supplementing stores with syrup if need be. Syrup incidentally needs to be just pure white sugar and water, nothing more. Any additives such as flavourings or herbal supplements, or impurities such as are found in brown or raw sugar, are almost certainly harmful to bees, they do not make the syrup more 'natural'.

One final comment before I reluctantly leave a subject I have barely started. Some recent research shows that well-fed bees may develop higher loads of Nosema ceranae spores than poorly-fed bees. This is not surprising because these parasites are competing with the bee for the food it consumes, so a well-fed bee has well-fed parasites. An infected bee requires more energy to function both to support this drain on its resources and also to mount an immune response.

Despite this, they may live longer than less well-fed bees with lower parasite loads. This may explain why foragers from apparently healthy colonies are frequently found with high Nosema spore counts. Good nutrition allows the colony to cope, and even prosper, despite having to fend off serious challenges to colony health.

Chris Strudwick, VAA Member, Email: cstrudwick@mindworkweb.net


Last update 3-Mar-2017
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