The bee's fat body: why one bee lives six weeks and another half a year

Ask yourself a question few people can answer: why does a summer bee live about six weeks, while a winter bee from the same nest, with the same genome, makes it to spring, that is half a year and more? It is not magic and not a different species. It is one organ and one protein: the fat body and the vitellogenin stored in it. Whoever understands how this works stops thinking of feeding as topping up fuel and starts thinking of building a bee that survives the winter.

This article takes the fat body apart: where it comes from, what exactly it does, why it decides lifespan, and when a bee loses most of it. And since it loses it mainly through feeding larvae, this is also the story of why autumn feeding and the moment the colony is relieved of brood weigh so heavily on wintering.

What it actually is

The fat body is an organ spread through the bee's abdomen, a loose, pale tissue lining the inside of the exoskeleton. The name misleads, because it is not a plain fat store. It is the insect's centre of metabolism, something like a cross between a liver, a larder and part of the immune system. Fats, proteins and sugars are stored here, detoxification happens here, and from here the colony draws building material when fresh food is short.

The most important thing the fat body accumulates is vitellogenin. It is a large storage protein, about 90 percent protein with a little fat and sugar, which in other insects serves mainly to make eggs, but in the worker bee has been co-opted for completely different tasks. The level of vitellogenin in the fat body is the single best indicator of whether you are looking at a short-lived summer bee or a long-lived winter one. A bee with a well-developed fat body and high vitellogenin is a bee that will survive. A bee with a depleted fat body is a bee running on empty, counting its days.

Where it comes from: it all starts with pollen

The fat body does not come from sugar or honey. It comes from protein, and the bee has protein from one source: pollen and the bee bread made from it. This is the crux that gets lost in discussions of feeding, where the talk is mainly about sugar and syrup. Sugar gives energy, but it will not build a single gram of fat body. Only protein from pollen builds it.

The mechanism is this. A young bee in its first days of life eats pollen intensively, and its fat body grows and loads up with vitellogenin. From that protein store the nurse bee then produces the jelly it feeds the larvae, and from the same store it draws for its own immunity and longevity. In other words, pollen eaten by young bees turns into fat body, and the fat body is the capital the colony lives on when pollen is short. That is why young bees' access to abundant pollen, especially in late summer, decides how strong they enter winter. A pollen shortage in that period is the quiet cause of many collapses, later blamed on frost or varroa.

How the fat body extends life

Now the most interesting part, the mechanism of longevity, because it is not simply that a fat bee has more reserves for later. Vitellogenin does something far cleverer.

First, it acts as an antioxidant. Work by Amdam and colleagues showed that vitellogenin scavenges free radicals and protects cells from oxidative stress, the very process that drives ageing in all animals. A bee loaded with vitellogenin ages more slowly, literally at the cellular level. That is why it is sometimes called the bee's elixir of youth.

Second, vitellogenin is part of the immune system. It supports cell-based immunity, and a high level means a bee better defended against pathogens. In winter, when the colony sits in a cluster for months with no generation turnover, that immunity is priceless.

Third, vitellogenin is in balance with juvenile hormone, and this is the key to the whole switch. When vitellogenin is high, juvenile hormone is low, and the bee stays a homebody, long-lived and calm. When vitellogenin falls, juvenile hormone rises, the bee shifts into forager mode, flies out into the field, wears itself out and dies quickly. This seesaw between two states governs the whole life of the worker.

The numbers are striking. A summer bee in the colony's growth phase lives on average about three weeks. A winter bee can live over one hundred and fifty days, and in a cold climate longer. The same genome, the same bee, and the difference in lifespan is five or sixfold. The fat body and vitellogenin make it.

When a bee loses the most fat body

And here we reach the thing that translates directly into what you do at the hive. The fat body is not given once and for all. The bee uses it up, and there are two main ways it loses it.

The first and most important is feeding larvae. A nurse bee, producing jelly for the brood, burns its vitellogenin and depletes its fat body. The more larvae it feeds, the faster it wears out and the shorter it lives. This sounds trivial and has enormous consequences: a bee that works hard as a nurse shifts into the short-lived, summer form, regardless of the season. Feeding brood literally shortens its life.

The second way is the shift to foraging. When a bee starts flying to the field, its vitellogenin drops, the fat body slims down, juvenile hormone rises, and the bee quickly wears out in flight and work. A forager is a bee on the wane, burning its last reserves.

Now combine these two things with what I wrote about wintering and it all clicks. A winter bee arises when a young bee eats its fill of pollen, loads its fat body, and then does not have to squander that store on feeding larvae. If in late summer the colony has a lot of brood to feed, the young bees burn their vitellogenin on jelly and enter winter as exhausted, short-lived workers. If, on the other hand, brood rearing naturally tapers off at that time, the young bees keep a full fat body and pass into the long-lived winter form. That is why the natural tapering of brood in autumn, and flooding the nest with thick stores that limits the queen's laying space, both favour the making of a good winter bee. You relieve the young bees of nursing, and they save the fat body for winter.

What this means for feeding

Since the fat body is built from pollen, not sugar, the way you feed the colony looks different once you grasp this mechanism.

First, pollen is as important as sugar, and for the longevity of the winter bee more important. A colony heading into late summer must have access to abundant pollen so the young bees can build their fat body. If pollen is short in nature, it is worth supporting the colony with a protein feed, because sugar alone gives energy but not longevity. That is why goldenrod, though its honey is troublesome for winter, is at the same time a valuable late source of pollen for the fat body.

Second, sugar stimulation and brood production are a double-edged sword. In a dearth you stimulate the colony so the queen lays in the window of winter-bee production, but then comes the moment to shut down brood rearing, precisely so the young bees stop burning their fat body on feeding and keep it for winter. Flooding the nest with thick stores at the end of the season does this for you by limiting the queen's space.

Third, sugar is still needed, but for something else. It provides energy to survive winter and to hold the cluster. But do not confuse the roles: sugar is fuel, pollen is body-building material, and the fat body is the winter bee's life. Good wintering is a colony that has both, and whose young bees entered winter with a full, untouched fat body.

Take care of pollen in the area: what to sow and plant

Since the fat body is built from pollen, and it is needed most in late summer and early autumn, it is worth taking care of that pollen not only through feeding but at the source, that is in the ground around the apiary. A few coins on seed can pay back in a healthier winter bee.

For a quick effect, from seed, annuals work. Phacelia is the classic, it grows fast, and with a late sowing it still flowers in late summer, giving both nectar and good-quality pollen. Mustard and oilseed radish from a late sowing add pollen just when it turns bare in nature. Borage also pollinates willingly and flowers long. Pumpkins and courgettes in the garden give abundant nectar and pollen right up to the first frosts, and their large loads are a solid protein boost, so they are a free addition not to be sniffed at. Leave buckwheat out of this role, because although it flowers long, its pollen is poor and makes a weak building material for the fat body.

Among perennials and wild plants on which bees load the fat body before winter, the most important are goldenrod, heather, asters and Michaelmas daisies and common ivy, which flowers really late and is often the last pollen of the season. Goldenrod and heather are at the same time the flows whose honey is troublesome for winter, but as a pollen source for the winter generation they are priceless, and that is their other, good side. Heather in fact has pollen counted among the most nutritious, so for the winter bee it is all but ideal.

It is also worth thinking long-term about shrubs and trees, because they give the most valuable, protein-rich pollen, only at different times of the season. Working directly for the winter bee are the late ones, ivy and heather. But a healthy, strong colony that then produces a good winter generation is built across the whole season, so if you plant for years, bet also on the early and summer pollen powerhouses. Goat willow and hazel give, right after winter, an early, protein-rich pollen known as bee bread that kick-starts spring development. Fruit trees, maple and lime add abundant, good pollen in spring and summer. Among shrubs it is worth having raspberry and blackberry, hawthorn, blackthorn and alder buckthorn, which flowers long and feeds steadily for half a season. It is a generational investment: a shrub or tree planted once works for pollen every year, without re-sowing.

And one thing you have to know so as not to waste space and money: not all pollen is equal. The pollen of willow, fruit trees or clover is rich in protein and really builds the fat body, whereas the pollen of conifers is essentially empty calories. When planting or sowing for the bee, aim for species with valuable pollen, not just for those that flower prettily. The same rule as with buckwheat: what counts is pollen quality, not the mere presence of flowers.

Supplementation when pollen is desperately short

Sometimes, though, the wintering site has a pollen shortage so bad it screams: a monoculture around, a dry autumn, no wild flows. Then sowing will not help in time for the season, and you have to give the bee protein from outside. It is a real tool, not a last resort, provided you do it with your head.

Best is natural pollen or bee bread, given back to the colony, because it is the food closest to what a bee would gather itself. If you have a surplus of pollen set aside from the season, it beats any substitute. The trouble is that pollen from other apiaries is not given without thought, because it can carry disease, so use a sure source, best your own and frozen.

When you have no pollen, protein cakes come in, that is patties with a pollen substitute. These are ready preparations based on yeast, soy protein isolates and similar sources, or homemade mixes, formed into a patty and placed straight over the nest, just above the cluster, exactly like a sugar cake. The colony eats them and turns them into building material for the young bees.

Two rules decide whether this helps or harms. First, timing. You give a protein patty for the winter bee in late summer and early autumn, in the window when the winter generation's fat body forms, not in the depth of winter. Given too early or at the wrong time, it will crank up brood rearing and you will get the opposite of what you intended, because instead of saving the fat body, the young bees will burn it on feeding. Second, quality. A good substitute must have the right amino acid profile, because a bee fed on any old thing will build a poor fat body. Substitutes are convenient, but none fully matches real pollen, so treat them as a rescue for a lean year, not as a permanent replacement for nature.

Curiosities and controversies

A winter bee is simply a nurse that ran out of larvae. So goes one of the more interesting theories: the winter bee is not a separate type but a young nurse that did not use up its fat body, because in autumn it had no one to feed. It did not give its vitellogenin to larvae, so it was left with a full store and passed into the long-lived form. Newer research adds pollen, day length and the colony's age structure to this, so the amount of brood alone does not explain everything. But the direction is simple: the fewer larvae a bee feeds, the longer it lives.

Vitellogenin is the insect's elixir of youth. Amdam showed that vitellogenin acts as an antioxidant, scavenging free radicals and slowing ageing. It is a rare case where a single protein is responsible for something we chase with supplements in humans: a real delay of the ageing process. Hence the half-joking name the bee's fountain of youth. It also explains the queen, who lives for years precisely thanks to a gigantic level of vitellogenin.

Royal jelly is liquid fat body. When you eat royal jelly or hear of its health properties, you are in fact talking about a secretion the nurse bee produces from its fat body and vitellogenin. That is why producing jelly so exhausts nurses: they give the larvae, literally, a piece of their own store of life.

Feeding larvae shortens life, and strongly. Against intuition, it is not flight and field work that are the bee's first killer, but the earlier grind at the brood. A bee that fed a lot enters the rest of its life already burnt out. This overturns the common notion that a bee first works calmly in the hive and only wears out in the field. The wearing out starts already at the larvae.

Most common mistakes

Confusing sugar with body-building material. Sugar gives energy but will not build the fat body. Only protein from pollen builds it. A colony with sugar to the brim but no pollen will not produce a good winter bee.

Ignoring a pollen shortage in late summer. This is the period in which the winter bee's fat body forms. A lack of pollen then means a depleted, short-lived winter generation, however much sugar there is.

Forcing late brood. A colony forced to feed a lot of larvae in late summer burns the young bees' fat body on jelly. The natural tapering of brood and flooding the nest with stores favour a good winter bee.

Treating the fat body as plain fat. It is not a lard store, but a metabolic and immune organ, the seat of vitellogenin. Its condition decides the colony's longevity and immunity.

Sources and further reading

• Amdam G. V., Omholt S. W. (2002). The regulatory anatomy of honeybee lifespan. Journal of Theoretical Biology. Basis of the vitellogenin and winter-bee longevity theory.

• Seehuus S. C., Norberg K., Gimsa U., Krekling T., Amdam G. V. (2006). Reproductive protein protects functionally sterile honey bee workers from oxidative stress. PNAS 103: 962-967. On the antioxidant role of vitellogenin.

• Corby-Harris V. et al. Work on winter-bee physiology and the effect of pollen and brood feeding on its formation.

• Keller I., Fluri P., Imdorf A. (2005). Pollen nutrition and colony development in honey bees. Bee World 86. On the role of pollen in fat body development.

• Randy Oliver, the Fat Bees series, scientificbeekeeping.com. An accessible account of the fat body and vitellogenin from a practitioner's view.

• Sulborska A., University of Life Sciences in Lublin, and materials from the Pasieka24 portal on pollen plants and filling forage gaps in late summer and autumn. Basis of the plant list and recommendations for Polish conditions.

• Works on the nutritional value of pollen and protein substitutes in bee feeding. Background for the supplementation section.