What are winter bees and what do they do?
As beekeepers, we tend to underestimate the importance of winter bees. We are especially unconcerned late in August, just when the colony is on the brink of producing these winter wonders. On a sultry August afternoon when the cat is long and the air is too hot to move, the next brew may seem more important than the next bee. But that next bee may be the one to shepherd your colony into spring, long after the brew is forgotten.
So what about winter bees makes them so important? And what makes them different from any other bee? According to Remolina and Hughes, winter bees are workers that emerge near the end of the foraging season . Rather than living six weeks like most of their summertime sisters, winter bees may live six months, or even longer. These are the bees that determine whether our colony will survive the winter. And because of that, we beekeepers need to pay them more attention .
What makes a winter bee special?
The main difference between a winter bee and any other bee is the presence of enlarged fat bodies in the abdomen. According to Rosanna Mattingly in Honey-Maker, “the fat body puts together, stores, and breaks down not only fats but also proteins, carbohydrates, and other molecules.”  Fat bodies also produce vitellogenin, an amazing substance that allows a nurse bee to secrete brood food even in the absence of fresh pollen. Vitellogenin also enhances the immune system and increases lifespan.
Winter bees spend their lives within the nest where they care for the queen, help the colony with temperature regulation, and raise the brood that will inherit the colony in spring. Biologists believe that winter bees evolved as honey bees began to migrate into colder climates. In areas where cool temperatures prevented year-round collection of pollen, honey bee colonies needed a system that could see them through the shortage.
Winter bees can be considered a caste
Just as any fertilized egg can become a queen, so can any fertilized egg become a winter bee. Their genetics are identical. In fact, some authors refer to winter bees as a separate caste. A caste in the traditional sense is defined as “a physically distinct individual or group of individuals specialized to perform certain functions in the colony.” The winter bee caste is physically distinct because of the enlarged fat bodies, and those fat bodies have a special function. They produce large amounts of vitellogenin which can supplement or replace a winter pollen supply.
Winter bees are produced when pollen becomes scarce. Just as a queen can be raised by feeding a larva a special diet, a winter bee can be raised by feeding a larva a special diet. But the diet that triggers a winter bee is not extra rich like a queen diet, instead it is extra lean. Larvae fed a diet deficient in protein can trigger the development of winter bee traits .
Because winter bees are produced when pollen is lacking, winter bee production is dependent on local conditions. If you live in an area with plenty of summer rainfall, your winter bees will develop later than someone who has a significant summer dry spell. The important point is that pollen, not temperature, regulates winter bee development .
Without pollen, a colony is nothing
As any good beekeeper knows, pollen is the currency of a beehive. While nectar provides energy, pollen provides everything else. You cannot raise bees or children on sugar alone; you also need protein, fat, lipids, vitamins, minerals, antioxidants, and trace elements. Pollen contains all of these and more.
Without a diverse source of high-quality pollen, a colony will collapse. Such a colony cannot produce healthy offspring and cannot perform the many functions necessary for day-to-day life, let alone prepare for lean times.
However, everyday life in a bee colony is fraught with lean times. Pollen can become scarce during protracted wet weather, dry spells, and certainly over winter. Yet a colony does not store pollen on the same scale as it stores nectar. Most pollen storage is used almost as quickly as it is collected, so how does a winter colony survive?
Enter the winter bee
The answer lies within the winter bee. Although we think of pollen storage as occurring in the combs surrounding the brood nest, winter storage of protein actually occurs inside the winter bee. The enlarged fat bodies, along with enlarged hypopharyngeal glands, provide a vast storehouse for vitellogenin and other materials needed to produce brood food.
This hidden treasure is the reason a healthy colony can produce a batch of spring bees long after the last pollen flow has ceased and long before the new one begins.
Even winter bees have their limits
But just as a pollen cell is limited in size, so is a winter bee. As winter bees begin to feed brood, their fat bodies shrink and the glands produce less. Eventually they can run dry. In many situations, they have enough to get them into spring. But if conditions are bad, if a dry spring follows a harsh winter, the protein may eventually run out.
The possibility of running out has a lot to do with the strength of the colony going into winter, the amount and quality of stored food, the mite load, and the winter weather. For those reasons, many beekeepers find that a supplemental pollen source can make a world of difference in the strength of overwintered colonies.
In an interesting twist, Mattila and Otis found that feeding supplementary pollen to colonies in late summer or early fall did not boost the quantity of winter bees but merely increased the length of the normal brood rearing season . This makes sense if you consider that that lack of high-quality brood food is what stimulates the production of winter bees. As long as good brood food remains available, normal “summer” bees will be produced.
Their findings suggest that supplemental feeding may be more beneficial after the winter bees have emerged. Many beekeepers follow this pattern, waiting until mid-winter before giving supplements. But even giving supplements early can have benefits because increasing the brood rearing season decreases the length of time winter bees need to survive.
Let’s not forget about Varroa mites
While we’re thinking of winter bees, remember that a weak or virus-infected winter bee will be useless to the colony. I cannot stress enough how important it is to have mites well under control before the winter bees emerge. If a winter bee starts life with a viral infection, that bee’s vitellogenin resources will be lost to the colony. In most of temperate North America that means mites need to be treated before the end of August.
Honey Bee Suite
 Remolina SC and KA Hughes. 2008. Evolution and mechanisms of long life and high fertility in queen honey bees. PubMed.gov. doi: 10.1007/s11357-008-9061-4.  Some authors refer to winter bees as “diutinus,” which is a pretentious word for “long-lived.” Use diutinus if you want to be pompous, but use “winter bees” if you want to be understood.  Mattingly RL 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Beargrass Press.  Mattila HR and GW Otis. 2007. Dwindling pollen resources trigger the transition to broodless populations of ling-lived honeybees each autumn. Ecological Entomology. 32:496-505.  Not everyone agrees on what triggers winter bee development. Other theories include the presence or lack of various pheromones in the brood nest.  Mattila HR and GW Otis. 2007. Manipulating pollen supply in honey bee colonies during the fall does not affect the performance of winter bees. Canadian Entomologist. 139:554-563.