Wintering success in beekeeping is dependent upon a multitude of factors. Good overwintering colonies start with good queens, a healthy, relatively mite-free worker population, and adequate honey stores which bees need during the coldest weather and to begin rearing brood after the winter solstice. Bees that age evenly over the winter, and disease-free early spring brood, are additionally important to winter survival.
Overwintering success also improves with the size of the population, at least to a degree. Colonies can be too large as well as too small. This concept is the Goldilocks effect — we want colonies to be “just right” to survive, whatever the winter has in store for them.
Studies of colonies in central Pennsylvania (at Penn State) found overwintering success not influenced by a particular stock or region of stock origin; the weight and population size the colonies reached prior to winter was most critical. Higher colony weight was a strong predictor of overwintering survival. Although the number of locations used in this study was limited, the difference in average colony sizes may be attributable to the abundance and diversity of seasonal floral resources (landscape nutrition) within the flight range of colonies. (Döke, et al. 2018 Jour, Econ. Entomol. 112 DOI:10.1093/jee/toy377)
In a different study, beekeepers across Pennsylvania (1,429 honey bee colonies within 257 apiaries over three seasons) supplied information on colony losses. The four most important variables in predicting winter colony survival were growing degree days (a measure of heat accumulation in the environment), maximum temperature and precipitation during the warmest seasonal quarter and precipitation during the wettest quarter. The bottom line: Winter success, at least in the Northeastern U.S., is most strongly related to summer temperatures and precipitation in the prior year. As to how, the researchers concluded it was most likely through floral resource (landscape nutrition) availability. (Calovi, et. al. 2020. Scientific Reports 11 Article 1553)
How’s your weather?
Wintering in many areas means changing weather. Winter can bring cold windy spells, snow or freezing rain, and hopefully intermittent bright sunny periods. Beekeepers express this as their “normal” weather — “If you don’t like the weather, wait 10 minutes and it will change.” Such changes make for tougher wintering by colonies. Wet winters, with widely fluctuating temperatures, are more stressful on bees than dry seasons with a more consistent temperature.
During mild spells, the bees may be too active and heavily consume stored honey and pollen before fresh pollen and nectar become readily available. A “false” spring might jump-start colonies into premature brood rearing expansion. Just as we might not know how to dress for weather changes, the bees too might struggle with frequent or rapid changes in temperature and moisture.
Fat bees/fat colonies
To enable bees to suspend the aging process to live through winter, they raise special wintering bees (we call them “fat” or diutinus bees). These bees are physiologically different from summer bees. We cannot physically see that bees are fatter — the term refers to bees having more fat body stores. The bee’s fat body is a tissue, somewhat analogous to the human liver. It is found beneath abdominal tergites and in the bee’s head. In addition to breaking down toxins and storing nutrients, honey bee fat bodies produce antioxidants and help to manage the immune system.
The fat bodies also play a key role in the process of metamorphosis, regulating the timing and activity of key hormones, and even are responsible for producing the wax that covers parts of a bee’s exoskeleton, keeping water in and diseases out. Dr. Sammy Ramsey, the discoverer that varroa mites target fat body as their food source while on the adult bee body, says, “Losing fat body tissue impairs a bee’s ability to detoxify pesticides and robs them of vital food stores. The fat body is absolutely essential to honey bee survival.” (Ramsey, et. al. 2019 Proc National Academy Sciences)
A major component of fat body is vitellogenin, a phospholipoglycoprotein, meaning it has properties of sugar, fat and a protein. It is used as a biomarker for honey bee health. Vitellogenins are widespread in animals as a female-specific precursor protein of egg yolk. Vitellogenin is stored in fat body of the bee adult; bees use it as a food storage reservoir — to store energy.
Fat body is an important antioxidant to prolong queen bee and worker life — the fat fall worker bees extend their life for months compared to the normal adult worker lifespan of only 4-6 weeks. It functions like a hormone, with elevated levels in nurse-age bees; as bees age it decreases so it likely plays a physiological role of lifestyle change from nursing to foraging.
It also influences what foragers collect. A nurse bee’s vitellogenin titer develops in the first four days after emergence and affects its subsequent age-related duties. If young workers are short on food their first days of life, they tend to begin foraging early (we sometimes label them precocious foragers) and they preferentially seek nectar. If moderately fed, they forage at normal age on nectar while if abundantly fed, their vitellogenin titer is high and they begin foraging later in life and are more likely to be pollen collectors. (Nelson, CM, KE Ihle, MK Fondrk, RE Page Jr., GV Amdam, 2007, The Gene vitellogenin has multiple coordinating effects on social organization. PLoS https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0050062. Reviewed by Randy Oliver in “Fat Bees Part 1,” ABJ August 2007)
There additionally is a mutually inhibitory interaction between vitellogenin and the systemic endocrine factor juvenile hormone (JH), which is central to honey bee reproduction. Vitellogenin and juvenile hormone work antagonistically, regulating honey bee development and behavior. Suppression of one results in high titers of the other. One may surmise that swarming bees would want to pack along vitellogenin to extend their lifespan and to be able to quickly build a new nest.
One of our measures of healthfulness in bee colonies is seeing early 1- to 3-day-old larvae floating on a generous pool of worker jelly. Nurse-age adult bees must be healthy to secrete enough jelly from their hypopharyngeal glands to feed their larvae in August and September. That depends on their having a low varroa parasite level. See 3-part article series on fat bees by Randy Oliver, https://scientificbeekeeping.com/fat-bees-part-1/ (and part-2 plus part-3, which has an update related to sugar syrup feeding), originally published in ABJ (2007).
In addition to fat bees, colonies need to be “fat” on colony stores. A “fat” colony is one that has adequate stored honey, and enough bee bread reserves to sustain the colony during the winter. The adult bees form a “thermoregulatory cluster” around diminishing brood and their queen. Powered by honey stores, they shiver their muscles to produce heat, keeping temperatures at the center of the cluster a comfortable 75 to 93 degrees Fahrenheit, 22-30 Celsius. To compact the brood area, bees backfill cells used in