By: DD Burlin
This article originally appeared in the Autumn 2020 issue of BEEKeeping Your First Three Years
There appears to be some debate about the key to longevity in the Winter bee, and studies of differences in bee lifespans have even been conducted and reported in gerontology journals as scientists seek to understand the human aging process. One trusted online source (Rusty Burlew at Honey Bee Suite) described Winter bees as another caste (in her January 2017 blog post), but the scientific journals do not entirely support this notion. So, what are Winter bees? (Let’s leave the human search for the fountain of youth to the gerontologists).
Winter bees can live 100 days on average, as compared to Spring workers who live on average 30-40 days, and Summer bees who live on average a significantly shorter 25-30 days. Within a colony, all of these workers are female, and all are sisters, bred from the same queen, yet their different lifespans vary greatly. With little to no brood production during the Winter, and in order for the hive to survive months of cold temperatures in northern climates, the development of healthy Winter bees with their longer lifespans is critical.
The physiological differences between Winter and Spring and Summer bees include the following:
- The level of Juvenile Hormone is lower in Winter bees than in Summer bees, (a lower JH is generally associated with in-hive tasks);
- Winter bees have enlarged fat bodies;
- Winter bees have enlarged hypopharyngeal glands; and
- Winter bees have a higher level of vitellogenin in their hemolymph.
The fat bodies in bees produce vitellogenin (a female egg-layer specific lipoprotein), which both enhances the immune system and increases the lifespan of bees. Vitellogenin is used in brood food production in the production of royal jelly, as well as in the regulation of foraging behavior. Both queens and newly emerged workers have high levels of vitellogenin at emergence, and while the level drops as worker bees age, it remains high in the long-lived queen. As the level of vitellogenin drops for worker bees a chain reaction occurs in which the level of Juvenile Hormone begins to rise, and the hive bees evolve into foragers.
The gradual replacement of Summer bees by Winter bees has been shown to correlate less to environmental conditions (e.g. temperature, and day-length) and more to conditions within the hive. Winter bees develop while the hive is in a broodless state during the late Summer and early Fall months. But their development must not be solely during broodless states or we would see them develop in hives that are not queen-right, and during dearths when some queens will stop laying.
One can see the effect of the cycle of the seasons: when there is little pollen coming in, foraging decreases (and its related forager pheromone increases as the foragers are all in the hive, and not out foraging), brood rearing decreases as do its attendant pheromones, and Winter bees are produced with their high level of vitellogenin. In the Spring the reverse occurs, and brood development triggers a reduction in vitellogenin due to brood pheromone, which in turn increases the level of Juvenile Hormone which then stimulates pollen foraging.
Specific experiments on the influence of pollen in the development of Winter bees suggest 1) that a reduction in pollen resources triggered the onset of Winter bee production, and that conversely, supplying additional pollen delayed the development of Winter bees (and though there was no impact on the number of Winter bees produced, the additional pollen would result in a somewhat shorter lifespan of the Winter bees), and 2) whether hives were supplemented with pollen, deprived of pollen, or kept as a control, there was no difference in the performance of the Winter bees, and they all had similar physiology and brood-rearing efficiencies. These studies suggest that a beekeeper could prolong the season of Summer bees, and reduce the duration of the Winter bee life-cycle through providing late Summer pollen with no real adverse effect to the number of Winter bees produced, nor an adverse effect in their actual performance as Winter bees in heating the cluster, surviving until Spring, caring for Spring brood, and transforming into foragers once the brood pheromone catalyzes that transformation.
Vitellogenin is used by nurse bees in the production of royal jelly, and since there is no significant brood to feed over the course of the Winter, perhaps rather than thinking of Winter bees as a separate caste (as suggested by Ms. Burlew) we may want to consider that Winter bees are simply nurse bees in a state of suspended animation who have not yet depleted their vitellogenin stores. (Again, we’ll let the experts in gerontology determine whether there is a human parallel in which a woman caring for her children becomes haggard and ages that much quicker than a woman without such duties… I volunteer to participate in the haggard control group of such a study).
While the presence of brood pheromone on its own (with or without the presence of actual brood) has been shown to reduce the level of vitellogenin and the longterm lifespan of a honey bee, all Winter bees die at around the same time, regardless of their emergence date, suggesting an elevation in Juvenile Hormone at roughly the same time among these bees as they begin brood rearing. Furthermore, there is still some uncertainty as to whether Winter bees differentiate themselves from Summer workers during the larval state or at the time of emergence. Doke, et al. (2015) describe the increase in Juvenile Hormone in the Spring as correlating with the decrease in vitellogenin and hemolymph protein levels, and suggest that it is as if the Winter bees return to a forager state in the Spring. So, it appears that Winter bees simply remain in a less mature state while the colony is in a broodless state during the Winter months, and then progress along the typical maturation timeline as brood rearing begins in earnest.
So, what does this mean for a beekeeper in a northern climate trying to keep his/her bees alive until Spring?
When we read that Varroa destructor mite negatively impacts vitellogenin titers, reduces abdominal proteins and carbohydrate levels, and reduces lifespans, and take that information in the context of Dr. Samuel Ramsey’s findings that Varroa feed on fat bodies rather than on hemolymph, we can begin to understand how imperative it is to control mite populations before Winter bees are developed.
A 2001 study conducted by Heather Mantilla in Manitoba, Canada described the gradual replacement of Summer bees with Winter bees in a control hive as beginning on August 31 (suggesting the eggs were laid on August 10), and brood rearing concluding on October 30 (so last egg laid on October 9), whereas hives that were requeened on July 26 did not have Winter bees emerge until September 12 (eggs laid August 22) and brood rearing ending altogether between November 11-23 (laid between October 21 and November 2). Randy Oliver (located in California) suggests getting mite levels in control before August 15, in order to ensure the health of the Winter bee population.
ls there a way to buy yourself more time if the mite load within a hive is high, or you just haven’t quite gotten your beekeeping work done before mid-August? Could raising Winter bees a little later in the season provide a shorter window during which you need to keep those Winter bees alive?
From the research cited above, I propose two possibilities: 1) feeding a pollen supplement in the late Summer could encourage a somewhat longer brood rearing season which would in turn delay the production of Winter bees without negative impact; and 2) because younger queens tend to lay later into the Fall, and the continued existence of brood pheromone delays the development of Winter bees, re-queening a hive with a queen bred during that Summer may shorten the lifespan requirements of the Winter bees.
Am I grasping at straws in my attempt to over-winter my hives? Maybe, but I do treat for mites, and I have seen colony dead-outs in Winter that surprised and dispirited me. I’m ready to try anything.
i. Winkler, et al., “Transcriptional Control of Honey Bee (A. mellifera) Major Royal Jelly Proteins by 20-Hydroxyecdysone,” Insects (2108).
ii. Sedal, Brynem, et al., “Brood Pheromone Suppresses Physiology of Extreme Longevity in Honey Bees,” Journal of Experimental Biology (2009).
iii. Matilla, Harris & Otis, “Timing of Production of Winter Bees in honeybee (A. mellifera) colonies,” Insectes Sociaux (2001).
iv. Doke, Frazier, and Grozinger, “Overwintering honey bees: biology and management,” Current Opinion in Insect Science (2015).
v. Matilla, “Dwindling pollen resources trigger the transition to broodless populations of long-lived honeybees each Autumn,” Ecological Entomology (2007).
vi. Matilla and Otis, “Manipulating Pollen Supply in Honey bee Colonies during the Fall does not affect the Performance of Winter Bees,” The Canadian Entomologist (Aug. 2007).
vii. (Smedal, 2009).
viii. Matilla, (2001).
ix. Doke (2015).
x. Ramsey, et al, “Varroa Destructor Feeds Primarily on honeybee Fat Body Tissue and not Hemolymph,” PNAS (2019)
xi. Matilla, (2001)
