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Homestead News:
Thanks so much for the great response to my Hive Jive interview!! I am so delighted that people enjoyed it, and I’m really happy to have new followers. Welcome! I hope you find my podcast interesting and maybe even useful.
Not sure my beans are going to make it. One set eaten by deer, the other eaten by some kind of insect. Sigh! Maybe one or two will make it.
Corn is progressing well. Did you know where the cobbs grow on the plant? I sure didn’t!
Mulching of the beds continues. I started tackling the much neglected side bed, probably to my neighbour’s delight.
Agatha is back on pain meds for the time being. The chickens are slowly beginning to moult and I could tell she was uncomfortable. The meds make a big difference and she seems comfy again. I am still really struggling with controlling lice on her, though. It got really bad again, and I’m getting to the end of my rope. I finally tried applying a very small dose of Frontline Plus (like what you use on dogs for flea/ticks). Fingers crossed it works! I keep considering asking my local crazy chicken ladies if any of them would be interested in adding to their house chicken flock, as I think being inside is the only way to stay 100% lice free for sweet Aggy. But I feel bad essentially pawning her off on someone, especially considering the fact that she requires medication. Also, I’d miss her!
Hive news:
Fall flow has started but the girls are still spicy as heck. I got stung through my suit, which has never happened before; she was so determined to get me! So I’m still behind on all their checks and mite tests.
I was able to see that Hive #1 has had a brood break due to the dearth; I found the queen and she is laying again. I found 1 frame of capped brood and 2 frames of eggs about 10 days ago. Not too bad considering how bad this dearth has been.
09/12. Did a full inspection of hive #2 (Macha, southern queen). Broke the hive down box by box. Girls were defensive but not too bad; no stings (that I noticed), although by the end I had a handful of very determined bees who followed me a good distance from the hive. Things going much better than I had anticipated! More brood than I saw in hive #1, very solid pattern, lots of pollen, and good honey stores as well, even despite the dearth. Found the queen, moved her aside, and did a mite check. 1/300!! Super low for this time of year. I thought it was a mistake at first so gave the sample a longer rinse but that was it; just the one mite. I really hope my other colonies are doing so well. Had to postpone further checks as I had other responsibilities. Hope to get the rest done this coming week.
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The book review that never ends continues!
Chapter 9: Temperature Control
No warmth, no cheerfulness, no healthful ease,
No comfortable feel in any member -
No shade, no shine, no butterflies, no bees,
No fruits, no flowers, no leaves, no birds,
November!
-Thomas Hood, ‘November’,1844
While many insects perish/end their life cycle before winter hits, or lie dormant underground until Spring, honey bees cluster together within their nest, shivering to maintain a steady temperature. When broodless, the core of the winter cluster will rarely fall below 18C/64F, while the outermost layer stays above 7C/44F. The ambient temperature can be as low as -20C/-4F or even colder, and still the cluster temperature remains steady.
When the bees are rearing brood (starting in late winter and ending in early autumn), the brood nest is maintained between 34.5C and 35.5C (94-96F). This temperature will vary by less than 0.5C/1F, even if the outside temperature drops below 0C/32F, or raises above 40C/104F. This level of temperature control is absolutely remarkable!
Studies have shown that honey bee brood are extremely sensitive to even small changes of temperature outside the normal range of 35.5C-35.5C mentioned above. One study conducted by Jurgen Tautz and colleagues examined the effect of small changes in brood nest temperature on the brood and subsequent behaviour as adult bees. They incubated capped brood in incubators set to 32C/90F, 34.5C/94F, and 36C/97F. Each group of bees was carefully labeled upon emergence and introduced to a foster colony housed within an observation hive. A sugar water feeder was placed 300 meters/980 feet from the hive, and the foragers were observed. It was noted that a bee raised at the lower temperature of 32C/90F performed just 10 circuits of the waggle dance after returning to their hive. Those bees raised at the higher temps of 34.5C/94F and 36C/97F performed 50 circuits on average. Interestingly, the bees raised at the lower temperature also demonstrated less precision while transmitting directions via the waggle dance.
Later work looked at the effect of brood rearing temperature on bee brains. Results showed that temperature had a direct effect on the connections between neurons in the centers of information integration (known as ‘mushroom bodies’). These connections were highest in bees that matured at the normal brood nest temperature, and were significantly lower in bees raised at temperatures just 1C/<2F above or below normal.
To fully understand how honey bees maintain a stable temperature within the brood nest, we need to examine how they produce heat, and how heat is lost. Both are equally important when looking at the whole. The processes that affect heat production and loss are fundamentally the same within wild colonies and those managed but how much effort is required for each differs greatly depending on nest structure. Therefore, we see a difference in wild colonies and our managed ones. Overall, colony thermoregulation (or temperature control), is generally easier for wild colonies due to the nature of their nest cavities (the thickness of the tree cavity, the use of propolis to seal all cracks and coat the walls and ceiling, etc).
Those of us who have overwintered our colonies in cold areas already know how important insulation and wind access/draftiness is when it comes to overwintering success. This is because these two things (insulation and draftiness) influence the microenvironment of the nest colony. Increasing insulation and decreasing cold drafts directly slows the heat flow between this microenvironment and the macroenvironment of the outside world. In simple terms, even if the outside air temperature is bitterly cold, a wild colony living in a deep-walled, well-insulated tree cavity will need to produce less heat to maintain the brood nest temperature.
Evolutionary Origins of Colony Thermoregulation
How do bees generate heat to warm their brood nest? They use their flight muscles. Flying is tremendously energy intensive; a honey bee consumes large amount of energy to fly, and in turn produces a certain amount of heat through metabolic processes. Seeley points out that as much as 80% of energy used for honey bee flight appears as heat in the muscles. Interestingly, the rate of heat loss during flight is relatively low, with a honey bee’s thorax reaching temperatures usually 10-15C/18-27F above the ambient temperature.
Not only does flight generate heat but, conversely, heat is essential to allow the process of flight. A honey bee must maintain a thorax temperature above 27C/81F in order to fly. Any cooler and the flight muscles cannot achieve the high rate of wingbeats needed to allow the bee to takeoff and fly. Due to this, honey bees have evolved the ability to warm up their muscles in order to reach the temperature needed for flight. They do this by activating the wing-levator (lift) and wing-depressor (down) muscles simultaneously. These muscles contract isometrically (ie the muscles are active but not changing in length), which results in heat production but very few, or even no, wing vibrations.
It is these very same muscles that are used to heat brood comb. Studies that recorded the temperature of foragers preparing to fly and nurse bees heating capped brood showed identical temperature rises of 2-3C/4-5F per minute in thorax temperature. In both settings, the wings of the bee remain motionless. To warm brood comb, a honey bee will either press her thorax onto the capped cells of sealed brood, or sit in an empty cell surrounded by sealed brood, remaining there for as long as 30 minutes with her thorax heated to 41C/106F, warming the pupae in adjacent cells. She basically functions as a carefully positioned little space heater!
The ability of the colony to thermoregulate has evolved simultaneously with the bee’s unique social life. A group of bees has a greater capacity to produce heat than an individual, and also has a reduced loss of heat per individual bee, as each bee is insulated in part by the temperature of her sisters. In simple maths terms, it’s an issue of surface area. A single bee has an average surface area of 3.8 square centimeters (0.6 square inches), whereas the surface area of some 15,000 bees (average wild colony size going into winter) condensed into a tight winter cluster is about 1000 square centimeters (155 square inches). When a bee is huddled within a cluster, her surface area is effectively reduced to 0.067 square centimeters (0.01 square inches), which is 60 times smaller than when she is standing alone.
Benefits of Temperature Control
The ability to heat or cool a nest cavity provides many benefits to a honey bee colony. We have seen how even tiny changes in brood nest temperature can dramatically affect an adult’s bees foraging and communication behaviours. High temperatures can also weaken beeswax comb, causing the heavy honey-laden combs to collapse, which can lead to many deaths of bees within the nest. The optimal temperature for full activity of honey bees is 35C/95F. Increasing this temperature by just 10-15C/18-27F, for a temperature of 45-50C/113-122F, will be deadly to the honey bee within just a few hours time. This low tolerance for high temperatures demonstrates that honey bees have not evolved enzymes that maintain stability within these higher ranges. This makes sense if we consider that any enzyme that was stable in these temperatures would likely function far slower at the lower, usual temperature that the bees require for maximum, productive activity.
Quick terminology note: what is an enzyme? Defined as: “a substance produced by a living organism, which acts as a catalyst to bring about a specific biochemical reaction” by Oxford Languages.
In simpler terms, an enzyme is a substance (usually a protein) that acts as a little helper molecule. This molecule is fragile and therefore will fail at high temperatures as high temperatures means more activity. High temperatures speed up reaction rates but this in turn puts more stress on the enzyme’s structure, eventually leading to failure/the enzyme breaking apart.
We can see that the significance of adapting to avoid nest overheating is obvious (death) but Seeley asks what then has favoured the evolution of nest warming?
Arguably, the main benefit is faster brood development, as this will allow for rapid colony growth, which is beneficial whenever a colony’s overall population has dropped (after winter, following predation, swarming, etc). Studies, such as one by Vern G.Milum, have found that brood development slows significantly at cooler temperatures. Milum found that brood at the edges of the nest, where temperatures average approximately 31.5C/89F, needed 22-24 days from egg laying to emergence. Brood at the center of the nest, where temperatures were on average 3C/4-5F degrees warmer, only required 20-22 days to complete their development.
Elevated brood nest temperatures also help prevent and manage disease. A study by Anna Maurizio in the 1930s demonstrated that chalkbrood (fungal disease) cannot infect brood maintained at a steady temperature of 35C/95F. Allowing the nest to cool to 30C/86F resulted in successful transmission and infection of chalkbrood.
A more recent study by Phil Starks identified a ‘brood comb fever response’ by honey bees when the brood nest is exposed to chalkbrood spores. Dead larvae covered in the fruiting bodies of the chalkbrood fungus were ground down and added to a sugar solution. When fed to colonies, it was noted that the exposed colonies raised their brood nest temperature by nearly 0.6C/1F. Considering that the normal range of brood comb temperature has a 2C variation (34C-36C), we can see that this seemingly mild increase in temperature is actually highly significant. It also appears to have been effective, as Stark reported that none of the colonies exposed to the fungus were infected.
Sadly, the effect of high brood rearing temperatures on viruses is currently unknown. However, studies of other insects have shown that increased temperatures can assist in preventing viral transmission so it is possible that this adaptation by honey bee colonies also assists in their resistance to viruses and disease overall.
Finally, as previously discussed, we know that the ability to heat the nest cavity has hugely benefited honey bees in their move to less temperature climates. The honey bee is a tropical insect but the ability to maintain a steady heat during even bitterly cold external temperatures has allowed this remarkable insect to survive all over the world.
Warming the Colony
As mentioned previously, a winter cluster of honey bees maintains a core temperature of 34-36C/93-97F when they have brood, and 18C/64F when they do not. The outer layer of the winter cluster is maintained steadily above 8C/46F. These lower temperatures are critical, as bees at temperatures below 18C cannot activate their flight muscles to produce more heat, and bees cooled below 8C become unable to move entirely and enter what Seeley describes as a ‘chill coma’. A bee subjected to temperatures at or below 10C/50F will perish within 48 hours.
Fundamentally, a colony maintains a study nest temperature by controlling both heat production and heat loss. The winter cluster loses heat by:
Conduction (through ceiling of the nest cavity and via the combs)
Convection (by air currents moving through and within the cavity and cluster)
Evaporation (respiratory evaporation from the adult bees, surface evaporation of the brood, and any damp combs)
Thermal radiation (heat radiates out from the cluster through the unoccupied combs surrounding it, as well as the nest cavity’s walls)
A diagram of heat loss from a colony overwintering in Langstroth hive (as seen above) demonstrates these modes of heat loss. Interestingly, it also shows that the temperature of the very bottom of the hive was the same as the outside (-21C/-6F), while the air around the cluster at the top of the hive was much warmer (-1C/30F).
A honey bee colony has two primary ways to reduce heat loss to the surrounding environment:
By reducing heat loss from the colony to the nest cavity
Reducing heat loss from the nest cavity to the outside environment
The rate of heat transfer from the cluster and the nest cavity increases proportionally to the difference in temperature between the inside and outside. Knowing this, we can see that insulation provided by the nest cavity plays a key role in minimizing heat loss. Thick, well insulated walls will have a lower rate of heat transfer to those that are thin or cracked.
Honey bees also decrease this rate of heat transfer by the mere act of clustering. When the temperature within the nest cavity drops below 14C/57F, the cluster is formed. If the temperature continues to fall, the bees press together more closely thereby shrinking the overall size of the cluster. At around -10C/14F, however, the cluster contraction reaches its limits and no further reduction in size is possible. Between the onset of clustering (at 14C) and the limit of cluster size reduction (-10C), the honey bee cluster shrinks almost 5 times its original size! That’s an exceptional reduction in size and therefore surface area. This decrease in surface area helps decrease heat loss by thermal radiation. The tightly pressed together bees also reduce heat loss by convection (air currents) since there is less space for air to move through the cluster, and the outer mantle of said cluster forms a tight barricade of little bodies to block air from entering the inner layer. This method of reducing heat loss by clustering is so effective that a study by Edward E.Southwick demonstrated that this rate of low heat conduction actually matches, or is sometimes even less, than that of birds and mammals of equal weight to the cluster. This means that the outer layer of bees is as effective at preventing heat loss as the feathers of birds and fur of mammals. Incredible!
As mentioned previously, the thickness of tree cavities used as nest sites provides valuable insulation to the colony. The thick bark of the tree conducts heat very slowly so the warm air of the internal cavity is slow to cool. One benefit of this slow rate of heat loss is that the bees inside the nest cavity can remain active through more of the winter months, making it easier for them to have access to the honey stores needed for their survival. Conversely, bees that nest in such environments might be slower to wake in the spring as the thick walls of the tree are also slower to heat; once they become cold, it takes longer for them to warm up, which could delay spring emergence of the colony.
In contrast to these thick walled nest cavities, we know that our man made hives provide very little insulation for the bees, who must cluster tightly in order to survive the cold temperatures outside, which are more quickly affecting the internal temperature. This means that the internal microclimate of a man made hive versus that of a wild colony’s nesting cavity will be affected differently.
To further explore this difference in microclimates of the hive, Seeley prepared a study with his colleagues, Robin Radcliffe and Hailey Scofield, to monitor the temperature of two different nesting cavities; one with thick walls, and one with thin. This study was actively ongoing at the time of publication so the following is all in present tense, although it is likely this study has since been concluded.
One cavity was built using standard pine lumber, and the other was cut into the trunk of a large super maple. Both cavities are identical in size and shape (tall and narrow), volume (50 liters/13.2 gallons), and entrance size (5cm/2in). The key difference between them is the thickness of the walls; one has walls a mere 2cm/0.75in thick, and the other walls that are 36cm/14in thick (like those of the average nest cavity). They are located next to each other, each containing two temperature sensors and recorders, which are positioned in the center of the cavity. A sensor and recorder is also placed between the two colonies to measure ambient temperature. The goal of the study is to measure the internal temperature of the nest cavities over a period of two years; one year with no heating, and one year where each cavity contains a 40W heating element to simulate that heat generated by a 2kg/4.4lb colony of honey bees.
Initial results taken during two weeks in April 2018, show interesting differences in the two different nesting cavities. The tree cavity internal temperature is overall more stable; growing less cold at night and less hot during the day. The thin walled hive box, by contrast, had an internal temperature that exceeded that of the ambient on warm days, and showed sharper drops in temperature at night. Seeley notes that it’s too early to tell what overall results this study will yield but he already feels that it is showing promise in helping us better understand how a colony’s nest cavity aids in its winter survival.
Great insulation alone, however, cannot keep a living system warm; some heat must be generated. We now know that a worker bee generates heat by isometrically contracting her flight muscles, raising the temperature of her thorax to as much as 40C/104F. A honey bee colony uses both heat production and heat loss reduction via clustering. As ambient temperatures drop from 30C/86F to around 15C/59F, heat production increases. It then remains relatively stable until the ambient temperature drops below 10C/50F, at which point the bees once again increase their heat production. However, once the ambient temperature drops below 15-10C, heat production declines. Why? Because this is the temperature at which the bees form a well-insulated, tightly pressed cluster that resists further heat loss. Since the cluster continues to shrink down until the temperature is around -10C/14F, reducing heat loss continues until this temperature is reached, while heat production is generated. Seeley posits that perhaps the reason that colonies do not cluster at higher temperatures is due to it limiting colony activities such as foraging and food storage.
Cooling the Nest
Earlier in the chapter, we learned how sensitive honey bee larvae is to even small changes in temperature. Just as the brood nest needs to be warmed at times, it also needs to be cooled. If the brood is subjected to temperatures just 2-3C/3-5F above 36C/97F, their development will be disrupted and they can perish. One cause of colony heating is simply the metabolic processes of all the bees, including those incubating. Thin walled, man made hives also face the threat of high outer temperatures heating up the internal cavity during the warm/hot months.
Honey bees are just as good at cooling a nest as they are of heating it. A study conducted by Martin Landauer in Southern Italy demonstrated just how stable honey bees can keep their internal nest temperatures even when subjected to extremely hot conditions. He placed a colony of bees within a thin walled, wooded hive on a lava field in Salerno, Italy, that receives a large amount of sunlight each day. He found that the colony’s internal hive temperature never rose above 36C/97F, even if the outside temperature soared to a whopping 140F!! Landauer had also placed an empty hive nearby and recorded that its internal temperature reached 106F so we can see that the bees are actively cooling their nest.
How do bees do this? They use 3 primary cooling mechanisms:
Spreading out the adults within the nest, and partially evacuating it (reducing internal heat production, increasing heat loss via convection)
Fanning (forced convection)
Spreading water on the combs (evaporative cooling)
Amazingly, fanning bees are deployed through the nest in ‘chains’ that are aligned such as to drive the air along existing currents. Fanning bees will also stand outside the opening of the nest, abdomens pointing away, to pull the warm air from the nest to the outside. This seems so clever and sophisticated to me!
Jacob Peters and colleagues at Harvard University have studied the velocity of airflow at the hive entrance, finding that it can be as high as 3 meters per second (10 feet per second)! This high velocity is only present when the brood nest temperature is reaching dangerous levels. The volume of airflow has also been studied. Engel H. Hazelhoff constructed a hive with two openings (one at the top and one at the bottom), connected to an airspeed indicator (anemometer). With this set up, he was able to measure airflow through the hive produced by the fanning bees. At one point, he observed 12 bees spaced evenly across the 25cm/10in wide hive entrance, all fanning steadily. The airflow rate at this time was up to 1.0-1.4 liters/0.26-0.37 gallons per second!
Hazelhoff also discovered that high levels of carbon dioxide within the nest cavity will trigger fanning behaviour, which means that fanning is also a behaviour that benefits the colony’s overall respiration (not just as temperature control). The average carbon dioxide levels within the nest cavities when bees are not actively fanning is 0.7-1.0%, which is 20-30 times greater than normal air percentages (0.03-0.04%). This means that the internal nest cavity is much ‘stuffier’ than one might expect, and further demonstrates how incredibly well adapted the honey bee is to live in these densely populated social structures.
Previously, we learned how the entrance of wild honey bee nests are relatively small, about 10-20 square centimeters (1.5-3.0 square inches). This would seemingly provide little natural airflow so how do bees ensure they ventilate their nest cavity as needed? Jacob Peters and his colleagues sort to answer this question and found that fanning bees position themselves asymmetrically around the nest entrance so that the air enters and exists continuously around its perimeter. Honey bees sense the air temperature (highest where the air exits) and position themselves with the direction of the airflow wherever the air is hottest. This means the bees use the airflow, not other fanning bees, to maintain the inward and outward flow of air through the nest entrance.
At times when increasing airflow is not enough to cool the nest, honey bees use evaporative cooling by dispersing water droplets across the comb. To turn from a liquid to gas, water uses a great deal of heat, thus removing it from the nearby environment. Think about the process of sweating; it is triggered when we are hot and aids in cooling our skin and, over time, our core temperature.
Just as foragers will gather pollen and nectar, they will also seek out water to bring back to their colony. In fact, recent studies have shown that some foragers specialize in water collection, and will travel as far as 2 kilometers/1.2 miles to a water source. Water is essential to a honey bee colony, not just for cooling, but to dilute stored honey, produce brood food, humidify the nest to prevent developing brood from becoming too dry, and as a source of hydration for the bees themselves.
Seeley recounts a time in winter when he noticed the colony that lived in his office observation hive making multiple trips out and back on a mild day. At first, he thought they were simply going on cleansing flights until he noticed a few bees performing highly vigorous waggle dances. He saw at least 339 repetitions of the waggle dance; the most he’d ever witnessed. He was able to discover that these excited bees were transmitting the coordinates to puddles of melted snow in the parking lot, and water collectors were soon zipping back and forth, only to be mobbed upon return to the hive by many thirsty bees eager for a drink.
Is this level of winter thirst normal for bees? It seems so. A beekeeper in Scotland, Ann Chilcott, recorded bees collecting water in January and February, even on overcast days, as long as the temperature was above 4C/39F. Similarly, Helmut Kovac and colleagues (at the University of Graz, Austria) measured the thorax temperatures of wintertime water collections using an infrared camera. While loading up on water, the forager activates her flight muscles to keep her thorax temperature consistently above 35C/95F. This was observed even in temperatures as cold as 3C/37F! Of course, when we consider that flight is inhibited in bees when their thorax temperatures drop below 25C/77F, this shivering behaviour to maintain heat makes perfect sense and demonstrates a key adaptation during the honey bee’s evolution.
Knowing that bees grow thirsty over the winter months, Seeley wondered if condensation within the nest cavity might be beneficial to the colony, as opposed to the detriment that beekeepers often see this as. Perhaps this would explain why so few wild colonies have nest cavities with upper entrances, which would allow warm, moist air to escape. One researcher, Derek Mitchell, notes that a well insulated nest cavity without a top vent hole will not have cold condensation drip down upon the cluster. The temperature of the ceilings and walls above the bees is above the dew point so condensation will not form here but on the lower, cooler walls below the bees. Perhaps this condensation then acts as a source of fresh water for the thirsty, over wintering bees. This could also explain why bees cover the walls of their nests with propolis as the water droplets will not soak into the wooden walls, and instead slide off and down.
Seeley decided to investigate how water collectors within a colony are galvanized to begin their vital task. With the help of two of his students (one undergraduate, one PhD), he moved a colony into a glass walled observation hive, which was placed into a greenhouse, allowing them to completely control the bees’ access to a water source. Only one source of water was provided and was placed on a set of scales so that they could monitor its drop in weight as the bees collected water for the colony.
To stimulate water collection, they heated the hive with an incandescent lamp. As bees began to visit the water, they were collected and marked to allow for easy identification during the study. By watching these marked water foragers, it was noted that water collection began approximately one hour after heat stress (the lamp being turned on) began. This indicates that the bees were not responding to the internal nest temperature, as this is insufficient time for it to have heated considerably. Instead, it appears as if the bees are activated by personal thirst and/or the thirst of other bees, begging them for drinks of water.. A previous study by Seeley and Susanne Kuhnholz found that an active water collector keeps informed of her colony’s water requirements by what she experiences upon returning to the hive. When she returns, she looks for receiver bees to take her water load. In times of need, there are more bees to receive and few, if any, rejections of the water. In times when water is not as vitally required, she will face a longer time finding a receiver bee or experience more rejections. In this way, she can easily address her colony’s water needs.
Do honey bees store water for later use? It is not common but some studies have reported finding small amounts of water stored in comb during periods of drought in Australia and South Africa. Bees can also store water in their crops (honey stomachs). O.Wallace Park documented his observation of bees acting as water reservoirs during Spring in early Iowa when water sources were scarce. Water collectors were noted to fly out en masse, returning to give these reservoir bees their water loads. Seeley adds that he has also found water-filled bees standing quietly on combs after a day of heat stress. It is likely that this temporary storage of water in both reservoir bees and comb plays an important role in colony function and survival.
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And that’s it for chapter 9! Next up, we will learn all about defense, and then we will be at the final chapter on ‘Darwinian Beekeeping’.
Thank you so much for reading along with me, and for tuning in. I hope you are all staying safe and happy out there. Please remember to wear your mask in public and avoid crowded areas! We can only get through this as a community working together so it’s especially important to follow health guidelines and not become complacent.
You can find me on all the social media platforms, or email me at homesteadhensandhoney@gmail.com I love to hear from you!
As always, hug your hens and then wash your hands. Take care!
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