Introduction
It’s the middle of April and we have not had consistently warm weather yet this year up in Nairnshire. One day the temperature reaches 18 °C then drops to 8 °C the next. The sun is bright today but it is nearly noon and only 6 °C. I can’t be alone in fantasising about moving to a warmer country. A bit late for that though. I have vivid memories of living in Australia and walking to work in summer when 40 °C felt like being assaulted on all sides by fan heaters sapping the energy out of you.
I did nurse training in Wagga Wagga and moved out of the nurses home fairly early on to a newly built little apartment on a housing estate. I can remember the address which was 8, Veale St. Even more vividly, I remember that there was no air conditioning and one night in summer it was impossible to sleep. Flatmates Kath, Lesley and I pulled the sheets off our beds and soaked them in the shower before hanging them over the open door and windows. We hauled our mattresses over near the door and got a little bit of comfort from evaporative cooling. I don’t want to be that hot again but a bit of heat here would be lovely.
I was going to post this article last week, but somehow it disappeared and I couldn’t retrieve it from the depths of the ether. Kevin Dolan, from Ontario, has helped me reconstruct my post, since he just happened to be studying the same topic at the same time. So, consider this a joint effort. Thank you, Kevin, for contributing and sending over your gleanings from Dr Keith Delaplane’s Honey Bee Social Evolution.
How Honey Bees Thermoregulate in Heat
What about honey bees? How do they cope in the heat? For starters, they are careful with their choice of nests in the wild and usually choose well insulated tree cavities with thick walls and small entrances. This choice was developed over millions of years when bees moved from ground tunnels/nests to cavity structures to allow them better control over their environment. They arrange their comb in such a way as to assist them with thermoregulating the brood nest. In early spring when brood rearing starts, they maintain the temperature of the brood area at between 30-35 °C which is the optimal range for this time of year1. In summer, the brood nest is kept within a range of 33.2- 36 °approximately with an optimal temperature of 34.5 °C and very narrow fluctuations. Starks et al.2 report increased brood mortality when they are exposed to nest temperatures above 36 °C. Development time is delayed and can result in malformed, legs, wings, stingers and proboscises.
We know that a colony can spread out when it gets hot and some honey bees will beard on the outside of the hive. Bearding is sometimes mistakenly seen as a precursor to swarming but it is a natural way to open up the brood nest and dissipate built up heat. The colonies above are hot despite being up on a Scottish moor to make heather honey. It is one of the hottest days in August and the air is unusually still so they are bearding.
Heat Shielding
Like me in Australia with the wet sheets, honey bees employ evaporative cooling strategies. They collect more water and paint it on combs and walls and spread it about the hive. When it gets really hot specialised ‘water bottle’ bees will bring back water to store in their honey crops and release to nest mates as needed. Honey bees fan their wings and circulate the air which also helps cool the hive.
Starks et al.2 discovered a fascinating nest cooling strategy back at the end of last century called heat shielding. We know that specialised heater bees are employed to raise the nest temperature by placing themselves inside empty cells in the middle of the brood area. They isometrically contract their wing muscles and create heat. We can think of the roles of heater bees and heat shielding bees as being the opposite in terms of outcome.
What happens in heat shielding is equally fascinating. Young, and otherwise unemployed, worker bees are activated to move onto the brood and protect older larvae and pupae from excessive heat. They move to the hottest parts of the brood nest and place the undersides of their abdomens (ventral surfaces) next to the heated areas so they are between the heat source and the brood and acting like heat-absorbing protective barriers. Adult worker bees can cope with temperatures up to 50 °C, whereas the brood must stay within a very narrow range. Drones cannot cope with excessive heat and succumb easily to overheating. They actively avoid areas of the brood nest that requires heat shielding.
Heat shielding bees are aged between 12-14 days and their numbers increase with the rise in temperature and size of the colony. It is thought to be an adaptive behaviour to protect temperature sensitive brood. But how do they cool off themselves and shed the heat? Bonoan et al.3 discovered that heat shielding bees move away to cooler areas of the hive and transfer the heat by conduction.
Another form of evaporative cooling that is exhibited in very hot situations is known as ‘tongue lashing’. It is the closest thing to sweating in humans. It happens when a bee regurgitates some water or nectar from her crop and spreads it over her expanded proboscis (tongue) and exposes it water loss by evaporation to surrounding air. This is similar to Ann soaking the sheets in water to cool the room in Australia. Bee use tongue lashing to convert nectar into honey by reducing the moisture, but in this instance, it is used to cool the nest4.
Beekeeping Management Strategies
Although some parts of the UK may experience high summer temperatures, it is unlikely that too many colonies in the north will need to employ heat shielding bees yet. But who knows about the future and how climate change will affect us. We know for sure that many parts of the world already experience tremendous heat waves and high summer temperatures and beekeepers will be dealing with this somewhere right now. What can they do to help their bees? Firstly, they can take heed of how bees in the wild usually select their nests to ensure well insulated hives with small entrances all year round. It is worth remembering that air conditioning systems work most efficiently when nobody opens a window and lets hot air into a room. The bees are working flat out to air condition their nests so we mustn’t let the hot air in either by opening up extra ventilation holes other than the usual hive entrances. It will also help to only inspect colonies when absolutely necessary during heat waves, and only during the least hot part of the day. Providing shelter from the sun and a water source is also essential for managed colonies in a static apiary but may be more difficult to do in migratory beekeeping.
Beekeeper Care
Beekeepers need to remember to look after themselves during summer heat and wear lightweight protective clothing, carry drinking water and only inspect a colony if absolutely necessary in the coolest part of the day. My bees and I can cope with Scottish summers but my assistant during the pandemic found it all a bit too much!
Summary of Practical Tips
Kevin has summarised the practical tips below. Honey bees have developed the ability to keep the brood nest warm in winter and cool in summer for maximum brood rearing and overall survival of their colonies. As beekeepers facing increasing climate challenges, what are some additional things that we can do to help our bees cope with summer heat?
- Ensure the bees have access to adequate water sources, natural or provided, ideally within 50 feet of apiary.
- Hive placement planning; with sun in the morning and some shade in the afternoon a good option in hot areas.
- Place two inches of foam board insulation on the top of the hive year-round to help with cooling in summer and heat preservation in winter.
- Consider a top entrance in the heat of summer to aid in bee fanning and air flow.
- In really hot weather, if possible, providing some sort of shade is helpful, but not always practical. White lids can reflect some sunlight off the hive.
- Ensure bees have enough space to avoid overcrowding—honey supers help with this.
- Monitor bee bearding if it looks excessive as it may indicate the need for better shading or ventilation.
References
1Winston, M. 1987. The Biology of the Honey Bee, Harvard University Press
2Philip T. Starks, Rebecca N. Johnson, Adam J. Siegel, Meridith M. Decelle, Heat shielding: a task for youngsters, Behavioral Ecology, Volume 16, Issue 1, Jan./Feb. 2005, Pages 128–132, https://doi.org/10.1093/beheco/arh124
3Bonoan, R.E., Goldman, R.R., Wong, P.Y. and Starks, P.T., 2014. Vasculature of the hive: heat dissipation in the honey bee (Apis mellifera) hive. Naturwissenschaften, 101(6), pp.459-465..
4Delaplane, K. 2024. Honey Bee Social Evolution: Group Formation, Behaviour and Preeminence. John Hopkins University Press.
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Great piece, good tips at the bottom.
In my study (20+ yrs ago) I compared a hive insulated with 4 inches of Kingspan to an uninsulated hive. I found that the insulated hive maintained a temperature variation of only 2 degrees Celsius between day and night on the crown board. In contrast, the uninsulated hive exhibited a temperature fluctuation of 19 degrees Celsius over the same period.
This significant discrepancy suggests that the insulated hive experiences greater thermal stability. Consequently, the uninsulated hive likely requires the bees to expend more energy to regulate the internal temperature, particularly during periods of extreme heat. This increased metabolic demand may lead to heightened stress on the colony.
Moreover, my anecdotal observation about the temperature of the hive roof on warm days reinforces the importance of adequate insulation. When inspecting hives on warm days, it is beneficial to assess the underside of the hive roof, as you may find that it is significantly hotter in uninsulated hives compared to those with proper insulation. This simple check can further illustrate the critical role that insulation plays in hive temperature management, ultimately affecting bee health and productivity.
Keep insulation on 24/7/365 it’s a good tip.
Hello, David, and thank you for commenting and contributing. You have been ahead of most of us with the insulation theory and it is fascinating to me that you experimented on this long before Derek Mitchell’s work shed new light on colony thermoregulation and hive management around 2016. Hive insulation is still a poorly understood topic in some quarters, and you still hear the mantra “it’s not the cold that kills the bees, it’s the damp” parroted at gatherings of beekeepers. That and the matchsticks under the crown board for winter. It would be great to have a blog post about your experiments if you have time?
I’ve been thinking back to some experiments I did years ago, which started as thought exercises but mostly ended in disappointment. I never documented them, but with more time on my hands now, I’m considering how to monitor thing’s. If you have any suggestions, let me know!
One key observation I made while exploring insulation was about mildew on crown boards. I noticed that the black mildew on crown boards that appears at the corners—where cold meets heat-laden moisture during winter—completely disappeared with well-insulated crown boards.
I’ve since switched to using polycarbonate crown boards made by a colleague at the Bee Sanctuary. Unlike the clear (uninsulated) crown boards I used initially, which showed a lot of condensation, the polycarbonate (insulated) ones show none, which is a huge improvement.
I’ll work on a plan to monitor and document temperature differentials and will get back to you!
Hello, David. This is indeed good news that you will experiment,document and share the information with us. What you mention is the topic that I am currently most interested in. Despite insulating the crown boards, beekeepers are telling me that they still get mould on the outside frames. I also have converted to clear polycarbonate boards because I can easily see what is happening without too much disturbance. Nice too when you just want to show children some bees at work without opening the hive up.
One of the things that I would like to better understand is hive humidity and the internal dynamics of its maintenance, and the significance of measuring it because I notice that hive humidity levels rise noticeably during a nectar flow.
I like Theresa Martin’s Edward Deming quote, “without data, you’re just another person with an opinion.”
Best wishes, Ann.
Thank you, Ann, for sharing the information about workers performing “heat shielding.” I did not know about this. The behavioral flexibility of worker bees is amazing.
Hello, Tom. It is pretty amazing isn’t it that we continue to discover more about honey bees when so much work has already been done. This is probably why so many of us are so drawn to these fine little animals and can never say we know it all.
I was lucky enough, when starting my beekeeping 20 years ago, to receive a gift of 4 sets of good solid WBC lifts and roofs from my uncle who’d decided to give up.
They were easily cleaned, sterilised and painted. I bought 4 sets of national boxes from our annual bee equipment auction; they fitted very well inside the WBC lifts, and that’s what I’ve been using since then with a couple of woollen carpet squares on top in winter.
An elderly beekeeper I met told me that he always stuffed a few old jumpers between the lifts and the boxes in winter. In Somerset damp is a bigger problem and the lifts and the space round the boxes keep them beautifully dry all year. No damp, no mould, a few spiders overwintering under the roof, but I’ve never found them a problem.
Thank you for this blog – it’s delightful, and very helpful.
Hello, Judy. What a brilliant idea. Thank you for your kind comments on the blog and for sharing your excellent tips. I have one very pretty ornamental WCB hive so I might try that myself. Best wishes, Ann.