Temperature Control Part 2: A. mellifera Subspecies & Vs. A. cerana
Variations in temperature control within Apis mellifera subspecies arose in response to their local climate. They still belong under the umbrella term of “A. mellifera: the Western/European honey bee”, and have received more precise classification on the basis of their habits, environment, and/or beehaviour.
For example, honey bees adapted to the Arabian deserts (e.g. A.m. jemenica) have better heat tolerance than those in the Austrian mountains (A.m. carinca), and both are less aggressive than the African savanna bee A.m. scutellata.
This part 2 covers the thermoregulation in A.melllifera subspecies and compares it with A. cerana (spoiler: they share similar approaches to temperature regulation) Go read part 1 for the basic principles of cold clustering, evaporative cooling & fanning- this article is about special adaptations.
There are two shirts that go with this article – designed & pressed by Yours Truly.
Adaptations of Three Apis mellifera Subspecies to Local Climates
To simplify the evolutionary hypotheses: Apis mellifera emerged out of Asia and drifted into Europe after the last ice age some 12,000 years ago1. Since then, they’ve “naturally” spread to different parts of Europe, Africa, and the Middle East. As they adapted to local climates and environmental pressures, minor variations arose in their thermoregulatory physiology and tolerance limits (amongst other non-temperature-related variations that are no covered here). Within I describe three subspecies2: A. m. ligustica (Italian bee; Southern Europe), A. m. jemenitica (Arabian desert bee; Middle East) & (A. m. scutellata (savanna bee; Africa).
A.m. ligustica: Hot, Dry Summers & Mild Winters in the Mediterranean
The Italian bee first sub-speciated (“became its own subspecies”) in the Italian peninsula of the Southern Mediterranean; hot, dry summers and mild winters. As a result, these bees handle warm conditions well, and extreme cold… less well.
Physiologically, they have a (slightly) higher heat tolerance than their counterparts in the Austrian alps, A.m. carnica (Kovac et al., 2014). In the study of Kovac and colleagues, Italian bees endured ~51.7 °C (8-hour LT50, or when 50% population survive after 8 hours “a 50%-lethal temperature”), marginally higher A.m. carnica at 50.3 °C. Their metabolic rate was elevated at higher temperatures, suggesting more activity for performing colony cooling beehaviours; at the individual level more heat shock proteins (HSPs) were activated for heat endurance.
The brood rearing season is longer in their native climate, and A.m. ligustica will continue rearing brood into the autumn, even if the forage conditions are poor (Source: a senior beekeeping club member). They continue focusing on brood rearing, which results in lower honey yields for beekeepers and/or starvation of the colonies due to increased stores consumption. Heating the same number of brood in colder weather is energetically costly, and their winter clusters are either not-tight-enough or too small (remember, Mediterranean winters are mild).
A.m. ligustica’s strength is in warmer weathers – once the spring season starts, they forage intensively, managing overheating by fanning and evaporative cooling, and overall cope better in hot summers than their other commercial counterpart, A.m. carnica.
Survival in the Arabian Desert: A.m. jemenitica
In Rutner’s Biogeography and Taxonomy of Honeybees, he describes A.m. jemenitica as having a (relatively) smaller body than other honey bees, a higher capability to forage at extreme temperatures and ability to withstand drought conditions with minimal food stores (Alqarni et al., 2011). Alghamdi and Alattal suggest that it is a subspecies-specific increase in heat shock protein expression that aides A.m. jemenitica in surviving desert summers (Alghamdi & Alattal, 2023).
The Arabian foragers start accumulating HSPs at (relatively) lower temperatures of ~31°C during the morning periods. The authors state that this sets them up to tolerate the higher daytime temperatures, and minimises foraging disruption (Alghamdi & Alattal, 2023). This adaptation is A.m. jemenitica-specific, as the study compared them to the imported A.m. carnica (recall: better suited to cooler climates). The Austrian bees also had increased HSP expression at different temperature timepoints, however their response was not as strong as A.m. jemenitica’s.
Another adaptation of the Arabian bees is in maintaining osmolarity (a measure of nutrient transportation in the haemolymph), as this increases under dehydration and heat stress (i.e., water loss leads to more concentrated bee blood). Ali and colleagues report that A.m. jemenitica had better control of their osmolarity compared to A.m. ligustica & A.m. carnica (Ali et al., 2017). This suggests that A.m. jemenitica has better heat adaptations, and under such conditions will lose less water than exotic species.
Another study found that A.m. jemenitica queens were better egg layers than A.m. ligusitica under the same heat (Alqarni, 2010). Could this possibly be due to high temperatures making it difficult for the queen to conduct and return from her mating flights? Maybe. Since Her Majesty is genetically identical to her sisters, an A.m. jemenitica queen would have the same water-retentive and heat shock protein-induced adaptations as the worker bees, increasing the chance of successful mating flights. Don’t forget the drones, who are also better adapted to such conditions.
Tropical Climates Enable Brood Rearing Year-Round: A.m. scutellata
The African honey bees A.m. scutellata are adapted to tropical-subtropical climates: the wet, the dry, the hot and the humid. Pure A. m. scutellata workers survive heat better than European bees (Heinrich, 1979). They also commence foraging at a higher temperature threshold, suggesting better tolerance to warmer hive conditions (Heinrich, 1979… yes he is describing A.m. adansonii instead… still an African bee, right?).
Unsurprisingly, this gives them poor cold tolerance, and this was determined by examining their hybridised counterparts, the Africianised honey bees (molecular-level studies on pure A.m. scutellata are hard to come by). They cannot form a tight, long-sustaining winter cluster like European bees do (Felciciano-Cardona et al,. 2020). A. m. scutellata (and its Africanised hybrids) are not naturally found in regions with harsh winters as they do not have survival mechanisms for these extended cold, forage-less periods… Relief for those worried about the spread of Africanised honey bees in the “New World”- they are naturally restricted by the Earth’s latitude, and their survival decreases the further away they are from the equator. Of course, there is the unknown factor of Climate Change™, and contrary to the hysteria, is probably not an issue for my dear readers’ lifetime.
*Ahem*We know that A.m. scutellata’s behavioural adaptation to extreme or extended stressors (rather than heat alone) is absconding. Rather than endure the heat or poor forage, the whole colony leaves in search of better conditions- the African colonies are sized with this escape mechanism in mind, tending to be smaller than their European counterparts, and also produce drones sooner (e.g. A.m. capensis, as described by Spiewok et al., 2006). The colony holds fewer honey stores and they nest in smaller cavities, relying on mobility to cope with environmental fluctuations.
Like A.m. jemenitica, A.m. scutellata is suited for hot climates- they forage in high heat, survive weather and dry periods that would stress a European colony to the point of collapse. Their brood’s heat tolerance is slightly above European ranges (but still within the 32-36°C sweet spot). This points to a trade-off between heat and cold tolerance: those genes that produce heat tolerance in A.m. scutellata make them weaker in cold climates. Pick your battles.
Comparing A. mellifera to A. cerana: Bigger is Better?
A. cerana & A. mellifera share the same methods of temperature management (clustering, shivering, fanning, water evaporation). The Asian honey bee is found across southern and eastern Asia like A. mellifera, also lives in cavities, forms colonies with a single queen, and cooperates in brood rearing. Their body size, colonies, and honey production are smaller, which produces differing approaches to temperature regulation.
A. cerana workers have a larger extreme heat tolerance than A. mellifera, in one study enduring ramping temperatures to 55 °C versus 48°C for A. mellifera (Li et al., 2019). The opposite was true when temperatures were “high”: A. mellifera had better survival, despite losing more water weight. On average, A. mellifera was mostly intolerant at 54-60°C, and A. cerana at 57-60°C. We know that temperature responses are subspecies specific in response to their environment, and this aligns with A. cerana’s natural habitat: tropical and subtropical Asia.
Asian honey bee workers also have better water retention at high temperatures compared to A. mellifera (Li et al., 2019; also recall that insects tend to lose body water as they age). Under identical hot and humid conditions (e.g. 45 °C at 60–70% RH; Li et al., 2019), A. cerana had significantly lower rates of body water loss per hour than A. mellifera. A reduced rate of dehydration helps A. cerana avoid desiccation in hot conditions, contributing to their (higher) heat endurance. Could this be a behavioural change, in that they reduce activity to conserve water and energy, or a physiological shift, such as a modification in the cuticle (“exoskeleton”) to reduce water loss?
A. mellifera has Better Cold Tolerance
After reading the above and taking into account their geography, it would not surprise you to know that A. mellifera has better cold tolerance than the Asian honey bees. Colonies of A. cerana are smaller (few thousand workers versus tens of thousands in A. mellifera) and thus have fewer bees to generate and store heat. They do form winter clusters in the colder parts of their range (like northern China, Korea, Japan), but these clusters are smaller and cannot survive as long without a break in the weather (as reported in Seely’s The Wisdom of the Hive).
From Seeley’s aforementioned work we know that A. cerana times its brood cycle to climate: in Japan, he observed that A. cerana colonies would raise their cluster core to ~34 °C around the winter solstice to start a small early brood nest, producing around a hundred bees, and them ramping it up as the spring progresses. Come late spring, they’ll reach full size. This constant egg laying is a small commitment on the part of the hive in case of a sudden cold snap- simply drop the small investment and start over. Is this conservative approach to brood rearing in alignment with its lesser capacity to heat a large cluster? Maybe, I would also look at it from a resource (foraging) and defence perspective: smaller colonies, smaller output.
Heat as Defence: Bee Balling in A. cerana
A “famous” beehaviour unique to A. cerana (and related Asian Apis species) is their defence against hornets via heat balling. When a hornet invades an A. cerana hive, hundreds of workers swarm the predator and collectively vibrate to heat it up; they can raise the temperature inside the ball to ~48 °C, lethal for the hornet, tolerable for the bees3. A. mellifera lack this adaptation and are vulnerable to hornet attacks when used in areas where they are present. As mentioned above, they have a lower heat tolerance than A. cerana, so the ~48°C needed to kill a hornet would also be lethal to them (one study reported that A. mellifera bee balls only achieve 44°C; Papachristoforou et al., 2011).
For A.mellifera, Bigger Colonies Better Withstand Temperature Shifts
In A. mellifera, it is the smaller colony that is more energy efficient than the larger ones, however it is the larger colony that better survives winter, despite being “less efficient”4( Norrstrom et al., 2021). In this Swedish study, smaller colonies of A.m. carnica & A.m. mellifera had the lowest winter weight loss (i.e. honey stores consumption), however they point to other studies whereby larger overwintering colonies led to stronger spring activity (e.g. Free & Racey, 1968), and better winter survivability (Doke et al., 2019).
The aforementioned work of Doke and colleagues suggests that colony size matters more than the subspecies type for overwintering survival, and this is also true for high temperatures (Jhawar et al., 2023). Heat stress causes workers to move away from the brood area and either into the honey comb or outside of the hive. It removes the division of labour: rather than maintaining standard task specialisation (nursing, guarding, undertaking, foraging etc…), the bees shift to heat management – fanning, water collection, water distribution and so on.
Thermoregulation and Beekeeping: Does the Bee Subspecies Matter?
I end with a “no”. Ideally, you’ll want to beekeep with the subspecies that is native to your area. Don’t want to? That’s fine too. From a pure temperature regulation perspective, it does not matter what subspecies you choose as you, the beekeeper, can manipulate the hive environment to better suit the bees. Too cold? Use boxes with thicker walls. Too wet? Provide extra ventilation via hive lid, consider using an inner lid to absorb extra moisture (e.g. woodchips wrapped in a cloth), position hives so that they are sloping a few degrees downwards. Insufficient forage? Provide supplementary sugar and pollen feeds.
Where subspecies selection may matter: the desert regions of Arabia, or the mountains of Austria. I can assure you that “can my bees thermoregulate” is not high up on the beekeeper’s list of concerns when picking a queen to work with. More pressing matters like honey production, brood rearing and aggression take precedent. No point in keeping aggressive African bees in a suburban neighbourhood… Unless you don’t like that neighbourhood!
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Footnotes
- I know, I know – it’s been contested/disproven/fake news-ed and that honey bees are originally from Africa or the Middle East (e.g. Wallberg et al., 2014 or Cridland et al., 2021)- keeping it simple here: A. mellifera, the European honey bee first came to Europe after the last ice age, capeesh? ↩︎
- I originally wrote “The Three Buzzkateers” but no one laughed. ↩︎
- It is not the high temperatures, rather the increased carbon dioxide concentrations that kill hornets. Sugahara & Sakamoto, 2009 reported two phases of temperature ranges in A. cerana bee balls: a rapid rise followed by a plateau at 0-5 minutes, and then a maintenance of that plateau with slight decreases in the second half, at 5-10 minutes. No hornet survived after ten minutes. The authors suggest that these phases are deliberately timed by the bees, who sense the physiological condition of the trapped hornet, maintaining the latter phase as a CO2 endurance test which the hornet does not win. ↩︎
- Your boss wouldn’t be happy to know that “more efficient” and “more effective” are not the same thing. ↩︎