Honey Bee Icarus: Temperature Limitations of Bees in Flight

Text of the article

A summary of recent research

I have often thought of bees in flight on hot days like it must be like riding a motorbike for them, the refreshing air rushing past. But there’s a crucial difference in that they don’t have a gas motor doing the hard work for them, so it may be more like going on an energetic run. Not so fun on a 100-degree plus day. Bees are well able to regulate the temperature inside hives, and as individuals can generate heat by “shivering” their wing muscles, but what happens when they need to use those same wing muscles and don’t want to generate excess heat? How do bees fly on 100+ degree days without overheating? That was the question Dr Jordan Glass at the University of Wyoming recently set out to answer.1

Other insects can mitigate heat stress by being more active at dusk or night, but bees need light to navigate, and need to work throughout the day to keep the hive’s material needs met — especially since on hot days they require a liter or more of water per hive for evaporative cooling, and it’s not going to collect itself. Reducing the frequency of wing beats can reduce metabolic heat production,2 but typically would be assumed to significantly reduce lift capacity, and there’s little point in going on a flight if you can’t

bring anything back. Honey bees routinely carry 20-35%, and sometimes as much as 80%, of their own weight in foraged materials.3

sweeping stage, while others generate more on the rotational stage. Normal flight of honeybees is about an even balance. [See also “The Dynamics of Honey Bee Flight,” by Bill Hesbach, May 2024 ABJ.]

Studies have shown that bees can stay alive inside a hive in extreme temperatures (in one example a colony was surviving at regular external temperatures of 60ºC (140ºF) on an (inactive) lava field, with the internal temperature never exceeding 36 C (97 F),4 but if individual bees’ body temperature reaches 49-50 C (120-122 F) they will die.5 If bees become unable to leave the hive and forage, then the hive will be on a crash course with running out of resources and failing, so how bees’ flight performance is affected at high temperatures is worth having a look at.

The first set of experiments was conducted at Arizona State University in the Sonoma Desert. Researchers used bees from three hives maintained by the university to study the effect of air temperature on body temperature, metabolic rate and water balance. They captured bees leaving the hive for foraging runs (who would therefore be unloaded) and weighed them, finding an average mass of 70 mg with very little variation. They then fed these bees between 0 and 45 microliters of sugar solution in a temperature-controlled room.

Dr Glass and his colleagues set out to study exactly this. They measured flight muscle temperatures, flight metabolic rates, and water loss rates of honey bees carrying nectar in a controlled setting at 20 C (68 F), 30 C (86 F) and 40 C (104 F). They also used high-speed video to analyze wing movements at 25 C (77 F) and 40 C (104 F). Wing movement (“kinematics”) in flight consists of two pertinent movements: a sweeping motion (“translational force”), and the creation of rotational vortices when rotating the wing before reversing direction. Some insects generate more lift during the

Immediately after feeding, the bees were transferred into cylindrical flight chambers, rested for two minutes, and then encouraged to fly by shining a light at them, tapping on the cylinder or inverting it. Carbon dioxide and water content (the results of respiration) of the outgoing air from the cylinder was monitored to calculate the bees’ respiration. What I’m giving you here is of course the extremely simplified overview of the process laid out in the paper — see the paper itself if you particularly want details about “LI-COR LI-7000 CO2/H2O analyzers” and “soda lime

Figure from Glass et.al., 2024 Figure from Glass et.al., 2024

Fig. 2. Water loss rates of honey bees increased with nectar load at 40 °C, but not 20 or 30 °C air temperature. Total body mass is the mass of the unloaded bee plus the nectar load it is carrying. Each point represents an individually measured bee. The regression line and its corresponding 95% confidence limits denote a statistically significant effect of total body mass on water loss rate.

Fig. 4. The length of time an unloaded forager (average: 70 mg) can fly at a given air temperature before reaching critical water content (CWC) when flying in dry air (blue line). The red dotted line represents the average foraging trip for a honey bee (30 min; 36). The red arrow denotes the upper critical thermal limit for honey bees at rest (alpproximately 49 °C; 38, 39).

columns” — but apparently you can calculate evaporative heat loss with this data. Within two seconds of cessation of flight, they then took the bees’ flight muscle temperature and weighed them.

The next experiments were to look at the effect on temperature of wing motions (“kinematics”), and took place at the beautiful campus of University of California Davis located in the agricultural countryside of northern California. They collected nectar foragers (excluding bees with loads of pollen) and weighed them. Since the unloaded weight of bees varies little from 70 mg it was assumed all weight over that was nectar cargo. These bees were placed in a temperature-controlled flight chamber and their flight was analyzed with high-speed cameras.

The various experiments generally showed little difference between bees flying at 20 or 30 C (68 or 86 F). At 40 C (104 F), however, wingbeat frequency significantly decreased (i.e., they’re “flapping” more slowly), and stroke amplitude increased (i.e., slower, big-

(which has a high water content) during their flight which will extend their flight time. However that’s based on extrapolated calculations and it is not yet known if they experience serious health harm before reaching the point that would kill them. I think it would be surprising if they don’t give a very wide margin to that limit — we ourselves certainly aren’t going to put ourselves in a situation nearing a few minutes or degrees of suffering a heatrelated death, and I don’t think we should expect bees to either.

Dr. Glass also notes that all these experiments were conducted in lowhumidity conditions; high-humidity air such as one might experience in tropical parts of Australia may have a significant effect — humidity may eliminate the evaporative loss that is the current limiting factor, but then again it may also severely limit their ability to shed metabolic heat and overheating may become the limiting factor. When I kept bees in the subtropical Bundaberg area it was very often 40 C (104 F) and 100% humidity, which I can tell you wasn’t very pleasant as a human and I’m very curious how it would affect the bees. I hope some follow-up research will look into this specifically.

I was wondering why it seemed to be the bees simply began flying more efficiently at 40 C (104 F), — there surely must be a tradeoff? — and then I was struck by this sentence by Dr Glass:

ger strokes). This reduces their metabolic rate (i.e., they’d feel like they’re working less hard). Interestingly, this did not appear to impact lift capacity. The most significant difference, however, was that bees don’t lose much water during flight below the mid-30s C, but do so increasingly (and rapidly) beyond that. Bees as you know, can’t sweat, which we do to benefit from evaporative cooling, but they can instead regurgitate the contents of their honey stomach over their own head to accomplish the same thing.6

Of course! This was a revelation — bees fly most efficiently at 39 C (102 F), they are known on cold days to shiver their wing muscles to get up to this temperature, and so when flying at less than that temperature they are essentially flying less efficiently to generate extra heat to reach that temperature! It’s not that they suddenly get “magically” more efficient at and above 104 F, it’s that they’re intentionally flying inefficiently below that.

At 40 C (104 F) bees are losing a net 0.21 mg of water per minute. A honey bee will die if its water content reaches less than 74% of its mass,7 i.e., if it loses 18 mg of its 70 mg weight. So if a bee in flight loses more than that amount in flight through metabolic processes and/or evaporation, it will die. At 40 C (104 F) that gives them about 85 minutes of flight time, well in excess of the average 30-minute foraging trip. It’s only at 46 C (115 F) that the survivable flight time drops below 30 minutes, though they may find water or nectar

The research conclusions seem to confirm my observation that bees really seem busiest and “happiest” on days in the 90s. Water seems to be the

Bees flying at 25° C displayed high wingbeat frequencies that were independent of total body mass, suggesting that bees use a less efficient kinematic strategy when flying in cool air, perhaps to generate additional metabolic heat and warm themselves toward 39°C, the flight muscle temperature associated with maximal flight metabolic rate. 8

limiting factor, which is all the more argument for providing water sources to your bees on hot days, perhaps both near the hive and if possible along their path to foraging. I thank Dr. Glass for conducting this useful research, and particularly for sharing it with me so I can share it with you.

A version of this article first appeared in the August 2024 issue of the Australasian Beekeeper.

References 1 Glass, J.R. et al. (2024) Flying, nectarloaded honey bees conserve water and improve heat tolerance by reducing wingbeat frequency and metabolic heat production, Proceedings of the National Academy of Sciences, 121(4). https://www.pnas.org/ doi/abs/10.1073/pnas.2311025121.

2 Dudley, R, C.P. Ellington, Mechanics of forward flight in bumblebees: II. Quasisteady lift and power requirements. J. Exp. Biol. 148, 53–88 (1990). 3 Winston, M.L., The Biology of the Honey Bee (Harvard University Press, Cambridge, MA, 1991).

4 Lindauer, M. (1955). The water economy and temperature regulation of the honeybee colony. Bee World 36, 81-92. https:// doi.org/10.1080/0005772X.1955.11094876 5 Kovac, H., H. Käfer, A. Stabentheiner, C. Costa, Metabolism and upper thermal limits of Apis mellifera carnica and A. m. ligustica. Apidologie 45, 664–677 (2014).

6 Berndt Heinrich, “Keeping a Cool Head: Honey Bee Thermoregulation” SCIENCE, 21 Sep 1979 Col 105, Issue 4412 pp. 1269- 1271

7 Burdine, J.D., K.E. McCluney, Differential sensitivity of bees to urbanization-driven changes in body temperature and water content. Sci. Rep. 9, 1–10 (2019).

8 Glass, J.R., J.F. Harrison, The thermal performance curve for aerobic metabolism of a flying endotherm. Proc. R Soc B 289, 20220298 (2022).

Kris Fricke is the editor of the Australasian Beekeeper, Australia’s national beekeeping magazine. He is originally from Southern California.

This is the plain-text version, extracted from the printed pages for readers who use screen readers, text-only browsers, or prefer reflowable text. Captions and sidebars appear where they fell in the page layout. The typeset pages are available in the reader.