Sometimes it’s just satisfying to see things for yourself. That opportunity was given to me last week during the great heat wave of the Pacific Northwest (June 25-29, 2021), during which the temperature rose to 116F (47C) in Portland, Oregon, on the hottest day. I have always read that honey bees maintain their brood chambers between 85 to 95 degrees Fahrenheit (30-35C) throughout the year, regardless if the outside temperature is below freezing or in the triple digits. In the summer, the brood temperature range is even narrower, often between just 92 to 95F (33-35C). Temperatures above 96F (36C) have been shown injurious or fatal to developing brood.
Last year I installed a hive monitoring system in one of my backyard honey bee colonies, just for fun and education. A partner of mine, Ed Bunch, did the same. These systems record temperature in the brood box, humidity and hive weight over time. These are not necessarily the most accurate instruments that can be used, but accurate enough to show trends and enable decision making. I knew the heat wave was coming and that record temperatures (>110F/43C) were forecast so I anxiously waited to see how the bees would cope. Could they manage to actually lower the inside air temperature of their hives by 15F degrees (8C) or more? And could they do this while the walls of their hives were being blasted by the baking sun, making the task even harder?
I was not disappointed for excitement – the bees put on a show and we got our temperature data. Yes, all colonies but one1 held the upper limit in the brood chamber to less than 97 (36C) for the entire period. Meanwhile, I was able to admire thousands of HVAC engineers working diligently to maintain equilibrium. There was lots of bearding, fanning, shuffling, water collecting and very prolonged days of flight activity. The bees kept foraging for the duration and even made several pounds of honey (which actually helped cool the hive with the evaporative process).
It has been known for some time that honey bees, despite being “cold blooded animals”, are able to modify their individual temperatures as well as their collective nest temperatures. Heat generation is done primarily by shivering their flight muscles, but cooling uses several techniques such as fanning inside and outside the hive, formation of ventilation channels, dispersal (keeping distance from one another), evaporative cooling by fanning water and nectar placed on internal surfaces (e.g., comb), and “tongue lashing”, the repeated extension and retraction of the proboscis holding a thin film of fluid. During flight, honey bees have been shown to “tongue lash” which serves to cool their hot thorax muscles. This type of flight cooling has also been reported in other bees such as Bombus vosnesenskii, Xylocopa varipuncta, and Centris pallida.
I used to work in the Nevada desert and I was amazed how well evaporative (swamp) coolers work to lower air temperature. They were so much less expensive to run than refrigerated air conditioners. All you need is water flow over a substrate, air flow, and an air temperature to dew point gradient; the latent heat of evaporation does the rest. Honey bees have most likely been utilizing these principles for millions of years – the amazing HVAC specialists they are.
Hive3 of Ed showed a temperature excursion over 100F (38C) for about 1 1/2 hours at the beginning of the heat wave (June 26 around 2pm), but those levels never returned even as temperature rose. Instrument capability might also have played a role as data in this period showed some anomalies.
Mark L. Winston. The Biology of the Honey Bee. 1987. Harvard University Press
Bernd Heinrich and Harald Esch. Thermoregulation in Bees. American Scientist, Vol. 82, No. 2 (MARCH-APRIL 1994), pp. 164-170
Roberts, S. P., & Harrison, J. (1998). Mechanisms of Thermoregulation in Flying Bees. American Zoologist, 38(3), 492-502.
Edward Southwick, The Hive and Honey Bee. 2015 Edition. Chapter: Physiology and Social Physiology of the Honey Bee. Dadant Publication
Placement of the single temperature sensor in the brood chamber may not always have been ideal. Brood nest centers often move so sensor placement can be sub-optimal. In our situations, temperature sensors were deemed to properly placed “in the brood nest” upon initial installment.
Granted, the temperature measurement accuracy of the instruments used in in this study could be greatly improved. The MG hive had a sensor (DHT11) accuracy of +/- 3.6 degrees F. Ed’s hives were better outfitted with a sensor (BME280), accuracy of +/- 1.8 degrees F. Nevertheless, I believe all measurement data is valid at the “gross reality check” level and acceptable for evaluation.
PDX temperature data was provided by the Weather Underground and measured at the Portland International Airport Station. The bee hives measured in this study are located in Beaverton and Hillsboro, about 13 to 16 miles from PDX, respectively. It is possible that temperature data may have differed by a one or two of degrees (F) depending on location. Data was measured and collected by BeeCorders and transmitted to ThingSpeak for storage and graphing.
BeeCorder Hive Monitor: Rev0. Brainchild of Steen Larsen.