Honey bees are known for their ability to carefully control the temperature inside their hives, but new research shows that extreme summer heat can overwhelm this natural cooling system. A study published in Ecological and Evolutionary Physiology found that prolonged heat waves can disrupt hive temperature control and lead to noticeable declines in colony populations.
The study, titled "Negative Effects of Excessive Heat on Colony Thermoregulation and Population Dynamics in Honey Bees," followed nine honey bee colonies during a particularly hot summer in Arizona. Over a three month period, temperatures frequently climbed above 40°C (104°F). The findings suggest that increasingly intense heat waves around the world pose a serious risk to honey bees and the essential pollination services they support.
"Honey bee colonies have well-documented mechanisms to cope with heat exposure," write authors Jun Chen, Adrian Fisher II, Gloria DeGrandi-Hoffman, Cahit Ozturk, Brian H. Smith, Jennifer H. Fewell, Yun Kang, Kylie Maxwell, Kynadi Overcash, Keerut Chahal, and Jon F. Harrison. "However, there have been no studies to date that have assessed the limits of such thermoregulation or how natural heat waves affect the capacity of honey bees colonies to thermoregulate and grow."
Dangerous Temperature Swings Inside the Hive
Researchers found that although colonies were able to keep average brood temperatures within the ideal range of 34-36°C needed for healthy development, temperatures inside the hive still fluctuated widely throughout the day. Bees developing in the center of the brood experienced about 1.7 hours per day below optimal temperatures and roughly 1.6 hours above that range.
Conditions were far more severe near the edges of the brood. Young bees in these outer areas spent close to eight hours each day outside the safe temperature window, exposing them to stressful and potentially harmful conditions.
These repeated temperature swings had clear biological consequences. Colonies exposed to higher peak air temperatures and greater internal temperature variation experienced declines in population size. According to the study, "excessive heat, with maximal temperatures exceeding 40°C, can reduce colony populations by impairing the thermoregulation of brood or by exposing adults to temperatures that shorten their lifespans."
Why Larger Colonies Fare Better
Colony size played a major role in how well bees could protect themselves from heat. Larger colonies were better able to maintain stable internal temperatures. In the smallest hives, temperatures at the outer brood edges fluctuated by as much as 11°C each day, compared with swings of about 6°C in the largest colonies.
Because of this added stability, both developing bees and adult workers in larger colonies spent much less time exposed to extreme temperatures that could threaten their survival.
Climate Change and Rising Heat Risks
The researchers warn that the challenges seen in Arizona may become increasingly common elsewhere. "Climate projections indicate that global average temperatures could rise by approximately 2.7°C by the end of the century, with potential increases up to 4°C under higher emission scenarios," the authors note. Such warming is expected to make heat waves more frequent and more intense across many regions.
Humidity could worsen the problem. The authors also point out that "high humidity significantly reduces the effectiveness of evaporative cooling -- the primary mechanism honey bees use to regulate hive temperatures -- potentially making thermoregulation even more difficult."
What This Means for Beekeepers and Agriculture
The findings have important implications for beekeepers and for agricultural systems that rely on honey bee pollination. The researchers suggest that management practices such as providing supplemental water, placing hives in shaded areas, improving hive design and insulation, and ensuring access to high quality forage may become increasingly critical. These steps could help reduce heat stress and support colony stability as temperatures continue to rise.
