Extreme heat waves disrupt honey bee thermoregulation and threaten colony survival

Although honey bees have the ability to regulate hive temperatures, new research published in Ecological and Evolutionary Physiology shows that extreme summer heat can overwhelm these critical pollinators' cooling systems, leading to significant colony population declines.

The research in “Negative Effects of Excessive Heat on Colony Thermoregulation and Population Dynamics in Honey Bees,” conducted during a hot Arizona summer, monitored nine honey bee colonies through three months of temperatures that frequently exceeded 40°C (104°F). The results indicate that intensifying heat waves worldwide represent a significant threat to honey bees and the pollination services they provide.

"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."

The research team discovered that while colonies maintained average brood temperatures within the optimal 34-36°C range necessary for healthy development, significant daily temperature fluctuations still occurred. Developing bees in the brood center experienced nearly 1.7 hours below optimal temperatures and 1.6 hours above them each day. Conditions were even more extreme at the brood edges, where young bees spent almost 8 hours per day outside the optimal range.

These temperature swings had measurable consequences. Higher maximum air temperatures and greater temperature fluctuations within hives led to population declines. The study found that "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."

Colony size emerged as a critical factor in thermal protection. Larger colonies maintained more stable internal temperatures, with the smallest hives experiencing daily temperature swings of up to 11°C at the outer edges compared to 6°C in the largest colonies. This enhanced stability meant that developing bees and adult workers in larger colonies spent far less time exposed to potentially harmful temperature extremes.

Beyond Arizona, "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 would intensify heat wave frequency and severity worldwide. Additionally, high humidity may compound these challenges in many regions. The authors note 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."

The research has practical implications for beekeepers and agricultural systems that depend on honey bee pollination. The authors suggest that implementing effective management strategies, such as supplemental water provision, shading of hives, improved hive structure and materials that provide greater insulation, and ensuring high-quality forage will become increasingly important to mitigate impacts of high temperatures and maintain colony stability in a warming climate.

Ecological and Evolutionary Physiology primarily publishes original research examining fundamental questions about how the ecological environment and/or evolutionary history interact with physiological function, as well as the ways physiology may constrain behavior. For EEP, physiology denotes the study of function in the broadest sense, across levels of organization from molecules to morphology to organismal performance and on behavior and life history traits.