Insect pollinators, including honey bees (Apis mellifera), are essential for agriculture and terrestrial ecosystem function, yet their responses to warming conditions and heat waves remain poorly understood. Honey bees have well-documented mechanisms to cope with heat exposure that could help them adapt to extreme heat. However, there have been no studies to date that have assessed how natural heat waves affect the capacity of honey bee colonies to thermoregulate and grow. To test the hypothesis that excessive heat impairs honey bee colony growth by exceeding the thermoregulatory capacity, we studied how variation in summer temperatures affected hive temperature regulation and colony growth during a desert summer in which maximal shaded air temperatures intermittently exceeded 40°C. We monitored the growth of nine colonies biweekly for three months and recorded temperatures at the center and edge frames of the brood nest, as well as on combs at the outer edge of the hive body. Average temperatures in the brood center and edge were quite stable and within the optimal range of 34-36°C necessary for healthy brood development throughout the summer. However, all hive locations exhibited cyclic, diurnal thermal fluctuations, and brood experienced considerable portions of each day (14% for the brood center, 33% for the brood edge) above and below the optimal temperature range. Higher maximal air temperatures and greater temperature fluctuations within the hive led to declines in colony population. These findings suggest 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. If excessive heat periods occur more frequently as predicted due to climate change, this could limit regions where colonies can successfully survive the summer.
Negative effects of excessive heat on colony thermoregulation and population dynamics in honey bees
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