Negative Effects of Excessive Heat on Colony Thermoregulation and Population Dynamics in Honeybees

AbstractHoneybees (<i>Apis mellifera</i>) are essential for the pollination and yield of many crops and have experienced significant mortality in recent years. While climatic warming has been suggested as a possible cause of honeybee losses, their responses to warming conditions and heat waves remain poorly understood. Honeybee colonies have well-documented mechanisms to cope with heat exposure. 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 honeybee colonies to thermoregulate and grow. To test the hypothesis that excessive heat impairs honeybee colony growth by exceeding colonial 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 3 mo and recorded temperatures at the center and edge frames of the brood nest and 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°C-36°C that is necessary for healthy brood development throughout the summer. However, all hive locations exhibited cyclic, diurnal thermal fluctuations, and broods 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 broods or by exposing adults to temperatures that shorten their lifespans. If excessive heat periods occur more frequently as predicted as a result of climate change, this could limit regions where colonies can successfully survive the summer.