Negative effects of excessive heat on colony thermoregulation and population dynamics in honey bees

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.