Data from: Larger pollen loads increase risk of heat stress in foraging bumble bees

Global declines in bumblebee populations are linked to climate change, but
specific mechanisms imposing thermal stress on these species are poorly
known. Here we examine the potential for heat stress in workers foraging
for pollen, an essential resource for colony development. Laboratory
studies have shown that pollen foraging causes increased thoracic
temperatures (Tth) in bees, but this effect has not been examined in
bumblebees nor in real-world foraging situations. We examine the effects
of increasing pollen load size on Tth of Bombus impatiens workers in the
field while accounting for body size and microclimate. We found that Tth
increased by 0.07°C for every mg of pollen carried (p = 0.007), resulting
in a 2°C increase across the observed range of pollen load sizes. Bees
carrying pollen were predicted to have a Tth 1.7–2.2°C hotter than those
without pollen, suggesting that under certain conditions, pollen loads
could cause B. impatiens workers to heat from a safe Tth to one within the
range of critical thermal limits that we measured (41.3°C to 48.4°C).
Bumblebees likely adopt behavioral or physiological strategies to
counteract the thermal stress induced by pollen foraging and may have
limited foraging opportunities as environmental temperatures continue to
increase.