Survival of extreme heat related to circadian cycle and sex in bees

The pollination services provided by insects have been a crucial part of
evolution and survival for many species, including humans. For bees to be
efficient pollinators they must survive the environmental insults they
face daily. Thus, looking into the short- and long-term effects of heat
exposure on bee performance provides us with a foundation for
investigating how stress can affect insect pollination. Solitary bees are
a great model for investigating the effects of environmental stress on
pollinators because the vast majority of insect pollinator species are
solitary rather than social. One of the most pervasive environmental
stressors to insects is temperature. Here we investigated how a one-hour
heat shock affected multiple metrics of performance in the alfalfa
leafcutting bee, Megachile rotundata. We found that a short heat shock
(1hr at 45ºC) can delay adult emergence in males but not females. Bee
pupae were rather resilient to a range of high temperature exposures that
larvae did not survive. Following heat shock (1hr at 50ºC), adult bees
were drastically less active than untreated bees, and this reduction in
activity was evident over several days. Heat shock also led to a decrease
in bee survival and longevity. Additionally, we found a connection between
starvation survival after heat shock and time of exposure, where bees
exposed in the morning survived longer than those exposed in the
afternoon, when they would normally experience heat shock in the field.
These data suggest that there is an unexplored daily/circadian component
to the stress response in bees likely similar to that seen in flies,
nematodes, and plants which is constitutive or preemptive rather than
restorative. Taken together our data indicates that single heat shock
events have strong potential to negatively impact multiple life history
traits correlated with reproduction and fitness.