Increased overwintering temperature reduces the reproductive success of solitary bees

Wild bees play a key role as pollinators of wild plants and crops, but
they are increasingly at risk through anthropogenic global change, such as
climate warming. However, how increased temperature during overwintering
of solitary wild bees affects their reproductive success remains largely
unknown. In a semi-field experiment, we assessed the individual life-long
reproductive success of 144 marked females of solitary bee
species Osmia bicornis that have been wintered at 0°C, 4.5°C, and
8°C. Overwintering mortality of bees was on average 32% higher at winter
temperatures of 8°C compared to 4.5°C or 0°C, at which almost all bees
successfully hatched. Moreover, female bees wintered at 4.5°C and 8°C
produced less offspring than those overwintered at 0°C (26% or 36% less
offspring, respectively). Longevity or daily offspring production rate was
not significantly affected by overwintering temperature. However, the
nesting duration of females wintered at 0°C tended to be longer (+2.5
days) than that of bees wintered at 4.5°C, which likely contributed to the
decreased offspring production at warmer overwintering temperatures. The
mortality and sex ratio of offspring was not significantly affected. These
findings indicate that increasing mean overwintering temperatures could
pose a so far largely overlooked threat to wild solitary bee populations.