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.

Overwintering of solitary bees under controlled conditions:

Solitary bee (Osmia bicornis) females were overwintered under controlled conditions at three different temperature treatments: 0°C, 4.5°C, and 8°C.

Semi-field experiment to assess consequences on the reproductive success of hatched female bees:

To study the consequences of overwintering temperatures on fitness-related parameters, a semi-field experiment using flight cages was conducted with hatched bees. From the day of release until the last bee was observed in the flight cages the number of females roosting inside the nesting units during the night was counted as an estimate of females alive inside each cage. This data was also used for the estimation of longevity (i.e., the time from hatching until the bee was last observed alive). Longevity data was complemented by analysis of recorded videos. Software analysis of video recordings was used to link individual nests with produced brood cells (offspring) to individual females. One week after the introduction of bees into the flight cages when bees had started to nest a high-resolution video camera was set up at a distance of 1 m at a height of 1.5 m in front of each nesting unit using a tripod to film the nesting female bees. Nesting units in each cage were filmed for at least 1 hour daily (usually 1.5 hours) during suitable foraging conditions between 10:00 and 17:00 hours. Nesting units were filmed roughly simultaneously with separate cameras in each cage. Videos were analysed with the novel machine-learning-based software “BeeTracker” [1]. The software is able to identify the ID tags (colour-digit marking) of individual nesting female bees and their nests (the cavity ID a female bee is nesting in). During the same period as the video recordings, progress in brood cell production was monitored, every day between 17:30 and 18:00 hours. To this end, newly produced brood cells were photographed and marked daily with a permanent marker on the acetate foil covering the drilled nesting cavities of each wooden layer. The photographs along with the markings on the acetate foils were later used to determine the number of newly produced brood cells by each female per day.

After the semi-field experiment was finished in mid-October 2021, it was assessed for each brood cell whether the offspring reached the cocoon stage and metamorphosis was completed (i.e., the adult stage reached in the analysis of offspring survival), whether it was still alive, and what sex it had.

Abdomen volume measures using micro-CT examinations:

To gain a better mechanistic understanding of the pathways underlying potential impacts of overwintering temperatures on hatching rate and reproductive success of females, micro-CT examinations were performed to track potential changes in fat bodies and abdomen volumes during overwintering of 12 bees (cocoons) overwinter at each the 0 °C and 8 °C overwintering temperature treatment. Scans of all monitored cocoons were carried out at four measurement dates during the overwintering period on 1 October 2020, 1 December 2020, 1 February 2021, and 1 March 2021.

References:

[1] Knauer, A. C., Gallmann, J., & Albrecht, M. Bee Tracker—an open‐source machine learning‐based video analysis software for the assessment of nesting and foraging performance of cavity‐nesting solitary bees. Ecol.Evol. 12, e8575 https://doi.org/10.1002/ece3.8575 (2022).