DOI: 10.5061/dryad.3tx95x6pt
title: Increased overwintering temperature reduces the reproductive success of solitary bees

description: 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.
description: <strong>Overwintering of solitary bees under controlled
 conditions:</strong> Solitary bee
 (<em>Osmia bicornis)</em> females were overwintered under
 controlled conditions at three different temperature treatments: 0°C,
 4.5°C, and 8°C. <strong>Semi-field experiment to
 assess consequences on the reproductive success of hatched female
 bees:</strong> 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.
 <strong>Abdomen volume measures using micro-CT
 examinations:</strong> 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.
 <strong>References:</strong> [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. <em>Ecol.Evol.</em> 12, e8575
 https://doi.org/10.1002/ece3.8575 (2022).
description: # Increased overwintering temperature reduces the reproductive success of
 solitary bees
 [https://doi.org/10.5061/dryad.3tx95x6pt](https://doi.org/10.5061/dryad.3tx95x6pt) ## Description of the data and file structure The Excel file "DATA Mueller et al. Overwintering Temperature Affects Bee Reproduction" contains the data used for the analyses presented in: Increased overwintering temperature reduces the reproductive success of solitary bees by Sarah Müller, Jana Collatz, Henning Richter, Robert Zboray, and Matthias Albrecht Corresponding author: Matthias Albrecht: [matthias.albrecht@agroscope.admin.ch](mailto:matthias.albrecht@agroscope.admin.ch) The Excel data file contains several sheets. The first sheet is a Read Me file describing all variables used in the different sheets containing different sets of data used for the analyses. The sheet "hatching rate" contains all relevant data for the analyses of hatching rate and emergence duration of bees; the sheet "weight" contains the data for the analysis of female bee weight at hatching; the sheet "offspring, nesting, longevity" contains the data used for analyses of different aspects of female bee reproductive success (e.g., the number of offspring produced), aspects of nesting (e.g., nesting duration) and longevity; the sheet "offspring survival & sex ratio" contains the data used for the analyses of bee offspring survival and sex ratio; the sheet "CT abdomen volume" contains the data used for the analysis of the development of the abdomen volume of bees during overwintering.