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Nesting biology and social organisation of the allodapine bee Exoneura angophorae (Hymenoptera: Apidae): montane environmental constraints yield biased sex allocation yet phenology is unhindered

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Abstract

The life cycle, nesting biology, and social organisation of a progressive provisioning allodapine bee, Exoneura angophorae, are detailed for the first time in the Greater Sydney region at the upper extreme of its altitudinal distribution. Nest collections of this montane population were undertaken monthly over an annual period to ascertain the life cycle and explore evidence for reproductive division of labour and sex allocation patterns in a lineage of facultatively social stem nesting bees. Most nests (68.8%) contained multiple adult females (\(\overline{x }\) = 2.3 ± 0.07 s.e., n = 591; modal colony size = 2; max. colony size = 13), predominantly collected from Alsophila australis australis tree-fern fronds. Ovarian enlargement and egg-laying commenced in autumn with respective peaks in winter and spring, the larval brood first appears in mid spring and pupate through summer until mid autumn. In summer, callow adult emergence coincided with feeding-stage brood providing opportunities for alloparental care and generational overlap—consistent with eusocial definitions of social organisation. Spring nests showed evidence of per capita benefits to brood production with multi-female nests containing the maximum numbers of brood (total and feeding-stage) during this time. Multi-female nests consistently contained more brood than single-female nests across multiple sampling periods, reinforcing implied benefits to group nesting. Within multi-female nests, reproductive dominance hierarchies existed in all seasons and were associated with body size in summer colonies—the largest individuals exhibited the greatest ovarian enlargement (accounting for allometric scaling). Sex allocation was extremely female-biased, based on pupal resource investment and numerical ratios (population-level SRi = 0.21; colony-level SRn = 0.18), with no evidence for split sex ratios over time or colony size. This montane population of E. angophorae experiences the coldest and longest winters of any exoneurine population studied to date. These environmental constraints lead to unhindered phenological patterns but may select for female-biased sex allocation that would promote the evolution of eusocial organisation.

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Acknowledgements

We acknowledge the Dharug people, traditional custodians of the lands on which this research was conducted. We thank Lisa Vella, Éliette Reboud, Lachlan King, Scott Nacko and Laura Brettell for field and lab assistance. We are grateful to Adam Cronin, Scott Nacko and two anonymous reviewers for constructive comments on earlier versions of this manuscript, and we thank Adam Cronin and Mike Schwarz for permitting us to incorporate their previous data into our comparisons. Research was supported by a Hort Frontiers Pollination Fund grant (PH15001) to J.M.C. and Western Sydney University general research funds to O.M.B. and S.M.T.

Funding

Western Sydney University and Horticultural Innovation Australia Pollination Fund project PH15001: Healthy bee populations for sustainable pollination in horticulture.

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SMT conceived and designed the study. OMB and SMT collected and analysed the data. The first draft of the manuscript was written by OMB and all authors commented on subsequent drafts of the manuscript. All authors read and approved the final manuscript.

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Correspondence to O. M. Bernauer.

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Bernauer, O.M., Cook, J.M. & Tierney, S.M. Nesting biology and social organisation of the allodapine bee Exoneura angophorae (Hymenoptera: Apidae): montane environmental constraints yield biased sex allocation yet phenology is unhindered. Insect. Soc. 68, 337–349 (2021). https://doi.org/10.1007/s00040-021-00832-6

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  • DOI: https://doi.org/10.1007/s00040-021-00832-6

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