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

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 (\n $$\\overline{x }$$\n = 2.3\u2009±\u20090.07\xa0s.e., n\u2009=\u2009591; modal colony size\u2009=\u20092; max. colony size\u2009=\u200913), 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\xa0autumn. 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\xa0SRi\u2009=\u20090.21; colony-level SRn\u2009=\u20090.18), 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.