Bees visiting unopened flowers: bumbling burglars or sneaky pollinators?

Abstract

Most studies of angiosperm pollination have been conducted on fully opened flowers, while interactions between animal pollinators and unopened flowers are less well-known. In December 2015, I observed aggregations of small-bodied hylaeine bees swarming between inflexed stamens of unopened blossoms of pink and red flowering gums ('Corymbia' spp.) planted as municipal street trees in southern New South Wales, Australia (Albury: 36.0737° S, 146.9135° E) (Fig. 1a). The bees appeared to be predominantly 'Hylaeus (Prosopisteron) perhumilis' Cockerell 1914 (expert identification provided via high resolution photographs), but there were also occasionally other small bees from the Halictidae and Colletidae families. All bee visitors appeared to be feeding on pollen-laden anthers. I saw this behavior on more than 15 individual flowers across five trees at various stages of opening between the initial split of the operculum and full extension of stamens. Individual bees were constantly arriving and departing, but a single bud had between 3-7 individuals visiting at any one time. 'Corymbia ficifolia' is native to southwestern Australia, but the species and its hybrids are popular as street trees in urban areas around the country. There is little published empirical work on animal pollination in 'Corymbia' spp., but its pollinators are traditionally assumed to be birds or large-bodied insects (Phillips et al. 2010).