Attraction of nocturnal scarab beetles by unusual floral volatiles in a Banksia (Proteaceae) with functionally diverse pollinators.

Pollination by beetles has evolved multiple times in flowering plants but with relatively few plant species adapted specifically to pollination by scarab beetles (Coleoptera: Scarabaeidae). However, some plant species may produce floral scents and offer floral rewards that attract scarabs alongside other pollinator functional groups. In Banksia attenuata (Proteaceae), nocturnal observations led to the discovery of extensive floral visitation by beetles, alongside birds, mammals and diurnal insects. Given the distinctive melon-like floral scent of B. attenuata, we predicted long-distance attraction of beetles by chemical cues. We undertook floral visitor surveys aided by camera traps and video recordings, and quantified insect pollen loads. We used GC-MS to identify candidate beetle-attracting compounds, which we then synthesised and tested in field bioassays. Honeyeaters, Honey possums and diurnal insects visited the flowers of B. attenuata. However, the most frequent visitors were nocturnal scarab beetles: Pachytricha minor and Phyllotocus occidentalis (Scarabeidae: Melolonthinae). Both beetle species fed on nectar and pollen and mated on the flowers. Beetles caught in the bioassays carried pure B. attenuata pollen. Floral volatiles of B. attenuata were dominated by two unusual compounds: 3,6-nondien-1-yl acetate and 3,6-nondien-1-ol that were not detected in 22 congeners. A synthetic mixture of these compounds proved strongly attractive to Phyllotocus occidentalis in the field. Nocturnal scarab beetles are a prominent component of the functionally diverse pollinators of B. attenuata. The attraction of Phyllotocus occidentalis by unusual floral volatiles suggests a case of adaptation to pollination by beetles among Australian Proteaceae, with remarkable parallels to some other plants pollinated by scarabs.