Neglecting nocturnal pollinators has long masked hawkmoth pollination in Rhododendron

Abstract

Insufficient investigation of nocturnal floral visitors may bias our understanding of floral diversification in many plant lineages. Here we re-examined the pollination ecology of Rhododendron excellens, which lacks a narrow floral tube characteristic of many hawkmoth flowers and has been considered a bee specialist with daytime observations alone. We used five temporally sequential proxies (i.e. visitation rate, pollen deposition, fruit production, seed production, and seed viability) covering the entire process of reproduction to track the relative importance of the two floral visitor groups that were active by day and by night respectively. We then quantified the floral syndrome and tested the mating system with hand pollination treatments. Both bumblebees and hawkmoths regularly visited R. excellens in two flowering seasons. Hawkmoths’ relative importance increased step-by-step from being inferior to bumblebees (floral visitation and pollen deposition) to making over three times the contribution by bumblebees (seed viability). This is probably because they differ in the ability to deliver outcross pollen for this partially self-compatible species. Correspondingly, R. excellens exhibits a floral syndrome associated with a particular subdivision of sphingophily. We revealed a distinctive pollination mode that mainly involves nocturnal pollinators and is likely to have repeatedly evolved in Rhododendron. Our findings also highlight the need to consider the ‘quality’ of pollen deposited onto stigmas, whose effect on plant fecundity may significantly expand over the course of sexual reproduction.