Hawkmoth pollination of the scented South African fynbos endemic Erica cylindrica Thunb. (Ericaceae)

Abstract

The predictive power of pollination syndromes has been demonstrated for many South African plant species with specialized pollination systems, but has rarely been tested in the florally diverse genus Erica. Here we evaluate the hypothesis that Erica cylindrica is moth-pollinated, based on its tubular, pale, and strongly scented flowers. Pollinator observations revealed exclusive nocturnal flower visitation by several hawkmoth species, and assessment of pollen loads confirmed that two hawkmoth species carried Erica pollen grains on their proboscises. Floral scent showed no clear periodicity in emission, but was dominated by benzenoid scent compounds including benzaldehyde, benzyl alcohol, benzyl acetate, and eugenol that are known to elicit antennal responses in hawkmoths. The pale cream flower colour was characterized by uniform reflectance across the human-visible light spectrum and a small secondary peak in the UV region. The volume of nectar produced was small, but the intermediate sugar concentration and sucrose:hexose ratio of 0.7 are consistent with hawkmoth pollination. More than three-quarters of flowers in an inflorescence are oriented upwards, which is highly unusual among South African ericas but has been shown to be adaptive in other hawkmoth-pollinated flowers. Anther-ring disruption indicated that at least 62% of flowers received visits and was associated with pollen deposition in 68% of visited flowers, suggesting that visitors are effective pollinators. Our study provides evidence for hawkmoth pollination of an Erica species, a system hitherto rarely recorded in Ericaceae. Several floral traits are consistent with the hawkmoth pollination syndrome, but further work is required to confirm the functional significance of these traits and the level of specialization in this hawkmoth pollination system.