Evidence for moth pollination in a rhinomyiophilous Erica species from the Cape Floristic Region of South Africa

Abstract

Contrasting pollination syndromes in closely related species suggest that floral trait divergence is associated with differences in pollination system, but empirical observations are required to confirm syndrome-based predictions. We present a comparative study of two closely related Erica species with contrasting pollination syndromes from the Cape Floristic Region of South Africa. Erica cylindrica has narrowly tubular pale and strongly scented flowers and is known to be hawkmoth-pollinated. The closely related Erica infundibuliformis has bright flower colours and appears to lack scent, traits that are suggestive of pollination by long-tongued nemestrinid flies (rhinomyiophily). Floral trait measurements revealed that both species exhibit predominantly upright flower orientation and elongated floral tubes, although tube length of E. infundibuliformis is consistently greater than that of E. cylindrica. For both species, petals are brighter than floral tube surfaces, but flowers of E. cylindrica lack the strong UV reflectance found in E. infundibuliformis. Nectar of E. infundibuliformis is more concentrated and produced in larger volumes. Scent composition, but not evening scent emission rates, differed between the species: scent of E. cylindrica is dominated by aromatic compounds, whereas scent of E. infundibuliformis is dominated by (E)-ocimene and other terpenoid compounds and is emitted at higher rates during the day than the evening. Pollinator observations contradicted trait-based predictions: although a single nemestrinid fly captured in the vicinity of E. infundibuliformis did carry Erica pollen, almost all other diurnal flower visitors were nectar-robbing Hymenoptera which did not carry Erica pollen. Contrary to predictions, at two sites and over two flowering seasons, flowers were consistently visited in the evenings by several species of settling moths and hawkmoths which carried pollen, almost exclusively of Erica, on their proboscides. Our findings thus suggest that, despite objective differences in key floral traits between the closely related hawkmoth-pollinated E. cylindrica and E. infundibuliformis, moths are also important pollinators of E. infundibuliformis. A bimodal pollination system involving predominant pollination by moths and occasional visits by long-proboscid flies could partially reconcile findings with predictions. Our study further suggests that hawkmoth pollination may be more widespread in both Erica and the broader Cape flora than has hitherto been assumed and emphasises the importance of nocturnal pollinator observations.