Pollination of seven Plectranthus spp. (Lamiaceae) in southern Natal, South Africa

The genus Plectranthus (Lamiaceae) shows remarkable radiation on the sandstones of southern Natal and northern Transkei in South Africa, where six endemic species occur. Two of these endemic species, P. hilliardiae and P. oribiensis, are included in this study, as well as P. reflexus, for which only limited data are available. The other species that were studied are P. ambiguus, P. ciliatus, P. ecklonii, P. madagascariensis and P. zuluensis; Four of these taxa, P. ambiguus, P. hilliardiae, P. reflexus and P. saccatus var. longitubus, have uniquely long corolla-tubes (20-30 mm) and this is related to pollination by nemestrinid flies of the genus Stenobasipteron that have proboscides of similar length. Other nemestrinid species of the genus Prosoeca have shorter proboscides and pollinate two species of Plectranthus with shorter corolla-tube lengths (6-15mm). Acrocerid flies, tabanid flies and anthophorid bees are also important visitors to these species. This study on the pollination of seven species of varying corolla-tube lengths shows a correlation between floral tube length and proboscis length of insect visitors, many of which are recorded for the first time as pollinators of Plectranthus.