The March fly and the ant: the unusual pollination system of Eustegia minuta (Apocynaceae: Asclepiadoideae)

Abstract

Pollination studies of South African Asclepiadeae (Apocynaceae: Asclepiadoideae) have mostly examined species in the moist summer-rainfall grasslands, with limited studies of the early-diverging groups occurring in the drier winter-rainfall habitats. This study examined the pollination and floral traits of Eustegia minuta, an unusual species endemic to the winter-rainfall Greater Cape Floristic Region and representing an early-diverging clade within the Asclepiadeae. Observations of floral visitors in Jonkershoek Nature Reserve showed that this species is visited primarily by a species of March fly, Bibio turneri (Bibionidae). These flies moved actively between plants and carried pollinaria on their mouthparts. In addition, an ant species, Camponotus vestitus (Formicidae), and a single honey bee Apis mellifera capensis (Apidae) individual carried pollinaria and may contribute to pollination. Bagging experiments confirmed that flowers require pollinators for reproduction. Flowers produced small amounts (1.2 µl per flower) of concentrated (32.5% sugar) nectar. Pollination success was low (14.5% of flowers were pollinated and 3.4% of flowers developed fruits). Pollen transfer efficiency (PTE) was 5.2%. The corolla reflectance was similar to that of green leaves, but the gynostegium exhibited a relatively bright human-white spectral curve. Floral scent comprised over 50 compounds, but was dominated by various aromatics along with 2,3-heptandione, (E)-4,8-dimethylnona-1,3,7-triene and several unidentified compounds. We conclude that E. minuta is pollinated primarily by the March fly B. turneri, although ants and possibly honey bees may make a lesser contribution. Pollination by bibionid flies has not previously been reported in asclepiads and is extremely uncommon amongst angiosperms.