Generalist, settling moth pollination in the endemic South African twig epiphyte, Mystacidium pusillum Harv. (Orchidaceae)

- The epiphytic orchid, Mystacidium pusillum, has small green flowers with short spurs.
- Five species of settling moths from three families pollinate these flowers.
- Insect proboscis length match nectar depth in spurs but are shorter than spur length.
- Flower colours approximate that of the leaves of the host tree.
- Support Vogel's (1954) hypothesis of settling moth pollination in these orchids.

Mystacidium Lindl. is a small, near endemic genus of South African epiphytic orchids. Three species have relatively large, pure white flowers with long nectar spurs while the remaining species have small, green flowers and relatively short nectar-containing spurs. Using a pollination syndromes approach and based on floral traits, Stefan Vogel suggested these were pollinated by hawkmoths and settling moths, respectively. To date, only a single observation supports the settling moth pollination hypothesis in the small, green-flowered species of Mystacidium. To test Vogel's predictions for these species of Mystacidium, we systematically observed plants of M. pusillum Harv. and recorded visits by a number of species of settling moth from the Noctuidae, Crambidae and Erebidae, with at least five species across the three families bearing pollinaria. Settling moths were collected at a light trap in the vicinity of M. pusillum flowers. Their tongue lengths were on average shorter than spur lengths of the flowers, but matched the nectar depth within the spurs, allowing moths access to the small amount of nectar present in each flower (mean volume: 0.13 μl; mean sucrose concentration: 24.2%). A survey of flowers showed that moths removed pollinaria from 14% of flowers and deposited pollinia on 6% of stigmas, with a pollen transfer efficiency of 21% across the population. Finally, we measured the spectral reflectance of M. pusillum petals and the leaves of the principal host trees Dovyalis caffra (Hook. f. & Harv.) Warb. which showed that the colours were similar. These data support Vogel's hypothesis of settling moth pollination in M. pusillum and suggest that scent is likely to be the primary pollinator attractant.