Abstract:

Appendages on orchid floral labellum that lack floral rewards for pollinators are considered attractive signals to potential pollinators. Eria coronaria, which has an unusual autumn flowering season in some locations, has a bright yellow spot on its labellum. This spot has been hypothesized to function as a visual attractant to potential pollinators because its color may be attractive to insects. We tested this hypothesis using field observations between October and November of 2006 in the Yachang Nature Reserve, Guangxi, southwestern China. Honeybee (Apis cerana cerana) was the only pollinator of this orchid. Generally, honeybees landed directly on the yellow spot on labellum, and then adjusted their position and entered the flower. When honeybees retreated from flowers, the pollinaria were adhered on their thorax, or the pollinaria carried by honeybee were stuck on the stigma. The anther cap, however, did not separate from the column when the pollinaria were carried away from the flower. This orchid did not provide any rewards to honeybees. The flowers of a co-blooming plant,Pittosporum glabratum, with abundant nectars and pollens, attracted numerous honeybee visitations during this period. The flower color and size of P. glabratum were similar to that of the spot on the labellum of E. coronaria. Based on the behaviors of honeybees on the flowers of bothP. glabratum and E. coronaria, we speculated that the bright yellow spot on labellum of this orchid functions as an attractant to honeybees. Hand-pollination experiments showed that E. coronaria was self-compatible, but that reproductive success depended on pollinators. Under natural conditions, the rate of fruit set of E. coronaria was 20.72%, which is very close to the average fruit set of other food deceptive orchids (20.7%).

Key words: yellow spot, labellum, attractive signal, Apis cerana cerana, anther cap, co-blooming plants