Two orchids attract different pollinators with the same floral odour compound: ecological and evolutionary implications

Abstract

1. The sexually deceptive orchids Chiloglottis trapeziformis Fitzg. and Chiloglottis valida D.L. Jones are pollinated by the thynnine wasps Neozeleboria cryptoides (Smith) and Neozeleboria monticola Turner, respectively, during attempted mating with the flower labellum. The orchids and their pollinators occur mostly in allopatry; however on the overlapping edges of their ranges they can attract each other's pollinator, leading to rare hybrids. 2. Using gas chromatographic analyses with electroantennographic detection (GC-EAD) we demonstrate that 2-ethyl-5-propyl-1,3-cyclohexandione ('Chiloglottone'), the previously known sex pheromone of N. cryptoides and pollinator-attractant of C. trapeziformis, is also used by C. valida to attract its pollinator N. monticola. 3. In behavioural field experiments, equal numbers of males of both species responded to, and attempted copulation with, a dummy scented with synthetic Chiloglottone. In dualchoice experiments where dummies were presented at different heights, N. cryptoides males preferentially attempted copulation with dummies a few centimetres above the ground, while N. monticola males significantly more often landed on the soil and attempted copulation with dummies on the soil. 4. These behavioural differences corresponded with differences in floral height in the orchids, C. trapeziformis being about twice as tall as C. valida. We suggest that this contrasting pollinator behaviour imposes selection for floral height in the orchids.