The evolution of beetle pollination in a South African orchid

The pollination biology of the orchid Ceratandra grandiflora was investigated in order to determine whether the partial loss of a specialized floral reward (i.e., oil) was the result of an incomplete shift from one specialized pollinator to another. In the three-species clade of section Ceratandra, there has been a progressive loss of the oil-secreting callus. lt is always present in C atrata, sometimes present in C. grandiflora, and never present in C. globosa. Thirty-nine to 67% of individuals in populations of C. grandiflora bear the callus gland, but gland presence has no signifikant effect on the proportion of flowers pollinated. Pollinator observations show that the shift in pollinators is complete and that the oil-secreting callus is a vestige of the ancestral oil-bee pollination system that no longer plays a role in pollination. C grandiflora is pollinated almost exclusively by a single species of hopliine beetle (Scarabaeidae). Experiments with artificial flower traps indicate that color alone can explain the attraction of beetles to C. grandiflora, despite the absence of a floral reward. The proportion of C. grandiflora flowers pollinated (50.2 and 61.1%; N = 524 and 324 flowers, respectively) is unusually high for a plant that relies on generalized food deception and is probably due to the use of inflorescences as mating sites (i.e., rendezvous pollination).     

Specialized pollination by spider-hunting wasps in the African orchid Disa sankeyi

Disa sankeyi Rolfe was found to be pollinated almost exclusively by Hemipepsis wasps (Pompilidae) at sites across its range in the Drakensberg mountains of South Africa. Wasps of both sexes appear to locate the cryptic yellow-green flowers primarily by using scent cues, as they show a classic zigzag flight pattern when approaching inflorescences. The sweet-spicy fragrance emitted by the flowers is strong enough to be discernable by a human from several metres away and consists of a blend of at least 65 volatile compounds, with (E)- cinnamic aldehyde and eugenol as the major constituents. On settling, wasps lick the exposed droplet of nectar that is secreted from the mouth of the reduced floral spur on each flower. Pollinaria become attached to the front feet of the wasps as they grasp flowers adjacent to those on which they are feeding. Pollen transfer to stigmas similarly takes place on flowers adjacent to those that are used as a nectar source. Almost 100% of flowers in the study populations were found to have been pollinated. Specialized pollination by pompilid wasps appears to be a rare phenomenon that has been reported only in southern African plant species. It appears to be associated with cryptic green-yellow flowers and highly accessible nectar. Behavioural assays are required to determine whether the basis of the specialization in this pollination system lies in a particular chemical composition of the floral fragrance.