Orchid Sexuality

Orchid sexuality is a very interesting topic that has been studied for decades.  It’s quite difficult to study the pollination patterns, however it’s interesting to note that certain elements can often produce great results.

Studying pollination in natural populations of sexually deceptive orchids can be difficult because of the brevity of each wasp visit and the often low frequency of pollinators. However, the populations of wasps and flowers are not congruent, and when “bait” flowers are introduced into populations of the appropriate wasp, males locate the flowers very quickly, and efficient experimental work is possible. Using this experimental approach with the deceptive orchid Drakaea glyptodon, Peakall (1990) identified two features that are critical to experimental design. First, following an initial rapid response at a single location, the number of visits declines dramatically and few visits are observed after 5 min. Relocation of the same flowers from several to many meters causes a renewed response. It is therefore necessary to move bait flowers regularly to maintain flower visits and to avoid trials near natural populations of the flower. Second, individual flowers can vary significantly in attractiveness, probably because of differing concentrations of emitted pheromones. Therefore, an attempt to use more or less equally attractive flowers is critical in pollinator-choice experiments. This is preferable to using many flowers of varying quality, which quickly deplete natural populations.

Although some pollination had occurred in these orchid populations, pollinators were uncommon. Therefore, each morning whole plants with fresh flowers were collected and transported in vials of water to an open eucalyptus woodland site (Felled Timber Road, Porters Retreat) 30 km west of Kanangra Boyd National Park, where the orchid was rare but the wasp pollinator was abundant. To test the hypothesis that floral height affects pollinator visitation rate, we conducted a series of choice experiments by synchronously presenting two flowers 15 cm high and two flowers at an alternate height of either 2, 8, 22, 30, 50, or 100 cm. In 1992, the alternate heights were restricted to 8, 22, 30, and 50 cm. The standard height of 15 cm was chosen because it approximated the maximum natural height of the flowers. Alternate heights ranged from ground level (2 cm) to 100 cm, which represents the maximum floral height observed for any sexually deceptive orchid in Australia. Thus, the experiments were restricted to biologically realistic heights even though thynnines frequently fly to and forage on shrubs and trees well over 100 cm high (Ridsdill Smith 1970a,b; Peakall 1990). To minimize the possible bias associated with unequally attractive flowers, we first selected flowers that each were attractive to the wasps in a preliminary presentation. In addition, each height choice experiment consisted of a set of 4, 6, or 8 trials in which different pairs of flowers were displayed together at each trial, and each flower was represented equally at both heights. At least three replicate experiments were conducted for each height choice with a different set of flowers. Each trial was conducted for 5 min at different locations and the time of day, number of wasp visits, and behavior during each visit were recorded. Many wasps circled the flowers, but a “visit” was recorded only if the insect landed on the plant.

Excerpt: Handel, Steven N., and Rod Peakall. “Pollinators discriminate among floral heights of a sexually deceptive orchid: implications for selection.” Evolution 47.6 (1993): 1681+.