Pollination by deception

By Jenny Cullinan and Karin Sternberg

Disperis capensis (Cape witch orchid) which has no nectar or other reward for bees, uses deception to attract the male carpenter bee to its flowers for pollination.

Mild winter days of late July, bring the first flowerings of Disperis capensis. The witch orchid times its display in this section of Cape Point Nature Reserve with the similarly coloured Muraltia (purplegorse). Overcast days make the purple of both Muraltia and the orchid stand out in an otherwise flower-barren patch. The orchid has a gentle, but beautiful sweet scent, reminiscent of both a component of Serruria villosa’s fragrance as well as Wurmbea hiemalis. Muraltia has little to no fragrance perceivable to the human nose.

Xylocopa rufitarsis visits Muraltia for nectar and sometimes mistakenly visits the delicately-scented orchid. Realising that there is no nectar to be had, he immediately flies off, the sticky viscidium adhering to the visiting bee and a pollinarium is withdrawn; it becomes immediately coiled so that the pollen massulae (individual pollen grains) become outwardly orientated so as to be correctly positioned to break off onto the stigmatic surface of the next Disperis visited by the bee. These are visible as individual grains on the sticky orchid stigma (Bill Liltved). The carpenter bee seems quite irritated with the pollinaria stuck under his thorax, but continues collecting nectar from Muraltia in a methodical way, only to make the same mistake with another orchid depositing the pollen in this way. 

Once D. capensis is fertilised it fades from its purple-pinks to a burnt orange and her bonnet folds in on itself.

It was previously thought that Disperis capensis only mimicked the nectar-secreting shrublet Polygala bracteolata, (Johnson 1994; Pauw & Johnson 1999), but these observations show how fascinating the unknown is and how much there still is to discover.

In a completely different biome in Cape Point Nature Reserve, we have found pockets of D. capensis resembling none of the other flowers surrounding them, neither in colour, shape nor fragrance. The only flowers in close proximity are Metalasia compacta, Diastella divaricata and Lobelias. However, the orchids in this location still have to be visited by a bee.

Disperis capensis and Muraltia. Both displaying similar colour combinations.

X. rufitarsis, realising that there is no nectar to be had, quickly flies off from the orchid carrying the pollinaria under his thorax

The pollen grains are outwardly orientated as he flies from flower to flower sipping nectar

An unpollinated orchid

A pollinated orchid

After fertilisation the orchid’s bonnet folds in on itself and turns a burnt orange

As a small exercise to show how a pollinarium is released, we mimicked a bee’s arrival by touching the sticky viscidium with a sterile tool, as a carpenter bee would make contact with the orchid with its thorax, and slowly withdrew the spring-loaded pollinarium from the anther sac while photographing it. Once released it immediately coiled.

We then took the pollinarium to a separate orchid, touching it on the sticky stigmatic surface as the carpenter bee would come into contact with it. Immediately the pollen grains broke off from the caudicle, the stalk to which the pollen masses are attached, leaving individual grains of pollen on the stigma.

Further reading:

The Cape Orchids (Liltved & Johnson 2012)

 

The authors at work:

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