The Fascinating World of Roridula

I recently had the pleasure of attending a talk by Leon Kluge on interesting pollination mechanisms in fynbos plants. In the course of the talk he mentioned that Roridula gorgonias is pollinated by the juveniles of the assassin bug Pamiridea roridulae. I found this information intriguing. Why? Let me start with some background about Roridula.

Roridula is the world’s largest “fly catcher” plant. There are two species in the genus Roridula: R gorgonias that grows in the mountains of Hermanus and R. dentata that grows in the Ceres mountains. Roridula occurs nowhere else on earth. What I’m going to say applies to both species.

Roridula in the wild – Photo by Joseph D.M. White

Roridula grows high up in Fernkloof in boggy seeps. There’s enough water, but – as is common in fynbos soils – very little in the way of nutrients. They need to supplement to survive. There are many examples of fynbos plants which form symbiotic relationships with fungi or bacteria that assist in sourcing nutrients. A well-known example is the sundews (genus Drosera) that have ‘hairs’ on their leaves that exude a sticky mucus to trap insects. The leaves themselves secrete enzymes which ‘digest’ the prey and absorb the nutrients through their thin cuticle.

Roridula is also carnivorous – but has a problem in that it has no digestive enzymes in its leaves. Its leaves also have glands that secrete extremely sticky ‘glue’ drops on the end of the trichoma ‘hairs’, so it traps loads of insects, but cannot digest them for their nutrients.


Photo right: Assassin Bug Pameridea roridulae – Photo Joseph D.M. White

Photo left: Assassin Bugs killing the plant’s ‘catch’


Instead, the plant relies on a symbiotic relationship with a little assassin bug (called Pameridea roridulae) that scurries across the leaves without getting stuck to the glue – thanks to a ‘lubricant’ on its exoskeleton and powerful legs that can unstick themselves. The bug attacks any insects caught on the leaves.

Being an assassin bug, it has piercing mouthparts and injects poison and digestive enzymes into the unfortunate prey and then sucks up its juices. Having had its meal, the bug then defecates on the plant. The digested nutrients can be absorbed either by the plant’s leaves or by the roots if the dung should fall into the water. A win-win for both bug and plant.

So the plant has sufficient nutrients to grow well, but still needs to be pollinated. Why was I surprised to hear that the juvenile bugs pollinate the flowers? The bug nymphs are tiny and flightless, meaning that they would not be able to get from one flower to another to cross-pollinate the flowers. Self- pollination produces inferior seeds because there is no mixing of genetic material.

Roridula flowers – Photo Candide

Moreover, Roridula invests precious energy and resources into producing large pink flowers to attract insect pollinators like bees that would do this job. Why would the plant develop a relationship with a bug to pollinate itself as well? The truth is that flighted pollinators are few and far between high up in the mountains where Roridula grows, so cross-pollination, which would produce’ fitter’ seeds, would be likely to have a low success rate. Both belt and braces are needed.


Roridula grows in fynbos which is characterized by frequent fires that maintain species richness and diversity, and a wide variety of plant responses have evolved to cope with fire. Although many plant species resprout after fire, others, including Roridula, do not. Instead, they grow again from seed. Fire stimulates seed germination, and a single cohort of seedlings springs up after each fire event. So reliable seed set between fires is crucial.

Reliance on scarce flighted pollinators is dicey, even if the seeds are fitter. Self-pollination or “selfing” is more reliable in the circumstances, but there is still a chance that pollen grains produced by the stamens do not find their way to the stigma of the same flower left to their own devices. Getting the help of a little baby bug pollinator in what is called “facilitated selfing” would make pollination more certain – and therefore ensure the production of large numbers of seeds, even if they are not top quality. But cross pollination is still first prize, so the stamens on the flowers ripen a day after the stigmas become receptive to pollen, giving cross pollination by any flying pollinators that may be around a slight advantage. Facilitated selfing does not preclude cross-pollination. But in the long run, facilitated selfing can provide reproductive assurance and even be a strong selective pressure for the evolution of self-compatibility and self-fertilization.

The large showy Roridula flowers have a secondary function besides attracting the odd wayward bee: the flowers close at night and provide a shelter for the baby bugs inside the flowers.

Mutualism between Roridula and Pameridea – Photo Joseph D.M. White

So it appears that the extremely species-specific mutualism between Pameridea and Roridula may well extend beyond simple nutritional benefits. This mutualism also ensures that Roridula will rise again after a fire. This plant has all its bases covered, and shows that having the odds stacked against you doesn’t necessarily mean defeat – joining forces with what may seem like an unusual ally can mean that you both survive.