Biological control as a means to fight water loss

Black wattle
Black wattle

Rhodes University’s Communications Division recently sat down with Distinguished Professor Martin Hill from the Centre for Biological Control, to discuss the history of biological control at Rhodes University and how this method of fighting alien plant species is helping drought-stricken areas like ours replenish water loss.


Q: What is the history of biological control at Rhodes University?

A: Biological control in this country goes back to 1913, when South Africa started working on cacti. At the Centre for Biological Control here at Rhodes University, we still work on cacti, but we also work on trees, and one of our big thrusts is aquatic weeds.

South Africa has a series of floating aquatic plants from South America that were introduced mainly as pond ornamentals. And they, just like the trees, are basically water pumps. We estimate that a plant like water hyacinth is increasing water loss from water body by about 40%. We've done to two studies to quantify this, in partnership with the Departments of Economics and Economic History here at Rhodes University [under the supervision of Professor Gavin Fraser].

First we looked at New Year's Dam, which is in Alicedale, and found that through biological control, we took that 80 hectare dam from 100% covered with water hyacinth to 5% cover and the water saving there was tremendous. It saved millions of litres of water per hectare annually.

We then compared that to a bigger system on the Vaal River (Vaal Hart Irrigation scheme) and we did exactly the same - we looked at what the scenario would be with 100% cover and through biological control we took it down to about 20% cover, and then we looked at the water saving.

Interestingly, the value of the water raw water Alicedale was 26 cents per cubic meter (26 cents per thousand litres). On the Vaal River, the water was valued at over R30 per cubic metre and the reason for that is what the water was used for – high-end irrigation for pecan nuts and maize, for instance. What this illustrates, is that the value of a resource depends on what it is utilised for. 

 Q: Which plants are our biggest water threat here in Makhanda?

A: Here in the Eastern Cape, a dry area, Black wattle (Acacia mearnsii) are very invasive, but they also have value. Thus the biological control efforts have been limited to using insects that attack the reproductive parts of the plant, and in doing so, reducing its spread.  South Africa has recently released a tiny fly which attacks the flowers of the Black wattle. So instead of the plant producing seeds, it produces galls in which the larvae of the fly develop. This has essentially sterilised this plant in the Western Cape and is well established in the Eastern Cape. 

Silky hakea (Hakea sericea) are still a massive issue, and long leaf wattle is still problematic, although lesser now, and pine trees.  Those would probably be the main culprits.

But sometimes, new things appear. Recently on Mountain Drive, it seems like there's an invasive grass that's got in. We don't know a lot about it and we know less about how to control it, but we carry on fighting.

Q: How does biological control help with the current water crisis, directly or indirectly?

A: The water crisis that we are currently experiencing is multifaceted. We know there are certain challenges within the municipality. Also, we know with global climate change, this area is likely to become drier. However, invasive plants are definitely also one of main players and in many cases biological control can offer a sustainable solution contributing to water security.


You can read Prof Hill's first Q&A here: