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Hunting aliens from space

- Shaun Smillie

Wits researchers are using high-tech imagery and biological agents to save our water resources and economy from invasive alien plants.


Lerato Molekoa scans satellite images from high-definition cameras and sensors orbiting Earth. She is hunting for aliens. Not little green men with three fingers. Molekoa, an MSc student in the School of Animal, Plant and Environmental Sciences (APES) at Wits, is hunting for alien vegetation here on Earth. She calibrates the satellite data to detect subtle light changes reflected in the leaves of salt cedar trees. This helps identify alien plants that are using our water. 

Molekoa is trying to work out the distribution of three different species of salt cedar trees. One of the trees, Tamarix usneoides, is indigenous while two other close relatives are exotics. The problem is that, up until now, only genetic analysis of the three different species can differentiate them. 

Alien invasion 

Mining houses use all three species to rehabilitate mines by removing contaminants from the soil and water – a process called phytoremediation. Salt cedar trees planted in paddocks at the base of storage dams leech sulphates and heavy metals from the water, and are important to combat acid mine drainage. 

“The mines have been using the alien species, which we are not happy about,” says Professor Marcus Byrne from APES. “Then we got drawn into finding out how to identify an alien species and what is not.” 

The two exotic types of salt cedar are invasive alien species, which severely affect South Africa’s water resources by consuming ±4% of the annual mean water runoff. 

“This is the water that makes its way off the land into rivers and dams, for our use, and instead it is used up by invasive species,” says Byrne. 

Invasive species also influence the economy, with an estimated annual cost of around R6.5 billion in 2007. 

A further problem specific to the genus Tamarix is that the exotic species interbred with the local species, resulting in hybrids. This reduces the local biodiversity by diluting the genetic integrity of the native species. 

“We knew we had these three species, but we didn't know how to identify them until we did the genetics,” says Byrne. Working out the distribution of the three species would be harder work without satellite imagery. 

“DNA is very expensive and time consuming, but with satellites you can basically tell the image to go and find where the species is located within a certain region,” says Molekoa, who downloads satellite images free and then uses software to set search parameters to find the colour signature of the trees within the image.


The jury’s still out whether it is the exotics or the local species of Tamarix that are best at phytoremediation. Sipho Mbonani, an MSc student in APES, believes it is the local species. 

“I remember going into the field to look at phytoremediation and we noticed that there were indigenous and alien plants together, and we could tell the aliens by the way their leaves were yellowish in colour. So I don’t think they are doing so well,” says Mbonani. 

Put a bug in it 

The next step to control alien species is to introduce insects as biocontrol agents. The search is on for an insect that eats only the exotic and not the local species. Satellite data also help scientists fight invasive aquatic species like water hyacinths, which choke waterways across Africa. 

 “A blanket of water hyacinth across a water body blocks out light and oxygen, and uses water. Consequently, the whole nature of the water system changes, and many native animals and plants disappear from that system,” says Byrne. 

Eight biocontrol agents have been introduced to control water hyacinth, but no one knows how effective these agents are and how local conditions influence their lifecycles and efficacy. 

Water pollution, for example, affects one of these agents, the water hyacinth weevil, Neochetina eichorniae. Jeanne D’Arc Mukarugwiro, a PhD student in APES, uses satellites to monitor the extent and movement of water hyacinth, and the wellbeing of the water hyacinth weevils in rivers with high turbidity, in Rwanda. Turbidity is the cloudiness of a fluid. High turbidity results from the suspension of tiny soil particles in water, which might affect weevils living on the plants. 

Scientists theorise that the high concentration of particles suspended in the water suffocates the weevil in its underwater pupal state. Further research is required to establish if the weevil faces the same problem in South Africa. Byrne points out that, despite the effectiveness of biocontrol agents (because insects don’t make mistakes), only 2% of South Africa’s weed control budget is allocated to research in this field. 

The water hyacinth might have the upper hand over its biological control agents currently, but the poster-child of biological control in South Africa is Stenopelmus rufinasus, the frond-feeding weevil that eradicated red water fern. 

Death by mail 

In the 1940s, the red water fern came from South America, just like the water hyacinth. By the 1980s, red water fern had become a widespread problem. Wits University, the Agricultural Research Council, and Rhodes University collaborated to find a solution. By 1997, they had found the bug that they thought would do the business. 

Death came to the red water fern via the post. 

Farmers who suspected a red water fern infestation sent a photograph to the researchers. On confirmation of red water fern, the scientists posted a polystyrene box containing the weevil. 

“We would tell them to open the box and just throw it in the dam, and in six months it will be gone, and we promised if it was not gone, we would come and clear it from them. It worked every single time,” says Byrne, of what is considered one of the world’s most successful biological control programmes. 

Back then, the postal service helped eliminate one invasive species that threatened South Africa’s water systems. Now, with the help of satellites and their software, the fight has become high-tech. 

“My dream one day is that we have such fine resolution that we will be able to tell from the satellite photo whether the plant has biocontrol agents or not.” 

Read more about the research conducted across faculties, disciplines and entities to help secure humanity’s most important resource for survival: water, in the fourth issue of Wits' new research magazine, Curiosity.