Mesquite (Prosopis species) were brought into South Africa by the government in the late 1800s and promoted for planting until the 1960s, according to the Agricultural Research Council. Photo: Supplied
The thorny mesquite tree with its yellow flowers and long pods was brought to South Africa to provide food and shade for livestock and for firewood but this useful tree has become invasive, reducing grazing and groundwater.
Mesquite (Prosopis species) were brought into South Africa by the government in the late 1800s and promoted for planting until the 1960s, according to the Agricultural Research Council.
Professor Brian van Wilgen, of the Centre for Invasion Biology at Stellenbosch University,
says, “The mesquite comes from arid areas in the Americas and was brought to South Africa to provide shade for livestock in dry areas, as a fuel wood and also because the pods are highly nutritious and can be used as a food source by livestock.”
It also provides nectar for honey production and timber for furniture.
The main Prosopis species that have been introduced are P glandulosa (honey mesquite) and P velutina (velvet mesquite) and P chilensis (Chilean mesquite). It looks similar to an acacia but it’s only a distant relative.
This tree is defined as invasive because it spreads rapidly and is harmful to the environment, affecting people’s livelihoods. It forms large stands, uses huge amounts of water and because nothing grows under it, it eliminates grazing, he says.
“The introduction of mesquite was a big mistake,” says Van Wilgen, adding that the government knew that the trees were invasive 50 years ago, but there weren’t programmes put in place to manage it.
Mesquite is found in drier parts of South Africa, particularly the Northern Cape, Western Cape, Free State and North West.
Philip Ivey, a researcher at the Centre for Biological Control at Rhodes University, added that Prosopis invasions also have serious negative effects on biodiversity. Numerous African countries including Ethiopia, Kenya, Malawi, Namibia, Botswana, Uganda and South Africa have problems with this invasive species. It is also considered a serious invader on several islands, the Middle East and parts of Asia and Australia.
Research by the Council for Scientific and Industrial Research (CSIR) shows that “Dense stands of Prosopis trees use more than five times the amount of water (544mm a year) than sparse indigenous trees (91mm), substantially reducing groundwater resources on which people are almost entirely dependent in these arid areas. The invasion also changes vegetation structure.”
The CSIR found that the problem has reached a stage where the invasions are set to grow exponentially, which could result in the collapse of sustainable agriculture across vast tracts of land unless adequately addressed.
The invasions are also a significant threat to biodiversity; native species decline sharply as a result of direct competition or because the habitat becomes unsuitable for indigenous
species to grow.
Seed-feeding beetles
In the 1980s beetles that feed on mature Prosopis seeds were introduced in 2020 as biological control agents in South Africa, but don’t have a meaningful effect on the rate of invasion, according to Ivey.
After extensive discussions with farming communities, researchers studied another biological control agent — the podlet weevil from South America — that damages immature pods and Prosopis leaves respectively.
The department of agriculture, land reform and rural development, approved the release of both the seed-feeding beetle and the podlet weevil.
In 2022, Rhodes University, in collaboration with the universities of Cape Town and the
Witwatersrand as well as the Fundación para el Estudio de Especies Invasivas (FuEDEI) in Argentina, facilitated the first releases of the podlet weevil.
UCT researcher Catharina Kleinjan, who manages the project, said the weevil attacks immature pods and kills the developing seeds. Because the pods are attacked while they are still immature they are not yet attractive to livestock and wildlife, she said.
Taking seed out of the system early could make a substantial contribution to the management of Prosopis because the consumption of mature pods with viable seeds is one of the factors considered responsible for the rapid spread of the tree.
Regarding the status of the project Kleinjan said: “We are now at the point where 1) tests confirming that the podlet weevil only feeds on Prosopis have been completed 2) permission to release this agent has been granted, and 3) the first releases of the podlet weevil have been successfully achieved. This is a significant milestone, but it is still just a very early step in the overall development of this biological control agent.”
The department of forestry, fisheries and the environment’s natural resources management programme has provided support and funding for the research into development of biological control agents for many invasive alien plant species in South Africa, including Prosopis.
MannaBrew
A new use for the mesquite has been found by two brothers, Aam and Brandt Coetzee, in the Northern Cape. Their company, MannaBrew, is using the seed pod to brew coffee after first developing it as a health supplement.
“We started Manna Health in 2005, making supplements from mesquite pods. Manna Blood Sugar Support was our first product. Then later we started with the shakes. About three years ago we started experimenting with making coffee with mesquite pods. After many tests and trials, we landed with MannaBrew,” Aam Coetzee says.
The brothers say the presence of this tree has created jobs for people in Prieska. The people in the area collect the pods which are then weighed and they are paid according to the weight of the produce collected. This helps to provide them with seasonal work and supports their local economy.
“We create seasonal jobs where the products are harvested. Between 500 and a 1 000 people at a time are involved with the harvest. The processing is done in Somerset West and we employ local people there. It is a win-win for everyone involved,” says Brandt Coetzee.
The brothers say it would be impossible to completely remove the plant and that the mesquite’s invasion could be limited if it were managed.
“We are working with different universities and the government as part of the management of the mesquite. It would be impossible to totally eradicate the plant, but there is a lot of potential when you start to use different parts of the plant in the management process, creating numerous jobs,” says Brandt Coetzee.
Harvesting the seed pods helps to reduce further infestation, protecting valuable farmland and scarce underground water reserves which sustain people’s livelihoods.
“There is good and bad in every plant/tree and one just needs to think out of the box regarding the value that can be added,” says Brandt Coetzee.
Not a total solution
Van Wilgen says that the way in which MannaBrew is dealing with the invasive tree will not do any harm but it also won’t solve the problem.
“The problem is too big. The value of the coffee they produce is tiny compared to the damage that this plant does.”
He says there is no way the removal of the mesquite can be held off because of the damage it does.
The best way to deal with it is to clear it in certain areas by getting some insects to attack it to make the job a little bit easier” says Van Wilgen.
Lesego Chepape is a climate reporting fellow, funded by the Open Society Foundation for South Africa