‘Setting a thief to catch a thief’ – new biological control agent (weevils) released to tackle invasives at Hartbeespoort
Deputy Minister of Water and Sanitation Sello Seitlholo (left) and Professor Julie Coetzee release the new biocontrol agent on the invasive aquatic plant common salvinia at Hartbeespoort Dam. (Photo: Julia Evans)
By Julia Evans | 19 Mar 2025
A new biotype of weevil has been released at Hartbeespoort Dam, marking its first appearance in South Africa. Speaking to leading experts in the field, Daily Maverick unpacks what exactly biological control is, why it is needed, and how it works.
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“It’s setting a thief to catch a thief,” said Dr Martin Hill, head of Centre for Biological Control and entomology professor at Rhodes University. He was explaining how tiny insects imported from abroad are being used to control alien invasive species that also originate elsewhere.
On Monday morning, 17 March 2025, the Florida biotype of a type of weevil (Cyrtobagous salviniae) was released in South Africa for the very first time, after travelling from labs at Louisiana State University in the US.
These tiny weevils (1.5-2mm) are a biocontrol agent for common salvinia or water spangles, a small floating plant that grows on freshwater surfaces. Native to South and Central America and the West Indies, these plants are invasive in South Africa.
Deputy Minister of Water and Sanitation, Sello Seitlholo, had the honour of releasing these weevils on to a mat of common salvinia, on the Crocodile River, which flows into Haartbeesport Dam on Monday — marking a new chapter in the fight against invasive aquatic plants.
But what exactly is biological control, why is it needed, and how does it work?
Daily Maverick unpacks these questions with insights from leading experts Professor Julie Coetzee — an authority on biological control and freshwater invasive species management, deputy director at the Centre for Biological Control at Rhodes University, and principal scientist at the South African Institute for Aquatic Biodiversity (SAIAB) — and Professor Martin Hill.
“I think that if people are informed, they accept it a lot more,” said Hill.
The problem: invasive aquatic plants
Q: Why are invasive aquatic plants a problem?
A: Invasive species like water lettuce and common salvinia threaten aquatic ecosystems and human activities. They form dense mats that block sunlight, reduce oxygen levels, and kill fish, crabs, and insects. This degradation affects biodiversity and affects recreational activities like fishing and boating.
“If they block out the sunlight into the water body, there can be no oxygen… no fish, no crabs, no insects,” explained Coetzee. “You completely destroy or alter the aquatic ecosystem.”
Q: What species are a concern in Hartbeespoort Dam?
A: Common salvinia is now a major concern, following historical issues with water hyacinth. Other invasive plants like parrot’s feather, Mexican waterlily, and yellow flag iris also pose threats, especially in wetlands.
Q: Has water hyacinth been controlled in Hartbeespoort Dam?
A: Water hyacinth has plagued Hartbeespoort Dam for decades, becoming a problem by the late 1960s. Biological control began in the 1990s but was hampered by herbicide use (which kills the weevils as well as the plants they’re trying to eat). A turning point came in 2019 when the Centre for Biological Control launched a new approach without herbicide control — involving community rearing programmes for the biocontrol agent, Megamelus, a planthopper that prevents water hyacinth from flowering, reducing seed production and slowing its regrowth. By 2020, Hartbeespoort Dam saw significant improvement, with water hyacinth levels dropping dramatically.
However, as water hyacinth declined, common salvinia surged (2021), clogging boat motors and water pumps. Water hyacinth resurges annually due to long-lived seeds. This cycle repeated for five years until 2023, when neither species saw a major resurgence.
Graph showing time series of water hyacinth and Salvinia minima cover on Hartbeespoort Dam, derived from satellite data. Over 2021 and 2022, Salvinia minima dominated the water surface during winter. Water hyacinth declined due to biological control, which allowed Salvinia minima to take advantage of the available nutrients and space in Hartbeespoort Dam. (Source: Centre for Biological Control)
Q: But I still see water hyacinth in Hartbeespoort — is biocontrol failing?
A: Biocontrol is working; however, water hyacinth seeds remain viable in the sediment for years, leading to recurring infestations. Biological control agents damage water hyacinth before flowering, depleting these seed reserves over time.
The good news is that biological control agents damage water hyacinth before they have a chance to flower (and produce seeds), which means that over time, these reserves will deplete. “Everybody sees (water hyacinth still around) and says, oh typical government stuff… but it is long term,” said Hill. “We’re depleting the seed source and it’s not flowering as much,” he said.
Common salvinia — the burgeoning new threat
Q: What is common salvinia?
A: Common salvinia (Salvinia minima) is an invasive aquatic plant from the Amazon. Identified near Hartbeespoort Dam in 2012, its spread has accelerated since 2021, threatening aquatic ecosystems with rapid growth and dense mats. Unlike water hyacinth, common salvinia reproduces vegetatively, with fragments growing into new plants, lacking persistent reproductive propagules.
Q: Where is common salvinia found in South Africa?
A: It has spread across several major water bodies, including Hartbeespoort, Roodekoppies, Bronkhorstspruit, and even the Limpopo River, posing a threat to neighbouring Botswana, Zimbabwe, and Mozambique.
Q: How did common salvinia get to South Africa?
A: The exact pathway is unclear, but possibilities include introduction via ornamental ponds, aquarium trade, or unintentional transfer by birds and boats.
Below the surface: What’s fueling the problem?
Q: What causes these alien invasive plants to multiply so fast?
A: The issue at hand is eutrophication — a process where water becomes overloaded with nutrients like nitrogen and phosphorus, triggering excessive growth of alien plants. These excess nutrients, often from fertilisers, sewage, and industrial waste, act as fertilisers, allowing invasive species to outcompete native plants. At Hartbeespoort Dam, one of the major contributors is malfunctioning wastewater treatment works, such as Percy Stewart, which release untreated or partially treated sewage into the water.
Addressing the pollution in the water (causing eutrophication) is vital to stopping this problem.
The solution: biological control
Q: What is biological control?
A: Biological control uses natural enemies like insects to control invasive plants sustainably, reducing the reliance on harmful chemicals.
Q: How do biological control agents kill the plant?
A: Weevils like Cyrtobagous salviniae are host-specific, targeting only salvinia. They feed on the plant, causing it to become waterlogged and sink, reducing its ability to regenerate.
Q: Are these weevils safe?
A: Extensive testing confirms that these weevils feed exclusively on common salvinia. “This is highly targeted control,” says Coetzee. They won’t feed on other native plants and will die if common salvinia is not available. Coetzee noted the biological control of prickly pear in South Africa, introduced years ago, shows how effective and safe this method can be. The agent introduced to control prickly pear continues to keep the plant at a low level, without affecting other plant species.
Q: Why is biological control better than other methods?
A: Biological control is cost-effective, sustainable, and environmentally friendly. The Department of Water and Sanitation spends R22-million annually on physical removal and has invested another R15-million in nanotechnology, while herbicides are costly and potentially harmful. Biological control agents remain in the system, naturally managing plant populations by preventing flowering and slowing regrowth. Initial research requires investment, but ongoing costs are minimal.
This tiny biocontrol agent released in South Africa this week, at just 1.5mm in size, was the Florida Biotype of Cyrtobagous salviniae, a weevil originally found in Florida, US, that is used to for the biological control of Salvinia minima, an invasive plant. (Photo: Julia Evans)
Professor Julie Coetzee, deputy director at the Centre for Biological Control at Rhodes University and principal scientist at the South African Institute for Aquatic Biodiversity, speaks at the official release of the new biocontrol agent Florida Biotype for the biological control of Salvinia minima in South Africa, at Hartbeespoort Dam on 17 March 2025. (Photo: Julia Evans)
The Crocodile River, which flows into the Hartbeespoort Dam, where common salvinia has taken over since water hyacinth has declined in 2021. (Photo: Julia Evans)
Common salvinia (Salvinia minima) is an alien invasive aquatic plant that originates in the Amazon region of South America. It poses a significant threat to aquatic ecosystems due to its rapid growth and ability to form dense mats on the water surface. (Photo: Julia Evans)
Q: Why does biological control seem to take so long?
A: Biological control is a long-term process involving cycles of plant growth and insect feeding that can take years to reduce plant populations. Seasonal cycles play a role, with plants growing rapidly in spring and control agents catching up later. To enhance effectiveness, the Centre for Biological Control uses an inundative approach, rearing control agents for mass release, increasing early season feeding pressure.
Q: Is biological control expensive?
A: Costs are mainly during research, ensuring agents target only invasive species safely. The Centre for Biological Control manages invasive species at Hartbeespoort Dam with an annual budget of just R1.5-million. The Centre’s Sisonke Programme employs people with disabilities in Makhanda to rear control agents.
“Currently there is very little financial support for biological control of weeds from the government,” said Coetzee.
Q: When can we expect to see results?
A: It may take two to three years for the weevils to control Salvinia minima, depending on their adaptation. The success of controlling Salvinia molesta with the Brazil biotype of the Salvinia Weevil (introduced in the 1980s) is promising. Rearing stations support this effort around Hartbeespoort Dam and at the Sisonke site in Makhanda, involving community members in rearing and releasing agents.
Biological control is a long-term process, requiring patience. Despite ongoing battles, the shift to biocontrol has shown clear improvements, and continued efforts are crucial for sustained success. DM
