Weevil species released into the Crocodile River to combat invasive alien aquatic plant

By Maile Matsimela

In an effort to combat the growing threat of the invasive alien plant Salvinia minima to South Africa’s freshwater ecosystems, Professor Julie Coetzee, the DSTI/NRF-SAIAB SARChI Chair in Biological Control and Freshwater Alien Invasive Species Management, has overseen the release of the biological control agent Cyrtobagous salviniae into the Crocodile River in Hartbeespoort.

The South African Institute for Aquatic Biodiversity (NRF-SAIAB), a research facility of the National Research Foundation (NRF), plays an important role in the conservation of the country’s biodiversity. 

Prof Coetzee was joined by the Deputy Minister of Water and Sanitation, Isaac Sello Seitlholo, in releasing the Cyrtobagous salviniae, a weevil species, into the river on Monday, 17 March 2025. 

The weevil will target Salvinia minima, commonly known as common salvinia, an invasive alien aquatic plant native to South America that presents a substantial threat to South Africa’s freshwater ecosystems.

Initially recorded at Hartbeespoort Dam in 2011, salvinia has since spread across various water bodies in the North West and Gauteng provinces and has extended its range to the Limpopo River, which forms South Africa’s national border with Botswana.

“If we don’t control this plant and if it is allowed to spread, we will see it moving into more and more water bodies across the country. Unfortunately, it thrives in polluted systems,” said Prof Coetzee.

She added that this plant, and other invasives like it, block out the sunlight into the water body, which reduces oxygen. 

“Plants in that water body won’t be able to create oxygen through photosynthesis. If there’s no oxygen, there are no fish, no crabs, no insects. It completely destroys or alters the aquatic ecosystem.”

Through Rhodes University’s Centre for Biological Control (CBC) as well as in her capacity as DSTI/NRF-SAIAB SARChI Chair, Prof Coetzee has conducted extensive research over several years to evaluate the efficacy of Cyrtobagous salviniae (Florida biotype) as a biological control agent for salvinia. She is also the Principal Scientist at the South African Institute for Aquatic Biodiversity (NRF-SAIAB), where she leads the development of sustainable management strategies for invasive aquatic species.

The Cyrtobagous salviniae released into the Crocodile River was imported from Louisiana, a state in the US that also battles Salvinia minima. 

“We’ve released a small starter population while we are rearing them at rearing facilities. We have one at Rhodes University in Makhanda, and we’ve also set up community rearing stations around Hartbeespoort Dam, Vaalkop Dam and Bronkhorstspruit. We have a lot of community partners who have realised the value, the sustainability and the cost-effectiveness of these insects,” says the Prof.

Seitlholo applauded Prof Coetzee and her team for tackling the salvinia problem at what he termed the beginning of a value chain. 

“Water pollution in this country is not something that people want to talk about,” he said, adding that every single day, “My mantra remains the same: If we don’t tackle the beginning of a value chain, we’re going to have a crisis in terms of the quality of what comes out of taps.”

He warned, “If we don’t tackle the beginning of a value chain, we will have so many Hammanskraals in this country,” referring to the Tshwane community that experienced water supply issues. 

“We all know it was due to a deeply polluted river that the Temba Water Treatment Plant could not treat the water to potable standards.”