Julie Coetzee, the deputy director of the Centre for Biological Control at Rhodes University, oversaw the release of the Cyrtobagous salviniae biological control agent into the Crocodile River.
A tiny weevil species has been released into the Crocodile River near Hartbeespoort to tackle the spread of the common salvinia, an invasive alien aquatic plant that is a growing threat to South Africa’s freshwater ecosystems.
On Monday Julie Coetzee, the deputy director of the Centre for Biological Control at Rhodes University, oversaw the release of the Cyrtobagous salviniae biological control agent into the Crocodile River.
The 2mm-long weevil species is a subaquatic herbivorous insect that feeds on the common salvinia (Salvinia minima), which is native to South America.
Since first being recorded at Hartbeespoort Dam in 2011, salvinia has spread across various water bodies in North West and Gauteng and has extended its range to the Limpopo River, which forms the national border with Botswana.
“If we don’t control this plant, 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,” noted Coetzee, who was joined by the deputy minister of water and sanitation, Isaac Sello Seitlholo.
This plant, and other invasives like it, block out the sunlight from the water body, which reduces oxygen, she said. “Plants in that water body won’t be able to create oxygen through photosynthesis. If there’s no oxygen, there are no fishes, no crabs, no insects. It completely destroys or alters the aquatic ecosystem.”
Salvinia is on the move in the country’s waterbodies.
“We first recorded it around Hartbeespoort Dam and we’ve gradually been finding it in more of these Highveld dams. It was on Bon Accord Dam near Pretoria, Roodeplaat Dam, Bronkhorstspruit Dam and then we found a huge infestation on Bospoort Dam, which is on a different river — the Hex River, just north of Rustenburg,” Coetzee said.
“The Hex river flows into Vaalkop Dam and Vaalkop now has a huge salvinia minima problem at the Hex River itself and then Vaalkop overflows into the Elands [River], which joins the Crocodile, after Roodekoppies Dam, so now Roodekoppies Dam has got some salvinia minima from Harties.”
Vaalkop and Roodekoppies are near each other and join in the Crocodile River. “Then it flows all the way to the Limpopo River on the border with Botswana and we found it in the Limpopo River as well.
“And the problem with that is that it’s on our border with Botswana; it’s going to flow into Zimbabwe, so it’s a bit of an issue … We should have a mandate to control anything coming downstream from our invaded systems.”
In part through Rhodes University’s Centre for Biological Control, Coetzee has conducted extensive research over several years to evaluate the efficacy of Cyrtobagous salviniae as a biological control agent for salvinia. The centre was granted approval for release of the insect late in 2024.
“It’s used in Florida [United States] quite extensively and they don’t really regard salvinia minima as much of a problem plant anymore. We know from that success, that we are highly likely to get success in South Africa as well,” she said.
The weevil was imported from Louisiana in the US, which also battles Salvinia minima.
“We’ve released a small starter population,” Coetzee said of the 150 insects that were unleashed on Monday. “But what we will be doing is rearing them at rearing facilities. We have one at Rhodes University in Makhanda.
“We have 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.”
Another batch of 100 insects was put into a rearing station at Lakeland, an estate at Hartbeespoort that is “already rearing the water hyacinth control agents [the water hyacinth hoppers] for us”.
Seitlholo applauded Coetzee and her team for the way they were tackling the salvinia problem “at the beginning of a value chain”.
