Following a barrage of public comments on the state of Hartbeespoort Dam in the past weeks, Keneilwe Sebola, from Rhodes University’s Centre for Biological control (CBC), said this week the public perception is being fuelled by inaccurate information on the cause of the pollution which has led to the water being covered by invasive weeds.

Big and small have joined the fight against the salvinia

“In instances where frustrations are high, everyone who has a few facts will jump in to try to explain what they know or spread some more disinformation. I realised that it is crucial for us as scientists to use platforms such as social media to disseminate information while being flexible enough to engage the public or get them involved in the work we do,” she said.

Currently, Hartbeespoort dam is infested with Salvinia Minima which started covering the dam in April of 2021 following the clearing of water hyacinth as a result of the planthopper insect released by CBC in the past four years. In the absence of water hyacinth, common Salvinia exploded. It forms a layer over the water surface. “Now the litter in the dam is more visible as it sits on top of the plant. CBC is working on an insect to deal with the new alien plant. The insect is currently under quarantine and once it has passed all the regulatory requirements, will be released to combat the common salvinia,” she said.

“Many people who visit Hartbeespoort Dam do not know that water hyacinth has been an issue for Hartbeespoort Dam since the 1970s. It has been controlled using herbicides until 2017. The herbicides killed all plants when sprayed and the quick death of the plants led to the pea-soup state of the dam caused by algal blooms. The Department of Environmental Affairs, at the time, then embarked on a process to find environmentally friendly ways to control the alien plant. CBS started releasing the biological control agents in 2018 when the hyacinth covered almost 50% of the dam. Keneilwe has been closely involved in releasing the plant hopper, Megamelus scutellaris, on a weekly basis at different points around the dam from 2018 to 2019. By January 2022 the coverage of water hyacinth had reduced to less than 7%.

Boat owners, estates, individuals and a team of Working for Water, as well as Rotary Brits Hartbeespoort and WESSA, are trying their utmost to manually remove the invasive plant.

CBS said since the die-off of water hyacinth earlier this year, due to biological control efforts, salvinia minima has benefitted from an abundance of nutrients and an open dam. “On 1 May macrophyte cover on the dam was 1.5%, and currently it stands at 47.8%. The graph and satellite image cannot distinguish between water hyacinth and S. minima. It is a mixture, but mostly S. minima. This has been confirmed by our researchers who regularly visit the dam,” said Rosali Smith of CBC.

Salvinia is a fre-floating invasive aquatic plant native to South America.

“Throughout this year we have been working hard to complete the risk assessment of Cyrtobagous salviniae, a semi-aquatic weevil, native to South America and a natural enemy of common salvinia. It is an effective biological control agent of common salvinia in North America. We will be finished with our risk assessment experiments very soon, after which we finalise our application for the release report. The report will be peer-reviewed by a scientific panel. Although the common salvinia infestation is extensive, the state of aquatic weed invasion on the dam will change over the next couple of weeks. Water hyacinth outcompetes common salvinia. With the change in season, water hyacinth seeds within the sediment will start to germinate, and water hyacinth will restart its invasion of the dam.”

In anticipation of this, CBC has already started releasing the water hyacinth planthopper for the new summer season. There are four biological control agent mass-rearing stations around the dam. The caretakers of these facilities rear the planthopper throughout the winter, and they will soon start to release their supply of biological control agents onto the water hyacinth in the dam.

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