New invasive aquatic weed on Dam

There is a new invasive aquatic weed on Hartbeespoort Dam, and it comes in the form of a floating fern called Salvinia minima, commonly known as common salvinia or small salvinia. The plant is native to South America, and is also highly invasive throughout southern America.

Common salvinia is covering the dam.

According to Dr. Julie Coetzee, deputy director at the Rhodes University’s Centre for Biological Control, this plant was first spotted by entomologist and biocontrol scientist Dr Carina Cilliers, when she noticed an unusual floating plant on the banks of Hartbeespoort Dam. The floating plant was later identified by fern taxonomist Dr Ronell Klopper of SANBI as the invasive Salvinia minima.

“Since then, common salvinia on the dam has been largely inconspicuous due to the presence of water hyacinth. The proliferation of common salvinia, just as water hyacinth came under biological control through the combined efforts of the Centre of Biological Control (CBC) and invested parties around the dam, is no coincidence. In areas that experience high levels of nutrient inflow (pollution), the control of one aquatic weed opens resources to other potentially invasive plant species creating a ‘secondary invasion event’. In the case of common salvinia, the ‘secondary invasion event’ is taking place just as resources such as light availability, space and nutrients, otherwise used by water hyacinth, have become available.”

Common salvinia is very easily confused with the invasive Kariba weed that has invaded Lake Kariba, and many water bodies in South Africa. Similar to Kariba weed, common salvinia is a small, rootless aquatic fern that floats across water bodies’ surfaces.” However, common salvinia is slightly smaller in size, and the leaves are round, measuring only 1.5-2 cm in diameter, with the ‘root-like structure’ extending just under 2.5 cm below the water’s surface. The upward-facing parts of the leaf have a dense mat of distinctive hairs or ‘trichomes'”.

The main form of spread of common salvinia is through vegetative growth. If a single frond with a healthy ‘root’ breaks off, it can effectively grow a whole new plant. In perfect conditions, where temperatures are high and the water column is nutrient-rich, typical of Hartbeespoort Dam, common salvinia can double its biomass in under two weeks. “Although the situation looks dire, there is a promising biological control agent for common salvinia.

Research by the United States Department of Agriculture (USDA) has shown that a smaller ‘Florida’ ecotype of the salvinia weevil successfully controls common salvinia in the southern United States. This weevil will be undergoing the rigorous host-specificity testing required for it to be released in South Africa at Rhodes University’s CBC quarantine facility. However, current travel restrictions are making it difficult to import a culture of the insect from the USA to South Africa. Nevertheless, the CBC hopes to successfully test the new biological control agent and get it approved for release by the relevant Government Departments (DFFE and DARD) in the coming months so that we can mass rear it and release it to control the invasive common salvinia,” Coetzee said.

She advised that it is vitally important that the spread of this weed is prevented. Boating, fishing, and other recreational water sport gear must be cleaned if they are to be used on other waterbodies in South Africa. There is already confirmation that common salvinia is downstream of Hartbeespoort on Roodekoppies Dam, as well as on Roodeplaat Dam. All boats, kayaks, fishing equipment and other recreational equipment should be washed after use and dam users are urged not to move the plants to private residences or other water bodies.

People around Hartbeespoort Dam are urged to report any new sightings outside of Hartbeesport Dam to