Salvinia covers over 35% of Hartbeespoort Dam

The invasion of the salvinia plant (Salvinia minima) on Hartbeespoort Dam has reached over 35% cover, and the Rhodes University’s Centre for Biological control described it as alarming, seeing that it is winter.

“We wouldn’t expect to see such fast growth at this time, but due to the high nutrient status, the plants are growing out of control,” Professor Julie Coetzee of the CBC said.

“The CBC is working hard to import a new biological control agent that is already in use in the USA. The weevil, Cyrtobagous salviniae, has successfully controlled common salvinia in Florida and Louisiana, without damage to any other species. The Covid-19 travel restrictions are preventing us from getting an agent into our quarantine facility at Rhodes University, where we will test the species against indigenous sub-Saharan African aquatic plants, to satisfy the safety requirements of the Department of Forestry, Fisheries and Environment, and Department of Agriculture, Land Reform and Rural Development.”

She said international courier companies will not transport live insects, and entry into the USA from South Africa is currently prohibited. “We are hopeful that we will have a culture of the weevil in our Quarantine by mid-August. Once we have approval for its release, we will step up mass rearing of the agent, both at Rhodes University, and at rearing stations we have set up with partners around the dam.”

Towards the end of May, the CBC highlighted the new invasion by common salvinia, Salvinia minima, on Hartbeespoort Dam. Since then, the invasion has continued to expand despite winter, and has peaked above 35% cover, nearing the extent of the surface area covered by water hyacinth in the past.

A satellite image on 10 July of the salvinia coverage on the dam.

“This highlights the magnitude of water pollution in the dam, where one invasive species has been replaced by another because the cause of the invasion has not been addressed. Floating aquatic weeds thrive on high nutrient levels, and Hartbeespoort Dam is South Africa’s, and if not Africa’s, most polluted systems as the result of effluent from industry, urbanisation, and failing water treatment works,” she said.

“Following the successful biological control of water hyacinth in January 2020, we warned that common salvinia would be the next species to take over the dam, and once we have found a solution to this species, who knows what will invade next.”