Rand Water has appointed the Centre for Biological Control to assist with biocontrol on the Vaal River Barrage reservoir.
Rand Water is the implementation agent for the integrated control and management of the invasive alien plants - Pontederia crassipes (water hyacinth) and Pistia stratiotes (water lettuce) - on the Vaal River Barrage reservoir.
It said both species of plants were extremely invasive with water hyacinth being more resilient and aggressive than water lettuce.
“In early 2024, an explosion of growth of the two species took place on the Vaal River Barrage reservoir, fuelled by the perfect storm, namely spills of effluent filled water rich in nutrients, hot summer conditions, and reduced water flows in the system.
“At the peak of the spread, water surface cover had reached approximately 397 of a total 940 hectares monitored under the contract,” Rand Water explained.
Due to public and local community involvement with physical removal and pro-bono assistance from the Rhodes University - Centre for Biological Control (CBC) at the time, Rand Water said the mass was reduced to less than 100 hectares, and further work was undertaken by the entity under the contract from the Department of Water and Sanitation to physically remove further plants over the winter months when the plants are dormant.
The entity said the cover was currently sitting at less than 1 hectare at the beginning of spring.
Rand Water noted that work would have to be long term due to the nature of the plants and the seed bank that was already present in the water.
“It was also identified that, in order to undertake this work, an integrated approach was required. The integrated approach adopted includes the use of various mechanisms; physical removal, chemical spraying, curtaining in the water course and biocontrol agents.
“The CBC is the only institution in South Africa equipped with specialist qualified staff, biocontrol agent rearing facilities, and a network of teams available to provide on the ground assistance in the form of advice, implementation support, and long-term monitoring techniques,” the entity said.
The CBC has been appointed as a sole service provider for the upcoming season to assist in all aspects relating to biocontrol and provide support for the project. The team is being led by Professor Julie Coetzee and Dr Kelby English with additional support from Professor Martin Hill.
Four permits granted to rear biocontrol agents
Through the approval of Department of Forestry Fisheries and Environment, Rand Water said four permits have been granted to rear biocontrol agents at the entity’s nursery, as well as three community sites, with the funding for community rearing sites came directly from the community.
“The aim, where possible, is to use biocontrol agents for as much of the control of the two invasive alien plants integrated with physical removal, and only where other methods are not working fast enough will approved herbicide application be used.
“All biocontrol agents released have been rigorously tested and confirmed to be host specific before they were approved for release in South Africa. This means the agents solely feed and complete their life cycles on their target host plants,” Rand Water said.
The four biocontrol agents will be the main focus of the rearing facilities, including water lettuce weevil Neohydronomus affinis, water hyacinth hopper Megamelus scutellaris, and the water hyacinth weevils Neochetina bruchi and N. eichhorniae. – SAnews.gov.za
