The Department of Water and Sanitation (DWS) has designated Rand Water as the implementing agent for managing invasive alien plants on the Vaal River Barrage Reservoir (VRBR). These plants include pontederia crassipes (water hyacinth) and pistia stratiotes (water lettuce).

Rand Water has partnered with the Centre for Biological Control to assist in implementing biocontrol measures.

Both species of plants are extremely invasive, with water hyacinth being more resilient and aggressive than water lettuce. Earlier in the year, rapid growth of these two species took place on the VRBR, fuelled by a combination of 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 about 397 ha of a total 940 ha monitored under the contract.

At the time, owing to significant public and local community involvement with physical removal, and pro-bono assistance from Rhodes University’s Centre for Biological Control (CBC), the mass was reduced to less than 100 ha.

Further work was undertaken by Rand Water under the contract from DWS to physically remove further plants over the winter months when the plants are dormant. Cover is currently less than 2 ha at the end of spring.

From the start of the project, it was identified that the work would have to be long-term owing to the nature of the plants and the seed bank that was already present in the water.

It was also identified that, to undertake this work, an integrated approach was required. The integrated approach adopted includes the use of various mechanisms, such as 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.

As a result, Rand Water appointed the CBC as a sole service provider for the upcoming season to assist in all aspects relating to biocontrol and to provide support for the project.

The CBC team is being led by Prof Julie Coetzee and Dr Kelby English, with additional support from Professor Martin Hill.

Through the approval of the Department of Forestry, Fisheries and the Environment (DFFE), four permits have been granted to rear biocontrol agents at the Rand Water Nursery, as well as three community sites. The funding for community rearing sites has come directly from the community themselves. All rearing sites are already in full swing with agents being reared for both water lettuce and water hyacinth.

The aim, where possible, is to use biocontrol agents for as much of the control of these two invasive alien plants, integrated with physical removal. Where other methods are not working fast enough, an approved herbicide application will be used.

All biocontrol agents released have been 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.

Four biocontrol agents will be the main focus of the rearing facilities, the water lettuce weevil neohydronomus affinis, the water hyacinth hopper megamelus scutellaris, and the water hyacinth weevils neochetina bruchi and neochetina eichhorniae eichhorniae.

In South Africa, the water lettuce weevil was first released in 1985 and the first water hyacinth agents were released in the 1970s and the most recent in 2013. There are a few other water hyacinth agents, namely a moth niphograpta albiguttalis, a mirid eccritotarsus catarinensis and a mite orthogalumna terebrantis, which are not actively reared and released but may find their way into the system naturally as they move to new or different water hyacinth populations across the country.

The moth has been found in the Vaal Barrage system and is steadily increasing in abundance.

Biological control is considered the most sustainable method of control, but this method does require continuous monitoring and technical support. Owing to the very nature of these invasive alien plants the Vaal Barrage system will need to be carefully managed for many years to ensure the population is reduced and controlled.