What is next for the hyacinth on Hartbeespoort Dam?

Earlier this year, an effective and long-term biological control programme, led by the Centre for Biological Control (CBC) at Rhodes University, resulted in spectacular decrease of water hyacinth on the Hartbeespoort Dam. It is the first time since the 1970s, that the spread of the weed was significantly reduced without the use of herbicides.

“Currently the estimated total cover of water hyacinth is 2.5%, which is the lowest total cover of the weed ever recorded without the use of herbicides. Biological control, which is the use of safe and specific insects or fungi to control invasive species, has been a successful and sustainable method of managing weeds on land and in the water in South Africa for over 100 years. It has no negative effects and a proven safety record,” says Benjamin Miller of CBC.

On the Hartbeespoort Dam, six different biological control agents contributed to the successful management of water hyacinth this past summer. However, much of the success can be attributed to a small planthopper called Megamelus scutellaris which, like the water hyacinth, is native to South America. This insect feeds on the plant and causes the leaves to turn brown, wilt, and die.

“Since 2018, the CBC has been involved in the Hartbeespoort Dam’s water hyacinth management by sending regular shipments of biological control insects from our facility in the Eastern Cape and from smaller facilities set up in schools surrounding the dam to increase the size of the beneficial insect populations on the weeds. Although the water hyacinth can be managed using biological control, the total eradication of the weed on the Hartbeespoort Dam is unlikely. This is due to the large size of the dam, the rapid growth rate of the weeds, and the ongoing water quality crisis.”

Miller said it is possible that the insect populations may have declined as a result of the decrease in water hyacinth due to the effectiveness of the biological control programme. “As a result, there is less food for the insects at present and we should expect a significantly reduced population of biological control agents going into the warmer months. Because the plants will regrow at a rapid rate after winter, we will limit the reinfestation of water hyacinth by releasing more insects to maintain control. The plan going forward is to have a small mass-rearing facility set up at Redstone Estate by the Harties Foundation and the CBC, to mass-rear the biological control agents for regular release on the dam throughout the growing season.”

The CBC will provide scientific and infrastructure support to the Harties Foundation to ensure the sustainable control of the water hyacinth on the dam, and will also investigate other places and organisations to manage more mass-rearing facilities with support from the Department of Environment, Forestry and Fisheries.
For queries please contact CBC at cbcinfo@ru.ac.za.