The Centre for Biological Control (CBC) at Rhodes University began a campaign to inundate the Hartbeespoort Dam (HBD) with the water hyacinth planthopper Megamelus scutellaris, a biological control agent of water hyacinth (Pontederia crassipes), between 2018 and 2020.
The objective was to manage the infestation of the water hyacinth that was covering large areas of the dam, in the absence of herbicide control. Although the HBD is situated on the Highveld, and the water is highly nutrient-enriched from wastewater, an unprecedented reduction in the water hyacinth was recorded between January and March of 2020.
This was caused by the cumulative feeding from several biological control agents of water hyacinth, including M. scutellaris and the water hyacinth weevils (Neochetina eichhorniae and N. bruchi), all of which are host-specific herbivores of the water hyacinth plant.
Due to the effect of the biological control, the cover of water hyacinth fell to as low as 2.5% over the winter of 2020. However, the cumulative effects of the cold winter and the sparse populations of plants that the insects rely on for food meant that the number of biological control agents was greatly reduced over winter.
As a result, the plants are beginning to regrow from the large seed bank that exists within the sediment, as each water hyacinth flower can produce thousands of seeds which remain viable for many years and water hyacinth has been present on the dam for almost 50 years so there is an extensive established seed bank. The seedlings will not have been exposed to the insects as yet, and are therefore growing rapidly as temperatures increase, causing much concern for the residents of Hartbeespoort.
Biological control is the most effective and cost-efficient method of controlling invasive water weeds, as well as being entirely eco-friendly and sustainable. To encourage the use of biological control in the management strategies for HBD, the CBC has set up several facilities around the Dam with willing partners, including the Harties Foundation, who will mass-rear the insects nearer to the dam where they can be released strategically and regularly. However, because of the high nutrient status of the water which feeds the water hyacinth growth, rapid increases of the plants are expected from time to time while the insect populations work to catch up. The CBC is implementing an augmented release strategy whereby continuous releases of the insects to inundate the plants will be carried out as soon as the insect populations grow to high enough numbers to facilitate these releases from the rearing facilities.
There is no quick way to reduce water hyacinth at present and until the seed banks are depleted, which will take years, regrowth from the seedlings is to be expected every year.
Eradicating the plant is also highly unlikely due to the size of the dam and the extensive seed bank in the sediment. However, through the use of biological control, the weeds can be kept below a level that is damaging and the dam can be open for use. Biological control can also help reduce flowering, thus fewer seeds are introduced into the system.