Alien invasive aquatic weeds are a global problem and have negative impacts on biodiversity, human health, and the economies of countries around the world. Alien invasive aquatic plant species have also invaded freshwater lakes, dams and rivers throughout the City of Cape Town. There are currently 15 problematic aquatic weed species found in the nutrient enriched water bodies of the City, including South Africa’s five main aquatic weeds; water hyacinth (Eichhornia crassipes) which is the world’s worst aquatic weed, water lettuce (Pistia stratiotes), red water fern (Azolla filiculoides), Kariba weed (Salvinia molesta) and parrot’s feather (Myriophyllum aquaticum). All of these major weeds, except red waterfern, were introduced for horticultural purposes.
The City implements a long-term aquatic weed control programme to combat the problem of aquatic weeds using a combination of different control methods: manual control, mechanical control (long boom excavators), chemical and biological control.
Manual control involves the deployment of trained aquatic weed teams to manually remove aquatic weeds while mechanical control involves the use of heavy machinery. River wardens are deployed to follow up after the initial manual, mechanical, and chemical control. The nutrient enriched waters of the City’s rivers and vleis provide ideal conditions for aquatic weeds to proliferate. A once-off clearing is therefore not sufficient, on-going maintenance is the only option to ensure the water remains free of aquatic weeds. Biological control is a cost effective and sustainable option for managing aquatic weeds and this method is used extensively in the City.
Biological control agents are released regularly on water hyacinth, water lettuce, parrot’s feather and Kariba weed. Red water fern is considered to be under complete control by biological agents and it is therefore not necessary to release any more agents. The snout beetle Stenopelmus rufinasus, which controls red water fern, is self-dispersed or dispersed through waterfowl movement and spreads rapidly to other red waterfern infested water bodies. The beetle was first released in South Africa in December 1997 with dramatic results. Three years after the start of the biological control programme, red waterfern is no longer regarded as a significant weed in South Africa.