Of all the various measures and techniques adopted to manage the spread of water hyacinth, biological control remains the most cost-effective, environmentally friendly and sustainable. South Africa is one of the countries infested by this notorious aquatic weed which has seen a number of aquatic ecosystems across the globe severely degraded.
Biological control measures
In South Africa, Haartebeespoort Dam is perhaps an ideal site where the water hyacinth has taken over almost the entire dam’s surface. Known as a recreational mecca, the dam was compelled to suspend all popular activities due to weed’s invasion. One of the people who are involved in the biological control of the water hyacinth in South Africa, particularly at the Haartebeespoort dam, is Professor Julie Coetzee. A University of the Witwatersrand’s PhD graduate, Professor Coetzee’s primary interest is on plant-insect interactions and her current research focuses on the biological control of invasive aquatic plants.
Professor Coetzee is working closely with the Centre for Biological Control (CBC), where she serves as its aquatic weeds programme manager and deputy director. Through this partnership, she co-ordinates a mass-rearing programme of the planthopper (Megamelus scutellaris), which were then released into the dam throughout 2019.
Working collectively with the CBC’s Sisonke programme, various schools and community groups in the area surrounding Hartbeespoort Dam, they successfully reduced the water hyacinth coverage on the dam from 42% to under 2%. Although she believes more work still needs to be done, Professor Coetzee is upbeat that the results of this expanded augmented biological control programme. Not only has this far exceeded her expectations but it has also received a great deal of local and international attention.
Her work on biological control agents of water hyacinth dates back to 1998. She has recently shifted her focus to floating aquatic plants, submerged and emergent aquatic plants. Her other interests include insect physiology, species distribution modelling, aquatic plant ecology and biostatistics. Early this month, Professor Coetzee delivered her inaugural lecture at Rhodes University to celebrate her achievement of her recently acquired professorship. She used the occasion to share her work and the latest results of her research on the invasive water plant species Pontederia crassipes, commonly known as water hyacinth, and her efforts to understand and control it.
First outbreaks of the weeds
Titled: Hoping on a hopper: the silver bullet for water hyacinth control? Professor Coetzee’s lecture shared how a plant enthusiast accidentally spread the water hyacinth across the globe at the 1884 World’s Fair. The first outbreaks of the invasive water weed were recorded in the 1800s while its presence in South Africa was recorded in the early 1900s.
She also highlighted the importance of the relationship and interplay of bottom-up effects and top-down pressures in aquatic ecosystems. This, she explained, is where the amount of nutrients present in the waters of the ecosystem would usually limit the amount of plant growth from a bottom-up perspective. But she says this was not the case in South Africa.
Professor Coetzee and her peers conducted a research in 2005 which showed that in all the water bodies where they had observed water hyacinth growing, they found high amounts of nutrients such as phosphorus and nitrogen. This contributed to the rapid propagation of the weed in our country’s waters, they observed. She attributed this high nutrient saturation to a lack of water purification infrastructure and lax water policy that contributes to nutrient-rich environments ideal for the spread of the invasive water weed. In addition, she discovered that the water hyacinth has no natural competitors in our ecosystem and it is therefore able to feed on the eutrophic water conditions to proliferate rapidly.
Professor Coetzee said the solution in dealing with the aquatic weed lies in introducing competitors into the ecosystem to act as biological control agents and thus exert top-down pressure on the invader. But according to Professor Coetzee, there is a high amount of nutrients in the water and this, combined with the inability of most of the introduced control agents to survive the colder winters in South Africa, achieved minimal success. But her most recent work has yielded positive outcomes.