Why is there still so much water hyacinth on Hartbeespoort Dam?

28/01/2025News

Why is there still so much water hyacinth on Hartbeespoort Dam? The Centre for Biological Control (CBC) of the Rhodes University, this week addressed the answers to this increasingly asked question by the public:

hyacinth

Hyacinth plants are starting to show strain thanks to the biocontrol bugs.

• Water hyacinth thrives in polluted, nutrient-rich water.
• Hartbeespoort Dam is heavily polluted as a result of nutrient inputs from non-compliant wastewater treatment plants, non-source point sewage inputs, agricultural and industrial runoff which flow into the rivers feeding the Dam.
• Water hyacinth can double its biomass under ideal conditions which include warm summer temperatures and high nutrient waters.
• Water hyacinth has been on Hartbeespoort Dam since the 1960s. It has flowered and produced prolific numbers of seeds that can remain viable for around 25 years.
• Chemical spraying was stopped at the end of 2016, and the plants exploded under ideal growing conditions. In the middle of 2017, 45% of the dam was covered in the middle of winter.
• In 2019, the Centre for Biological Control (CBC) at Rhodes University, initiated a biological control programme, using the water hyacinth hopper, Megamelus scutellaris.
• The bug is host specific, meaning it can only feed on and lay eggs on water hyacinth. Permission to release it was granted by Department of Forestry, Fisheries and the Environment (DFFE) and Department of Agriculture, Land Reform and Rural Development (DALRARD) in 2013 after extensive testing under strict quarantine conditions.
• The CBC released hundreds of thousands of these bugs, frequently, throughout 2019, and in the beginning of January 2020, the mat collapsed, and by March 2020, less than 5% of the mat remained, and this went below 2% over the winter months.
• The plants returned in spring (October) 2020, because of the extensive seed bank lying dormant in the sediment. The CBC continued with its inundative release campaign over the rest of 2020, and in early summer, 2021, the mat collapsed once again.
• But the same pattern occurred in the following years, including this last season in September 2024.
• The CBC relied on DFFE funding for this biocontrol programme, but funding ceased in 2022.
• Despite this lack of government funding, the CBC continued to mass rear and release the bug onto the dam, and also established community rearing stations where local community partners reared the bugs for release onto the dam.
• We now have 10 rearing stations established around the dam which are self-funded and maintained.
• The main aim of the rearing programme is to release as many bugs as possible as soon as the new plants germinate early in the spring to prevent the mat from covering the entire dam.
• This January, we are experiencing the same phenomenon we have seen over the past five years – the plants germinated in spring, the bugs were released, December rains increased the nutrient runoff into the dam giving the plants more food, and so they exploded. But the bugs are catching up, in the same way that they have each year.
• Very importantly, the biocontrol damage prevents the plants from flowering and therefore producing seeds. This means no new seeds are added to the seed bank.
• According to the CBC, another factor driving this season’s growth is the very low water level at the beginning of the growing season which exposed seeds which have remained dormant at depths below their germinating capability.
• The biocontrol agent populations have increased as we expected, and we anticipate a reduction in cover this summer, much in the same way we have experienced in the past.
• Biocontrol does not aim to eradicate an invasive species, but to reduce its population to a manageable level.
• In the absence of water nutrient remediation, water hyacinth will continue to proliferate each season until the seed bank is depleted.”

hyacinthcoverage_85043_Claro1

Hyacinth coverage is at approximately 40%.

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Hyacinth covers about 40% but plants are showing bug strain