Water hyacinth plant hoppers (Megamelus scutellaris) pierce the leaves of the invasive plants, causing them to rot and lose buoyancy. Photos: Ihsaan Haffejee
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Water hyacinth, originally from South America, has infested the Hartbeespoort Dam since the 1960s, thriving on nutrient-rich, polluted water entering the dam.
The plants cover the water, blocking sunlight and depleting oxygen levels, which impacts the ecosystem. Tourism around the dam is also affected.
Until 2016, water hyacinth at the Hartbeespoort Dam was predominantly controlled with herbicides.
When this was stopped by the Department of Water and Sanitation because the resulting blue-green algal blooms contaminated irrigation water, new measures had to be adopted to control the rapidly spreading plant.
These measures, which include manually removing plants, releasing hyacinth-eating bugs, and other experimental techniques, now appear to be having an impact.
300 000 plant hoppers were released last year. The population may have grown over a billion during the summer months.
Dr Kelby English from the Centre for Biological Control (CBC), which oversees the biological control strategy at the dam, says the use of water hyacinth plant hoppers (Megamelus scutellaris) has led to progress. These are tiny insects about 3mm in size which pierce the tissue of the hyacinth, causing it to rot and lose buoyancy.
The CBC has been doing biocontrol since 2018. Hyacinth coverage during the winter of 2017 remained above 30%. Since spring 2019, coverage has been below 5% during the winter months.
During peak summer months, in which hyacinth thrives, more than half the dam can be covered. This January, although 52% of the dam was covered by hyacinth, satellite imagery shows that many of the plants were beginning to turn from green to brown, suggesting that they were not in good health and that the plant hoppers are having an effect. Some parts of the hyacinth “carpet” have lost buoyancy and started to sink.
350 000 plant hoppers were released into the dam by the CBC between September and December last year. English says the population probably exceeded a billion in the summer months.
Plant hoppers are causing large swathes of water hyacinth to turn brown.
In addition to the discolouration, fewer hyacinth plants are producing flowers. This indicates that the plant hoppers are successfully preventing the plants from producing seeds that spread.
“Biocontrol does work. It’s not a quick process, but it is effective over time. We have reduced the seed bank, and we have observed very few flowers, meaning that very few new seeds are being added to the soil,” said English.
