Water hyacinth, native to South America, is described as the world’s worst aquatic weed. It thrives in nutrient-enriched waters like Hartbeespoort Dam, forming dense impenetrable mats that affect boating, fishing and water sport activities, harms aquatic biodiversity. (Delwyn Verasamy/M&G)
It’s a warm October morning and Ian Hendrikse is scanning the expanse of Hartbeespoort Dam’s polluted waters for signs of his adversaries — invasive water hyacinth — floating on the surface.
A few clumps of the fast-growing superweed drift in on a gentle breeze. “The dam is looking much, much better now,” said Hendrikse, who manages the Hartbeespoort Boat Club on the banks of the dam. “Everybody predicted that by the end of October we would be closed completely [by water hyacinth] and we’re standing at just 5% coverage. It’s amazing.”
The credit, he said, must go to the insect army that has been deployed onto the dam by the Centre for Biological Control (CBC) at Rhodes University to tackle the dam’s decades-long infestation, in a biological control programme managed by the department of forestry, fisheries and the environment.
The inconspicuous biocontrol agent, Megamelus scutellaris planthoppers, are natural enemies of water hyacinth, and are host-specific. The planthopper responds well to mass rearing, reproduces rapidly, and recovers faster after periods of cooler temperatures than some of the other biocontrol agent species, according to the CBC. Importantly, they can be “exceptionally damaging” to water hyacinth.
“These are very powerful goggas; you can’t believe what they’re doing. People laugh at us because they come here and say, ‘How are you cleaning the dam?’, and we say ‘a little bug that high,” Hendrikse said, referring to the miniscule 3mm adults.
“But it’s working so well. If you saw last year how quickly the hyacinth died with the bugs and you check what’s actually happening on the plants, you can’t believe it. It kills that plant chop-chop and then it sinks.”
Water hyacinth, native to South America, is described as the world’s worst aquatic weed. It thrives in nutrient-enriched waters like Hartbeespoort Dam, forming dense impenetrable mats that affect boating, fishing and water sport activities, harms aquatic biodiversity.
But the sap-sucking insects are an effective weapon. According to the CBC, the feeding damage they inflict pierces the plant’s tissue, damaging cells.
This harms the petiole, which connects the leaf blade to the stem. This results in water logging, which reduces plant buoyancy and causes the tissue to rot. It can be seen once the leaves start to turn brown and a sooty mould develops on the leaves.
“And because it’s under so much stress, the plant stops putting energy out to producing flowers, which means less seeds, so that’s also another benefit of biocontrol is you’re reducing the seed banks,” explained Kelby English, a postdoctoral fellow at the CBC.
Each flower can produce thousands of seeds, which can last up to 25 years in the sediment.
Raising an army
The CBC’s latest annual report describes how for the third year in a row, water hyacinth was reduced to less than 5% cover at Hartbeespoort Dam through biological control alone. In 2022, it released nearly 400 000 Megamelus scutellaris from its Waainek mass-rearing facility for water hyacinth control around South Africa, while satellite rearing stations run by citizen scientists have released thousands more onto infested waterways.
At Hartbeespoort Dam, five satellite rearing stations were set up in March, bringing the total to nine. Hendrikse and petrol attendant Takalani “Happy” Mamathato run one at the boat club, and recently released an estimated 90 000 bugs in a single operation.
“We’ve got a magic breeding formula. The tunnels are full again and they’re breeding like mad … We’re at the frontline. We’re actually fighting a war here and these are the little soldiers that are working for us.”
Within a few weeks, the 90 000 planthoppers will have doubled in number, English said. “It’s a slow process because it’s a natural process; it’s not instant gratification but it does work. It keeps that hyacinth peak a bit lower and we get to that 5% coverage.”
At the end of last year, water hyacinth exploded on Hartbeespoort Dam, covering more than half its surface. “You couldn’t see water, it was green on this side and green on that side,” Hendrikse said. “We’re running a resort so people come in a boat. When we’re full of hyacinth, we don’t get the income. We’ve got a petrol station and we don’t get boats coming to fill up so it hit us hard.
“Now with the dam looking like this, there’s three weeks coming up where we’re fully booked — in December, we didn’t have a single caravan. People didn’t want to come here. They come here for the dam, not to look at hyacinth.”
Moments later, Mamathato waded into the dam, carrying a bucket filled with dying water hyacinth, crowded with adults and nymphs. “These are my babies,” he crooned, as he gingerly released them. “I’ve opened my heart to these bugs. In winter, I put on the heaters for them; I put fresh water and everything for them to thrive for them and to live. They’re working so hard for us.”
The volunteers who run the satellite rearing stations are “our champions around the dam”, English said. “Word is getting around that biocontrol is a really good option and I think that everyone’s really hopeful.
“We’ve got the scientific data to prove that it’s done really good work in previous years and now we’ve added five extra stations so the more the better … We were sitting at 1% [coverage] for May, June and July. By August, it started growing slowly, but now by October it is still only 5%.”
The CBC said its new approach to biological control of water hyacinth through “frequent inundative releases”, from both its Waainek facility and the satellite rearing stations, has changed the prospects for control of water hyacinth in the country.
“Feeding damage inflicted by the planthopper, Megamelus scutellaris, is reducing water hyacinth cover around South Africa at sites where we have historically faced difficulty in getting agents established.”
Adopting an augmentative approach, whereby the planthoppers are released often and in high numbers to “inundate and overwhelm” the water hyacinth, is most likely to lead to success. These augmentative releases can significantly reduce water hyacinth infestations over the summer growing season as the control agent populations continue to increase.
“However, the management of these programmes at key points of the invasion must be implemented timeously, with emphasis on releases during springtime to reduce the lag time between plant and control agent population build-up, as significant reductions in the populations of M scutellaris can be expected over winter or after floods.” For this strategy to work, insect mass-rearing plays a pivotal role.
Notching up wins
Julie Coetzee, the aquatic weeds programme manager at the CBC, added: “For some people, zero water hyacinth on the dam is a success, for others no water hyacinth over December is a success.
“With biocontrol we’re getting more success than we’ve ever had, following this inundative release, where the water hyacinth is dying far faster and it’s not persisting over winter, which is what it’s always done in the past. So for us, that is a success. We keep having to adjust the plan and to adjust our measure of success.”
English added that although biocontrol is an ecologically-friendly method of controlling water hyacinth, it is only fixing the symptom of a major problem — poorly-treated wastewater from Johannesburg and Pretoria that delivers a liquid fertiliser of nitrates and phosphates to the dam, enabling water hyacinth to flourish.
“If the sewage spills and the treatment works aren’t fixed, then this will continue. All the nutrients in the water [nitrates and phosphates] make Hartbeespoort Dam perfect for water hyacinth and we’ll just continue to treat the symptom.”
In May, the department of water and sanitation officially appointed Magalies Water for a 36-month period, to implement a remediation plan focused on improving raw water quality in the Crocodile West catchment upstream, including Hartbeespoort Dam.
Its spokesperson, David Magae, said a “synergistic approach” has been enacted through a holistic intervention plan where mechanical and manual removal of water hyacinth is supplemented by biological control. “Biological control is part of the programme and will be considered.”
Its service providers for the removal of debris and floating plants “are on the ground working”, he said. Water quality is monitored on a monthly basis using the state-of-the-art and SANAS 17025 accredited laboratory, owned by Magalies Water. “The main quest is to check improvement in quality and there is notable improvement.”
Hya-Matla Organics are removing the floating plants and debris using a hybrid approach of mechanical and manual harvesting while Blue Planet South Africa is “boosting the dam using nano-bubblers”.
“Nanobubbles boost the oxygen levels in the dam and animate dead zones in the dam. Most importantly, it depletes the nutrients, which are catalysts to hyacinth hence restoring the intrinsic values of the dam and its pristine conditions, through the restoration of native biogeochemical processes and destress the impoundment.”
The installation of the nets in two critical ingression points, the Crocodile River and Magalies River, is underway to trap debris and confine the operations in the dam. “The intervention is yielding positive results. The technology suppresses the growth of aquatic plants, ie, hyacinth and algae, and fosters the flourishing of aquatic life such as fish.”
‘Curbing nutrients at source’
The core focus is to harvest the hyacinth for the production of fertilisers/compost and “not recycling the problem” by letting them sink to the bottom “where a devastating effect ensues when they deplete the water dissolved oxygen through their aerobic decomposition”.
“Our premier approach comprises the removal of nutrients, main ingredients/catalysts, through a de-ammonification process and phosphate sequestration. The plants will suffocate and their growth will be retarded.
“This will enable our removal efficacy to outperform their prolific growth and this plan will curtail the problem … Upstream intervention is also our fundamental intervention plan since the dam is the symptom of the upstream catchment activities,” Magae added.