Insect army winning war against invasive superweed in South Africa’s waterways

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If dynamite does come in small packages, then living proof can be found hopping on the expanse of Hartbeespoort Dam’s polluted waters.

Since 2018, battalions of tiny bugs — biocontrol agents — have munched their way through the floating carpet of invasive water hyacinth that has clogged the dam for more than 50 years.

This 350 000-strong insect army of water hyacinth plant hoppers (Megamelus scutellaris) is winning the war on the fast-growing superweeds, reducing the cover of the problematic green plague to less than 5%.

The project’s success is now being replicated in other polluted waterways in South Africa. In total, 150 000 of the minuscule goggas have been deployed to tackle water hyacinth infestations elsewhere. 

The biocontrol blitz against the aquatic invaders is being run by the nonprofit Centre for Biological Control (CBC), a research consortium at Rhodes University. It has been joined in its mission by troops of citizen scientists, who are rearing and unleashing the hoppers in their thousands. 

The CBC said biocontrol agents such as the hopper are host-specific natural enemies, sourced from the country of origin of the weed, that can only complete their life cycle by feeding on their target weed, in this case, water hyacinth.

The adult hoppers, native to South America, measure just 3mm and pack a powerful punch. They devour the sap produced by water hyacinth. 

They pierce the plant tissue, damaging cells. Damage in the petiole (which connects the leaf blade to the stem) leads to water logging, which reduces plant buoyancy and causes the tissue to rot. This is evident once the leaves start to turn brown and a sooty mould develops on the leaves.

Dense, choking mats of water hyacinth, the world’s worst aquatic weed, choke waterways, harming aquatic biodiversity and costing millions of rands annually in control efforts.

The nutrient-enriched waters of Hartbeespoort Dam caused by poorly treated wastewater from the infrastructural collapse of sewage treatment plants lie at the root of the problem, said Julie Coetzee, the aquatic weeds programme manager at the CBC.

The dam’s waters, arguably the country’s most polluted water body, have been plagued by water hyacinth since the 1960s. Through the 1970s and 1980s, chemical control of the plant using various herbicides kept the plant populations at manageable levels, she said.

As ever-increasing nutrient pollution from Johannesburg and Pretoria was allowed to worsen, water hyacinth flourished in the presence of nitrate and phosphate contamination.

In the 1990s, biological control agents were released into Hartbeespoort Dam to reduce the rapid reproductive rates of the plants. But their effect was limited by the combination of cold winter temperatures and herbicide applications that continued until 2017, Coetzee said. 

“Cold winters and frost kill off the water hyacinth leaves and stems, which the control agents rely on as their food,” she said. “The crowns of the plants, which remain below the water surface, survive the frosts, enabling the regrowth of the plants with the onset of warmer weather.” 

But the recovery of the control agent populations lags, “so the water hyacinth has an earlier start than the biological control agents and tends to maintain this lead through the growing season.” 

Herbicides are detrimental to certain water hyacinths. “Once the herbicides are applied, the immature stages of the [biological] agents cannot escape the sinking plants and are lost. The combined effect of the winter damage and the regular herbicide sprays limited the control agent populations.”

Each water hyacinth flower produces thousands of seeds, which remain viable in the sediment for up to 25 years.

In 2018, the CBC adopted a different approach — inundating the dam with the hoppers. “This involved frequently releasing tens of thousands of the hoppers throughout the year to increase the establishment and population buildup, especially at the beginning of the summer season when water hyacinth proliferates,” Coetzee said.

This kind of release approach has the benefit of artificially increasing the biological control agent populations after a winter-induced population decline, “reducing the delay between the post-winter regrowth of the plants and the increase in biological control populations to damaging proportions”.

Megamelus scutellaris (Delwyn Verasamy/M&G)

Sentinel-2 satellite images were used to measure the reduction in water hyacinth cover from over 37% to less than 6% over three consecutive years since the hopper was first released on the dam. 

Coetzee said site surveys confirmed a corresponding increase in the hopper’s population density from fewer than 500 insects/m2 in October 2019, to more than 6 000 insects/m2 by March 2020. The CBC continued the release programme, without government funding, from the summer of 2021 to 2022. Water hyacinth was again reduced to less than 5% on the dam. 

“And now, for the first time, we have not seen reinfestation from seeds this summer,” she said. “It may still be too early in the season to tell, but it appears as though the action of the control agents has depleted the huge water hyacinth seed bank.”

Meanwhile, in the absence of the water hyacinth, another aquatic invader, the common salvinia or floating fern, has exploded on the dam. The CBC is developing a biological control agent, which is safe but damaging to the fern.

The CBC established a citizen scientist programme with stakeholders around Hartbeespoort Dam to continue the mass-rearing and releasing of the hopper, especially in spring after the large-scale germination of water hyacinth seeds.

While the CBC rears and releases the hopper from its facility in Makhanda, “we realised that establishing satellite rearing stations in close proximity to infestations of water hyacinth would be a key step in increasing the impact of this insect around the country”, she said.

All this approach needed was the buy-in from committed stakeholders, a greenhouse tunnel and containers to rear the plants and insects, and regular input from the CBC team. 

“The release effort of the hopper was increased significantly, allowing the CBC to focus our releases further afield where water hyacinth remains problematic.”

Based on the success of the Harties’ rearing programme, CBC has set up rearing stations in the North West, Gauteng, Mpumalanga, KwaZulu-Natal and Eastern Cape, all with local people’s input and support.

The CBC runs a Facebook page with regular updates about its hyacinth programme. “Through this social media platform, we are contacted by individuals and organisations who also struggle with water hyacinth invasions, and we are able to either send them insects for release, or assist in the set up of a rearing station.” 

This has been instrumental in establishing a management programme for the Ekurhuleni metro whose water bodies from Springs, to Benoni and Boksburg have been plagued with water hyacinth for years, she said. This month, hoppers were introduced at President’s Dam in Brakpan.

And, after an extended hiatus, the unit was once again funded by the department of forestry, fisheries and the environment. 

A new study by hydrogeologists at the University of Pretoria used isotopes to pinpoint the root of Hartbeespoort Dam’s water hyacinth problem — effluent from sewage works, mainly those servicing Johannesburg.

Ryno Germishuys, a MSc student in hydrogeology, collected plant and water samples from the dam, and from the inflowing Crocodile River and nearby boreholes. The dam’s surface water was found to contain high volumes of faecal bacteria such as E coli, typically found in mammal and bird waste and untreated sewage. 

He conducted a nitrogen isotope-related analysis of the plant material, with his results indicating that human faeces and manure, rather than industrial or agricultural pollutants, are the major sources of growth-stimulating nitrogen still flowing into the dam. 

He said this was worrying because the build-up of nitrogen and other nutrients in water causes eutrophication, which depletes oxygen levels and affects fish and plants.

The area on the dam covered by water hyacinth has been reduced to below 5% because of the Centre for Biological Control’s programme. At one stage, hyacinths covered almost 80% of the surface. 

“Results seem very promising, but continuous work and control is needed to keep the hyacinth in check and to prevent them from spreading rampantly again,” Germishuys said.

His supervisor, Roger Diamond, said such efforts are like “putting a plaster on a wound”, because they do not address the root of the water-quality problem — sanitation upstream. 

“The only way to really control Hartbeespoort’s water hyacinth problem is to maintain, upgrade, and expand municipal sewage treatment works, and to supply better sanitation facilities to informal settlements in the Crocodile River.”