SA government kills funding for best weapon against thirsty invasive trees

Centre for Biological Control (CBC) research fellow Dr Kim Canavan and CBC deputy director, associate professor Grant Martin, hang seed traps beneath the canopy of a silver wattle near Ficksburg in the Free State, in anticipation of releasing a new biocontrol agent here later this year. (Photo: Leonie Joubert)

By Leonie Joubert

20 May 2025

Water-greedy alien trees – especially pine, eucalyptus and wattles – are among the biggest threats to South Africa’s precarious water future. Infecting them with insects or diseases from their home countries is the most effective and affordable way to slow this form of pollution. But state funding disruptions have put a stop to any new research into novel biocontrol agents.

As far as the gear for a scientific experiment goes, this is about as low-tech as it gets. A few lengths of PVC piping from the local hardware store, about as long as a forearm and a little more stout. Upended into it is the top-half of a 2l plastic bottle, its downturned mouth funnelling to a porous white bag.

Five in all, hung from branches beneath the canopy of a mature silver wattle tree like homemade lanterns, with little more than roughly twisted fencing wire, gauge uncertain.

The most sophisticated bit of the experiment is the code, on a printed label: AC-FS-02-17.

“We’re trying to measure the seed-rain,” says Associate Professor Grant Martin, a researcher with the Department of Zoology and Entomology at the University of the Free State and deputy director of the Centre for Biological Control (CBC) at Rhodes University.

In just more than an hour, Martin and colleague Dr Kim Canavan, also a researcher with the CBC, have hung all their seed traps – five traps, five trees – but it’s heavy going, beating their way through the dense thicket. This should be open grassland, but there isn’t room to swing a cat, given how well these non-native trees and other shrubs have muscled in. The canopy is as tall as a building and closed in; the understorey a claustrophobic tangle of weeds.

“It’s almost all alien plants,” says Canavan. “Mostly lamb’s quarters and khakibos, not much else really.”

The indigenous ground cover has long been exiled.

Invasive trees crowd in along a river near Rhodes in the Eastern Cape grasslands. In spite of efforts to beat back the spreading water-greedy invasive plants across the country through ecosystem restoration projects under the state-funded Working for Water programme, recent aerial surveys completed in 2023 show that these tenacious weeds have spread by 10.6% in the past 15 years. Wattle, eucalyptus, pine and mesquite species make up nearly three-quarters of this growth. (Photo: Grant Martin)

Invasive trees along a river near Rhodes in the Eastern Cape grasslands. (Photo: Leonie Joubert)

Invasive trees near Rhodes in the Eastern Cape grasslands. (Photo: Leonie Joubert)

The farmer who recently bought this neglected piece of land 25km west of Ficksburg in the Free State wants it restored to open grazing. Even if that’s possible, it will take years to recover. But he seems willing to bet good money on the approach these two are recommending.

The solution is a diminutive Australian wasp that looks as though it’s stepped out of a Beatrix Potter story book – small, cute, none of the aggression of a typical hornet – but it might be a heavyweight contender to slow this wattle’s growth. If it does, it’ll be one of the most viable front lines of defence for the country’s heavily plant-invaded eastern grasslands, and the ecological water services they offer.

The wasp’s name is a tongue twister – Perilampella hecataeus – but back in its home country it’s a natural predator on silver wattles, attacking the flowers and dramatically reducing the number of seeds a tree can produce each season.

Martin and Canavan will release the wasp later this year, and monitor the trees’ seed production for as many years into the future as they can.

However, state funding disruptions threaten the future of this field study, along with other efforts to find new biocontrol agents to tackle the country’s barely controlled invasive tree problem.

The Mzimvubu catchment in the eastern grasslands is one of the country’s most important water sources, and starts about 200 km south-east as the crow flies from the Ficksburg wasp release
site. Recent aerial surveys show a 32 percent increase in the spread of invasive plants in just 15 years in this catchment. DFFE has not released any funding for ecosystem restoration work in the Eastern Cape in the past two years, bringing many projects to a halt.(Map: SANBI)

Closing the tap on a biological ‘oil spill’

To the untrained eye this spot might look like a pleasant woody grove – silver and black wattles ornamented with filigreed lichen and moss; the odd poplar and a pine tree or two – but invasive trees like these are more like an oil spill, a kind of pollution that lives, breathes and self-replicates.

The silver wattle is one of the most aggressive invaders in the country’s grasslands and has already taken over an estimated 400,000 hectares across the biome.

Around South Africa, these thirsty invaders typically congregate in water catchments, clogging up wetlands, streams and river banks. They crowd out indigenous species, dry out and hard-pack the soil, poison the ground so other plants can’t grow, add to a fuel load that makes normal fires lethal to indigenous veld, and allow soils to wash downriver.

Recognising the threat that invasive plants pose to the country’s water resources, agricultural production and grazing, biodiversity and ecosystem services, in 1995 the then water affairs minister Kader Asmal launched the Working for Water programme and offshoot initiatives aimed at restoring water catchments and creating jobs. These programmes have largely tackled the problem through mechanical clearing and herbicides, spending roughly R7.1-billion on this work between 1999 and 2020, according to the Department of Forestry, Fisheries and the Environment’s (DFFE) 2022 report on the state of biological invasions.

Grant Martin sifts through leaf litter to find silver wattle seeds, and studies fallen seed pods for signs of another biocontrol agent that is active in the area. If this specific seed-eating weevil has found its way to the site, he expects see pinprick-sized holes in the pad. After the weevil’s larvae have fed on the nutrients in the seed, the maturing insect will drill its way out of the pod. (Photo: Leonie Joubert)

A recent study, based on aerial surveys across the country, tells a grim tale, though. In spite of decades of investment to contain the spread of invasive plant species, their footprint continues to grow. This study – a collaboration between various research institutions, an NGO and the DFFE – used low-flying airplanes to map the extent of invasive alien plant infestations. Completed in 2023, it found a 10.6% spread of invasive plants in the past 15 years, with wattle, eucalyptus, pine and mesquite species making up nearly three-quarters of this growth.

The same study, though, shows that biological control agents are the most effective, affordable and sustainable way to get on top of these spreading plants. Where there has been a notable decrease in the spread of certain invaders – particularly certain wattle species, hakea and cacti – biocontrol agents were working to suppress the populations.

A small portion of the Working for Water budget – R62.7-million annually between 2015 and 2020 – has gone towards allowing experts like those at the CBC and its affiliate organisations at various universities around the country to find and test new biocontrol agents, run mass rearing facilities, manage the agents’ release and monitor effectiveness.

Centre for Biological Control (CBC) research fellow Dr Kim Canavan hangs seed traps beneath the canopy of a silver wattle near Ficksburg in the Free State, in anticipation of releasing a new biocontrol agent here later this year. (Photo: Leonie Joubert)

This is small change next to the amount needed to contain invasive plant invasions across the board: a 2024 estimate by, among others, the Centre for Invasion Biology at Stellenbosch University and the South African National Biodiversity Institute, puts the figure at R231.8-billion.

Where mechanical and chemical clearing are labour-intensive ways to mop up the pollution after it’s clogged up wetlands and rivers, biocontrol agents turn off the pollution at the source, before it can damage more water courses.

But following Treasury’s tightening of the purse strings after Covid-19, then environment minister Barbara Creecy cut Working for Water and related ecosystem restoration funding from R1.7-billion in the 2020/21 financial year to just R377-million in 2024. While the total budget for the DFFE’s environmental programmes has remained relatively constant over the past four years (R2.6-billion for 2020/21 and 2021/22, R3.2-billion for 2022/23 and R2.9-billion for 2023/24) — Working for Water has seen a 78% decrease relative to the 2020/21 budget.

Where the CBC previously accessed DFFE funding through a rolling grant system, a switch to an onerous and slow tendering process has resulted in a complete shutdown in funding to the CBC, which has partner institutions at several universities around the country.

“Funding was stopped in about 2020,” says the CBC’s director, Professor Martin Hill.

The most recent estimate for getting on top of the country’s invasive plant problem is around R231.8-billion, but the country has only spent about 4% of that since 1960. (Photo: Leonie Joubert)

A host-specific wasp from Australia that attacks the tree’s flowers has already shown success against the silver wattle in the Western Cape, and researchers expect a dramatic reduction in the tree’s seed production once it gets established at release sites in the Free State. But state funding cuts threaten the future of this field study, and others like it. (Photo: Grant Martin)

The centre was asked to submit a proposal in 2023, and amend it to reduce the total budget in response to the DFFE’s request, but that is the last the CBC has heard from the department, according to Hill.

“To this day we have not received any official notification from the department as to our proposals not being accepted, or why it was not accepted.”

In response to the recent aerial survey showing the steady bush encroachment from these trees, and Working for Water and CBC funding cuts and disruptions, Brian van Wilgen, co-author of the survey analysis and emeritus professor with the Centre for Invasion Biology at Stellenbosch University, raises the scientific community’s concerns with the new environment minister, Dion George.

“(B)iological control has brought some of the worst problem invasive species under complete control or has substantially reduced the effort that will be needed to control them by other means,” Van Wilgen says in a letter to the minister in August 2024.

“(T)he practice offers an exceptionally cost-effective option for long-term, sustainable control of invasive species.”

Several studies have been done to demonstrate the returns on investment from biological control agents.

“One study estimated that the costs of invasions in South Africa are currently reduced by up to R30-billion per year as a result of the historic and ongoing use of biological control,” Van Wilgen writes.

A case from the Cape: rooikrans gets a clobbering

John Hoffmann is animated as he recounts a tale that’s worthy of a screenplay, rather than where it’s mostly been told – buried in academic journals and the odd government report. It’s a story of field mice, fire, a flower-loving midge and a seed-eating weevil who have unwittingly been working together to beat back the bridgehead of rooikrans wattles, a tenacious invader in the Western Cape’s fynbos.

An experiment like this doesn’t deliver gold overnight. It needs years of patient watching, waiting and seed counting. The now retired entomologist and associate professor from the University of Cape Town’s (UCT’s) Department of Biological Sciences has been following the unfolding events since 1994 when a host-specific, seed-feeding weevil from Australia was introduced at some sites overrun with rooikrans. Then, in 2002, biocontrol experts introduced a midge that attacks the tree’s flowers.

One of the seed traps beneath the canopy of a silver wattle near Ficksburg in the Free State, in anticipation of releasing a new biocontrol agent here later this year. (Photo: Leonie Joubert)

A seed trap is prepared. (Photo: Leonie Joubert)

The battlefield advantage that wattles like rooikrans have is that they produce thousands of seeds a year. Once tucked away in the soil, seeds can remain viable for two decades, give or take. They also respond particularly well to fire, something that’s natural to a fynbos system. Clear a thicket of rooikrans, but be ready for a second wave of growth as soon as a fire runs through.

Except at these biocontrolled sites.

“There’s an accumulation of seeds in the soil, built up over many years,” Hoffmann explains. “When the fire comes, the heat and smoke stimulate the seeds to germinate. But contrary to popular belief, many of the seeds are destroyed by the heat of the fire.”

Those that do survive and germinate send up tender seedlings that now must survive the hot, dry Cape summer.

“There’s a huge mortality of seedlings at that stage.”

Two years after a fire, all the seeds have either been scorched or they’ve germinated but with few seedlings able to get to seed-bearing age.

“Without the biocontrol agents, the (adult) plants start producing seeds again.”

But once the biocontrol agents are introduced, the seed-fall drops.

“It’s not a dramatic effect, but (seed numbers) gradually decline.”

By shutting off the source of the pollution like this, when the fire does its thing, and seed-eating animals like field mice take their share, it’s enough to stabilise a rooikrans population or even drive it into decline, particularly if other clearing methods are used alongside the biocontrol agents. At one study site following a fire, Hoffmann and his team found only one or two seeds in the soil, nothing more.

“By the time the infestation got big enough to burn again, there was no store of seeds in the soil.”

There’s no silver-bullet biocontrol agent and the time-horizons for a successful agent to bring a population of plants under control can run into a decade or two. But it’s a control method that does the work itself, quietly in the background.

Each troublesome invader needs to be matched with its own natural predator or disease. Some will work in their new location. Some won’t. For instance, some biocontrol agents are working well against wattle species in the Western Cape, with its winter rainfall, but don’t work as well when released in the east of the country in summer rainfall areas. Biocontrol experts say this is why they need to continually search for and test new potential agents, to find the best solutions to the unique contexts in which these invasions are unfolding.

Breaking new ground

“I can’t stand the smell of khakibos,” says Canavan, a hint of her Irish origins gilding her speech, as she turns her nose up slightly at the objectionable aromas kicked up as she stomps through the undergrowth looking for the next adult silver tree to hang the traps beneath.

“I associate it with this, walking through a disturbed landscape,” she says, pointing to the botanical slum around her that reaches almost waist high.

This CBC team has already collected five years’ worth of seed-fall data from silver wattles in the area, which will give them a solid baseline against which to measure a change in seed production at this site.

The wasp was first trialled for its potential to suppress silver wattle populations in the Western Cape, with almost immediate signs that the agent had established itself. Batches of wasps were released at a few sites in December 2023 and by August 2024 the trees were growing tumour-like galls where their flowers should have developed into seed pods.

With this positive finding in the bag, the CBC team released a batch of these wasps at a spot near Clarens, about an hour’s drive east of this farm. Come flowering season later this year, they’ll know if the wasps have settled into their new home. The knobbly galls will be the first clue. Whether or not the CBC team will be able to continue monitoring the wasp’s success here in the eastern grasslands, though, and trial more releases, hangs in the balance as long as the funding disruptions continue. DM

This is part of the Oil Spill series for the Story Ark – tales from southern Africa’s climate tipping points project, which investigates the extent of non-native plant invaders, the fallout for the country’s natural systems, and how these collide with climate instability.

The series is a collaboration with the Stellenbosch University School for Climate Studies and the Henry Nxumalo Foundation which supports investigative journalism.

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