Scars on the bark of an oak tree tilting over the Liesbeek River, near a historic water mill in Cape Town, mark it as infected by a tiny, invasive beetle and its deadly fungus. Brittle leaves and stained patches on the trunks of nearby London planes are similar signs of infestation.
The polyphagous shot hole borer (PSHB) beetle, which has invaded eight out of nine provinces since it was first detected in Durban, threatens up to 65-million trees in South African cities by 2050. Thousands of trees have already been killed and the death of millions more — if not replaced — could make cities hotter.
“Cities are islands of high temperatures ... and they are heating up,” said climate risk researcher Alice McClure from the UCT Climate System Analysis Group, noting that vulnerable communities suffer the most from heat stress in the concrete jungle.
By far the most important agents reducing the rate of global warming are land and ocean ecosystems, which absorb human-caused carbon emissions, said professor Guy Midgley, acting director of the School of Climate Studies at Stellenbosch University.
“They have bought us some time. We would already be close to temperatures over 3° warmer (than pre-industrial times) without them.”
Fighting smart when outnumbered
Scientists are the next line of defence. They are investigating biological controls and technology to protect SA’s urban forests, indigenous and fruit trees against the virtually invisible and, for now, invincible beetle attacking them.
Wits and Stellenbosch researchers are using apps such as Google Street View and iNaturalist, satellite imagery and drones to map the battlefields in Johannesburg, Cape Town and Stellenbosch. Colleagues at universities including Pretoria and Rhodes are leading the charge to find the beetle’s natural enemies.
“It is like fighting a war against an invading enemy and we need to convert to intelligent strategies,” said Midgley.
Conventional strategies won’t work against a beetle which can breed at speed, without needing a male to kick off the reproductive cycle. A single female can lay eggs which hatch into males and she can mate with one of the offspring, starting an epidemic with millions of offspring.
This is one of the reasons the PSHB is hard to defeat. Professor Martin Hill, director of the Centre for Biological Control at Rhodes University, summed up its advantages this way: “First there is its biology, then the diversity of trees it attacks and the way it attacks them.”
About 50 species of trees, of about 130 host trees, can be so severely impacted by the beetle that they die. The beetles drill deep into the wood, making them difficult to detect or control, and feed on a fungus which hinders the movement of water and nutrients inside trees which they need to survive.
“The only way to stop the beetle spreading is by pulling out (reproductive host) trees because, at the moment, we have no chemical control,” Hill said of the urgency to find a biological control, like a parasitoid wasp to suppress the PSHB.
“We want to live in a country with fewer pesticides and biological control is inherently sustainable. A natural enemy does not go on strike,” said Hill. Nor is it subject to funding cuts, a challenge now facing SA’s biological control research programmes.
The hunt for biological control
Professor Brett Hurley, from the Forestry and Agricultural Biotechnology Institute at the University of Pretoria, is one of the front-runners in the hunt for the beetle’s enemies. Shortly before Easter, he flew back to OR Tambo International Airport with 150kg of infested wood from central Vietnam, one of the countries where the PSHB is thought to originate.
Hurley said of the recent road trip there: “We were scouting for infestations of the beetles so we could collect samples of wood. We went to 15 different sites across a wide area so we would have a reasonable chance to find a biological control, even in low numbers.”
They collected wood mainly from acacias “as they were easier to find and cut down” and they got a few castor bean samples. “We had to buy extra luggage to fly the wood back in sealed boxes,” says Hurley of the samples, in quarantine on the Pretoria campus.
It is like fighting a war against an invading enemy and we need to convert to intelligent strategies
— Professor Guy Midgley, acting director of the School of Climate Studies at Stellenbosch University
“This is not a sure hit game and we will have to wait and see,” he said of their efforts.
The samples the team collected in 2023 in North Vietnam did generate a “lot of very tiny wasps” but they couldn’t rear enough to infest the beetle, said Hurley. The parasitoid wasps which emerged were 0.2mm in size, about a tenth of a female shot hole borers which measure about 2mm.
The team would need to do DNA testing on the beetles after wasps laid eggs inside them, to prove it was a biological control. “We have done this with a number of forest pests,” said Hurley.
Nowhere in the world has any biological control to stop the polyphagous shot hole borer beetle yet been identified.
Mapping beetle battlefields
Scientists are racing against time to find one as the beetle spreads across SA. Wits professor of entomology and zoology, Marcus Byrne, said: “We need to know how big a problem it is so we’re counting the trees which are sick.”
Citizen scientists are supporting efforts to map the invasions and their data is helping verify research being conducted with Google Street View and satellite images.
Byrne said, for example, the Houghton Residents Association did an exemplary survey of 785 trees — a tiny fraction of the millions of trees in the city — which they “use as a benchmark against data from other methods” in conjunction with their own benchmark data.
“With Google Street View we can track hotspots not just in space but in time. It goes back in time to at least 2010,” he said. But Google Street View can be spotty in coverage, for example, not covering parks or all areas, such as Soweto.
“We have gone from people on the ground with paper, to Google Street View to survey trees in all areas (except Soweto), to remote sensing to drones to satellite imagery,” he said.
Johannesburg has an estimated 10-million trees in its urban canopy which, since 2017, are being decimated by the beetle.
Shot hole borer invasion takes high toll
Eerste River, on the outskirts of Cape Town towards Stellenbosch, is the latest place in the southern part of the country where the beetle has been discovered in 24 trees.
The City of Cape Town has cut down and chipped nearly half of all the infested trees sighted to March 28 said Eddie Andrews, the deputy mayor and mayoral committee member for spatial planning and the environment: 2,406 trees in the Somerset West region and 64 in the city’s southern suburbs.
In Stellenbosch and on nearby wine estates, centuries-old oaks are being cut down to stop the beetle wreaking havoc in the wine lands.
The PSHB can fly only a few metres so people moving infested wood, for example fire wood, for commercial and recreational purposes, are responsible for its dispersal over distance.
Byrne said: “The shot hole borer travelled a long distance from Southeast Asia via shipping routes around the Cape through the Suez Canal and into the Pacific. It turned up in Israel via the Suez Canal, and then in SA, and in California.”
South Africa now has the largest infestation in the world, according to Stellenbosch ecology professor Francois Roets, in the department of conservation ecology and entomology.
He was co-author on a study modelling what this beetle invasion could cost the economy through its damage to natural forests, urban trees, commercial forestry and the avocado industry over 10 years: an estimated R275bn (then $18.45bn).
Besides the major metropoles, Paarl, George, Knysna, Makhanda, Bloemfontein and Richards Bay are among the cities with PSHB infestations, according to a list kept by the Forestry and Agricultural Biotechnology Institute.
Public can help to protect trees
South African citizen scientists — anyone can contribute in this way — are helping by reporting data on at-risk trees over a vast area, Stellenbosch researchers noted in a recent study. More than 4,100 people have made 35,144 observations of at-risk trees on iNaturalist as part the “Reproductive hosts at risk of PSHB in SA project”, they reported.
Cape Town and Stellenbosch were used as case studies in this research, said lead author Dr Luke Potgieter, from the Centre for Invasion Biology.
Meanwhile, biological control experts are struggling to fund research since government tenders for this have not been renewed in the past two years. In early April they heard the department of forestry, fisheries & environment tender had been cancelled, said Hill.
This has implications for their research but also for the people working for their projects. For example, the Centre for Biological Control — a consortium of Rhodes, Wits, UCT and the University of KwaZulu-Natal — has two mass rearing facilities in the Eastern Cape and one in Upington in the Northern Cape where people with disabilities work.
The failure to fund these programmes does not align with the support they offer sectors like forestry and agriculture, their creation of jobs and protection of the greenery of cities.
Asked why the tender was not renewed, chief director of communications for the department, Peter Mbelengwa, replied: “The bids received were above the benchmark of the department and as such the tender could not proceed. The department is now exploring co-funding opportunities with other stakeholders to continue the biological control service.”
Planting fast-growing trees
UCT applied climatologist Peter Johnston said: “Not all cities have a central lung like Cape Town or New York; most have few trees. But Johannesburg has lots of trees and Pretoria too on its arterial roads. If they start losing their leaves due to disease brought on by pests and exacerbated by climate change, this will reduce their effectiveness, and planting trees, to replace those that are taken out, will leave a gap of 10 to 15 years.”
Cities tend to be heat islands because concrete and tar absorb heat and buildings reduce the wind flow, he said. The sudden loss of urban trees which provide shade, due to PSHB, would likely worsen heat island effects and add to SA’s carbon emissions, said Midgley. He said that 65-million trees could store roughly 30 megatons of carbon. Replacing historic old trees with young ones does not however spell doom for the urban tree carbon uptake, he said. “When you are planting new trees, you can boost the carbon uptake, which can be higher when trees are actively growing.”
Many endemic trees are slow-growing and planting non-endemic trees in cities is fine, if they are not invasives, in his view. “We must plant a new species mix of urban trees and lots of them, focusing on species resistant to PSHB.”
Midgley said that communities can influence how green their cities are by planting African endemic forest trees to replace the cutting down of old trees, like oaks.
“I don’t want to sound Pollyannaish,” he said, “but if we pull together, we can make a difference.”
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