/ 24 February 2024

Invasive beetle fells mighty oaks

oak tree beetle
An oak tree infested (left) with the invasive shot hole borer will eventually die.

Their history in South Africa spans nearly four centuries, but the era of oak trees may soon fall victim to a tiny invasive beetle, the polyphagous shot hole borer.

This could forever change the treescape of towns and cities such as Cape Town, George, Paarl, Stellenbosch and Swellendam, according to a team of researchers at Stellenbosch University.

In a research paper published in the South African Journal of Botany, ​​​​ecologists at the university’s Centre for Invasion Biology at the School for Climate Studies traced the history of the introduction of the oaks, genus Quercus, into South Africa, its status and the factors that are changing its distribution across the country’s landscapes.

They found records of 47 Quercus taxa, which were refined to a list of 22 to 34 species probably present. 

The first written record of the English oak (Quercus robur) dates back to 1656, when it was introduced under the authority of Jan van Riebeeck, said Christiaan Gildenhuys, a post-graduate student at the university’s department of botany and zoology and the first author of the article. 

“Dozens of other oak species were introduced to the Cape of Good Hope by early ​Dutch settlers​ and the British colonial government,” he said. 

“Many ​oaks were​ ​subsequently widely cultivated across the country and have since become​ one of the most widespread and recognised tree​ genera​ in South Africa today.” 

Gildenhuys found that three oak species — the English oak, pin oak, and cork oak — have become benign invaders along riverbanks ​and the urban-wildland interface ​in Stellenbosch and Cape Town. These oaks do not cause major problems now but may do so in the future.​  

At the same time, many species, including the most widespread species​,​ the English oak, are highly susceptible to diseases and invasive beetles such as the polyphagous shot hole borer. 

“Not only does this mean that many century-old oaks are at risk, but it also means that infected trees must be removed before the infestation spreads further,” Gildenhuys said.

“We don’t think that they [the oaks] are going to have any severe [invasive] impact any time soon, especially considering that they are super vulnerable to the polyphagous shot hole borer.”

When a shot hole borer infects an oak tree, it will inevitably kill it because there is no cure. “And it’s been spreading from Somerset West to Cape Town to Stellenbosch and Johannesburg.”

Gildenhuys told of how he recently walked down Stellenbosch’s Dorp Street, and noticed that half the oak trees were already infected. “So, it’s not looking good for the oaks in Stellenbosch so far. We expect they will, five years from now, will be dead or dying. That’s the bigger issue that needs to be addressed right now … it’s becoming a rapid problem.”

Stellenbosch street
The trees have for centuries lined the streets of Stellenbosch, which is also known as Eikestad.

The story of oaks in South Africa is a classic example of how global change is rapidly changing the roles and perspectives of species in urban areas​, said​​​ Dave Richardson, an ecologist at the Centre for Invasion Biology and a co-author of the article.

“We must accept that the potential impact of the polyphagous shot hole borer is a game changer. As a result​ of this invasion​, the treescapes of many towns in South Africa are going to change rather radically. 

“Landowners and authorities who may decide to replace infected Q  robur trees with less susceptible tree species must also consider the potential negative impacts of these species.”

Infected trees should be replaced with indigenous species, which are less susceptible to diseases and pests​ such as the polyphagous shot hole borer. But​ people’s attachments to their oak-lined streets may inhibit replacement efforts and induce conflict​s​ between management and stakeholders, Richardson said.

Trees make a vital contribution to lessening the effect of climate change by reducing heat stress in urban areas, said Guy Midgley, the interim director of the School for Climate Studies. But the way that thousands of diseased trees are disposed of may significantly affect carbon emissions. 

Gildenhuys added that a vital tool in the study and for his continuing work is the citizen science platform iNaturalist

“It is a downloadable app where users can upload pictures of plants and animals that can be used by researchers like myself for all kinds of projects,” he said. 

“It gives ordinary people an opportunity to contribute to science and interact more closely with researchers doing these projects.”

Then there’s the debate about the cultural value of oaks. The research paper describes how oaks are characteristic sights in many South African landscapes, particularly in the Western Cape. 

Stellenbosch, the second oldest settlement in South Africa, is known as the “Eikestad” or “Oak City” because of the oaks planted along its streets and in gardens and parks, the researchers said, noting that oaks are present in many towns and cities outside the Western Cape. 

“For example, there is a well-known avenue of Q  robur in the town of Potchefstroom [in North West]. Oaks adorn the streets of large cities such as Pretoria and many small towns on the South African Highveld where they are valued for their aesthetic appeal. They are also associated with several species of edible mushrooms,” the authors noted.

Oaks were first planted in Potchefstroom in 1868. The historical oak avenue in the town was planted in 1910 and declared a national monument in 1977.

Many oak species became culturally important after their introduction. Several individuals of multiple Quercus species are listed as “Champion Trees”, which are considered culturally significant and are afforded special protected status under the National Forests Act.

The authors said the significance of Q  robur in South African literature, art, and architecture is well documented. 

“Oak symbology, including acorns and leaves, features in the logos of many local organisations and schools,” said Gildenhuys. “However, the use of oak symbology has become contentious recently. Some have argued that oaks (particularly Q  robur, being one of the first widely planted European tree species) are a symbol of South Africa’s painful colonial past.”

This topic featured in discussions that lead to the replacement of the oak leaf as the logo of Stellenbosch University in 2022, he said.

Oak trees provide many benefits, according to Gildenhuys. “I like to look at them objectively in a way, like they add a lot of value aesthetically and in terms of services like shade and cooling. 

“There are a lot of great things about oak trees and trees in general but the sad thing is now with the polyphagous shot hole borer you also now need to consider the downside that when these trees get infected they have to be removed and that has enormous costs associated with it.”

Gildenhuys said he had read a recent scientific paper, which estimated the cost of removing diseased oak trees throughout South Africa over the next decade would accumulate to about 0.6% of South Africa’s GDP. 

“All we can do is hope that it [the polyphagous shot hole borer] doesn’t spread further than it already has and try to contain it,” he said.