A recently released study found that the potential economic impact of the polyphagous shot-hole borer in South Africa may amount to a massive R275 billion over the next decade, with municipalities having to carry the costs if nothing is done to plug the hole.
The study titled “Invasion of the Polyphagous Shot-Hole Borer Beetle in South Africa: A Preliminary Assessment of the Economic Impacts,” conducted by a research team comprising of expert economists, ecologists, entomologists and biologists from the Stellenbosch University (SU) and the University of Pretoria (UP).
Instead of basing their findings on existing data, the team used a modelling approach based on forecasted impacts, seeking to simulate possible future impacts of this invader if nothing is done to prevent it from spreading further.
The shot-hole borer beetle was first detected in South Africa in 2012 and has since spread to eight of our nine provinces, making it the largest current outbreak of this invasive pest globally.
Shot-hole Borer Invasion Awareness from CapeNature on Vimeo.
Although most of South Africa’s most notorious invasive species are problematic in rural areas, this aggressive invader will have the largest impact on trees in urban areas, as Durban’s Berea knows all too well.
The findings were published in the article “An assessment of the potential economic impacts of the invasive polyphagous shot-hole borer in South Africa” in the Journal of Economic Entomology last week.
Professor Francois Roets, an ecologist in SU’s Department of Conservation Ecology and Entomology and one of the co-authors, said, in the research report, that a tree-rich town such as Stellenbosch stands to lose an estimated 20 000 of its big, old oak trees and plane trees which adorn its streets.
In Somerset West, more than 10 000 trees have already been infected, with some of the oak trees now dying, only four years after being discovered in the area.
Roets said that trees in urban areas are generally more susceptible to the beetle’s effects as these trees are usually already under some stress in an urban environment compared to those in a natural forest rich with biodiversity.
People in urban areas also tend to plant more non-native tree species – many of which are cloned and lack the genetic diversity necessary to fight off new pests.
Professor Martin de Wit, an economist at SU’s School for Public Leadership and the main author of the article, said that if nothing is done to prevent the further spread of this invader, municipalities country-wide will have to remove and safely dispose of an estimated 65 million urban trees between 2020 and 2030. This excludes trees on private land outside urban areas.
“We need a national policy and coordinated strategy for municipalities to stop this beetle in its tracks,” he warns. “To date, the polyphagous shot-hole borer is not yet listed under the Alien and Invasives Species Regulations, making it difficult for municipalities to react effectively.”
Prof Brian van Wilgen, an ecologist and co-author of the study, says the options for managing this invasion are limited given how widespread the beetles have become.
“Eradication is impossible, and management will have to focus on reducing further spread and mitigating its impacts. A first step, and likely the most economical, will be to restrict the free movement of potentially infected planting material, wood and wood products.”
The polyphagous shot-hole borer is only able to fly short distances from 500 metres to 2 kilometres. Its rapid spread in South Africa is, therefore, mainly due to people moving wood across municipal and provincial borders.
“The bad news is that a single virgin female can establish a new colony by producing all-male offspring and then mating with them. This “perfect invader” also carries its own food in the form of the fungus, an alien species that was co-introduced with the beetle,” explains Prof Roets.
Research is underway to find a biological control agent for the fungus and the beetle, but it may take up to a decade before one can be found and released and even longer before any significant reduction in spread could be expected.
To date, there are no thoroughly tested and approved insecticides or fungicides registered in South Africa to treat infestations of the shot-hole borer effectively, at least not for urban trees.
“Anyone who tells you they will save your tree with chemicals and fungicides is likely lying and will be breaking the law,” warns Prof Roets.
“A coordinated strategy to deal with the invasion in South Africa will require a revision of legislation and the creation of policies relating to biological invasions. Currently, there is no coordinated management of invasive species in urban ecosystems, a critical oversight,” the authors conclude.
IOL