The invasive bug could unleash R275-billion in economic damage in next decade and urban areas stand to lose 65 million trees. (Photo by Ilan Godfrey/Getty Images for Lumix)
If nothing is done to stop a tiny invasive black beetle in its destructive tracks in South Africa, it is set to cause a whopping R275-billion in potential economic damage over the next decade.
And 65 million urban trees are likely to die and will need to be safely removed and disposed of.
This is according to an estimate on the unmitigated economic effect of the invasive polyphagous shot hole borer (Euwallacea fornicatus) in South Africa between 2020 and 2030.
The research is the result of a collaboration between economists at the Stellenbosch University’s School for Public Leadership, ecologists at the DSI-NRF Centre of Excellence for Invasion Biology (CIB) and the department of conservation ecology and entomology, and the Forestry and Agricultural Biotechnology Institute at the University of Pretoria. The study was published in the Journal of Economic Entomology this week.
“This beetle is going to cause a lot of damage, I’m afraid,” said Brian Van Wilgen, an ecologist at the CIB and co-author of the study. “At the moment there is no effective way of killing this beetle. We might be able to develop something but it takes a few years, and to go through all the processes of getting it registered takes time. I think, for the next while, we’re going to have to deal with trying to slow its spread.”
The ambrosia beetle, which is no bigger than a sesame seed, is native to Southeast Asia and bores into tree trunks and branches, laying its eggs inside woody tissue. The female carries its own food in the form of a fungus (Fusarium euwallaceae), which is also an alien species, from tree to tree, and grows in their tunnels. This disrupts the flow of water and nutrients to the tree, causing branch die-back, which can kill susceptible trees.
The shot hole borer was first detected in South Africa in 2012 and has been found in eight provinces, with some infestations separated by more than 1 000km, “making it the largest current outbreak of this invasive pest globally”, according to the study.
In South Africa, the fungus has been established on more than 100 plant species, many of which die as a result. The hosts include major crops such as avocado, lemon and orange, fig, macadamia, pecan nut and guava; important plantation forestry trees, and large numbers of native and urban trees, with urban trees particularly vulnerable.
“Given that the invasion in South Africa is at an early stage, we know very little about both its current and potential future spread and impact,” the authors state. Given how widespread the invasion is, “eradication is impossible and management will have to focus on reducing further spread”.
Forecasted impacts
The researchers used a modelling approach based on forecasted impacts, seeking to simulate possible future effects of this invader if nothing is done to prevent it from spreading further. They estimated that there are 255 million urban trees in South Africa and this is expected to decline by about 65 million over the next 10 years.
The culprit in question. (Rachel Osborn)
Forest trees are expected to decline by about 50 000 hectares and wattles by roughly 140 000 trees. Avocado trees are expected to increase initially as a result of the ongoing establishment of new orchards, but will decline by about 30 000 trees at the end of the simulation period.
The “striking finding” from the analysis is that the loss of urban trees would be by far the largest cost component at a national level. “Most of the costs in our model are for the removal of impacted urban trees, but they also include a loss in ecosystem services provided by urban trees.”
These services include atmospheric carbon dioxide reduction, air quality improvement, storm water runoff reductions, as well as aesthetic, property value, social, economic, and other benefits.
Big losses for Stellenbosch, Somerset West
A tree-rich town like Stellenbosch stands to lose 20 000 of the big old oaks and plane trees lining its streets, Francois Roets, an ecologist at the department of conservation ecology and entomology and a co-author, said in a statement. In Somerset West, where the shot hole borer was first detected four years ago, more than 10 000 trees have already been infected and some oaks are now dying.
Urban trees are more susceptible to the beetle’s effects because they are more stressed than their counterparts 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 novel pests.”
With municipalities struggling with limited resources and weak accountability, it is unrealistic to expect that budgets for urban tree management could be increased substantially without affecting the provision of other municipal services.
Much of the unmitigated costs of invasion will be borne directly by citizens through expenditure on tree removal, and by a loss of urban tree ecosystem services, while dead and affected trees are replaced by trees that would not be suitable hosts.
Van Wilgen said the government has been “slow to act” because there are “no obvious solutions” to the problem, while for dysfunctional municipalities, “things like trees dying, would probably be shifted down the municipality’s priority list”.
“One would hope that citizens would start to take charge of this when they see trees dying in their gardens or on their streets. That will probably work better in the richer suburbs than the poorer ones, because in poor suburbs people have other priorities — and often they don’t have trees.”
A strategy needed
While research is underway to find a biological control agent for the fungus and the beetle, “it would take at least a decade before the agent could be released, and even longer before any significant reduction in spread could be expected,” the authors found.
To date, there is no thoroughly tested and approved insecticide or fungicide registered in South Africa to treat infestations effectively, at least not for urban trees.
The borer beetle can only fly short distances — from 500m to 2km. Its rapid spread is mainly a result of people moving wood across municipal and provincial borders. But a single virgin female beetle can establish a new colony by producing all-male offspring and then mating with them.
“As a developing country, a large proportion of households in South Africa are heavily dependent on wood for their energy needs; this presents a probable pathway for long-distance spread. Recreational barbecue fires may arguably present an even greater long-distance dispersal risk, as firewood is often transported over hundreds of kilometres, even into pristine natural environments.”
Professor Martin de Wit, an economist at the School for Public Leadership and main author, said: “We need a national policy and coordinated strategy for municipalities to stop this beetle in its tracks. 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.”
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