For all the facts known about South Africa’s Johannesburg, one little known fact is that the city of gold is also home to one of the world’s largest urban forests. But this urban forest is now under threat from the most unlikely of sources, a tiny beetle called the shot hole borer.
The beetle, which is polyphagous, is believed to have arrived in South Africa through packing crates from Southeast Asia, or through the trading of plant materials. Polyphagous means the beetle is capable of eating various kinds of food. Trudy Paap, a forest pathologist at the University of Pretoria, discovered the beetle in the Pietermaritzburg Botanical Gardens last year. She published her discovery in the journal Australasian Plant Pathology, calling it part of "the surge in the global spread of invasive forest pests" because of globalization.
The beetle has since moved to Johannesburg, which is some 320 kilometres away from Pietermaritzburg. The beetle has since spread across Johannesburg’s urban forest. According to the Massachusetts Institute of Technology initiative Treepedia, Johannesburg has the world's sixth-largest green canopy cover.
Today, many of Johannesburg's estimated 6 to 10-million trees are dying. It is a crisis that has been obscured by the current winter season. Some of the infected trees have the telltale holes in the bark made by the 2-millimeter-long beetle. "This beetle doesn't actually eat the trees," said Paap. Instead, it carries a fungus that blocks the vessels that transport water and nutrients, "which ultimately leads to die-back and death of the tree."
Though scientists don't know just how many trees have died from the beetles' invasion, the outlook for Johannesburg is grim: "The city is going to lose a lot of trees." The trees in Johannesburg do not have an evolved resistance to the polyphagous shothole borer, unlike in Asia where the beetles naturally occur.
It is the older, more established trees that are at risk, said arborist Neil Hill. "So that's going to leave a gap in the landscape. And if we don't start to plant straight way with new trees, that gap is going to become more and more of a concern as far as urban blight, pollution, aesthetic beauty." Hill has been experimenting with organic and chemical fungicides and pesticides. He intends to continue when spring arrives, which means the trees will no longer be in their dormant winter phase.
According to the Associated Press, many city residents have expressed concern, including Markus Scheuermaier, who is chair of the Johannesburg Urban Forest Alliance. "It's a real crisis for the city because the trees do provide shade, do provide pollution control, do provide all sorts of services that will disappear if the shot hole borer starts spreading across all the trees in Johannesburg," he said.
Just like any other major city, Joahnnesburg is a major emitter of carbon dioxide. The trees within the city provide an important function in storing the carbon. Johannesburg City Parks' arboriculturist Adelaide Chokoe acknowledges the shot hole borer is a big problem, with multiple hurdles in dealing with it effectively.
How will residents from poorer suburbs view the city spending its limited resources on saving trees in the wealthy predominantly white northern suburbs? And how can the beetles be stopped? "So far there is no remedy that has proven to control this pest," Chokoe said.