A tiny beetle is threatening one of the world’s top urban forests

A tiny beetle, smaller than a matchstick, found in the suburb and potentially the wider Johannesburg area threatens one of the world’s largest urban forests, with more than 10 million trees, a phenomenon which could potentially affect the ecology of the city.

The polyphagous shothole borer, or Euwallacea fornicatus, was discovered in the country for the first time in 2017 by Dr Trudy Paap, a postdoctoral fellow at a biotechnology institute at the University of Pretoria.

Paap discovered the beetle which has its origins in Asia 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.

According to The Conversation, during a survey for diseases in the KwaZulu-Natal Botanical Gardens in Pietermaritzburg, the forest pathologist found a lane of infested plane trees. The identity of the beetle was subsequently confirmed and the tiny beetle – each about 2mm long – was found at work in gardens and roadsides in Johannesburg, about 500 km from Pietermaritzburg.

The beetle carries several fungal species with it when it infests living trees. One of these, Fusarium euwallacea, seems to be a permanent associate of the beetle and can eventually kill a beetle-infested tree.

The polyphagous shothole borer is tiny – but a fungus it’s commonly associated with can be deadly for trees. Wilhelm de Beer/The Conversation

The polyphagous shot hole borer is thought to have made its way to Johannesburg from Southeast Asia on packing crates or through the trade in plant materials. The beetle has since moved to Johannesburg, 320 kilometers (198 miles) away, and spread across its urban forest, which according to the Massachusetts Institute of Technology initiative Treepedia has the world’s sixth-largest green canopy cover.

Today, many of Johannesburg’s estimated 6 to 10 million trees are dying. Some of the infected trees have the telltale holes the 2-millimeter-long beetle makes in their bark. “This beetle doesn’t actually eat the trees,” Paap said to AP. 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.”

Insecticides aren’t effective because the beetles bore deep into the wood. The only known method of managing the spread is to cut down infested trees and burn them. But research is underway to find more effective methods.

The tree species affected in the Sandton area of Johannesburg include non-native ornamental trees such as Japanese and Chinese maple, London plane, kapok, and liquid amber. Several paper bark trees, native to South Africa, were also heavily infested and dying.

But it’s a different story in Californiathe US and in Israel. Paap’s work has confirmed that the South African beetle and fungus are the same genotypes as those found in Israel and California. The beetle and its fungus were introduced in these countries during the past 15 years and have caused serious damage, especially on avocado trees.

Though scientists don’t know just how many trees have died from the beetles’ invasion, the outlook for Johannesburg is grim. The trees do not have an evolved resistance to the polyphagous shothole borer, unlike in Southeast Asia, where the beetles naturally occur.

It is the older, more established trees that are at risk, said arborist Neil Hill to AP. “.. 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.

Like any other big city, Johannesburg and its population of well over 4 million is a major emitter of carbon dioxide, and its trees provide an important function in storing the carbon. There are also reports of shothole borers in Limpopo.