Saving one of the world’s largest urban forests from a killer pest
The infestation of both street and private trees by a borer (Poly-
phagous Shothole borer) beetle in Joburg is reaching crisis point. Between 100000 and 500000 trees are expected to be lost in the next five years as a result.
The spread of the borer beetle is fast being identified across the city, with the trees outside the Johannesburg City Parks and Zoo (JCPZ) headquarters in Braamfontein found to be infected this week.
The city is reported to have about 10million trees and is well known for being one of the largest urban forests in the world.
However, experts predict that this may soon come to an end due to the borer beetle infestation. This crisis is likely to hit the agricultural industry as well, with reports that some avocado trees have already been infested. There are also reports from Knysna that the pest has invaded trees there.
Some 200 species of trees, many of which are indigenous, have been identified as carriers. In recent months, many trees in the northern suburbs of Joburg, including entire streets, have been identified as infested with the shothole borer beetle, and stakeholders are now in urgent meetings to find a cost-effective solution to prevent their spread.
Private tree companies charge about R12000 to treat a single tree.
One of the problems is that the number of infestations isn't known as residents often don't report the affected trees because they don't know what to look for.
The challenge, says Andrea Rosen, co-founder of the Johannesburg Urban Forest Alliance (JUFA) - which is lobbying to save the trees - is that the JCPZ budget will not cover a mass treatment of the trees or the mass felling of affected trees and their proper disposal, which, if not done correctly, will spread the infestation.
While some chemical solutions have been found, nothing is guaranteed yet to stop the infestation, said Rosen.
“Some chemical combination solutions to stop both the beetle and the fungus which accompanies it, and which actually cause the trees to die, impact negatively on both the life cycle and food supply of the beetle, and will likely not result in eradication. An effective strategy for addressing this problem in an urban setting requires the collaboration of all stakeholders.
“Given the cost of treatment, it may be the case that only selected trees can be saved,” she said.
In South Africa, the borer beetle was discovered in Pietermaritzburg in 2016, and was first identified in Joburg in April last year. It is described as a small beetle which bores tiny holes in trees, less than the size of a sesame seed or rice grain, which develops fungus around it.
The problem, said Rosen, is that the holes are so small that many people don't identify the problem until it is too late, and large portions of the trees are infested.
Most of the London plane trees in Craighall, Emmarentia, Rosebank and Hurlingham have been hit particularly hard, with almost all the street trees affected.
Since they are such tall trees, one can clearly see the signs of their infestation across the suburb, Rosen said. The trees have wilted brown leaves and the leaves are thinning at the top of the tree and on infested branches.
The most obvious sign of infestation is that branches have brown stains around each hole where the borer has penetrated the tree, but each species displays its own symptoms, said Rosen.
“The discovery of this beetle and fungus in South Africa is of major concern to foresters, farmers and landscapers, as these organisms are known as aggressive tree killers.”
Another Craighall resident, Hilton Fryer, who has had many trees infested in his garden, has conducted his own research into the pest.
“The beetle is often not identified as it is so small and not known in the region. It creates massive colonies within the fungus-lined tunnels inside the tree,” he said.
Scientific research has shown that the optimal treatment to control the beetle is a combination of surfactant insecticide, injected insecticide and injected fungicide.
“I tried some pesticides but they were negatively impacting on the soil of my neighbours, especially after rain.”
Fryer has now imported organic chemicals and the equipment to treat the trees from overseas, but will have to wait to see if they’re effective.
Research that was conducted shows that the beetle originates in Vietnam, where it isn't causing serious damage because tree species have evolved with the beetle-fungus complex and have developed resistance towards them.
However, the beetle and fungus were somehow introduced into Israel and California during the past 15 years, where they cause serious damage, especially to avocado trees.
The University of Pretoria’s Forestry and Agricultural Biotechnology Institute (FABI), under the supervision of professors Wilhelm de Beer, Brett Hurley and Noelani van den Berg, as well as JUFA, City Parks and private chemical companies, are working together in a bid to find a solution.
Dr Trudy Paap, who first identified the infestation in Pietermaritzburg, will, along with a team of experts, continue to monitor the spread and host range of the beetle in South Africa.
They will also assess the risks posed to crop trees, avocado trees, commercial forests such as eucalyptus and wattle trees, and ornamental trees such as planes and maples, to investigate possible control measures.
The member of the mayoral committee for community development, Nonhlanhla Sifumba, said the likelihood of the infestation being more widespread was a possibility.
“JCPZ is working closely with institutions such as the FABI to survey the extent of the spread of the infestation and is set to start a chemical-controlled trial in Hurlingham. We are yet to identify the extent of the shothole borer infestation in Joburg,” she added.
Residents are urged to look out for diseased symptoms, which vary in different tree species, from patches of white powdered wood called frass, to blotches of oozing resin or gumming along the holes in the tree.
“Pesticides are not effective because the beetle bores deep into the wood. The only known method of managing the spread is to cut down infested trees hosting the beetle and to dispose of them in a controlled manner, through burning,” Sifumba explained.
Through the JCPZ, Sifumba has established a committee to source the necessary resources to:
* Undertake a thorough assessment to understand the extent of the infestation.
* Train personnel to monitor, dispose of trees, and identify the borer and other fungal hosts that accompany the beetle.
* Put in place a designated area for the controlled dumping and burning of trees.
* Ensure that a tree replacement strategy is in place.
Residents are urged to examine trees in their gardens and on their streets for signs of infestation, and to include photos of symptoms, tree species, location and contact details, which should be emailed to [email protected] See Fryer’s website www.pshb.co.za for details.