How to save Joburg’s trees
Johannesburg - Julian Ortlepp inspects a diseased boxelder tree on Hume Road, examining its trunk closely. It’s riddled with clusters of tiny pen-sized holes. Shotgun-like scars run like tears down its peeling limbs.
“Look there, see right in that tiny crack?” he exclaims, as he finds one of the culprits. “There’s a little bugger’s backside sticking out right there.” He scrapes the tiny tree-killer out forcefully.
There’s no hope of saving this street tree in Dunkeld West which, like so many other dead and dying trees in the suburb, has been ruined by a veritable army of highly invasive beetles, each no bigger than a sesame seed.
Together with Andrea Rosen, of the Johannesburg Urban Forest Alliance (Jufa), Ortlepp is on a borer information session with the SA Landscapers Institute around Hugh Wyndham Park, which has several infested tree species.
“This beetle is causing havoc,” Ortlepp tells the group of the invasion of the polyphagous shothole borer, which is native to south-east Asia.
Its presence in South Africa was first detected early last year by researcher Dr Trudy Paap of the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria.
She noticed small shotgun-like lesions on the stems and branches of mature London plane trees planted along the street outside the KZN Botanical Gardens.
The beetle carries several fungal species, one of which is Fusarium euwallaceae, when it infests new trees. It bores through the bark to the sapwood and inoculates the fungus into living wood.
“The fungus grows in the galleries (tunnels) of the beetle and serve as ‘vegetable garden’ for the beetle larvae, but in susceptible trees the fungus can spread through the sapwood, causing disease or even death of the tree,” says FABI.
What’s alarming to Paap’s colleague, Dr Wilhelm De Beer, is how fast the beetle is spreading in South Africa. It is now attacking 80 tree species, 35 of which are native. Around a quarter of the 80 species are reproductive hosts in which the beetle inoculates its fungus and then multiplies.
De Beer, who together with colleagues, is monitoring the spread of the beetle, says what sets it apart is the sheer variety of tree species it threatens: agriculture, commercial forestry, natural forests and urban trees.
“Never in the country’s history has any insect attacked and killed trees in all these sectors... Typically, we deal with beetles attacking pine trees or avo trees but this beetle is attacking everything. It’s really quite scary. The good part is that it’s not killing all the trees, but certainly some species are very susceptible.”
No one knows quite how it got here, but it most likely hitched a ride on timber products.
“We suspect it came in on timber; either used for pallet construction or on the blocks of wood used to stabilise pallets on container ships,” says Prof Marcus Byrne, an Ig Nobel prize winner and entomologist at Wits University.
“Global international trade is almost certainly responsible for movement of the beetle around the world.”
De Beer points out that ornamental and street trees most affected in cities include the London plane, boxelder, Japanese maple, Chinese maple, English oak and liquidamber, while native trees such as coral trees, wild olives, yellow woods and Natal figs are also highly at risk.
This week Johannesburg City Parks and Zoo stated that some London planes along Jan Smuts Avenue in Saxonwold had developed a “fighting-back mechanism,” which “hopefully signals the tail end of the outbreak”.
“We’re guided by research findings to confirm that this outbreak is on its way out,” remarks Community Development MMC Nonhlanhla Sifumba.
But this irks De Beer. “No, I don’t agree. The beetle is spreading fast in South Africa. We have not seen the end of this beetle yet. There are certain species we are going to lose. Some will get sick and recover, but a small group of trees, including some native trees, will die quite rapidly. If there’s drought stress, too much water and wind damage - trees are more susceptible.”
Byrne adds that what may be happening with the plane trees in Saxonwold is to be expected. “Presumably the plane trees in question have a natural degree of resistance, which is allowing them to hold out against the fungal attack.”
How cities dispose of infected trees is another major issue.
“We’ve seen in Joburg how some of these trees are being cut down and sold as firewood, moving the beetle to other parts of the city such as Soweto and Lanseria,” says De Beer. “There, the beetle starts invading again. We recommend cities have dedicated places where people can dump these trees.”
Ortlepp agrees. “This little beetle can fly 1.6km in her lifetime. When we transport infected wood, we can transport it 10km like this,” he says.
“It’s really disappointing that the City of Joburg has not come up with a dedicated disposal site. We met them in April about this and we’re still waiting. The big problem is that this beetle is not being contained.”
Rosen, too, of Jufa, which works to protect Joburg’s man-made forest, which is home to an estimated six to 10million trees, concurs the city is not out of the woods yet.
She stresses the importance of public awareness and reporting infested trees to authorities. “It’s certainly not our intention to create hysteria but we feel the city has done little, if anything. This has to be done if we are to truly attempt to contain the spread of this infestation.”
There have been repeated outbreaks of the shothole borer in California and Israel, she says, citing a recent US Forest Service survey which showed how the borer could kill as many as 27million trees in Los Angeles, Orange, Riverside and San Bernardino counties, roughly 38% of all trees in the urban region.
“If you look at it practically, you can comfortably say that without treatment in 10 years’ time there won’t be any English oak trees and boxelders left in the older suburbs of Joburg,” says Ortlepp. “There may not be any plane trees, maples and acacias.”
Nor is the beetle going anywhere, cautions Byrne. “We can’t eradicate it. That’s one of the many problems with invasive species - there’s no going back... The beetle and its fungus are here to stay.
“We need to find out what trees will be attacked by the beetle. What trees are susceptible to the fungus... resistant to the fungus. And try to understand how the beetles move around, with or without the help of humans. We should also test insecticide options to see if the beetle can be controlled by insecticides.”
This, he says, is unlikely as the beetle tunnels quite deeply into the tree, where insecticides can’t reach. Biological control options using its natural enemies such as host-specific parasitic wasps in its country of origin are another option.
“Once we know lots about the beetle and how it behaves in South Africa we can formulate management plans on how to control it. In the interim, we need to contain the infestation by not spreading the beetle around. This must be done as soon as is possible.”
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