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Saving Joburg's urban forest: Shot hole borer treatment pilot project

14 Oct 2019

It’s not the beetle: it’s who it hangs out with that’s the trouble. And now the party is getting out of control, so much so that South Africa’s urban forests are under threat.

During the upgrade of Emira’s Hyde Park Lane Office Park, it became apparent that the attractive wooded grounds were heavily infected with the shot hole borer.

In 2017, the polyphagous shot hole borer (Euwallacea fornicatus) – or PSHB – was detected for the first time on London plane trees in the KwaZulu-Natal National Botanical Gardens, in Pietermaritzburg.

Since then, evidence of the beetle’s presence has been found in 151 species within the country, and the invasive little borer from Southeast Asia has spread all the way to Johannesburg, bringing with it a picnic of its favourite fungi.

And this is the real problem.

As they bore into the wood, the beetle relies on the fungus (Fusarium euwallaceae) to feed its adults and larvae. It is this fungus that slowly kills the tree, not the bug itself – the fungus grows along the beetle’s tunnels, blocking the tree’s vascular system, causing the dieback of the terminal branches and leaves, and eventually the death of the entire tree.

The advancing beetles have also been noted in Durban, Richard’s Bay, Pietermaritzburg, George, Knysna, and Hartswater.

However it is Johannesburg, with its dense urban forest that has been hit particularly hard.

To date, there had been no single successful treatment of the infestation: a heavily infected tree needed to be urgently treated or removed, as the contagion easily spreads. But now Johannesburg is hitting back hard too, thanks to a pioneering partnership between Emira Property Fund and the newly-formed Beetle Busters, who have successfully registered a ground-breaking treatment for the infestation.

During the upgrade of Emira’s Hyde Park Lane Office Park, it became apparent that the attractive wooded grounds were heavily infected with the shot hole borer. One infected tree can contain over 100 000 beetles, and the females can fly up to 1km, although most beetles only fly to the surrounding trees.

“Our initial concern was that we would have to fell all of the infected trees in the park, which would substantially change the environment of the office park,” says Emira Senior Development Manager, Justin Bowen. “Fortunately, we were introduced to Beetle Busters, who are partnering with Emira and using Hyde Park as a test case for the treatment and eradication of the borer – hopefully saving most of the trees in the park.”

Historically, the treatment for PSHB has been to poison the beetle itself, but this often proves toxic to the host tree. Beetle Busters’ new treatment targets the fungus instead, technically starving the beetle while killing the fungus before it can kill the tree.

“If successful, this will be a ground-breaking intervention for South Africa, which could arrest the nationwide infestation and save our trees,” says Tim Conradie, of Beetle Busters. “The treatment at Hyde Park Office Park has been completed and we hope to be able to confirm the results as we move into spring and the trees start their re-growth cycle.”

“Emira is exceptionally proud to be part of this ground-breaking test case and cannot wait to see the results in the coming months,” says CEO of Emira Property Fund, Geoff Jennett. “We really are hoping that this will be the ‘silver bullet’ that we need to keep our treed cities.”

Justin Bowen adds: “The response continues Emira’s environmental leadership in SA’s property sector. Emira was the first company in Africa to have our Science-Based Carbon Reduction Targets approved by the SBTi. The country’s trees are one of the largest carbon sinks that SA has as a weapon to combat climate change, and we are hoping that our pilot will be the first of many successful treatments to eradicate the devastation that the shot hole borer is having on our urban forests.”

Image supplied: Polyphagous shot hole borer

Trees to watch:

Twenty-one tree species – both exotic and indigenous – have been identified as “reproductive hosts” within South Africa, meaning trees where PSHB can successfully reproduce, and which will eventually die due to the fungus. Other “non-reproductive” trees may also be attacked by the beetle, but PSHB reproduction is not successful. The fungus might or might not eventually kill these trees.

Reproductive hosts – exotic trees
Australian Blackwood (Acacia melanoxylon)
Avocado (Persea americana)
Black wattle (Acacia mearnsii)
Boxelder (Acer negundo)
Castor bean (Ricinus communis)
Chinese maple (Acer buergerianum)
English Oak (Quercus robur)
Honey locust (Gleditsia triacanthos)
Japanese maple (Acer palmatum)
Liquidambar; American sweetgum (Liquidambar styraciflua)
London Plane (Platanus x acerifolia)
Pin oak (Quercus palustris)
Pink flame (Brachychiton discolor)
Southern magnolia (Magnolia grandiflora)

Reproductive hosts – indigenous trees
Fever (Acacia zanthophloea)
Flat crown (Albizia adianthifolia)
Coast coral (Erythrina caffra)
Common coral (Erythrina lysistemon)
Natal fig (Ficus natalensis)
Wild plum (Harpephyllum caffrum)
Pigeonwood (Trema orientalis)
Paper bark thorn (Vachellia sieberiana var. woodi)
Wild frangipani (Voacanga thouarsii)

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