Cape Town races to save trees from destructive beetle borer

2019-05-16 12:48
The damage to trees caused by the shot hole borer is seen. (City of Cape Town)

The damage to trees caused by the shot hole borer is seen. (City of Cape Town)

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The City of Cape Town is racing to deal with a newly identified infestation of the Polyphagous shot hole borer in the metro.

The tiny beetle was first identified in KwaZulu-Natal by Dr Trudy Paap of the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria.

It has since spread to Cape Town.

"The first sighting was confirmed on 3 April 2019 in Somerset West," Cape Town mayoral committee member for spatial planning and environment Marian Nieuwoudt told News24.

The beetle is native to Asia and has a symbiotic relationship with ambrosial fungi, which are inoculated into host trees.

Infestations

According to a 2016 paper titled "Polyphagous shot hole borer and Fusarium Dieback in California" by Colin Umeda, Akif Eskalen and Timothy D Paine, beetle infestation is deadly for the trees.

"High levels of infestation of susceptible host trees have resulted in high levels of mortality. The currently recognised host range for the beetle-fungus complex includes more than 200 tree species that can be attacked by the beetle, more than 100 species that can support growth of the fungus, and 37 species that can be used as a reproductive host by the beetles," they write.

So far the identified infestation in Cape Town is limited.

"At this stage the only positive sightings have been confirmed in the Somerset West area. Recently a sighting 1km away from the first positive sightings has been confirmed," said Nieuwoudt.

She added that the City's Invasive Species Unit was monitoring the area for further infestations and urged the public to report infected trees here.

"We are unable to confirm immune trees as new host trees are being added to the original list. However, ongoing research and monitoring will be able to assist us in the near future. We are requesting that the public do not plant host trees," said Nieuwoudt.

Shot hole borer

Symptoms of infestations are seen. (City of Cape Town)

But the problem may already be bigger than thought.

News24 reported that the beetle is spreading throughout SA.

"The beetle has now been found in eight provinces, all except Limpopo," FABI Professor Wilhelm de Beer told News24.

Shot hole borer

Workers remove infected trees. (City of Cape Town)

Lures

According to the National Environmental Biodiversity Act, all invasive species should be removed and the City of Cape Town advised the public not to plant, sell, or transport invasive species.

"The Invasive Species Unit has an annual budget of R14m to implement the EPWP Kader Asmal Integrated Catchment Management Programme across the city," said Nieuwoudt.

Shot hole borer

Trees are chipped on site. (City of Cape Town)

US researchers Christine Dodge, Jessica Coolidge, Miriam Cooperband, Allard Cossé, Daniel Carrillo and Richard Stouthamer used a semiochemical (pheromone) quercivorol as a lure for the beetle in 11 experiments over two years.

They found that quercivorol was significantly attractive for the beetles, but added that the fact that females spent most of their lives protected in trees made eradication difficult.

Symptoms of infected trees include:

  • gum or sap oozing on the bark
  • visible entry and exit holes
  • sugary exudates
  • sawdust or frass visible around holes
  • fungal staining on sapwood or outer bark
  • dieback of part of the tree or the entire tree

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