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GARDENING: Keep your eyes peeled for a beetle invasion

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Local authorities are urging residents to look out for shot hole borers in trees, and to do all they can to improve tree health

The polyphagous shot hole borer or PSHB (Euwallacea fornicates), a tiny Asian beetle about the size of a sesame seed, has invaded, damaged and killed thousands of trees across the country.

Gauteng are urged to be on the lookout for PSHB and Fusarium dieback following unconfirmed reports of the beetle elsewhere.

Cape Town’s Recreation and Parks Department and its Invasive Species Unit have operational plans in place to deal with PSHB should the city’s trees show symptoms of infestation.

Councillor Zahid Badroodien, the mayoral committee member for community services and health, said the damage caused by the beetle should not be underestimated.

“An invasive species of this nature could go undetected as people don’t usually inspect trees to see if there are beetles,” said Badroodien. “That is why it’s important our City informs residents so they can report sightings.”

Beetles invade trees

Adult female beetles burrow into trees to establish brood galleries and lay eggs. The female introduces its fungal symbiont, Fusarium euwallaceae, into galleries as food for developing larvae and adults.

The fungus disrupts the flow of water and nutrients to the tree, causing Fusarium dieback which leads to branch dieback and possibly tree death. PSHB has been reported in cities and towns around the country, including Joburg, George, Knysna, Richards Bay, Bloemfontein, Durban and Pietermaritzburg, where the beetle was first discovered in 2017 by Forestry and Agricultural Biotechnology Institute (FABI) researcher, Dr Trudy Paap.

Trees affected

In both Israel and California, where it was first identified in 2003, the beetle has caused extensive and costly damage to urban and agricultural trees. PSHB attack a number of different trees, both exotic and indigenous species, but only reproduces in reproductive host trees.

In non-reproductive hosts, the fungus may or may not cause disease and kill the tree.

Safe disposal

When heavily infested trees are felled, the wood needs to be disposed of correctly and responsibly. Beetles can live for months in cut wood and researchers believe incorrect disposal is fuelling the spread of the beetle to other areas. Infected wood should be burned or wood chipped.

The wood chipping progress builds up heat with kills the beetles and the fungus. Solarisation, where wood chips and logs are placed in the sun and covered with black plastic for six to eight weeks, has also proved to be effective. If infected wood needs to be moved, it must be wrapped in plastic and sealed before transport.

Cities usually have a safe disposal plan in place. Infected wood will be chipped and held at the closest landfill site for solarisation.

What can be done?

“Where trees have been lost we do see a changing landscape and this will be in the case in the Cape over the next five years,” said Professor Wilhelm de Beer of FABI.

“While it is important to be concerned about our trees, we should also note that not all trees will die.”

So what can Gauteng gardeners do?

Improve tree health: Healthy trees may be able to fight off an attack. Ensure your trees have the right nutrients and water requirements. Add compost to the soil around the tree and add a layer of mulch. De Beer says it is best to treat the tree as it grows in nature. Overwatering can also cause stress, as can wind or extensive heat reflected from paving and walls.

Be vigilant: Treatment can be effective if infestation is detected early, especially on a minor branch.

Watch for these external symptoms

  1. Entry holes to the beetle tunnels.
  2. Look for dark shot hole lesions in the bark, thickening around entrance holes, watery sap staining, resin- or gelatinous-like drops.
  3. Dead or wilting leaves could indicate infection from Fusarium dieback disease.

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