Borer threatens trees
The famous plane tree avenue in the city’s botanical gardens may be felled by a tiny borer which is eating its way through trees all over the country.
The presence of the polyphagous shot hole borer (PSHB) had not been detected in South Africa until researcher, Dr Trudy Paap, noticed in February last year that it had infected the historic plane trees in the Pietermaritzburg Botanical Gardens.
Since then it has been detected in all provinces, and is a risk not only to indigenous and decorative trees but also to agricultural crops such as avocado and macadamia.
The plane tree avenue was planted in 1908.
The beetle is two millimetres in size and has already been identified as the cause of massive tree deaths in the Johannesburg urban areas, George and in Knysna. The beetle (Euwallacea nr fornicatus) is an ambrosia beetle (of the weevil family) and is native to Southeast Asia.
What makes it so destructive is the mutually beneficial relationship it has with three species of fungi, including the pathogen Fusarium euwallaceae.
The adult female beetles bore through the bark into the sapwood and inoculate the fungus, which grows in the tunnels the beetles have created.
The fungus then serves as food for the beetle.
Trees that are more susceptible to the beetle can develop the disease Fusarium dieback, which disrupts the flow of water and nutrients within the tree and can eventually lead to death.
Paap, of the University of Pretoria’s forestry and agricultural biotechnology institute, said that when she first discovered the beetle on one of the trees in the Botanical Gardens avenue, she realised that it had already spread to most of the other plane trees.
She said the trees, London Plane Trees, seemed to succumb less easily than other trees but that there was some concern that the trees would be lost. “Most of the trees have been infected but we are holding out hope that we can keep these trees and try some sort of chemical treatment.
“At the moment nothing really can be used to kill this.”
The beetles attack a very wide range of exotic and indigenous trees in urban, agricultural and natural landscapes.
The beetle, however, can only reproduce in certain tree species.
According to Paap, important reproductive hosts confirmed to date in South Africa include various species of oaks and maples, willows (exotic and indigenous), coral trees, avocado and castor bean.
Paap said that if the borer was picked up early enough, it was possible it could be eradicated. However, she said “it has become evident that the beetle has been prevalent in South Africa for five or six years prior to detection”.
“We don’t really know at this stage the impact on all the various trees but trees in urban areas have been mostly affected such as English oaks and maple trees.”
She said there was also concern over coral trees, bush willows and monkey thorn.
The Department of Agriculture, Forestry and Fisheries (DAFF) said it had also had reports of infestations on pecan trees in Hartswater, Northern Cape province. “The department has since established a working committee with other relevant stakeholders to do more research on the pest and how it can be effectively controlled.”
The department also identified important crop tree species that can be infected such as avocado, macadamia, peach, orange, grapevine pecan trees and forest trees such as the cabbage tree, monkey plum, common coral tree and honey flower.
The department said the movement of infested wood is the main way the beetle is spread.
It said the appropriate disposal of infested trees (by chipping/composting, solarisation or burning) will be key to reducing the spread of this damaging pest.
“Surveys to monitor the spread of the beetle and fungus throughout South Africa are continuing, and chemical treatment trials and experimental trappings are planned. The public can assist by looking out for symptoms and reporting symptoms to the relevant authorities,” said the department.
KwaZulu-Natal National Botanical Garden spokesperson Siphuxolo Ngqasa said there have not been any recommendations on what to do with the trees yet. A Working Group has been formed in Gauteng.
The group consists of researchers from FABI (Forestry and Agricultural Biotechnology Institute at the University of Pretoria), members of DAFF’s Plant Health Early Warning Systems and Johannesburg City Parks and Zoo.
Ngqasa said the group was to co-ordinate research and monitor efforts with the aim to implement control and management strategies to limit further spread.
“The KZN National Botanical Garden is researching on tree species that will grow in the nursery for future purposes, should one day the plane trees die. This might still take years, and it’s not guaranteed that they will die.
“However, the researchers will give the garden feedback timeously on the results from their ongoing studies so that this can be controlled and monitored to hopefully save our plane trees.”
• Borer holes the size of a toothpick on the trunk or branches.
• Entrance holes with or without white wood powder (frass) — usually in the cracks of thick bark like on old oak trees.
• Compacted frass “noodles” begin emerging from entrance holes on the bark.
• Resinous or gelatinous drops oozing from entrance holes.
• Brown, watery sap begin staining the bark.
• When the bark is removed small dents in the sapwood, entrance holes, and/or fungal staining are visible.
• When the first 1-2 cm of sapwood is removed, beetle tunnels continue
• The beetle is around two millimetres long. Mature females are very dark brown to black in colour and are larger than their male counterparts.
— Witness Reporter.
What you can do:
• Chemical control (injecting infested trees with fungicides and insecticides) may prove to be effective in controlling the beetle and fungus, but may be expensive.
• Plant materials showing similar symptoms, or infested trees, should be reported to the relevant authorities or alternatively cut down and chipped.
— Witness Reporter.