Residents are being asked for their assistance in identifying and mapping the location of Boxelder trees in and around the southern suburbs in an attempt to limit the spread of the invasive Polyphagous Shot Hole Borer beetle (PSHB) in the Peninsula.
Last week, Eddie Andrews, the City of Cape Town’s deputy mayor and Mayco member for spatial planning and environment, confirmed that two trees (Boxelder) infested with the invasive PSHB were discovered on a private property off Kildare Road in Newlands.
READ | Concern as tree-killing beetle spotted in southern suburbs
“We are extremely concerned about this latest sighting in Newlands as to date we have managed to contain the invasive Asian borer beetle to the Somerset West area with the assistance and cooperation of residents,” Andrews said.
Paul Barker, a consulting arborist with extensive experience in managing PSHB, has since confirmed another 15 cases of PSHB infestation spread across Mowbray, Rosebank Newlands and Rondebosch.
PSHB is a tiny black beetle (2 mm in size) that, in symbiosis with a fungus called Fusarium euwallaceae, aggressively kills trees.
A paper published in 2021 (“Invasion of the Polyphagous Shot Hole Borer Beetle in South Africa: A Preliminary Assessment of the Economic Impacts”) conservatively estimates the mortality rate of trees (growing in a city, town or suburb) infested with PSHB at 25%. But Barker says researchers now estimate that percentage to be closer to 30%.
“So, if we have an estimated four million trees in Cape Town, we’re looking at 30%. That basically means we could be losing about 1,2 million trees over the next five to 10 years.”
Besides the Boxelder (Acer negundo), alien trees infested to date include London plane trees, Liquidamber, Japanese maples, Chinese maples, pin oaks, and English oaks. Indigenous trees invaded to date include the Coast Coral tree, Forest Bushwillow, and the Cape willow.
Barker explains that the Boxelder is a sentinel species (organisms, often animals, used to detect risks to humans by providing advance warning of a danger). He says it is one of the first species that PSHB attacks.
“And it is easy to spot the beetle on this tree,” he adds.
Barker says by mapping and monitoring the boxelders located in the southern suburbs, it is possible to predict the invasion pathway of PSHB.
“And it’s also aiding to control the disease among Boxelders, which are the primary food source for PSHB. By targeting this and isolating and observing where the beetle movement is, and removing infested trees, we are limiting the spread of the beetle. We are not going to stop it but it will limit it,” says Barker.
He encourages residents to download the iNaturalist app on their phone and to use it to map the location of a Boxelder wherever they may find it, be it on their property, on their streets or along the banks of the Liesbeek.
Described as a social network of naturalists, citizen scientists, and biologists built on the concept of mapping and sharing observations of biodiversity across the globe, the app allows people to record, identify and share the location of just about any fauna or flora specie. And it is as easy to use as taking a photo, literally.
“It’s just a really good mapping system. If we are to collate data, we’d like to collate it in one place. And this is the most effective place to do it,” he says.
Barker says residents should also inspect the trees located on their properties to ensure that the boxelders are healthy and live wood aren’t being attacked by the beetles.
“There are different types of beetles that live on dead material. PSHB only attacks live wood. If one of your trees is showing signs of branch dieback, take a closer look.”
If property owners suspect that a tree on their property is infested with PSHB, they should contact the City of Cape Town’s Invasive Species Unit.
According to the management protocol drawn up by the unit, the priority is the swift identification and removal of highly infested trees to reduce the beetle population.
The protocol requires that the wood from highly infected tree be chipped on site and carefully removed under cover of heavy-duty plastic and incinerated at an appropriate site.
“Tree fellers, or garden services are not equipped to handle this. This has to be handled by invasive species management specialists. There are not many of those, so it is best if residents go through the City,” Barker says.
Information about the borer beetle is available on the City’s website; included are photos of infected trees and what symptoms to look out for.
Use the following link: https://www.capetown.gov.za/City-Connect/Activities-and-programmes/Nature-and-environment/invasive-species-programme, click on ‘Invasive Tree Bug Awareness Notice’ for more information.
There is also a link to report a sighting to the City. Give your details and the location of the infected tree and officials from the City’s Invasive Species Unit will conduct an investigation.
