An invasive beetle with the ability to kill any tree it had burrowed into, had been sighted in the Stellenbosch Central Business District (CBD) recently. The presence of the Polyphagous Shot Hole Borer beetle (PSHB) was confirmed by Stuart Grobbelaar, communications manager of Stellenbosch Municipality.
The first tree infected with the PSHB was spotted in Church Street in February, according to Grobbelaar. Stellenbosch Municipality was quick to respond and the infected tree was removed within municipal protocols, stated Grobbelaar. This was “to ensure that the beetle cannot spread to other trees in the area”.
“In Stellenbosch we reckon anything between 15 000 to 20 000 trees are going to die within less than 10 years,” stated Dr Francois Roets, head of Conservation Ecology and Entomology at Stellenbosch University (SU).
The Polyphagous Shot Hole Borer beetle compared to a matchstick. PHOTO: Liam Voorma
“In 2019, we found the first [infected] tree in Somerset West and that spread rapidly – 3 km a year. The oak trees are busy dying now, four years later, and [local municipal managers] have taken down about 10 000 trees already,’’ said Roets.
No answers yet
“The use of pesticides and fungicides have not proven effective at eradicating PSHB from infected trees,’’ said Lizel Steenkamp, senior media liaison at the City of Cape Town government.
Methods of biological control are, however, in the works, stated Roets.
The PSHB beetle can infect about 130 species within the surrounding areas, but there are only about 10 species that are in trouble of dying out, said Roets. “At the top of that list would be Box Elders, Plane trees – like the ones in Victoria Street – and definitely Oak trees,’’ explained Roets.
An entry hole of a Polyphagous Shot Hole Borer beetle in a Plane Tree in Victoria Street, Stellenbosch. The hole is roughly the size of a sesame seed (2mm). PHOTO: Liam Voorma
‘It shuts down the tree’
The PSHB beetle uses its mandibles to drill into a tree, creating a tunnel system within the tree, explained Dr Roets.
“This already blocks some of the water flow to the tree,’’ said Roets. As they chew through the innards of the tree, they release spores and a fungus starts to grow into the vascular tissue, he said. The fungus, Fusarium Euwallaceae, concentrates nutrients into these tunnels allowing the beetle to feed on the nutrient rich fungus, stated Roets.
A tree in the Stellenbosch Botanical Gardens with multiple entry holes made by the Polyphagous Shot Hole Borer beetle. The liquid oozing from the holes shows the tree’s attempts to fight the infection, said Dr Francois Roets, head of department of conservation Ecology and Entomology at Stellenbosch University. PHOTO: Liam Voorma
“It shuts down the whole tree… if you look at the trees they seem to die from drought symptoms,’’ added Roets.