Protecting South Africa's Trees From The Shot Hole Borer

The polyphagous shot hole borer threatens over 150 South African tree species. If left unchecked, it could cause huge damage to citrus and avocado orchards, amongst others. Prof Wijnand Swart and Dr Gert Marais, who are striving to find a solution to the problem, spoke to Sabrina Dean.

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The polyphagous shot hole borer (PSHB) is a relatively new threat to trees in South Africa. Yet such has been its trail of destruction that researchers, scientists, horticulturalists, tree farmers and landscapers have been left scrambling for a cure. The problem is that once a tree is attacked by PSHB, little action can be taken to save it. All that remains to be done is to fell the entire tree, dispose of the deadwood appropriately, and hope that the pest has not spread to any other trees. Around the globe, the PSHB (Euwallacea fornicatus) has a wide range of hosts, and can infect over 200 species of trees.

THE BEETLE

Prof Wijnand Swart, head of plant pathology at the Department of Plant Sciences at the University of the Free State in Bloemfontein, is one of the researchers who has been alerting the public to the presence of the pest in Bloemfontein. According to him, the problem is not so much the vector, but the fungus Fusarium euwallaceae that it introduces into the tree.

“The beetle carries this fungus around, and after it bores a hole into the tree, it lines the gallery [nest] where it lays its eggs. It’s basically a fungus farmer,” he says.

The beetle damages the phloem and xylem of the tree with its hole-boring habit, but the real danger is the fungal infection: this eventually kills the tree.

PSHB, which originated in Vietnam in Asia, is believed to have been introduced into South Africa via wooden pallets used in the shipping industry. While the beetle can infect a large variety of tree species, only some are suitable as reproductive hosts. These include foreign trees such as the English oak (Quercus robur), box elder (Acer negundo) and London plane (Platanus x acerifolia), as well as indigenous trees such as the coast coral tree (Erythrina caffra) or fever tree (