Killer beetles in our trees
The very informative presentation by Dr Wilhelm de Beer from the Forestry & Agricultural Biotechnology Institute in Pretoria about the polyphagous shot-hole borer was attended by representatives of SANParks and interested and concerned members of the community. The necessity to keep track of the spread of this beetle, originating from Asia, was highlighted. It had initially spread to California and Israel before it was noted in South Africa in the early 2010s.
“Polyphagous” (feeding on a variety of foods) is a very apt description of this beetle and although it seems to favour the English oaks and plane trees, it has infected a variety of other exotic, as well as indigenous trees. The beetle itself does not kill the tree, but a fungus, which it introduces as food for its larvae, inhibits the tree’s sap flow and this eventually kills it.
Some trees, even though attacked, have shown resistance to the beetle and/or the fungus.
Evidence in Knysna
De Beer visited Belvidere and Knysna town and evidence of the beetle was found on the following trees:
Exotic: English oak (Quercus ruber); plane tree (Platanus x acerifolia); and butterfly tree (Phanera (Bauhinia) purpurea);
Indigenous: keurboom (Virgilia divaricata); essenhout (Ekebergia capensis); real yellowwood (Podocarpus latifolia); Outeniqua yellowwood (Podocarpus falcatus); common spike-thorn (Gymnosporia buxifolia); and sweet thorn (Vachellia (Acacia) karroo).
No effective control
In Johannesburg, the once forested area of Sandton is losing trees at an alarming rate and there is no way to stop the spread as there is currently no effective way of controlling the beetle.
Some trees in that area that have proved to be particularly susceptible to infection are the box elder (Acer negundo); common coral tree (Erythrina lysistemon); and paperbark (Vachellia (Acacia) sieberiana). Other species of concern include monkey plum (Diospryros lycoides); cabbage tree (Cussonia spicata); red elder (Cunonia capensis); forest elder (Nuxia floribunda); huilboerboon (Schotia brachypetala); avocado; pecan nut; macadamia; and orange trees.
The invasive castor oil plant (Ricinus communis) is of significance, as it serves as a reproductive host – a plant in which the beetle breeds very successfully – and the plant itself spreads and invades easily throughout South Africa.
Signs to look out for
The tell-tale signs that the beetle has infected a tree are as varied as the trees it attacks.
Dieback of the branches and obvious “shot holes” in the bark are good indicators. Other, more subtle symptoms on the bark are wet, weeping areas; oozing sap; resin drops; and “sugar volcanoes”, where sweet sap has evaporated to leave white marks.
One way to help control the spread is to cut down badly infected trees and burn or solarise them (cover with black plastic in the sun for a few days).