Written by Francois Roets, Associate Professor | Department of Conservation Ecology and Entomology at Stellenbosch University
The invasive Polyphagous Shot Hole Borer beetle (PSHB, Euwallacea whitfordiodendrus) is a well-known and devastating insect pest that, together with its symbiotic fungus Fusarium euwallaceae, can kill a wide range of native and exotic tree hosts and cause significant economic damage.
Originally from Asia, this pest now causes immense damage in Israel, the USA and in many parts of South Africa where it was first detected in February 2017. It poses a significant threat to both urban and agricultural trees in the country.
To date, more than 300 tree species have been found infested by the beetle, but not all develop Fusarium disease symptoms and die. A list of known host tree species for the beetle in South Africa is available at fabinet, but this list is rapidly expanding as more information becomes available.
How do these beetles cause infestation?
The female beetle enters a tree through the bark and drills into the sapwood. She releases its symbiotic fungus from special pouches at the base of her mandibles, which will start to colonise the walls of the tunnel system in the wood. In reproductive hosts, the fungus will grow well and the life cycle of the beetle can be completed. This is because the fungus, and not the tree tissues, is the main food source of the beetle. The female beetle will lay eggs and some of these will develop into flightless males while most will develop into winged females. The males and females mate within the tunnel system and mated females will leave the tree in search of new hosts.
Reproductive host trees usually develop Fusarium dieback disease symptoms and die. In many cases, the beetle may penetrate a host, and transfer the fungus to the host, but will fail to complete its life cycle. These host trees often survive as the fungus does not establish well. However, a few of these tree species may still be susceptible to the fungus and develop dieback symptoms. Importantly, some non-reproductive hosts may become reproductive if they experience stress.
Very susceptible garden trees include, but are not limited to English oaks, London planes, Liquidambar, Salix and Acer species.
Symptoms on trees vary between species but usually involve the presence of minute holes on living trees (<1mm) accompanied by frass or oozing gum. For pictures of most common symptoms refer to fabinet.
Is it treatable?
There are currently no chemicals registered to combat the fungus or the beetle and most seem ineffective over extended periods.
Current best practices for treatment involves removal of infested reproductive hosts and disposing of the material on site (chipping and composting or burning). This is because there are inherent risks in spreading the pest when moving infested material from one area to the next. The best option for dealing with the pest at the moment is, therefore, to prevent its spread in the first place. No affected wood material should be allowed to move from infested areas unless properly treated. This includes, but are not limited to, the movement of wood for fires and infected nursery material.
Numerous studies have now been initiated by members of the Polyphagous Shot Hole Borer Beetle Network in South Africa on understanding the pest and its associated fungus in commercial, urban and natural environments in the country. The help of the public is however needed to help monitor and document the spread of the pest.
Members of the public can report suspected infestations to their local municipality after consulting the very useful information pamphlets available at Fabinet.