Since it was discovered in KwaZulu-Natal in 2017, the Polyphagous Shot Hole Borer (PSHB) (Euwallacea fornicates) has spread rapidly into every province in South Africa except Limpopo, and has infested thousands of trees in urban areas, native forests and more recently in fruit crops.
This tiny invasive ambrosia beetle, native to Southeast Asia, has set alarm bells ringing as it is able to infest a wide variety of tree species – including seemingly strong, healthy trees - and is capable of spreading far and wide with the aid of human activity. It bores into the sapwood of trees and brings a damaging fungus, Fusarium euwallaceae, along with it.
The economic impact of PSHB in South Africa is still in its infancy, but is expected to run into the billions if allowed to continue unchecked. The environmental impacts are also potentially huge as a number of native trees are susceptible to infestation.
One of the management options being explored by scientists at the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria, is to identify a natural enemy of PSHB in its native range and then introduce it into South Africa where it could play a role in controlling the PSHB population.
This led to a recent trip to northern Vietnam by Prof Brett Hurley and Dr Wilma Nel of FABI and PhD candidate Garyn Townsend to collect material from trees infested by PSHB with the aim of finding, identifying, rearing and testing natural enemies. The visit was highly successful and the team brought PSHB-infested Acacia back to the FABI quarantine facility, after obtaining the required permits from South Africa and Vietnam.
The plan is to monitor the material for the emergence of natural enemies, specifically parasitoids, which could be reared and deployed in South Africa to serve as an effective biocontrol agent for PSHB. This is a lengthy process that involves extensive testing to ensure that the introduced species is sustainable and does not cause any collateral damage to native species in South Africa.
The project has been made possible through the funding of the Department of Forestry, Fisheries & the Environment and through collaboration with the Vietnamese Academy of Forestry Sciences, specifically Prof Pham Quang Thu.
According to a recent article in the South African Journal of Science by a group of FABI scientists, the South African PSHB invasion represents the largest outbreak of this beetle in its invaded range anywhere in the world. It has infested 130 plant species in urban, agricultural, and native ecosystems in South Africa, including 44 previously unreported hosts.
In South Africa, PSHB is currently mostly confined to urban environments, but its presence has also been confirmed in the natural forests around Knysna and George where it is feared it could do extensive environmental damage.
Virgilia oroboides, a native tree species found in the southern Cape natural forests, is a reproductive host for PSHB.
“It is an important pioneer forest species in the southern Cape that protects forests from severe climatic fluctuations and fire and houses a large number of native organisms. Elimination of this single species could have irreversible consequences for native forest integrity,” the scientists write.
PSHB control measures using pesticide sprays have been effective in laboratory conditions, but are currently not considered practical in the wild. Deployment of a biocontrol agent looks to be the best management option at this stage, provided a suitable candidate can be found, and the necessary authorisation for its release in South Africa obtained.
The only agricultural crops that appear to be affected by PSHB so far are pecan and macadamia, but they are not reproductive hosts and so the impacts are expected to be limited. Other orchard crops that may be vulnerable to PSHB are cherry, apple, citrus, peach, guava, olive, grape and prune crops.
Initial surveys in invaded urban areas of Johannesburg, Knysna, George and Somerset West indicate that a high percentage of English oak, London plain trees, box elder and other maples will die when infested by PSHB. The cost of disposing of these urban trees and replacing them will be considerable.
Urban trees play a vital role in providing shade, moderating temperatures and creating an attractive environment that enhances the quality of life and boosts the hospitality and tourism sectors. They also provide refuge for numerous birds and insects.
How to detect PSHB infestation
The first signs of PSHB infestation are the presence of tiny holes penetrating the bark of trees and leading directly into the sapwood. These random holes, less than 1mm in diameter, look a bit like they could have been caused by shotgun pellets – hence the common name ‘shot hole borer’.
Lesions, wet patches and gum exudation may be visible on the outside of the tree, and a pinkish-brown stain caused by the associated fungusmay be seen spreading from the gallery into the vascular tissues below the bark. PSHB colonisation of reproductive hosts often results in wilting and death of the infested branch - or the entire tree.
The FABI team has had a lot of success with the introduction of biocontrol agents for a number of tree pests in South Africa in the past, and this experience will be invaluable in finding an effective biological enemy for PSHB. The stakes are high as this tiny hitchhiking beetle continues to spread across southern Africa.
Article by W. Nel, B. Slippers, M. Wingfield, N. Yilmaz & B. Hurley in South African Journal of Science, April 2023
All photos courtesy of FABI.