In California, surveys in botanical gardens revealed that more than 200 tree species have been infested, damaged and often killed, by these organisms. An important distinction is being made between different types of infestations. Reproductive host trees are trees that the beetle infests and where it successfully establishes a breeding gallery in which the fungus grows, where eggs are laid, and larvae develop into mature adults, thus completing its life cycle. The majority of reproductive hosts eventually succumb to the disease symptoms caused by the fungus. Non-reproductive host trees are attacked by the beetle, but the beetles do not establish breeding galleries. The fungus may, or may not cause disease and these trees are generally not expected to die.

A problem when compiling host tree lists is that sometimes PSHB can infest a stressed tree (e.g. as result of drought, too much water, root damage, etc.). Such an individual, stressed tree might then become a reproductive host, whereas healthy growing individuals of the same species are barely affected. Thus, when trees are assessed for Fusarium Disease or whether it is a reproductive host, other stress factors on the tree should always be considered.

To date 161 tree species have been found infested in South Africa. These include 83 reproductive hosts and 78 non-reproductive hosts. The COMPLETE HOST TREE LIST (Last updated 2023-04-18 can be downloaded here: https://www.fabinet.up.ac.za/images/PSHB/PSHB_host_list_v6_20230417.pdf

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.

Sources:

FABI News;

Article by W. Nel, B. Slippers, M. Wingfield, N. Yilmaz & B. Hurley in South African Journal of Science, April 2023

https://saforestryonline.co.za/articles/in-search-of-a-biocontrol-for-the-shot-hole-borer/?fbclid=IwAR01umpqyTlAI9AvsVQ8F8_8zw-HHBNjbxJlDqDeAR_72bZbRNiOmHoCmKg

The shot-hole borer is ravaging the city’s trees from within, while bark-strippers attack from without
BL PREMIUM - UNLOCKED BY RMB
27 APRIL 2023 - 05:00
By: ANNAMIA VAN DEN HEEVER

Cape Town’s leafy suburbs are in danger of losing up to 40% of their canopies to the same insect predator that has infected trees across the land, the tiny shot-hole borer beetle. There’s also a threat from people who strip the Cape Peninsula’s trees of their bark for medicinal purposes.

The beetle was first spotted in Newlands, Cape Town, in January and is spreading rapidly, often in firewood or pallets. “It could potentially be one of South Africa’s largest ecological tragedies,” according to TreeKeepers, an association dedicated to conserving Cape Town’s urban forest, which gives the 40% tree-loss estimate.

Though international and local researchers are working on solutions, there is no known way of eradicating the pests, other than felling susceptible reproductive host species and disposing of the wood responsibly.

The City of Cape Town has begun a social media campaign to inform residents. About 260 infested trees have been identified in the southern suburbs, according to mayoral committee member Patricia van der Ross. The number is increasing daily. Infected trees can contain more than 100,000 beetles.

The city has asked residents to report infestations to its invasive species unit, which provides assistance. Sightings are also encouraged on the social media platform iNaturalist.

“I don’t think that it’s any exaggeration to say that this is the most calamitous thing that has happened to the urban treescape in South Africa’s history. It’s like the rinderpest of trees,” arborist Francois Krige tells the FM.

“Civil society needs to get behind this. People often realise the severity of what they are facing when it’s too late.

“There is so much we can do right now: stopping the movement of firewood from region to region; not moving plant material in and out of zones of infestation; and supporting planting programmes, so that we will not be remembered as the generation that cut everything down.”

Krige and others suspect the beetle arrived in Newlands when an out-of-town rugby fan brought in wood for a braai near the sports stadium.

He believes the most important management tool available is the pre-emptive felling of box elder trees, the beetle’s preferred host. “The beetle jumps from one box elder to another,” he says. “So the initial stage of infestation is primarily a disease of box elders.” Other common hosts include oak, plane, poplar and coral trees. These are all suburban favourites.

Krige’s work includes tree management in Kirstenbosch Botanical Gardens and he is the chair of Friends of the Arderne Gardens. The Arderne Gardens in Claremont, which have one of the most diverse and valuable collections of exotic trees in Africa, are home to what Krige considers “the most iconic tree in Cape Town”, a towering Moreton Bay fig that has been a backdrop for wedding photographs for many decades.

This “wedding tree” may be at risk. The beetles have been found within 100m of the Arderne Gardens and Moreton Bay figs have been infested in Australia, where the tree comes from. An experimental protection programme, if fully implemented, could cost about R350,000 a year. “The responsibility weighs heavily on me,” says Krige.

The other calamity, illegal bark stripping, has caused serious damage in Newlands forest. Krige estimates it would take 100 years of zero tree felling, coupled with a well-funded alien clearing programme, to bring the forest to a safe and healthy state.

The Table Mountain National Park has an “integrated strategy” to combat the stripping of bark, which traditional healers use for a variety of ailments. The strategy includes intelligence gathering, law enforcement and “proactive solutions” such as painting mature and seed-producing trees with grey, water-based PVA paint, to make the bark less attractive for consumption.

“[The park] will [also] continuously implement law enforcement patrols, facilitate educational awareness campaigns through workshops with traditional healers, and set up a nursery to grow forest species for replanting trees,” according to park manager Megan Taplin.

Indigenous trees that are commonly stripped of their bark include the Cape beech, Cape chestnut, wild peach, assegai and stinkwood. “This has a huge impact on forest diversity as trees die a slow death due to the interruption of the nutritional transport system,” Taplin says.

SANParks says rangers arrested three bark strippers “in the act” last week soon after midnight in Newlands forest.

Bark- and root-harvesting have increased throughout the city’s southern and northern suburbs, CBD and conservation areas, says deputy mayor Eddie Andrews. The city has also resorted to painting trees.

He says bark stripping seems to increase before religious holiday periods such as Easter and Christmas.

The city is considering a new tree bylaw to address this issue more effectively. “As this is an extensive exercise, it may take some time to introduce,” according to Andrews.

https://www.businesslive.co.za/fm/fm-fox/2023-04-27-bored-to-death-in-cape-town/