Farmer's Weekly (South Africa)
Many unanswered questions bugging the arbour industry
The alien invasive polyphagous shot-hole borer beetle, with its deadly fungus Fusarium euwallaceae, has created widespread havoc in South Africa. As multiple research teams study the insect, it is clear there is no single solution to deter it. Magda du Toit recently joined a media information session hosted by Hortgro, during which various aspects of a research project under the auspices of Stellenbosch University were discussed, with visits to trials in the pear orchards at Lourensford Estate in the Western Cape.
Almost six years have passed since the detection of the polyphagous shot-hole borer (PSHB) (Euwallaceae fornicatus) in trees in South Africa, and it seems the beetle is spreading at a fast and steady pace. According to researchers, it can now be found in nearly all parts of South Africa and in more than 160 tree species.
The PSHB is native to Southeast Asia. This tiny beetle, only 1mm in diameter, attacks its host by tunnelling into trunks, stems, and branches.
According to scientists, this particular beetle is arguably the most damaging alien tree pest ever to arrive in South Africa.
The presence of PSHB was confirmed in 2017 based on the findings of Dr Trudy Paap of the Forestry and Agricultural Biotechnology Institute (FABI) of the University of Pretoria, who had identified PSHB on London plane trees in the Pietermaritzburg Botanical Garden in KwaZulu-Natal.
The spread of PSHB was subsequently confirmed in trees across the country, including in Johannesburg, Cape Town, Stellenbosch, Bloemfontein, Durban and Pietermaritzburg. It can be found in all provinces except Limpopo.
EXTENSIVE OUTBREAK
This widespread PSHB invasion in South Africa constitutes the largest geographical outbreak of the beetle in the world. It is affecting trees in all sectors: the agricultural and commercial forestry sector, urban trees (public spaces, streets, gardens), as well as native trees in natural forests.
According to Wanga Ncise, senior horticulturist at Stellenbosch University Botanical Garden, they recently confirmed three cases of PSHB prevalence in the botanical garden in Platanus
acerifolia, Podocarpus microphylla and Acer palmatum.
“The team in the garden are aware of the problem and know how to identify the insect. At the end of November, we had our regular walk-through in the garden to specifically look for investigations and found that each of the trees had more than 20 holes. We, however, do not have dieback yet on these trees,” he says.
He adds that he earlier identified one tree outside the botanical garden premises that had been damaged by the shot-hole borer. “The tree leaves were falling, and the tree was slowing dying from the top downwards. The tree was cut down and covered with black plastic for more than six months.”
To address the problem, the team at the botanical garden are working in collaboration with the Department of Conservation Ecology and Entomology at the university regarding research, conservation and education. “We have a student who is doing research in our trees and setting traps to lure the shot-hole borer. Traps have been placed in affected trees,” says Ncise.
With the botanical garden being located in the middle of Stellenbosch, it is clear that many other trees in the town are under threat.
The problem with this invader is not so much the damage caused by the beetle itself but rather that it acts as a vector for the Fusarium fungus, which is introduced into the tree in the tunnels made by the female beetle.
Although the beetle causes damage to the phloem and xylem of the tree with its holeboring habit, the real danger lies in the fungal infection as this eventually kills the tree.
PSHB has a symbiotic relationship with three species of fungi, including Fusarium euwallaceae.
It carries them around and after it bores a hole into the tree, it lines the gallery of the nest, where it lays its eggs.
One could say that the beetle is a fungus farmer, farming with the fungi inside the tree to ensure a food source for itself and its larvae. In susceptible trees, the fungus kills vascular tissue, causing
Fusarium dieback and tree death.
PSHB is believed to have been introduced into South Africa through wooden pallets used in the shipping industry.
Around the globe, PSHB has a wide range of hosts and can infect over 200 species of trees. These include the English oak (Quercus robur), box elder (Acer negundo) and London plane (Platanus x acerifolia), as well as indigenous trees such as the coast and common coral tree
(Erythrina caffra and E.lysistemon), and important commodity trees such as avocado and pear.
While the beetle can infect a large variety of tree species, only some are suitable as reproductive hosts, says Matthew Addison, crop protection programme manager at Hortgro and a researcher at Stellenbosch University. Reproductive hosts are susceptible trees in which the beetle can reproduce. It lays its eggs in them and the larvae develop into mature adults to complete its life cycle.
The majority of reproductive hosts eventually succumb to the disease within a few years of the PSHB attack.
MORE DAMAGE IS CAUSED BECAUSE THE BEETLE ACTS AS A VECTOR FOR THE FUSARIUM FUNGUS
These tree species amplify PSHB population and increase the risk to surrounding trees.
The beetle also attempts to infest trees that are considered non-reproductive hosts. These trees are attacked by the beetle but it does not establish breeding galleries.
While PSHB will bore into these trees and introduce the fungus, they are not suitable for the rearing of its offspring. “Even when it has bored its way into a non-reproductive host and left the tree, it has already damaged the phloem and xylem of the tree and propagated its fungi,” says Addison.
The fungus may or may not cause disease and these trees are generally not expected to die.
Non-reproductive hosts include citrus (Citrus), grapes (Vitis) and most Eucalypts (Eucalyptus) species.
According to Addison, a problem when compiling host tree lists is that sometimes PSHB can infest a stressed tree due to drought, too much water, root damage or other diseases. Such a stressed tree might become a reproductive host, whereas healthy growing individuals of the same species are barely affected.
To date, 161 tree species have been found to be infested in South Africa. These include 83 reproductive hosts and 78 nonreproductive hosts. Some of these include: ȊȲMaple (Acer)
ȊȲBox elder (Acer negundo)
ȊȲJapanese maple (Acer palmatum)
ȊȲSycamore maple (Acer pseudoplatanus)
ȊȲSilver maple (Acer saccharinum)
ȊȲEnglish Oak (Quercus robur)
ȊȲPlane (Platanus)
ȊȲCoral tree (Erythrina)
ȊȲAvocado (Persea americana)
ȊȲWillows (Salix)
ȊȲCastor oil (Ricinus communis)
ȊȲWeeping willow (Salix babylonica)
ȊȲAcacia species: ȊȲLong-leaved wattle (Acacia longifolia)
ȊȲBlack wattle (Acacia mearnsii)
ȊȲBlackwood (Acacia melanoxylon)
Indigenous trees are also under threat and some of the species identified as acting as hosts include: ȊȲOuteniqua yellowwood (Afrocarpus falcatus)
ȊȲForest false nettle (Acalypha glabrata)
ȊȲPod mahogany (Afzelia quanzensis)
ȊȲDune false crowberry (Allophylus natalensis)
ȊȲCape chestnut (Calodendrum capense)
ȊȲRiver bushwillow (Combretum erythrophyllum)
ȊȲSickle bush (Dichrostachys cinerea) ȊȲErythrina species.
At Lourensford Wine Estate in Somerset West, the University of Stellenbosch together with Hortgro is conducting various research projects to gain more insight into the effect of borer infestation in orchards and to ascertain the impact of the borer on specifically agriculture.
One of the main problems with infestation is that once a tree is attacked by PSHB, little action can be taken to save it.
DIFFICULT TO DETECT
According to Dr Minette Karsten, a researcher in applied entomology at Hortgro, detection of PSHB is difficult as the insects are very small. The female PSHB are about 2mm in length and range from brown to black in colour. She says that only females have the ability to fly and disperse to other trees. Males are smaller at about 1,6mm in length and have no wings.
Mature larvae are about 3,5mm long and 1,1mm wide. The larvae are white, shaped like a ‘C’ and legless, with a reddish head.
There are, however, a number of symptoms that indicate that a tree might be infested, Karsten says. ȊȲThe prevalence of frass. Produced by the beetle’s tunnelling, frass or ‘noodles’ may be extruding from trees. If frass is detected, it may indicate that the infestation levels are high. ȊȲDieback. In susceptible trees the Fusarium fungus kills tree vascular tissue, causing branch dieback and/or tree death.
ȊȲSugar volcanoes or crystalline foam. This may exude from the entry and/or exit holes in
DO’S AND DON’TS
DO’S:
Remove primary host trees – alien or exotic trees – before they become infested.
Treat infested trees and plant material as soon after felling as possible.
Treat the material on site and do not move it to an alternative site.
If possible, fell infested trees in winter or during cool weather, as fewer beetles leave the tree in cold weather.
Do remove infested branches from secondary host trees and treat them, that is, chipping.
Monitor ornamental trees, windbreaks and orchards.
Thoroughly clean all equipment before it is moved to another area.
DON’TS
Do not remove infested plant material and firewood into or out of an area.
Do not move living plants with a stem diameter larger than 20mm. the tree trunk or branches, and is a common sign of infestation on avocado trees.
ȊȲTiny holes in the tree trunks or on the branches.
HOW TO MINIMISE IMPACT
Dr Steffan Hansen, a postdoctoral fellow at the University of Stellenbosch and part of the team that works at Lourensford, points out that the best solution at this stage is to fell the infested tree, dispose of the wood appropriately, and hope the pest has not spread to any other trees. “One should either remove the entire tree or remove the infested branches. The wood should be chipped and solarised under plastic to ensure that the insects die.”
Hansen points out that trees damaged by PSHB can quickly become a safety hazard and a constant source of beetles that can disperse to neighbouring trees.
“Early detection and the prompt removal or pruning of infested trees are key to containing PSHB spread and minimising further impact on neighbouring trees,” he says.
According to the Hortgro team, no chemical treatments are available to control the insect. Addison says: “Although we do trials to ascertain the level of treatment or to determine a possible chemical product that can be recommended, at this stage the only conclusion that we can come to is to say that treatments only delay the entire infestation process.”
Karsten adds: “At the moment we are looking at protecting the bottom part of pear trees in the orchard with a variety of treatments to stop the beetle infesting the lower part of the trees.”
She says the following steps should be taken to control the infestation:
ȊȲCut off the infested trees or branches. ȊȲChip the tree trunk and/or branches as small as possible.
ȊȲCollect the chippings and place them in a heap and cover tarpaulin or a sheet of plastic. Ensure that the beetles cannot escape. Leave the heap in full sun for six weeks in summer and six months in winter.
ȊȲThe material can be mulched after the said period.
Addison advises that one of the best solutions would be to remove exotic or non-native trees and alien invaders that are reproductive hosts. “We found high infestation in trees that form wind-breaks and although we would love to recommend removing these trees, the consequential problem would be with establishing windbreaks of non-susceptible trees.”
Movement of firewood can contribute to the spreading of the pest insect across the country. Addison therefore advises strongly against the movement of untreated wood and firewood from one area to another.
PREVENTION METHODS
To address the situation, industry should focus on the following:
Establishing a surveillance programme where all data can be collected in one central point to determine the distribution of PSHB within the country.
Containing the pest to prevent further spread to non-infested regions within South Africa, especially in the fruit production areas.
Providing advice and information to the general public, farmers, nurseries, conservationists, the agriculture industry, and all other stakeholders.
Ensuring that all response activities are conducted safely, consistently and efficiently.
At present researchers at FABI, in collaboration with colleagues from several other universities and institutes, are coordinating monitoring efforts and research on the
PSHB and its fungus in South Africa.
According to Ncise, the university and the team at the Stellenbosch Botanical garden are working with other industry role players and have been attending workshops and conferences dealing with the PSHB infestations and possible ways to manage it.
However, he confirmed that at this stage they cannot recommend one single solution.
“I am also part of a shot-hole borer team that is collecting data from affected trees and reporting on the extent of the infestation and the damage,” he adds.
The project is running on iNaturalist (inaturalist.
and is named ‘Shot-hole borer project’.
Visit fabinet.up.ac.za. Email Matthew Addison at mfa@sun.ac.za, Minette Karsten at minettek@ sun.ac.za, Steffan Hansen at shansen@sun.ac.za, or Wanga Ncise at wanga@sun.ac.za.