The Polyphagous Shot Hole Borer (PSHB) and its fungus in South Africa

FABI information pamphlets on the PSHB

PSHB - Information sheet   PSHB - Life stages of the beetle   PSHB - Symptoms

PSHB - External symptom types   PSHB - How to photograph for diagnosis   PSHB - How to sample

PSHB in the news

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Californian information pamphlets - Californian host tree list

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Background

As participant in the International Plant Sentinel Network (IPSN), Dr Trudy Paap had been tasked to do routine surveys for tree pests and diseases in and around the National Botanical Gardens of South Africa in collaboration with SANBI. It was during such a survey that she noticed small lesions resembling shotgun marks on the stems and branches of mature London plane (Platanus x acerifolia) trees planted along the street outside the KwaZulu-Natal Botanical Gardens. Upon closer inspection, she found that the lesions developed around entrance holes of small beetles. When she removed the bark, the sapwood was discoloured by a fungus. She brought samples back to FABI, and with the assistance of FABI team members, the beetle and fungus were identified based on DNA sequences as Euwallacea whitfordiodendrus (Polyphagous Shot Hole Borer or PSHB) and Fusarium euwallaceae, respectively. The results were published in the Australasian Journal of Plant Pathology (Paap et al. 2018).

The beetle-fungus symbiosis

The discovery of this beetle and fungus in South Africa is of major concern to farmers, foresters, landscapers, home owners and ecologists, as together, these organisms can be aggressive tree killers. The PSHB is a 2 mm long ambrosia beetle that is native to Southeast Asia. The female beetle carries several fungal species, one of which is Fusarium euwallaceae, with it when it infests new trees. It bores through the bark into the sapwood of the tree and inoculates the fungus into the living wood. The fungus grows in the galleries (tunnels) of the beetle and serve as 'vegetable garden' for the beetle larvae, but in susceptible trees the fungus can spread through the sapwood causing disease or even death of the tree. 

In its native environment in Southeast Asia, it seems as if the beetle and fungus do not cause serious damage because tree species have evolved with the beetle-fungus complex and have resistance towards them, and because there are most likely a suit of natural enemies of the beetle. However, the beetle and fungus were somehow introduced into Israel and California in the early 2000's where they caused serious damage on several ornamental trees as well as avocado trees.

Identification of the beetle and fungus

Until December 2018 the PSHB was known as Euwallacea nr. fornicatus. However, Gomez et al. (2019) showed with DNA sequences that 'Euwallacea fornicatus' is actually a species complex including four closely related, but distinct species. These four species of Shot Hole Borer are very similar in shape, and can only be distinguished by specialists under a microscope or with DNA sequences. The four species carry different fungal species, have different host ranges, and different geographical distributions (Gomez et al. 2018). Although they suggested the name Euwallacea whitfordiodendrus should be used for the PSHB, this was an error that was corrected in a subsequent paper by Smith et al. (2019). The correct names of four Shot Hole Borer species in the E. fornicatus complex are:

1) Tea Shot Hole Borer A [TSHB-a = Euwallacea perbrevis (Schedl 1951)]

{Distribution: Asia (American Samoa, China, Indonesia, Papua New Guinea, Samoa, Sri Lanka, Taiwan, Thailand), Australia, and introduced in the USA (Florida and Hawaii)}

2) Tea Shot Hole Borer B [TSHB-b = Euwallacea fornicatior (Eggers 1923)]

{Distribution: Asia (Malaysia, Papua New Guinea, Singapore, Sri Lanka)}

3) Polyphagous Shot Hole Borer [PSHB = Euwallacea fornicatus (Eichhoff 1868), previously referred to as Euwallacea nr. fornicatus and Euwallacea whitfordiodendrus (Schedl 1942)] 

{Distribution: Asia (China, Hong Kong, Japan, Taiwan, Thailand, and Vietnam) and introduced in USA (California), Israel, and South Africa} 

4) Kuroshio Shot Hole Borer [KSHB = Euwallacea kuroshio Gomez and Hulcr 2018]

{Distribution: Asia (Indonesia, Japan, and Taiwan) and introduced in Mexico and USA (California)}

In California, PSHB and KSHB were introduced separately and now co-occur, which poses a greater threat than only one species. With our borders being open for trade to Southeast Asia, the risk that one of the other species can be introduced is more than likely. Co-occurring species increase the chances for interbreeding which will enhance the adaptability of the beetles to new hosts and new environments, posing a greater threat. We thus need to confirm the identity of PSHB from each new location or host tree reported in South Africa, with a DNA sequence of either the fungus or the beetle or both, before the host is added to the official list below.

A large diversity of host trees

'Polyphagous' refers to the ability of the PSHB to infest many different tree species. In California, surveys in botanical gardens revealed that more than 200 tree species have been infested, damaged and often killed, by these organisms (Eskalen et al. 2013). 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 trees where the beetle attacks, penetrates and inoculates the fungus that then starts growing in the sapwood. However, the beetle either leaves or dies without reproducing in these trees. The fungus can eventually kill or damage reproductive hosts (referred to as Fusarium Disease), but many of the tree species on this list seem to be unaffected.

A problem when compiling these 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. When trees are assessed for Fusarium Disease or whether it is a reproductive host, other stress factors on the tree should always be considered.

The invasion in South Africa

Since its discovery in KwaZulu-Natal in 2017, the FABI team has confirmed the presence of the PSHB in eight of the nine provinces in South Africa. The only exception as of March 2019 is Limpopo, but that might merely be because appropriate samples have not been received from that province. Below is a list of all the host trees in South Africa on which the presence of the beetle and/or fungus have been confirmed with DNA sequences in FABI. 

Based on the experiences in California and Israel, avocado trees are among the most susceptible agricultural crop trees to PHSB infestation and FD. To date, it has been found on some backyard avocado's in Sandton and Knysna, and not in any commercial orchards in South Africa (Van den Berg et al. 2019). However, PSHB was discovered infesting pecan tree orchards in the Northern Cape and one pecan tree in Nelspruit. Other fruit trees in private gardens on which it has been found include lemon, orange, guava, peach, and grapevine. However, at this point there is no evidence suggesting that PSHB pose a threat to these crops, but producers should carefully monitor and report any infestations.

In addition to agriculture, commercial forestry is another sector that is concerned. We have only found small numbers of roadside wattle trees with PSHB infestations in the Southern Cape, with a single roadside eucalyptus tree infested in Sandton. No trees in commercial plantations, including pines, have been detected with PSHB in South Africa.

The most visible impact of the PSHB invasion in South Africa is in urban forests on street, park and garden trees, and this became the focus of many articles in the media. Many trees have been killed by PSHB in Sandton, George, and Knysna, while reports from Sedgefield, Bloemfontein, Ekhurhuleni, Jankempdorp, Hartbeesfontein, Pietermaritzburg and Durban suggest that the impact is becoming worse in those areas. The most common trees to be killed are English oak, Chinese maple, Japanese maple, boxelder, and sweetgum.

Of great concern is the recent discovery of PSHB on London plane and sweetgum in Somerset West in the Cape Peninsula. With the major impact of the disease on oak trees in especially the George and Knysna areas, it is inevitable that the famous oaks of Stellenbosch and the surrounding wine farms will be dramatically impacted.

Most unpredictable is the impact that the PSHB invasion will have on our native forests. Several native tree species were found to be infested in the gardens of Sandton, George and Knysna, with species like the coral tree, keurboom and Cape willow being particularly vulnerable and often killed. It is now known that the beetle is spreading from the urban areas into native forests close to the towns of George, Knysna and Durban. However, which species will be affected and to what extent, is unpredictable.

The PSHB Research Network

PSHB as an invasive is relatively new to science, and has only been studied since the outbreaks in California and Israel in the early 2000's. There are thus many unanswered questions about this pest that is essential to understand to be able to better reduce its impact. The research team at FABI under leadership of Prof. Wilhelm de Beer, is engaging with various government agencies, municipalities and industries to advise on policy and strategy, and to secure funding for research projects. In the process they have established a research network including academics from seven other universities who will collaborate on various aspects of the PSHB invasion in the different regions.

Projects that are fully funded at present include the monitoring and impact on avocados (Prof. Noelani van den Berg), macadamias (Dr Gerda Fourie), pecans (Prof. Wilhelm de Beer), botanical gardens (Dr Trudy Paap and Dr Mesfin Gossa), and forestry crops (Dr Trudy Paap and others from the Tree Protection Cooperative Programme (TPCP)).

Ongoing projects that are partially funded include an assessment of the impact of PSHB on native trees and forests in the Southern Cape (Profs Wilhelm de Beer, Francois Roets [Stellenboch University], and Martin Hill [Rhodes University]).

Envisaged projects include an exploration of biological control options and assessment of PSHB impact on native forests in KwaZulu-Natal, with funding committed by the Department of Environmental Affairs (DEA). In addition, the City of Johannesburg Metropolitan Municipality has committed for two projects on the impact and management of PSHB in urban forests. These projects will hopefully be launched before middle 2019.

What can be done?

There is no way in which the PHSB invasion in South Africa can be stopped. However, some treatments and management strategies can reduce its impact.

Municipalities should:

·       Train staff to recognize and cut down heavily infested reproductive host trees from streets and public areas

·       Infested branches can be cut if the main stem is not infested (unlikely, as PSHB usually infests the stem first)

·       Designate dedicated dumping sites where infested wood can be dumped as it poses a risk of spreading the beetle.

·       Chip wood to pieces finer than 2 cm at the dumping sites.

·       Provide a help desk (preferably online) where the public can report infested trees and get information.

Tree growers/home owners should:

·       Try to determine whether the symptoms are really caused by PSHB (see FABI brochures)

·       If unsure, ask help from municipal or other help desks, or your local arborist

·       If the tree is a heavily infested reproductive host, cut it down

·       Infested branches can be cut if the main stem is not infested (unlikely, as PSHB usually infests the stem first)

·       Dump the wood at a dedicated (by your municipality) dumping site

·       Chip the wood to finer than 2 cm, allow chips to compost by keeping it wet

·       Or burn the wood on site (some beetles will fly away when the wood becomes hot or when smoke appears, so do not burn in uninfested areas)

·       DO NOT MOVE FIREWOOD AROUND, ESPECIALLY NOT OUT OF THE INFESTED AREA WHERE YOU LIVE

·       Or solarize (leave in full sun) chopped wood under thoroughly sealed clear plastic sheets for at least one month in summer or several months during winter

·       At present no chemical product is registered (legal) to use on PSHB in South Africa.

Enquiries:

Western Cape: fill in the online report form at www.capetowninvasives.co.za 

Johannesburg: email trees@jhbcityparks.com or Whatsapp 0828030748

Other parts of the country, especially from towns where it has not been reported, and host tree species not on the list below: email pshb@fabi.up.ac.za (English please, some of our staff are foreigners who do not speak Afrikaans)

PLEASE DO NOT SEND SAMPLES TO FABI WITHOUT FIRST CONTACTING US AT pshb@fabi.up.ac.za

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Current host trees on which infestations in South Africa have been confirmed¹

REPRODUCTIVE HOST TREES2
Exotic species				Native SA species
Latin name	Common name			Latin name	Common name
Acacia melanoxylon	Blackwood			Combretum krausii	Forest bushwillow
Acacia mearnsii	Black wattle			Erythrina caffra	Coast coral tree
Acer buergerianum	Trident (Chinese) maple			Podalyria calyptrata	Water blossom pea
Acer negundo	Boxelder			Psoralea pinata	Fountain bush
Acer palmatum	Japanese maple			Salix mucronata	Cape willow
Brachychiton discolor	Pink flame tree			Virgilia oroboides subsp. ferruginea	Keurboom
Gleditsia triacanthos	Honey locust
Liquidambar styraciflua	American sweetgum
Magnolia grandiflora	Southern magnolia
Persea americana	Avocado
Platanus x acerifolia	London plane
Quercus palustris	Pin oak
Quercus robur	English oak
Ricinus communis	Castor bean
Salix alba	White willow

NON-REPRODUCTIVE HOST TREES3
Exotic species				Native SA species
Latin name	Common name			Latin name	Common name
Bauhinia purpurea	Butterfly orchid tree			Bauhinia galpinii	Pride of De Kaap
Betula pendula	Silver birch			Buddleja saligna	False olive
Camellia japonica	Common camellia			Calodendrum capense	Cape chestnut
Carya illinoinensis	Pecan nut			Calpurnia aurea	Geelkeurboom
Ceiba pentandra	Kapok			Combretum erythrophyllum	River bushwillow
Cinnamomum camphora	Camphor			Cordia caffra	Septee tree
Citrus limon	Lemon			Cussonia spicata	Cabbage tree/ Kiepersol
Citrus sinensis	Orange			Diospyros dichrophylla	Star apple
Eriobotrya japonicum	Loquat			Diospyros lycidioides	Monkey plum
Erythrina livingstoniana	Aloe coral tree			Ekebergia capensis	Cape ash
Eucalyptus camaldulensis	River red gum			Erythrina lysistemon	Common coral tree
Ficus carica	Common fig			Ficus natalensis	Natal fig
Fraxinus excelsior	European ash			Grewia occidentalis	Cross berry
Jacaranda mimosifolia	Jacaranda			Gymnosporia buxifolia	Spike thorn
Macadamia sp.	Macadamia nut			Halleria lucida	Tree fuschia
Melia azedarach	Syringa			Harpephyllum caffrum	Wild plum
Morus sp.	Mulberry			Melianthus major	Honey flower/ Kruidjie-roer-my-nie
Platanus occidentalis	American plane			Nuxia floribunda	Forest elder
Platanus racemosa	Californian plane			Olea europea subsp. africana	Wild olive
Plumeria rubra	Frangipani			Podocarpus falcatus	Outeniqua yellowwood
Populus nigra	Lombardy poplar			Podocarpus henkelii	Henkel’s yellowwood
Prunus nigra	Black plum			Protea mundii	Forest sugar bush
Prunus persica	Peach			Prunus africana	Red stinkwood
Psidium guajava	Guava			Rapanea melanophloeos	Cape beech
Schinus molle	Pepper tree			Schotia brachypetala	Weeping boerbean/ Huilboerboon
Taxodium distichum	Swamp cypress			Senegalia (Acacia) galpinii	Monkey-thorn
Ulmus minor = Ulmus procera	English elm			Vachellia (Acacia) karroo	Sweet thorn
Ulmus parvifolia	Chinese elm			Vachellia (Acacia) sieberiana var. woodii	Paper bark thorn
Viburnum sinensis	Viburnum			Virgilia divaricata	Keurboom
Vitis vinifera	Grapevine
1 Only tree species on which the presence of the beetle and/or Fusarium fungus have been confirmed with DNA sequences are listed here. Last updated 2019-04-24.
2 Host trees in which both the beetles and the fungus establish, and where the beetle successfully reproduce. In most cases the reproductive hosts will eventually be killed by the fungus.
3 Host trees that are attacked by the beetle and where the fungus establishes, but where the beetle does not successfully breed. The fungus might, or might not cause disease and kill the trees.

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