The polyphagous shot hole borer is 2mm long. The polyphagous shot hole borer is 2mm long. Pic: S. Bush (FABI)

Check your trees for the shot hole borer

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Who discovered the borer?

  • Last year, the Forestry and Agricultural Biotechnology Institute (FABI) undertook a survey of tree health in the botanical gardens of South Africa.
  • The project - entitled ‘Monitoring tree health at sentinel sites: botanic gardens and arboreta’ - was funded by the South African National Biodiversity Institute.
  • FABI is a post-graduate research institute was established in 1997, based on a recognition that the future of forestry and agriculture in South Africa.

Where did FABI first discover the problem?

  • FABI discovered the ambrosia beetle/fungal associate damaging an avenue of London plane trees (Platanus x acerifolia) in the KwaZulu-Natal National Botanical Gardens (KNNBG), Pietermaritzburg (Fig 1).
  • The polyphagous shot hole borer (PSHB)  is native to Asia.
  • Together with its fungal symbiont Fusarium euwallaceae, the borer was identified as the causal agent (Paap et al., 2018) of the significant damage to London plane trees in the KNNBG in Pietermaritzburg.

 PSHB Fig 1 Final

Fig 1. a-b. external symptoms of polyphagous shot hole borer attack on Platanus x acerifolia;   c. removal of bark and cambial tissue exposing symptoms caused by fungal colonisation associated with beetle entry hole;     d. longitudinal section through branch showing internal symptoms of discolouration around beetle gallery.

Why is this borer a problem for South Africa?

  • The PSHB is one of three cryptic species in the Euwallacea fornicatus species complex.
  • As adult female beetles burrow into trees to establish brood galleries, they introduce the symbiotic fungus Fusarium euwallaceae which colonises gallery walls, becoming a food source for developing larvae and adult beetles (Eskalen et al., 2012; Mendel et al., 2012).
  • The fungus then invades tree vascular tissue, causing cambial necrosis, branch dieback and death of a broad range of trees (Eskalen et al., 2013).
  • PSHB appears to attack trees of all age classes, the outcome varies from complete death of the tree to mild symptoms of branch dieback or sometimes no impact.
  • FABI believe this pest presents a very serious threat to the health of trees in urban, agricultural and natural environments.
  • It has become alarmingly apparent that PSHB is well established, thus has more than likely been present in South Africa for some years.

 

PSHB Fig 3 Final

Fig 2. Tree decline and death associated with polyphagous shot hole borer.    a. Quecus robur, George;   b. Acer negundo, Hurlingham Johannesburg;    c. Virgilia divaricata, Knysna;     d. Harpephyllum caffrum, Hurlingham Johannesburg and     e. Afrocarpus falcatus, Kareedouw.

 

Where in South Africa are trees being killed by the borer?

  • Since this first detection of PSHB in the KZN NBG, subsequent infestations have been confirmed in Durban, Hartswater, Bloemfontein, Johannesburg, George and Knysna.
  • The municipalities of George and Johannesburg are already facing removal of hundreds, potentially thousands of dead and dying street trees (Fig 2).
  • The suburb of Craighall Park is an epicentre of the invasion in northern Johannesburg. Entire streets of trees in Bedfordview (Talisman Rd) in Ekurhuleni are now also infected and dying.
  • It has become alarmingly apparent that PSHB is well established, thus has more than likely been present in South Africa for some years.

What tree species are affected?

  • The list of susceptible host species continues to grow.
  • Currently the list of host tree species observed in South Africa stands at around 60.
  • Confirmed highly infected tree species observed in the urban environment include Acer spp., Quercus spp. Platanus spp.
  • The growing list of indigenous tree species affected, include Erythrina spp., Combretum spp., Afrocarpus/Podocarpus spp., Harpephyllum caffrum and Vachellia spp. , Halleria lucida, Salix mucronata, Calodendrum capense, Psorelea pinata, Podalyria caliptrata, Nuxia floribunda, Grewia occidentalis and Virgilia divaricata.
  • Some of these observations come from semi-natural areas including the Garden Route Botanical Gardens (George) and Pledge Nature Reserve (Knysna).
  • In recent years, PSHB has emerged as an important invasive pest killing trees in Israel and California.
  • In these countries, it is causing significant and costly damage to urban trees, as well as presenting a major threat to the avocado industry (http://ucanr.edu/sites/pshb/).
  • In California PSHB was initially observed causing problems in urban and agricultural settings, however, as in South Africa, it has recently appeared in natural settings raising grave new concerns (Boland, 2016).

 How does the beetle spread?

  • The movement of the dead wood of infested trees - through chipping/composting, solarisation or burning – is the single biggest pathway for spread of the beetle.
  • Another potential pathway is nursery stock, we have recently observed PSHB attacking containerized trees in the nursery environment.
  • The potential for spread over long distances through the sale and movement of nursery stock is cause for serious concern (Fig 3).
  • Surveys to monitoring the spread of the beetle and fungus throughout South Africa are continuing, and chemical treatment trials and experimental trapping are planned.
  • Given the extent of the threat this pest poses to trees in urban, agricultural and natural environments, a well-developed and coordinated national action plan is required.
  • A well supported awareness campaign emphasising the risk associated with the movement of infested wood as a major pathway for continued spread of PSHB throughout the country will essential.
  • DAFF together with DEA and stakeholders have formed a Steering Committee to drive development of the action plan and awareness campaign.

Fig 4 Nursery Stock PSHB Final

Fig 3. Infested nursery stock.

 

References

Boland, J.M., 2016. The impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California. PeerJ 4, e2141.

Eskalen, A., Gonzalez, A., Wang, D.H., Twizeyimana, M., Mayorquin, J.S., Lynch, S.C., 2012. First Report of a Fusarium sp. and Its Vector Tea Shot Hole Borer (Euwallacea fornicatus) Causing Fusarium Dieback on Avocado in California. Plant Disease 96, 1070-1070.

Eskalen, A., Stouthamer, R., Lynch, S.C., Rugman-Jones, P.F., Twizeyimana, M., Gonzalez, A., Thibault, T., 2013. Host Range of Fusarium Dieback and Its Ambrosia Beetle (Coleoptera: Scolytinae) Vector in Southern California. Plant Disease 97, 938-951.

Mendel, Z., Protasov, A., Sharon, M., Zveibil, A., Yehuda, S.B., O'Donnell, K., Rabaglia, R., Wysoki, M., Freeman, S., 2012. An Asian ambrosia beetle Euwallacea fornicatus and its novel symbiotic fungus Fusarium sp. pose a serious threat to the Israeli avocado industry. Phytoparasitica 40, 235-238.

Paap, T., de Beer, Z.W., Migliorini, D., Nel, W.J., Wingfield, M.J., 2018. The polyphagous shot hole borer (PSHB) and its fungal symbiont Fusarium euwallaceae: a new invasion in South Africa. Australasian Plant Pathology 47, 231-237.

Source: FABI, University of Pretoria.