The Polyphagous Shot Hole Borer threat that most South Africans do not know about

Posted on 9 Oct 2019

The Polyphagous Shot Hole Borer (PSHB) needs to be further studied to ascertain the extent to which South Africa have been infested (Paap, De Beer, Migliorini, Nel & Wingfield, 2018) as there is not enough known about the crisis. According to Dick, Rous, Nguyen & Cooke (2017), there are various reasons why multidisciplinary approaches may be advantageous. By implementing these, the PSHB problem in South Africa can hopefully be solved. Each facet of the complex socio-ecological systems should be taken into account when assessing the PSHB problem. Local, district and national government, communities, activist groups and research institutions should come together and contribute to finding a solution. This brings more perspectives to the solutions table. By working together, results gathered from research may be implemented in practice to find a solution by compromising where required in reaching the common goal of solving the PSHB problem. Solving the PSHB problem will ensure food security and prevent a loss in biodiversity that could be caused by this alien invasive species.


Background of the PSHB


The Polyphagous Shot Hole Borer (PSHB) was first noted in 2003 in California in the United States of America (Grosman, Eskalen & Brownie, 2019). Paap et al. (2018) describes the PSHB as: “an ambrosia beetle native to Asia, has emerged, as an important invasive pest in Israel and in California in the United States. In these countries, it is causing significant and costly damage to urban trees, as well as presenting a major threat to the avocado industries” (Paap, de Beer, Migliorini, Nel & Wingfield, 2018: p. 2). Most of the work is done by female beetles as they make channels into the branches and the stumps of trees. Studies show that the beetle known as the PSHB works in symbiosis with a fungus, called Fusarium Euwallaceae with the purpose of feeding beetles and providing a place to lay eggs (Mayorquin et al., 2018). The same study show that the fungus vests itself into the veins of the trees causing their water transportation systems to be affected and resulting in branches dying and falling off, followed by the tree itself dying.


Dr. Paap discovered the beetle was present in a tree during a survey she was conducting while working with the South African National Biodiversity Institute (SANBI) in South Africa during 2017 (“Forestry and Agricultural Biotechnology Institute”, 2019). When discovered, the beetle and fungus was taken to the Forestry and Agricultural Biotechnology Institute (FABI) where it was recognised as the PSHB and its symbiotic Fusarium euwallaceae (“Forestry and Agricultural Biotechnology Institute”, 2019). It is the same team at FABI that discovered the PSHB and its associated fungus in all provinces in South Africa, except Limpopo.


PSHB in South Africa


The PSHB and the Fusarium euwallaceae has been identified in KwaZulu-Natal, Gauteng, Eastern Cape, Free-state, Mpumalanga, Northern Cape, North West and the Western Cape. The only province in South Africa where the beetle has not yet been located is Limpopo (“Forestry and Agricultural Biotechnology Institute”, 2019).


Tree Susceptibility


Eskalen et al. (2013) published a research paper called the Host Range of Fusarium Dieback and Its Ambrosia Beetle (Coleoptera: Scolytinae) Vector in Southern California. Within the research paper, results were provided of 335 tree species that were studied out of 5 families. 207 of these tree species were assessed to have been attacked by the PSHB. The 207 tree species from 58 families were indigenous to all countries except Antarctica. 128 tree species also from 58 families did not show any evidence of attack while 33 plant families could not be differentiated as attacked or not. In 113 of the original 207 species from 40 families, evidence of the symbiotic fungus could be found. Procreation was found to be feasible in 19 of these tree species (Eskalen et al., 2013). In South Africa, 21 species are viable for procreation of the beetles or described on the PSHB website as “Reproductive Hosts” (“ Polyphagous Shot Hole Borer in South Africa – Save your Tree”, 2019).

In addition, Umeda & Paine (2018) found that temperature plays a role in beetle activity.


Symptoms of PSHB


According to van den Berg, du Toit, Morgan, Fourie & de Beer (2019), the size of the beetle is between 0.8 and 1 mm and can therefore not easily be seen with the naked eye. The same study also indicated that signs of white coloured discharge and small beetle sized holes that can clearly be seen when removing the bark from the tree are symptoms with infection by PSHB. Other symptoms include discoloration of the bark, discharge of tree gum and branch dieback, which is when branches die and fall off at their joints (Eskalen et al., 2013).


Treatments investigated for trees with PSHB and Fusarium euwallaceae


A study by Eatough Jones & Paine (2015) concluded that chipping and solarisation are two effective methods for reducing the number of beetles in a tree, however it does not entirely remove the beetles. They describe chipping as the removal of the infected tree and cutting it into small pieces. Solarization is the process of placing dead trees under black or colour free HDPE plastic in the sun to decrease beetle activity by increasing the temperature. It was also found that the higher the temperature (i.e. summer months), the lower the beetle activity after treatment as in the case of dry logs compared to wet logs (Eatough Jones & Paine, 2015).


Another study found that insecticides are a possible means of treatment for PSHB, however the treatment is not as effective when sprayed on the outside of the trees as beetles predominantly stay within the trees they infect. The study also suggests that a fungicide would be effective for treatment by killing the symbiotic fungus serving as a food source for the PSHB. The treatment of trees affected with only beetles and trees with the symbiotic fungus show different tolerances to insecticides and fungicides, which also have a different effect on different trees with varying vasculature. The writers therefore recommended for further studies to be conducted (Mayorquin et al., 2018).


According to the Random Harvest website, the PSHB could lead to one of South Africa’s worst ecological disasters to date with plantations, forests and urban forests at risk (Viviers, 2019). The Institute of Environment and Recreational Management (IERM) is one group that can aid in identifying incidences of PSHB and preventing surrounding tree infection. IERM organised a workshop with a panel of experts to discuss the threatening PSHB epidemic this year (“IERM”, 2019). Another group calling themselves the Shot Hole Borer Action Group are trying to do something about the PSHB by creating awareness of the issue at large. These are only a few of the actions taken to address the PSHB issue, however, more needs to be done, inclusive of funding to research methods of treatment and prevention (“Shot Hole Borer Action Group”, 2019).

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