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Dive into Discovery: Understanding the Polyphagous Shot Hole Borer

The polyphagous shot hole borer, Euwallacea fornicatus, is seen as one of the most devastating environmental disasters of the last decade. It was first identified in South Africa by Dr. Trudy Paap in the KZN Botanical Gardens in Pietermaritzburg in 2017. However, DNA from a sample taken in Durban in 2012, suggests it may have been introduced to South Africa much earlier.

It is now believed that Polyphagous Shot Hole Borer (PSHB) has spread to 8 of the 9 provinces in South Africa, with Limpopo being the only province not recording its presence yet, most likely due to a lack of awareness. Originally from Southeast Asia, this tiny little ambrosia beetle has spread worldwide to the USA, Australia, Israel, and of course, South Africa.

Photo Credit: Pia Scanlon, DPIRD, WA.

This beetle is known to infest more than 200 different plant species from 28 different plant families worldwide. In South Africa, it has been noted to infest more than 100 different plant species. It is estimated that if nothing is done, the polyphagous shot hole borer can cause more than R275 billion worth of damage in the next 10 years. It poses an unmeasurable threat to our fruit production in South Africa, and it will without a doubt have lasting economic consequences for generations to come.

Identifying an E. fornicatus infestation has proven to be challenging. This small 2mm long borer beetle is easily looked over when inspecting a tree for infestation, which most likely aided in its initial rapid spreading. The females are the most problematic as they are winged and thus able to spread between hosts quite effectively.

The females are generally between 1.8mm to 2.5mm in length and black in colour. The males are smaller (1.5mm to 1.67mm in length) and are brown in colour. The males are wingless and therefore cannot fly. The mature female carries on average 32.4 eggs and during oviposition, she will penetrate the host bark and deposit her eggs in an egg gallery (tunnel) whereafter they will incubate for 4 days. The larvae will then develop in the egg galleries for 16-18 days, whereafter they will pupate for 8-10 days before maturing for a further 4-6 days to become an adult. 

The Department of Primary Industries and Regional Development (DPIRD)

Interestingly it has been noted that female beetles will only fly when ambient temperatures are higher than 20°C, limiting these beetles to environments where they find these favourable conditions.

The beetles cause extensive physical damage by tunnelling deep into the sapwood of a tree at a staggering speed. This will cause limbs to break off after a period, posing a potential falling hazard to anything below. The devastating part of the Polyphagous Shot Hole Borer infestation, however, is not the beetle itself.

Euwallacea fornicatus has an obligate symbiosis with a fungus Fusarium euwallacea, which is carried by the beetle between hosts where it will then colonize in the galleries of the infested wood. They do not primarily feed on the wood of the tree and use this fungus as its main source of food. The fungus is carried on the mouthparts of the female beetle and spread to new hosts when they start to bore into bark before depositing eggs.

F. euwallacea colonizes the vascular tissues of the host plant, which are responsible for translocation of water and nutrients. This then causes the typical wilting symptoms that accompany the infestation of the borer. Wilting is generally the most common symptom that is associated with infestation; however, symptoms can vary between different host species.

The Department of Primary Industries and Regional Development (DPIRD)

The most common physical signs that are associated with PSHB infestation are 2-3mm holes in the bark, typically in a shotgun-like pattern. There might or might not be leeching of sugary exudates out of these holes. Gummosis, or a formation of a gummy substance, might be visible in more severe cases. In all cases the woody sawdust-type “frass” was found to be present, as is with most wood borer infestations. This is generally the first sign that tree owners find when inspecting their trees.

Polyphagous Shot Hole Borer being a relatively “new” pest to South Africa, treatments are quite limited and generally focused on physical practices to inhibit the spread of the pest. In the case of ornamental trees, the primary defense strategy is removal and destruction of the affected tree. Trees are carefully removed by professional tree health specialists, whereafter the plant material is either burned or sanitized through solar exposure, which kills all the beetles and their eggs, thus inhibiting further distribution.

However, industry experts are investigating other avenues of control. Chemical intervention seems to be the logical answer, but due to the nature of the PSHB boring so deep into sapwood, chemical control is very limited as systemic insecticides translocate in the vascular tissue and do not penetrate deep into structural wood. Contact insecticides have the obvious problem of not being able to come into contact with the beetles. However, trials are being done on tree injections to possibly deliver the chemicals into woody tissue where these beetles populate.

Entomopathogenic microbes are another avenue of research being investigated. The products Bio-Insek and Eco-Bb, which make use of the entomopathogenic fungus Beauveria bassiana, have shown promising results by decreasing the survival rate in the PSHB, but has not shown high enough efficacy to be economically viable. 

The Department of Primary Industries and Regional Development (DPIRD)

Lastly, parasitic wasps and the use of entomopathogenic nematodes are also being investigated as a potential control measure Polyphagous Shot Hole Borer.

If you suspect a Polyphagous Shot Hole Borer infestation, the following steps should be taken as soon as possible: 

  • Take a clear photo of the complete tree, clearly showing the dead or dying branches. 

  • Identify the tree species, and if you are unsure, take a photo showing the leaves, bark, flower and/or seeds. 

  • If any external signs are visible, take a clear photo of this area.

  • Take a close-up photo of the suspected entry hole and scrape the bark off to reveal the underlying tunnel. 

  • Use a chisel and remove a square piece of bark, showing the underlying tunnel and possible fungal stains (darker stains on light wood). 

  • If possible, remove one of the dead/dying shoots and make a cross-section of it. 

With all this information at hand, contact your closest plant health care specialist for identification.



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