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.
Understanding the Economic and Environmental Threat of PSHB
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 the Polyphagous Shot Hole Borer: Characteristics and Life Cycle
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.
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.
How PSHB Infests Trees and Spreads Fungus
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.
Physical Signs of PSHB Infestation in Trees
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.
Current PSHB Control Methods and Challenges
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.
Lastly, parasitic wasps and the use of entomopathogenic nematodes are also being investigated as a potential control measure Polyphagous Shot Hole Borer.
Steps to Take if You Suspect a PSHB Infestation
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.
FAQ: Polyphagous Shot Hole Borer (PSHB) in South Africa
1. What is the Polyphagous Shot Hole Borer (PSHB)?
The Polyphagous Shot Hole Borer (PSHB), scientifically known as Euwallacea fornicatus, is a small beetle that infests trees by boring into the wood. It has a symbiotic relationship with a fungus (Fusarium euwallacea), which it spreads, causing severe damage to trees' vascular systems.
2. How did PSHB come to South Africa?
PSHB is believed to have originated from Southeast Asia. It was first officially identified in South Africa in 2017 but might have arrived much earlier. Its spread has been facilitated by human activity and a lack of early detection.
3. What are the signs of a PSHB infestation?
Signs of a PSHB infestation include 2-3mm holes in the bark, wilting of branches and leaves, gummosis (formation of a gummy substance), and sawdust-like frass (wood shavings) around the entry holes. These signs can vary depending on the tree species.
4. Can PSHB kill my tree?
Yes, PSHB can kill trees over time, especially if left untreated. The beetle and its associated fungus disrupt the tree’s ability to transport water and nutrients, leading to wilting, dieback, and ultimately death of the tree.
5. How does PSHB spread?
Female PSHB beetles are winged and can fly short distances, but the beetle is also spread through the movement of infested wood, such as firewood, timber, and mulch. This is why controlling its spread is challenging.
6. What should I do if I suspect a PSHB infestation?
If you suspect PSHB in your trees, take photos of the infestation, identify the tree species, and contact a tree health specialist. Avoid moving infested wood to prevent further spread.
7. Are there any treatments available for PSHB?
Currently, treatment options for PSHB are limited. Physical removal of infested trees is common, while research is ongoing into chemical injections, biological controls, and microbial treatments. Consulting with a professional arborist is essential for effective management.
8. How can I prevent a PSHB infestation?
To reduce the risk of infestation, regularly inspect your trees, avoid moving potentially infested wood, and seek guidance from tree health specialists if you notice signs of PSHB. Proper sanitation of tools and tree debris also helps prevent spread.
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The OA Team
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