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The Polyphagous Shot Hole Borer

  • Posted 2 weeks ago
  • Local Stories

An Imminent Threat to Swellendam’s Trees

Written by Dr Luke Potgieter 

The polyphagous shot hole borer (PSHB) (Euwallacea fornicatus), an invasive ambrosia beetle native to Southeast Asia, has rapidly spread to various parts of South Africa, causing extensive damage to urban and natural forests.

Identification

PSHB is a small, dark-coloured beetle, typically measuring 1.8-2.5 mm in length. The adult beetle bores into tree trunks and branches to establish galleries (tunnels) where it lays eggs. Unlike many other wood-boring beetles, the PSHB does not primarily feed on the wood itself but rather on a symbiotic fungus (Fusarium euwallaceae) it cultivates within the galleries. This fungus causes damage to the tree’s vascular system, disrupting the transportation of water and nutrients, leading to dieback and, in many cases, tree death.

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Photo 1: Polyphagous shot hole borer (PSHB, Euwallacea fornicatus) (photo credit: S. Bush)

Host Range and Impact

PSHB has a very broad host range. It affects over 160 tree species in South Africa, including commercially important trees such as avocado, macadamia, and pome fruit, as well as many native and ornamental species. Some of the most affected trees include box elder (Acer negundo) and many other maple species, castor bean (Ricinus communis), English oak (Quercus robur), London plane (Platanus x hispanica), and willows (Salix).

The damage caused by PSHB is often severe and can result in the death of the tree. In urban settings, PSHB infestations can compromise tree health, leading to losses in ecosystem services, increased management costs, and potential hazards from falling branches. In agricultural settings, infestations can result in significant economic losses due to reduced yields and increased management expenses.

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English oak (Quercus robur) planted in George in 1884, felled in 2021 due to PSHB infestation. Clear signs of Fusarium dieback (photo credit: Wilhelm de Beer)

Spread

The natural dispersal of the beetle is relatively slow, but the most important pathway for the spread of the beetle is through the movement of infested wood. For example, the movement of firewood and infested nursery material poses a significant risk for long-distance dispersal of the beetle. Another pathway is the disposal of infested plant material at waste facilities without following appropriate handling and sanitation measures, which also presents a major dispersal pathway for PSHB.

Symptoms to Look Out For

Susceptible tree species can either be reproductive or non-reproductive hosts for the beetle, and these often show different responses to infestations. Reproductive host trees are those in which both the beetles and the fungus establish, and where the beetles can reproduce. These reproductive hosts may eventually be killed by the combined impacts of the beetle and the fungus. Non-reproductive host trees are attacked by the beetle, often leading to the establishment of the fungus, but the beetle does not reproduce. In these trees, the fungus may or may not cause disease but is unlikely to kill the tree.

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Symptoms (staining) of PSHB attack on a London Plane tree (Platanus x hispanica) photo credit: Luke Potgieter

Different host trees show different responses to infestation. Many trees respond to the boring activity of the beetle with the following symptoms:

Entry holes: Tiny, round entry holes about the size of a ballpoint pen tip on the trunk or branches. These holes are usually the first visible sign of infestation.

Frass: The presence of sawdust-like frass (insect excrement and wood particles) around the entry holes.

Gumming and sugary exudate: The presence of gumming or sugary exudate (sugary volcano) around the entry holes.

Dieback: Dieback of branches, often starting from the top of the tree and progressing downward.

Fungal staining: Peeling away the bark will reveal dark staining or streaking around the entry holes, which is indicative of fungal colonization.

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Galleries (tunnels) made by PSHB in which the fungus Fusarium euwallacea has established (dark staining). Female PSHB is also visible (photo credit: Andre de Villers)

Management Strategies

Limiting the movement of infested plant material is crucial to avoid spreading the beetle to new regions. The best management approach is to reduce beetle numbers by physically removing highly infested reproductive host trees. The appropriate disposal of infested wood by chipping followed by either solarization, incineration, or composting is essential for reducing the spread of PSHB. Importantly, burning biomass that has not been chipped (e.g., logs or branches) is not an effective disposal method. PSHB beetles can easily emerge and fly away before the wood has reached lethal temperatures. Rigorous sanitation practices must be carried out after exposure to infested plant material (cleaning and sterilization of any clothing and equipment that comes into contact with infested plant material). Chemical treatment (a combination of insecticides and fungicides) can slow down additional beetle colonization attempts and fungal growth but does not reduce overall beetle numbers in successful colonies.

Regular monitoring (visual inspections for symptoms) of susceptible trees is essential. Early detection allows for more effective management and control measures. Research is ongoing into biological control options, including the use of natural predators or pathogens that specifically target PSHB. These methods offer potential long-term, sustainable solutions but require further development and testing.

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