The Dangers of Polyphagous Shot Hole Borer
Although these pests may be tiny, the presence of Polyphagous Shot Hole Borer (PSHB) in South Africa is a massive cause for concern. Geographically, this is the largest outbreak of PSHB in the world, affecting over 80 tree species in agricultural, commercial and urban sectors. The first local discovery was made in the KwaZulu-Natal National Botanical Gardens, and the beetle’s presence has since been confirmed in eight of our nine provinces.
Unfortunately there is currently no effective treatment available for shot hole borer, and once the beetle has made its way into a tree its demise seems inevitable. The deadly spread PSHB is native to Southeast Asia, but due to globalisation and increased interconnectedness the beetle has through international trade found its way to Israel, California and now South Africa.
As they are so tiny (about 2mm long) and burrow into the wood, they are very difficult to detect in early stages and are often only discovered as the host tree shows signs of dying. The problem has spread throughout the country with the movement of infested dead wood (like firewood). Current projections of the scale of the infestation reach up to half a million trees in Gauteng alone, and that number is set to grow. Without proper management and awareness the potential for the continued spread of PSHB through wood products is a huge risk.
A forecast by the website Tree Survey predicts that by 2022 all urban sites across the country will have a presence of PSHB, and the areas with current high levels of infestation will reach the maximum level of infestation of susceptible trees (20 – 30% of all trees).
The Science-y Stuff
The beetle itself (Euwallacea sp.) does not harm trees; it merely does the burrowing. The real issue is the fungal symbiont Fusarium euwallaceae, a fungus that feeds the beetle and its larvae. Once the beetle enters the tree, the passages in the wood are lined with the fungus, which eventually kills the tree by preventing the movement of water and nutrients through the vascular system. Adult female PSHBs can lay more than 30 eggs at a time, ensuring a quick spread between trees. Not all tree species are ‘reproductive hosts’, meaning PSHB cannot breed in every tree, but the fungus that they carry can still kill a non-reproductive host tree.
Management and identification
Research networks have been established to coordinate information and develop future solutions, but these processes are time consuming. Government departments have been informed and are becoming an integral part of removing trees infested with shot hole borer from public spaces and issuing warnings to the private sector and the general public. Local conservancies in a number of areas have also played a role in spreading the word about the dangers of PSHB, forming part of an important awareness campaign. Scientists say the best thing that the average Joe can do is to be on the lookout for the signs of infestation and report any concerns immediately.
The tell-tale signs of shot hole borer are:
- Wilting or drying leaves
- Branch dieback
- Shotgun-like marks around small entrance holes
- Dark, wet staining around holes
- Resin oozing from holes.
How to save your trees from shot hole borer
This outbreak is enough to terrify any tree owner, as trees are treasured and considered prized possessions. While there is no way to prevent an attack on your trees completely, there are steps you can take to limit your risk. Keep your trees healthy and happy, as PSHB tends to invade trees with compromised immune systems.
Place mulch around the base of the tree regularly and continuously check for signs of pests or diseases. Provide additional water if needed and add bio-stimulants to the surrounding soil. Make sure you focus more attention on the trees that are known hosts as not all trees will carry the beetle (the full list can be found on the PSHB website).
Check all wood you introduce onto your property for signs of PSHB and limit the movement of dead wood as much as possible to avoid exposing your existing trees. This includes firewood, which can be especially dangerous when burned as the beetles fly away and can spread to your own trees.
Apply reactive treatment to the trees that are already infected with shot hole borer. There is currently no proven treatment that can cure PSHB completely, but there is some good news. Tree injection, which is the most popular treatment being used, is the least dangerous to the surrounding environments, and it can limit the damage and spread of PSHB. Products can be found at treeinject.co.za, or contact your local arborist. As we’ve said, it is the fungus that actually kills trees, and a product developed by Pan African Farms has just been approved under emergency registration procedures to treat the fungus. The recommended application procedure is treating each tree three times with 2-week intervals between each trunk spray application. The PanAf PSHB Fungicidal is available for purchase at www.panafricanfarms.co.za/pshbproducts.html. A second product by the same company has been developed but is yet to be approved, so hope is on the horizon.
Spread the word and be on the lookout for the symptoms of PSHB. If you spot the beetle in your garden or area, inform your neighbours of the problem and make your friends and family aware. Contact your local municipality or representative and urge them to act.
Infested trees can be reported to FABI at firstname.lastname@example.org or logged using the Tree Survey app. Action is vital, as this beetle is not one we can ignore.
My tree is dead, what now?
If your tree has succumbed to the infection, proper removal is vital in limiting the spread of the beetle to other trees in your garden or your neighbourhood. Once the tree has been cut down there are a number of methods you can use to contain PSHB.
Burning the wood on site is not recommended, even if it is superficially effective. The problem is that some beetles might escape and then fly from the burning wood onto surrounding trees. Unless you are sure the surrounding area is already heavily infested, it is best to take another route.
Solarization is the safest method available. Leave the pile of chopped wood in a sunny area and cover completely with a plastic sheet, tucking the edges in to contain heat and moisture (and beetles). Within 6 – 8 weeks the beetles should be dead and the wood can be removed.
Wood chipping is another method proven to kill PSHB, but an industrial chipper is usually required.
The ideal solution would be the establishment of designated dumping sites by the municipality, but no PSHB dumping sites have been erected yet. Contact your municipality or City Parks to ask about the process and when a dumping site will be established. Thank you to Professor Wilhelm de Beer from the Forestry and Agricultural Biotechnology Institute at the University of Pretoria for the information and images. More information can be found at polyphagous-shot-hole-borer.co.za. This information was correct and up to date at the time of going to print.
Confirmed reproductive host trees:
• Persea americana (avocado) • Platanus x acerifolia (London plane) • Quercus robur (English oak) • Salix mucronata (Cape willow) • Acer buergerianum (Chinese maple) • Acer palmatum (Japanese maple) • Acer negundo (box elder) • Albizia adianthifolia (flat crown) • Erythrina caffra (coast coral tree) • Erythrina lysistemon (common coral) • Acacia melanoxylon (Australian blackwood) • Gleditsia triacanthos (honey locust) • Liquidambar styraciflua (American sweetgum) • Ricinus communis (castor bean)
Confirmed non-reproductive host trees:
• Carya illinoinensis (pecan nut) • Citrus limon (lemon) • Jacaranda mimosifolia (jacaranda) • Ulmus parvifolia (Chinese elm) • Erythrina lysistemon (coral tree) • Vachellia (Acacia) sieberiana var. woodii (paper bark thorn) • Buddleja saligna (false olive) • Camellia japonica (common camellia) • Ficus carica (common fig) • Plumeria rubra (frangipani) • Ficus natalensis (Natal fig) • Rapanea melanophloeos (Cape beech)
RootPro is a bio-stimulant that can improve the health of your trees. It contains the spores of a beneficial fungi that is fast growing and aggressive. This helps your trees by supressing root diseases and enhancing root growth, enabling it to better resist diseases and fungal infections.
- The shotgun-like marks of the PSHB on infested London plane trees.
- The beetles create breeding galleries stained by the Fusarium fungus that reveal themselves when the tree is cut down.
- An English oak tree that fell victim to a PSHB infestation.
- The ‘tree mafia’ living in a pecan tree. Several older (black) and younger (brown) females live together with a single (small brown) male in the gallery, feeding off the grey fungus on the gallery walls.
- A wild plum shows clear evidence of a PSHB infestation underneath bark.
- Dark, wet stains caused by the fungus often surround the shotgun-like holes.
- The internal symptoms on a native monkey thorn.
- The tell-tale fungal stain around beetle tunnels on English oak.
- Breeding galleries in the wood of Chinese maple trees.
- Breeding galleries are also present in the smaller branches of trees, evident in this Chinese maple.