The polyphagous shot-hole borer beetle (PSHB) has spread rapidly in the past few
months. The original infections were noted in Oldenland Street in Somerset West in
2019. Since then, infected trees have been identified in many areas of Somerset
The life cycle of the PSHB begins when a mated female finds a suitable host tree.
She then starts digging a gallery directly into the tree trunk or branch. The female
carries spores of the symbiont fungi (Fusarium euwallaceae) to start growing their
fungus gardens inside the gallery. Once the galleries grow rich with fungi the females
start laying their eggs. Most of the larva hatching from those eggs will be females.
Both larvae and adults feed on the nutritious fungi growing on the walls of the
gallery. After 3 to 4 weeks the larvae develop into adults. Siblings mate with each
other within the natal galleries. Shortly after this the newly mated females disperse,
leaving the males behind, and the whole process begins again. The PSHB
population can grow quickly, brood sizes are between 20 to 50, and in South Africa it
is suspected that there can be between 3 to 7 generations per season.
How to identify infected trees? There are several things to look for.
Small holes (flight holes) in the trunk or branches of the tree
Powder / sawdust or frass in the bark or under the tree around the trunk
Dry noodles, protrusions coming out of the trunk or branches that look like
Jelly drop, resin, sap drops or flowing sap on the trunk or branches
Wet spots (looks like a wet finger print)
Staining on the trunk or branches
Dieback of branches in your tree
Depending on the type of tree it may be difficult to see signs of the beetle. If your
trees have smooth or light-coloured bark, like London plane trees it is easier to spot
the flight holes of the beetle, the wet spots and sap coming out of the holes. In trees
with rough and dark coloured bark like English Oak trees it is trickier to spot. Look for
weeping sap or what look like dark wet patches on the trunk or branches. If you see
branches dying back, take a closer look at those branches.
Below is a list of non-reproductive host* trees that have been infected in South
Bauhinia purpurea (butterfly orchid tree); Betula pendula (silver
birch); Bougainvillea sp. (bougainvillea); Camellia japonica (common camelia); Carya
illinoinensis (pecan nut); Ceiba pentandra (kapok); Cinnamomum
camphora (camphor); Citrus limon (lemon); Citrus sinensis (orange); Eriobotrya
japonicum (loquat); Erythrina livingstoniana (aloe coral tree); Eucalyptus
camaldulensis (river red gum); Ficus carica (common fig); Fraxinus
americana (American ash); Fraxinus excelsior (European ash); Hakea
salicifolia (willow-leaved hakea); Hibiscus sp. (hibiscus); Jacaranda
mimosifolia (jacaranda); Macadamia sp. (macadamia nut); Malus
domestica (apple); Melia azedarach (syringa); Morus sp. (mulberry); Olea
europaea subsp. europaea (cultivated olive); Platanus occidentalis (American
plane); Platanus racemosa (Californian plane); Plumeria rubra (frangipani); Prunus
avium (sweet cherry); Prunus cerasifera (cherry plum); Prunus nigra (Black
plum); Prunus persica (peach); Psidium guajava (guava); Quercus rugosa (net leaf
oak); Robinia pseudoacacia (black locust); Salix babylonica (weeping
willow); Schinus molle (pepper tree); Taxodium distichum (swamp cypress); Ulmus
minor = Ulmus procera (English elm); Ulmus parvifolia (Chinese elm); Vitis
vinifera (grape vine).
Adansonia digitata (baobab); Afrocarpus falcatus (Outeniqua yellowwood); Albizia
adianthifolia (flat crown); Buddleja saligna (false olive); Calodendrum capense (Cape
chestnut); Cordia caffra (septee tree); Cussonia spicata (cabbage tree); Diospyros
dichrophylla (star apple); Diospyros whyteana (bladdernut); Dombeya
rotundifolia (wild pear); Dovyalis caffra (kei apple); Ekebergia capensis (Cape
ash); Erythrina lysistemon (common coral tree); Ficus natalensis (Natal fig); Ficus
sur (Cape fig); Grewia occidentalis (cross berry); Gymnosporia buxifolia (spike
thorn); Halleria lucida (tree fuschia); Harpephyllum caffrum (wild plum); Ilex
mitis (Cape holly); Leonotis leonurus (wild tobacco); Melianthus major (honey
flower/Kruidjie-roer-my-nie); Nuxia floribunda (forest elder); Olinia ventosa (hard
pear); Osteospermum moniliferum (bietou); Podocarpus henkelii (Henkel’s
yellowwood); Protea mundii (forest sugarbush); Prunus africana (red
stinkwood); Rapanea melanophloeos (Cape beech); Schotia brachypetala (weeping
boerbean/ huilboerboon); Searsia chirindensis (red currant); Searsia
lansea (karree); Senegalia burkei (black monkey-thorn); Senegalia (Acacia)
galpinii (monkey-thorn); Vachellia (Acacia) karroo (sweet thorn); Syzygium
cordatum (waterberry); Vachellia (Acacia) sieberiana var. woodii (paper bark
thorn); Virgilia divaricata (keurboom).
* Reproductive host trees are those in which both the beetles and the fungus
establish, and where the beetle successfully reproduces. In most cases the
reproductive hosts will eventually be killed by the fungus.
* Non-reproductive host trees are those that are attacked but the beetles do not
establish breeding galleries. The fungus may or may not cause disease. Trees are
generally not expected to die.
If you suspect that your trees are infected, please do report it. Reporting your trees
will not mean that they will automatically be removed. It is important that there is a
record of where the infected trees are located to monitor where the beetles have
spread to. The city is currently only removing the heavily infected Boxelder trees.
Report your trees on the Cape Town Invasives website
(capetowninvasives.org.za/pshb-form) by completing the form. The team from
Nature conservation will then contact you to inspect your trees.
For more information about the PSHB visit the FABI website
(www.fabinet.up.ac.za/index.php/pshb). This website is current and updated
regularly. There are many links to useful photographs to help identify the PSHB.