Is my tree infected?

JUDITH BRINK

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

West.

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

small noodles

 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

Africa.

Exotic Species

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).

Indigenous species

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.