A guide to PSHB biology. By Anna Mouton.
What are PSHBs?
Polyphagous shot-hole borers — PSHBs — are beetles in the weevil family. They form part of a group called ambrosia beetles. Ambrosia beetles feed on fungi that they establish inside burrows in wood. Most ambrosia beetles target dead or dying plants, but PSHBs can attack healthy trees.
PSHBs and their associated fungi can damage and kill trees. The economic implications for orchards and plantations of susceptible tree species are obvious. The beetles also impact the human and natural environment by destroying ornamental and indigenous trees. Loss of urban and natural forests has far-reaching consequences — the death of street trees is already changing the face of some South African suburbs.
Where did they come from?
PSHBs are native to Southeast Asia. Studies have identified distinct genetic PSHB subtypes. The dominant invasive subtype found in South Africa is thought to originate in Vietnam. The less common genetic subtype in South Africa occurs in China, Hong Kong, and Vietnam.
The beetle was officially detected in 2017, but when researchers returned to previous collections, they discovered that the first South African PSHB sample was collected in 2012 in Durban.
It currently occurs in every province except Limpopo. The beetles have also established in California, Hawaii, Israel, and Western Australia. They have occasionally been detected in — but eradicated from — European greenhouses.
Ambrosia beetles spread globally with the movement of infested wood, especially wooden packaging material and sawn timber. Infested firewood could be a significant vehicle for PSHB spread within South Africa.
Female PSHBs can fly but not far — probably less than a kilometre — unless blown by the wind. Their maximum speed in a laboratory trial was a paltry 2 kilometres per hour. Males are flightless.
What trees are at risk?
PSHBs attack hundreds of tree species. Reproductive hosts are trees in which both the beetle and its fungus can establish and reproduce. Non-reproductive hosts — as the name implies — are trees in which the beetles do not create breeding galleries. This could be because the tree is unsuitable for the fungus or the beetle.
Although the fungus can establish itself in non-reproductive hosts, the trees usually survive. Reproductive hosts are more likely to die. Highly susceptible reproductive hosts in South Africa include box elders, English oaks, and weeping willows. These trees generally die within a few years of PSHB infestation.
Among commercial tree crops, almonds, apples, apricots, avocados, pears, and pecans are confirmed reproductive hosts in South Africa, although some have only been found to be so when stressed. Cherries, figs, grapevines, guavas, lemons, macadamias, olives, oranges, peaches, and persimmons are currently considered non-reproductive hosts — this could change.
Preliminary observations indicate that plums may also be hosts, but their status has not been confirmed.
Signs of infection
The main signs are small round entry holes — about 1 mm in diameter — in woody trunks and branches. In deciduous fruit trees, beetles tend only to attack stems greater than 25 mm in diameter. It may be necessary to remove the bark to see the entry holes.
Depending on the host species, the holes may show weeping or gumming. The area around the hole might be discoloured. The beetles might push light-coloured powder or fine sawdust from the hole.
Branches of affected trees might wilt and die back. The entire tree may eventually die.
Suspected PSHB infection must be confirmed by laboratory analysis. Pome- and stone-fruit growers should contact Hortgro at firstname.lastname@example.org for information on sample submission. Members of the public should contact their municipality.