Shot hole borer in SA, but not yet in the Cape

The holes in the Yellowwood tree that alarmed Stéphan Pieterse. PICTURE: NORMAN MCFARLANE

When Stéphan Pieterse saw that one of his beloved Yellowwood trees opposite his home on the common property of the complex where he lives in Somerset West was dying, he leaped into action.

“On examining the tree, I saw that the bark was covered in tiny holes, so it was obvious that some insect had attacked it,” Mr Pieterse told Bolander last week.

After reading a newspaper article about Asian shot hole borer, an Ambrosia beetle that originates from Asia, and which has been identified in Gauteng, he was concerned that it might be this pest which had attacked the Yellowwood tree.

 His enquiries led him to Professor Wilhelm de Beer at the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria, who asked him to collect samples of the bark and, if possible, the beetle, package them and ship them up to Pretoria for analysis.

Professor De Beers’ initial investigation has confirmed that thankfully, Mr Pieterse’s bug is not Asian shot hole borer, but they will only be able to identify the species, once DNA sequencing is complete.

According to Professor De Beer, polyphagous shot hole borer (PSHB) – its proper name – is concerning because unlike most other borer species, it attacks a wide variety of tree species, and it also attacks living trees.

“We only confirmed in the last month, that PSHB is present in Gauteng, but indications are that it might have been present as early as 2015,”  Professor De Beer told Bolander on Monday.

Although the PSHB was described in 1923 as being from Malaysia, it only gained scientific attention when it started killing avocado trees after it was introduced into Israel and California.

In a study led by Dr Akif Eskalen of the University of California, Riverside, it was found that the beetle not only infested avocados, but 207 of 335 tree species observed in two botanical gardens in Los Angeles County.

The PSHB forms a symbiosis with a fungus, Fusarium euwallaceae, which plays a critical role in the propogation of the beetle.

In a reproductive host, a tree species that fosters the growth of the fungus and reproduction of the beetle, “adult female beetles burrow into a tree, creating brood galleries beyond the cambium, which are concurrently inoculated with the Fusarium euwallaceae. The fungus colonises the gallery walls and becomes the sole source of food for developing larvae and adult beetles. The fungus invades the vascular tissue of the tree, blocking water and nutrients from the roots to the rest of the tree, eventually causing branch dieback and tree death,” according to Dr Eskalen’s paper.

The first scientifically confirmed report of the PSHB and its fungus in South Africa, was published this month by Professor De Beers’ colleague, Dr Trudy Paap, and others from FABI in the journal of the Australasian Plant Pathology Society. They discovered it last year on plane trees in Pietermaritzburg. Professor de Beer recently also confirmed its presence on various tree species in Gauteng.

The FABI team pointed out that “several species native to southern Africa”, including the cabbage tree, common calpurnia, monkey plum, dwarf coral tree, common coral tree, huilboerboon, honey flower, red alder, forest elder and red orchid bush, were identified as susceptible in the Eskalen study.

“Most of these species showed some level of susceptibility to Fusarium dieback, except the last three that were infested by the beetle but did not develop disease,” says the FABI paper.

Worryingly, several commercial crop trees that are planted in South Africa, such as avocado, macadamia nut, pecan, peach, orange and grapevine, are susceptible to PSHB infestation and Fusarium dieback, according to the Eskalen paper, but none of these have been confirmed as a reproductive hosts.

Bolander spoke to wine industry producer organisation VinPro, and clonal vine material producer Vititec on Friday, and both confirmed that PSHB had not yet been identified in any vineyards.

But as Dr Paap points out, “Eskalen et al. (2013) also listed as susceptible several trees that while exotic to South Africa, are planted as ornamentals, including maple, holly, wisteria, oak and Camellia.”

“We now know that in Gauteng, oak trees are a reproductive host for PSHB, although it is uncertain at this time whether or not the beetle and fungus are causing the dieback of these trees,” Professor De Beer said.

“We don’t want to be alarmist but the public needs to be vigilant. PSHB will attack an exotic such as an oak tree, before it attacks a grapevine,” Professor De Beer said.

Although grapevines are not a reproductive host for PSHB, oak trees are, and with their popularity in a number of wine regions, they could become a vector for PSHB infestation in close proximity to vineyards.

“If any signs of infestation are seen, it is better to be safe than sorry. Collect samples and submit them for analysis,” Professor De Beer said.

Senior entomology researcher at ARC Infruitec in Stellenbosch, Dr Elleunorah Allsopp, concurs. “People should look out for any signs of borer in trees and if they spot it, specimens should be collected and submitted for identification. They can send it to FABI or hand it in at ARC Infruitec or at the University of Stellenbosch’s  Plant Pathology Department where Dr Pia Addison runs a plant pest diagnosis service. We can see that it gets to the right people for identification.” Drs Allsopp and Addison confirmed they had not yet received any PSHB samples.