Polyphagous shot-hole borer update

This invasive pest was featured in the September 2023 Fresh Quarterly. What have we learnt since then? By Anna Mouton.

Polyphagous shot-hole borers are wood-boring beetles that establish and cultivate fungal gardens in their host plants. The beetles feed on the fungi, which feed on the plant, which can deteriorate and die.

The borers are native to Southeast Asia. They were officially detected in South Africa in 2017 and have spread to every province except Limpopo. Subsequently, Hortgro funded surveillance in the Western and Eastern Cape, with 321 traps in the Berg River region, EGVV, Klein Karoo, the Langkloof, Stellenbosch, and the Warm Bokkeveld.

Various service providers monitored traps during the growing season in 2023 and 2024. Surveillance continues as part of a collaboration between Hortgro and the FruitFly Africa Centre for Excellence in Monitoring. The updated distribution map is available on the Hortgro polyphagous shot-hole borer website.

Infestation in fruit trees

A Hortgro-funded research project led by Prof. Francois Roets of the Department of Conservation Ecology and Entomology at Stellenbosch University assessed the threat of polyphagous shot-hole borers and their fungal symbiont to fruit trees. Karyn Engelbrecht, the MSc student on the project, recently completed her dissertation.

Engelbrecht surveyed borer infestations in 2022–23 in one apple orchard (Golden Delicious), four pear orchards (two each of Packham’s Triumph and Forelle), two plum orchards (Angeleno, Fortune, and Songold), and one vineyard (Cabernet Sauvignon) in Somerset West.

Infestation levels were generally low except for one Packham’s Triumph and one Fortune orchard, where 40% of the trees were infested. Of these, about a third had suffered more than 30 attacks. This Packham’s Triumph orchard became the subject of further study, which is described below.

Engelbrecht also conducted field experiments to test whether the fungal symbiont could establish in different fruit types and cultivars. Although the fungus could colonise all these, the beetles failed to breed in most of them.

The researchers concluded that polyphagous shot-hole borers could threaten apple, pear, and plum trees, but except for Packham’s Triumph pears, the beetles don’t readily form colonies in these plants.

Two years of Packham’s Triumph

Polyphagous shot-hole borers invaded the Packham’s Triumph orchard in the summer of 2022. The relatively high infestation levels in the 8-year-old block provided an ideal opportunity to study the effects of the invasive beetles and their fungal symbiont on a commercial orchard.

The Hortgro-funded project was led by Dr Casper Crous of the Department of Conservation Ecology and Entomology at Stellenbosch University. Emma Neethling, the student on the project, recently completed her MSc based on data from the first year after the invasion. The trees were also monitored for a second season.

To quantify infestation, the researchers ranked trees by the number of beetle entry holes, with more holes considered a sign of increased attack density. However, they pointed out that not all entry holes represent successful beetle and fungal establishment.

In the first year, densely attacked trees produced smaller, duller-toned fruit with higher sugar levels than uninfested trees. This variable effect didn’t increase linearly with increased beetle entry holes.

Leaf measurements in the first year suggested that densely attacked trees use more water. Data from the second year showed a weak trend of increased water conductance in densely attacked trees. Nonetheless, the trees had well-functioning photosystems and didn’t show stress.

In November 2024, Matthew Addison, previous crop-protection manager at Hortgro Science, reported that infested and uninfested pear trees had similar phenology and appearance. Many other infested trees, including beefwoods and English oaks, are dead and dying on the same premises.

These results suggest that pear trees are resilient to the polyphagous shot-hole borer, possibly because they aren’t good hosts for the beetle or its fungal symbiont.

A better beetle trap

Entomologist Gaylen Carelse investigated different traps under the supervision of Dr Annike Pienaar, then crop-protection portfolio manager at ExperiCo Agri-Research Solutions. The Hortgro-funded project aimed to improve the Baker-style traps widely used to trap polyphagous shot-hole borer in South Africa.

Baker-style traps tend to collect rainwater, only allow beetle entry from one direction, and need frequent servicing.

Carelse enclosed a folded sticky sheet in a coarse (2.6 x 2.6 cm) plastic netting cage to exclude large insects, small birds, bats and foliage. She deployed caged traps with or without a quercivorol lure to test different trap positions (0.4, 1.5 and 2.5 metres above the ground) and colours.

Unfortunately, the caged sticky traps were ineffective, so she found no significant differences in catches for trap position or colour.

She also compared the caged traps, Baker-style traps, modified Baker-style traps (with drainage holes and a dichlorvos tablet), yellow bucket funnel traps with extensions, and black panel traps. All worked equally well, except for the ineffective caged traps.

Hitchhiking mites

Once entrenched inside a tree, polyphagous shot-hole borers and their fungal symbiont are protected against biological and chemical control. Attempts to prevent or remove infestation with systemic products, even by injecting trees, have had little success.

Postdoctoral researcher Dr Steffan Hansen, under the supervision of Roets, is exploring a novel biocontrol strategy for the polyphagous shot-hole borer and its fungal symbiont based on fungi such as the entomopathogenic Beauveria bassiana and fungal-killing Trichoderma species.

Hansen thinks hitchhiking mites could be vectors for introducing biocontrol agents into the beetles’ galleries. Many species of these phoretic mites associate with bark beetles such as polyphagous shot-hole borer, living in or around the beetles’ galleries. Hansen has collected five mite species and is culturing them in the laboratory.

If the mites aren’t adversely affected by B. bassiana and Trichoderma species, he plans to see whether spore-carrying mites can infiltrate beetle galleries and infect the borers or their fungal gardens.

To stay current on polyphagous shot-hole borer research, contact minettek@sun.ac.za to sign up for the monthly Hortgro IPM meetings. Meetings can be attended on Zoom or in person.