The response to PSHB

South African stakeholders launch a counteroffensive against the invasive beetles threatening our trees. By Anna Mouton.

No one knows when polyphagous shot-hole borers — PSHBs — snuck into South Africa. Their cover was blown in 2017 thanks to a surveillance project that uses botanical gardens and tree collections as sentinel sites to identify new and emerging pests.

Dr Trudi Paap and colleagues from FABI — the Forestry Agriculture and Biodiversity Institute — at the University of Pretoria found the beetles holed up in a London plane tree growing in the KwaZulu-Natal National Botanical Gardens in Pietermaritzburg.

Subsequently, it turned out that PSHBs were already around in 2012 when they were sampled from the Iphiti Nature Reserve in Durban by researchers from the Centre for Biodiversity Genomics at the University of KwaZulu-Natal.

The genomes of the unidentified Iphiti beetles ended up online, and in 2018 they were exposed as PSHBs by a team under the guidance of Prof. Richard Stouthamer at the University of California.

Paap and co-workers reported that the beetles sampled in 2012 in Durban were genetically similar to those collected in 2017 in Pietermaritzburg. For those unfamiliar with KwaZulu-Natal, the Iphiti Nature Reserve is about 30 km inland from the Port of Durban. The KwaZulu-Natal National Botanical Gardens is another 60 km from the reserve. This suggests that the invaders were already well-entrenched by 2017.

By 2022, PSHBs had been detected in every province except Limpopo. Dr Anandi Bierman and Profs John Terblanche and Francois Roets of Stellenbosch University found that two genetically distinct subgroups of the beetles are present in South Africa.

The scientists concluded that the invaders probably entered South Africa more than once. They noted that one subgroup is more widespread than the other and may have been introduced first. Only the Western Cape and KwaZulu-Natal — coastal provinces — have the more common as well as the rarer subgroup.

PSHB comes to the Cape

In March 2019, homeowners in Somerset West stumbled on PSHBs in a dying London plane tree in their garden. The City of Cape Town deployed a plant-removal team to the affected street, but felling infested trees failed to purge the beetles. More sightings were soon being recorded.

The beetles have since spread as far afield as Stellenbosch and several Cape Town suburbs. Many streets are changing for the worse as the borers take their toll on trees such as oaks and planes. But it was the discovery of PSHB in pome- and stone-fruit trees in home gardens that jolted the fruit industry into action.

The first Hortgro-funded project on PSHBs in deciduous fruit trees started in July 2019, and research continues — more on the specifics below. Hortgro also formed a PSHB Focus Group to coordinate research and risk-mitigation priorities. They have developed a risk-mitigation strategy built on monitoring, containment, and eradication.

Hortgro deployed a total of 300 PSHB traps spread across the Berg River, Ceres, EGVV, Klein Karoo, Langkloof, Stellenbosch, and Wolseley production areas. A few traps were sent to the Free State and Limpopo Provinces.

In these areas — besides the infestations already mentioned — the beetles are known to be present from George to Plettenberg Bay, which is worryingly close to the Langkloof. Ornamental trees in Bloemfontein have also been hit hard.

As an industry body, Hortgro engages with various government bodies to step up funding and control. Furthermore, Hortgro continues communicating with the public, growers, and various stakeholders through awareness campaigns, field days, publications, and a dedicated website.

Deciduous fruit trees at risk

Once PSHBs attacked fruit trees in South African gardens, research to assess their likely impact on commercial orchards became imperative. The first Hortgro-funded project focused not on the beetles themselves but on their fungal symbiont.

Roets and postgraduate student Mignon de Jager used fungi isolated from borer-infested fruit trees identified in urban gardens to inoculate tree branches in commercial orchards. The fungal symbiont cannot travel without its beetle wingmen, so the experiments carried little risk of spreading disease.

The researchers tested Cripps Pink apples, unidentified pear cultivars, Alpine nectarines, and Angeleno, Fortune, and Songold plums. After giving the fungus several weeks to develop, they removed the inoculated branches and cut them open. All showed discoloured areas associated with the inoculation sites. Reisolation of the fungus from these areas confirmed it as the cause.

In this trial, the fungus grew better in nectarines than in plums, although the results could differ for other cultivars. The researchers observed that fungal growth in nectarines was so rapid that the infection could reach the trunk by the time dieback was noticed in the branch. Removal of visually affected branches may therefore fail to control fungal infections.

To facilitate their investigations, De Jager and Roets developed a technique for the molecular identification of the fungal symbiont from either infected wood or laboratory cultures. Although not tested on deciduous fruit trees, the method worked well in nine of the ten sampled tree species — English oaks were the exception. It can be used on fresh and dried wood and even on biopsies collected using a small drill, although this last method is more prone to false negatives.

While the fungal-inoculation study was ongoing, PSHBs were spotted attacking commercial apple, pear, and plum orchards at Lourensford in Somerset West, lending even greater urgency to researching the problem.

New Hortgro-funded projects

As described in our PSHB basics article, PSHBs feed on symbiotic fungi that the beetles cultivate inside trees. Therefore, the impact on any specific host tree depends on how well it suits both beetles and fungi. These relationships are the focus of a new Hortgro-funded project led by Roets.

Karyn Engelbrecht is the master’s student on the project. She will sample fungi from different pome- and stone-fruit cultivars and check the infected orchard trees for breeding colonies of PSHBs. This will indicate whether cultivars are reproductive or non-reproductive hosts.

Engelbrecht will then inoculate a selection of pome- and stone-fruit cultivars with fungal isolates to evaluate host susceptibility. She will also perform controlled exposure of different trees to beetles and measure how well the beetles can infect and reproduce in the trees.

Observations so far lead Roets to speculate that deciduous fruit trees are not preferred PSHB hosts, suggesting that beetle attacks on orchards represent spillover infestations.

Engelbrecht plans to conduct choice experiments to see which fruit trees are preferred. Beetles will be confined with different combinations of fruit-tree stumps to assess whether they infest some fruit types or cultivars rather than others or oak — experience teaches that PSHBs find English oaks irresistible.

A second Hortgro-funded project led by systems ecologist Dr Casper Crous of Stellenbosch University in collaboration with Roets has two components. One is to examine the effect of PSHB infestations on fruit trees, and the other is to locate possible sources of orchard infestations. Emma Neethling is the master’s student on the project.

Neethling has already started collecting data from a pear orchard at Lourensford in Somerset West. She is using the number of beetle entrance holes as a proxy for infestation severity and comparing the fruit quality and plant physiology of uninfested controls to trees with five categories of infestation ranging from mild to severe.

She is determining tree water use for the past season by measuring the ratio of different carbon isotopes in the leaves. The hypothesis is that the combined activities of beetles and fungi will disrupt water transport and increase stress in infested trees. Transpiration and photosynthesis are expected to drop in trees that respond by closing their stomata.

Additional collection of phenological and physiological data is planned for the coming season.

The other aspect of Neethling’s project is surveying PSHB infestations in orchard trees, windbreaks, and adjacent vegetation. She aims to tease out the role of various surrounding host trees on orchard infestations and to investigate edge effects — greater pest pressure on the edges than in the centre of orchards.

Her data can also shed light on seasonal fluctuations in beetle numbers and the effect of temperature and weather on beetle activity.

Monitor to manage

Besides the research described above, Hortgro will expand existing efforts to develop integrated management of PSHBs, led by Matthew Addison, Crop Protection Programme Manager, and Dr Minette Karsten, applied entomology researcher, both at Hortgro.

Areawide monitoring kicks off again in September when warmer weather will bring beetles out of hiding. Currently, trapping relies on quercivorol lures in Baker-style traps made from empty plastic soda bottles. The imported quercivorol is expensive and in short supply, whereas at times the traps catch more rainwater than beetles.

Chemical engineer Prof. Robbie Pott from Stellenbosch University has been developing ways to isolate and synthesise extracts from the fungal symbiont. PSHBs are attracted to compounds produced by their fungal food source, and the researchers plan to test whether the extracts are an alternative to quercivorol lures.

They will also be trying out a lure —a-copaene — that is used for other ambrosia beetles, as well as experimenting with different traps. Standard delta traps are ineffective, so a sticky sheet in a modified trap is in the pipeline.

Of course, effective monitoring is only the first step. Pome- and stone-fruit growers want to know how to protect their trees once the beetles are detected in their area.

The quest for control

There is currently no effective treatment for either beetles or their fungi in trees. Even extreme measures such as repeated trunk injections of potent chemical crop protectants have limited efficacy and cannot eliminate infestations.

As prevention is the only viable control measure, Addison and Karsten have been testing stem barriers in pear trees at Lourensford. They have collected data on the effect of different coatings and additives for one season. Unfortunately — depending on your perspective — new infestation rates were low during that season, so assessing treatments is challenging.

Even if stem barriers can keep PSHBs out of trees, protecting all trees might not be practical. At Lourensford, Addison and Karsten observed that pear trees only have entry holes in the trunk below about 1 m from the ground, but plum trees have entry holes much higher.

Trials carry on in the coming season. Hortgro also continues to make trial plots available to private crop-protection companies.

Biological control is an appealing alternative to traditional crop protectants and may have a better chance of reaching PSHBs and their fungal symbiont inside trees. An ongoing project involving Roets, FABI, and Rhodes University is on the hunt for natural predators, from South African forests to the beetle’s native Vietnam.

Other FABI research has identified bacteria and fungi that harm the beetles or their fungal food source. Dr Steffan Hansen, postdoctoral researcher at Stellenbosch University, and Dr Davina Saccaggi, acarologist at Citrus Research International, plan to explore whether phoretic mites could deliver these biocontrol agents to beetle colonies. Phoretic mites hitch a ride on beetles and can penetrate beetle burrows better than any chemical spray.

When not looking for ways to kill them, Hansen is establishing a breeding colony of the beetles that can generate insects for experiments.

Besides searching for control measures, Hortgro intends to develop a risk model for the Western Cape deciduous-fruit growing areas. The model will generate maps of PSHB spread under different conditions, including host densities and human-mediated movement.

For video content on the PSHB treatment trials at Lourensford, visit the Hortgro YouTube channel, where you will also find the recent Hortgro Research Showcase presentations on the borer in Western Cape deciduous fruit trees.