​Stellenbosch University (SU) entomologists are continually keeping an eye on the developing issue surrounding the spread of the polyphagous shot hole borer beetle (PSHB) and its associated fungus around the country, and the threat it holds for trees. Preliminary information from one such study on the Vergelegen Estate in Somerset West shows that there could be ways to try and curb its spread.

The work being done at SU in particular involves postgraduate students and postdoctoral fellows in the Department of Conservation Ecology and Entomology. They are among others monitoring how the beetle spreads, and which plants it is hosted by. Special attention is given to its potential economic impact, and the indigenous Tsitsikamma forests of the Southern Cape.

Postdoctoral fellow Dr Anandi Bierman is among others using population genetics to understand its movement throughout the country, and the intricate interactions between the beetle and a symbiotic type of fungus, Fusarium euwallaceae.

By boring into tree trunks, the PSHB introduces the fungus, which in turn weakens and can kill trees. It was first noticed in South Africa in 2012 and has since had a devasting effect on garden and even indigenous forest trees in many provinces.

“Modern approaches are being used by MSc student Madeleine Pienaar to understand the physiological boundaries of the beetle and its potential to spread, and MSc student Mignon de Jager  is evaluating the possible effect of the beetle and its fungus on grapevine and commercially grown fruit trees," says study leader Prof Francois Roets of the Department of Conservation Ecology and Entomology.

“These are all pieces of a greater puzzle to help understand the impact and future mitigation strategies needed to stop this invasion from reaching its full potential."

Related projects, including ones on the use of biological control options, are coordinated by the PSHB research network across the country.

One of the SU MSc students who has been working on the PSHB problem, Elmar van Rooyen, is set to graduate in December. He was recently the lead author, along with Prof Roets and other local experts, of a new review paper in the South African Journal of Science taking stock of the beetle/fungus in South Africa.

It states that the beetle has already been found on 130 plant species in urban, agricultural, and native ecosystems in South Africa, including 44 previously unreported hosts.

“The South African PSHB invasion represents the largest outbreak of this pest in its global invaded range," the experts wrote. 

​Vergelegen project

PhD student Heather Nependa is conducting studies at Vergelegen Wine Estate in Somerset West. The pestilence was confirmed in the town in April 2019, and at the estate in February 2020.

Initial findings from her three-year project, conducted under leadership of Prof Roets, have already generated some valuable insights on how to tackle this developing problem.

In a recent press release, Vergelegen's Risk and Commercial Manager Leslie Naidoo called the estate “an ideal natural research lab", as the 321-year-old estate is home to a vast collection of historic trees. Its history and biosphere are considered a microcosm of urban and peri-urban areas in South Africa."

Their work at Vergelegen has already provided the SU research team with a better idea of the seasonal distribution of the beetle. Nependa for instance found that the use of chemical lures to trap the insects could help keep beetle infestations low, while pesticide and fungicide injections into selected trees have further slowed down beetle infection rates.

According to Naidoo, more than ten infected trees on the estate – including an English Oak, two Boxelders, two Trident Maples and one Japanese Maple – have been treated to date.

Seasonality

The estate uses two types of traps to monitor for the presence of the beetles. One is a chemical lure in a plastic bottle, intended to “pull" beetles from trees, that is used in conjunction with chemical repellents that can “push" beetles away from valuable trees. The other is a 3D-printed trap secured over holes made by beetles on infected trees, with steel mesh over each opening to prevent the pests from escaping.

Based on counts held every two weeks at the chemical lure traps, there was a sharp increase in beetle numbers during April and May. Up to 500 beetles were collected at each inspection. These numbers began to decrease in June and July to about 10-20 beetles per inspection. Currently numbers are increasing rapidly due to the warmer weather, with more than 1500 beetles being collected at the site per inspection.

“This is significant as we now know that the best time for controlling active beetles in flight is during summer. It would be possible to control and remove infested plant material in winter when beetles are not active and when there will be less chance of accidentally transporting the insects to new locations," says Prof Roets.

At the 3D traps, the first beetles also emerged in April and May. These are carefully monitored to count the numbers of beetles that emerge from every breeding gallery.

“This data has been very valuable as we are now more informed about the seasonal distribution of the beetle, which will help inform how to manage them," says Nependa.

Chemical lures

To further unravel beetle biology and behaviour, 50 plots (15mx10m) were set up around each chemical trap. Each tree in a plot was surveyed and factors such as evaluating entry holes, sap flow and other signs of boring have since been recorded. These variables were first assessed in April and July, and are being repeated every three months.

“Early results show that the use of quercivorol and verbenone (chemical lures) has been crucial in keeping PSHB infestations low, with only three out of 50 plots having PHSB infestations," Nependa.

“Whether this positive influence will last is unknown, as we are currently seeing the highest numbers of active beetles ever recorded during our surveys. We will need to keep a very close eye on the developments," adds Prof Roets.

Pesticide injections

Nependa is also investigating the use of an insecticide, emamectin benzoate, and a fungicide, propiconazole.

The first set of these chemical injections was applied in February and March 2021. Injected trees have since been monitored every 28 days. The rate of beetle infections has slowed down in these trees, with not more than 10 new holes being observed since.

The remainder of the chemical trial started in late October, both at Vergelegen and in the surrounding area. English Oak and London Plane trees are being injected with either propiconazole, emamectin benzoate, or both.

Lab preparation

A good deal of prep work has also been taken place in Prof Roets' lab in the SU Department of Conservation Ecology and Entomology. This includes breeding beetles to test their colonisation of chemically treated trees. There are currently 10 active colonies from which live beetles are collected. These have been introduced into chemically treated trees and their colonization success over the next few months will be closely monitored.

The symbiotic fungus (Fusarium euwallacea) that goes hand in hand with the PSHB is also being grown in the lab. This was also inoculated into the chemically treated trees to measure the effect of the chemicals on fungus growth.

Satellite imagery

A largely desktop-based remote sensing study is being conducted to predict the economic impact of the beetle. For this purpose, satellite images from Somerset West and Stellenbosch were taken in 2019 and 2020 are being used.

“The imagery is used in machine learning models, which are being trained to distinguish between evergreen and deciduous trees, and infected and non-infected trees.

“This data is used to produce maps, after which a time-series analysis will provide more insight into how trees respond to various infestation levels, and how this ultimately impacts or changes tree functioning and the ecosystem services they provide. Monitoring these changes over time will help inform early detection procedures," says Nependa.

Measuring fungal growth and Isotope analysis

Further lab work involves experiments to measure fungal growth rates in various nutritional mediums, which are adjusted with carbon and nitrogen.

“Carbon and nitrogen were selected because they are readily available in host trees and their quantities in host trees reflect the health of the host. Carbon isotope measures usually indicate water stress, while nitrogen isotope measures indicate pollutants in the environment. They also affect how the fungus – and in turn the beetles – will develop," says Nependa.

The Stable Light Isotope Laboratory of the University of Cape Town's archaeology department is also involved. Wood samples have been collected from two beetle reproductive hosts, English Oak and London Plane trees, and two non-reproductive hosts, Wild Olive and Camphor trees.

“These trees represent different levels of stress and will hopefully give an indication of what kind of stress the trees are under. After the isotope evaluation, the selected trees will be used in beetle development and fungal growth experiments."

Research at Vergelegen continues until June 2022, says Naidoo.​