Borer beetle-busting research on wine estate hopes to curb further invasions

In February, an established wine estate in the Western Cape discovered with horror the polyphagous shot hole borer (PSHB) beetle was invading its trees. 

 Vergelegen wine estate the put strict measures in place to avoid further infestations. This included banning the transport of firewood to the estate, to installing monitoring traps and repellant on the farm’s perimeter. 

But luckily, Vergelegen is the research site for Stellenbosch University’s Heather Nependa a conservation ecology and entomology postdoctoral student in the AgriScience faculty. If anything can stem the invasion of PSHB beetles, it is the power of science and research. 

Doctoral student Heather Nependa shows a repellent pouch on a giant camphor tree at Vergelegen. Photo: Supplied

What is a polyphagous shot hole borer beetle?

The PSHB beetle first came from southeast Asia and was discovered in South Africa in 2017.

The beetle is invasive and deadly for trees. Females infest trees with a fungus the beetles feed off, which eventually kills the trees. 

From Durban to Bloemfontein and Sandton to Sedgefield, PSHB beetles measuring around 2mm have threatened to destroy natural and urban forests across the country. 

Vergelegen risk and commercial manager Leslie Naidoo with Nepanda by a giant camphor tree at Vergelegen. Photo: Supplied

The only province not to be hit by the PSHB wave yet is Limpopo, which also has its own measures in place to prevent the spread of the beetles. 

On 8 April, 2019 the beetle’s presence was confirmed in Somerset West.

Some of the most susceptible trees are American sweetgum, Japanese and Chinese maples, box elder and English oak. 

Hot and long summer days have exacerbated the infestations, despite estates such as Vergelegen and also residents, establishing certain measures against the beetles. 

For scientists such as Nepanda, a research site such as Vergelegen, rich in history and biodiversity, allows for extensive research on the beetle and its potential impact on South Africa. 

Research on Vergelegen estate 

Vergelegen wine estate in Somerset West is 320 years old. It was established as a model farm by Cape governor Willem Adriaan van der Stel in 1700 and has been used in agricultural and horticultural testing since then. 

In 1999, the estate led research with the University of Pretoria on the leaf-roll virus, which is specific to grape vines in SA.

In 2018, it completed the largest privately funded alien vegetation clearing project in the country. And it has been a site for numerous masters and doctoral theses on topics ranging from grasses to dragonflies. 

Due to its age and lush environment, the estate is the custodian of a number of special trees, including five camphor trees planted by Van der Stel himself. 

There is also an English oak on the property believed to be the oldest oak tree in South Arica, said Vergelegen risk and commercial manager Leslie Naidoo. 

Repellant pouches on an ancient English oak, believed to be the oldest oak tree in South Africa. Photo: Supplied

Naidoo said several of the estate’s tree species are potential PSHB hosts, including London plane, English oak, sweetgum, willow and maple trees. 

Nepanda’s research aims to establish a monitoring and early detection scheme, unravel the PSHB fungus-tree relationship, test the effectiveness of lures and repellants, test potential insecticides and fungicides and to collect and analyse data in the field for a custom, countrywide PSHB management plan.

She is being assisted by two fourth-year conservation ecology and entomology students and has already compiled an inventory of 1 010 trees at Vergelegen. 

Two types of traps are being used to monitor and collect the beetles. The simple design uses one and two-litre plastic bottles filled with a chemical lure  tied to 1.5m steel rods. There are 50 of these traps.

The second, more sophisticated traps have been 3D-printed and are secured over holes bored by beetles on the trees. Steel mesh is placed over each opening to stop them from escaping. 

All traps are inspected every two weeks, with content delivered to Stellenbosch University for inspection and identification. 

Researcher Heather Nependa and Vergelegen risk and commercial manager Leslie Naidoo in front of an infected English oak on the Vergelegen boundary. The plastic bottle trap on the rod lures the beetles away from the tree. The 3D-printed traps secured to the tree are checked every fortnight and the contents delivered to Stellenbosch University for inspection and identification. Photo: Supplied

“An important next step is to map out invasion pathways and determine what drives dispersal,” Nependa said. 

To help track how climate change affects dispersal, temperature logging buttons will be placed at each sampling site.

“Despite an established and expanding host list, the PSHB still seems to select hosts indiscriminately,” Nependa said. 

Fifty plastic bottle traps, fitted with a chemical lure, have been set up on the estate to attract the beetles away from the trees. Photo: Supplied

But in order to experiment on beetle and fungus biology and physiology, fungus will be grown in a lab and experimented on with plant nutrients and plant volatiles to determine what is needed for successful fungal establishment, she explained. 

“This information will also help further model the impact and potential distribution of the PSHB. This will ideally be modelled for the greater Somerset West area to include more hosts.”

Nepanda’s project will run until June 2022. All findings will be made available via the Stellenbosch University’s conservation ecology and entomology department. 

(Compiled by Nica Richards)

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