Page 14 - LandscapeSAIssue111

FEATURE

BORER BEETLE RESEARCH

FINDINGS FROM VERGELEGEN

An ongoing concern of tree lovers is the tiny polyphagous shot hole borer beetle
(PSHB) which bores into tree trunks, introducing a fungus that weakens and often
kills the trees. This phenomenon has been causing devastation since the beetle
was discovered in South Africa in 2017.

owever, initial research findings from Vergelegen Wine Beetle counts held every two weeks at the chemical traps revealed
Estate in Somerset West, the site of an intensive three- a sharp increase in numbers during April and May, when about 500
Hyear research project on the beetle, have generated some beetles were collected at each inspection. These numbers began to
valuable insights: decrease in June and July, with about 10-20 beetles per inspection.
• researchers are better informed about the seasonal distribution
of the beetles, which will help inform how to manage them; At the 3D traps, the first beetles also emerged in April and May.
• chemical lures have been crucial in keeping beetle infestations
low; and “This data has been very valuable as we are now more informed
• pesticide injections into selected trees have slowed down beetle about the seasonal distribution of the beetle, which will help inform
infection rates. how to manage them,” says Nependa.

This is according to a research report from PhD candidate Heather Chemical lures
Nependa, Department of Conservation Ecology and Entomology To further unravel beetle biology and behaviour, 50 plots
in the Faculty of AgriScience at Stellenbosch University. Nependa is (15m x 10m) were set up around each chemical trap. Each tree in
working under the guidance of Professor Francois Roets. a plot was surveyed and many factors recorded, such as evaluating
entry holes, sap flow and other signs of boring. These variables were
There are 14 infected trees at Vergelegen, says the estate’s risk first assessed in April and July and will take place every three months.
and commercial manager Leslie Naidoo. One English Oak, two Box
Elders, two Trident Maples and one Japanese Maple have been “Early results show that the use of quercivorol and verbenone
treated. (chemical lures) has been crucial in keeping PSHB infestations
low, with only three out of 50 plots having PHSB infestations,” says
Vergelegen is an ideal natural research lab as the 321 year-old Nependa.
estate is home to a vast collection of historic trees. Its history and
biosphere are considered a microcosm of urban and peri-urban Pesticide injections
areas in South Africa, says Naidoo. She is also investigating the use of an insecticide, emamectin
benzoate, and a fungicide, propiconazole.
Seasonal distribution
Vergelegen uses two trap types to monitor the beetles. One is a The first set of these chemical injections was applied in February and
chemical lure in a plastic bottle, intended to repel beetles from March 2021. Injected trees are monitored every 28 days, revealing
trees. The other is a 3D-printed trap secured over holes made by that beetle infection rates have slowed in these trees, with not many
beetles on infected trees, with steel mesh over each opening to more than ten new holes observed.
prevent the pests from escaping.
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 injected with either propiconazole, emamectin
benzoate, or both.

Lab preparation
In addition to these measures, a good deal of prep work has taken
place in a Stellenbosch University lab. This includes breeding
beetles to test their colonisation of chemically treated trees. There
are currently ten active colonies from which live beetles will be
collected. A symbiotic fungus (Fusarium euwallacea) is also being
grown in the lab. This will be innoculated into the chemically treated
trees to measure the effect of the chemicals on fungus growth.

Satellite imagery
Predicting the economic impact of the beetle also requires remote
sensing work. This is largely desktop-based, using satellite images
taken in 2019 and 2020.

“The imagery is used in machine learning models, which are being
Chemical injections on an A Box Elder at Vergelegen trained to distinguish between evergreen and deciduous trees, and
Oak tree at Vergelegen with chemical injections infected and non-infected trees.

12 Landscape SA • Issue 111 2022