People's Post

Polyphagous Shot Hole Borer: What does the science say?

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PSHB holes on an English oak.PHOTO: Supplied
PSHB holes on an English oak.PHOTO: Supplied

Reports that the Polyphagous Shot Hole Borer (PSHB) has arrived in Newlands, Mowbray, Rosebank, Rondebosch and, most recently, Claremont have left concerned property owners scrambling.

While the City of Cape Town addressed concerns during an online community meeting held on Tuesday 7 February, for every question answered, two more were raised. Chief among these was how the beetle was spreading, how it could be – if not stopped – slowed down and if there perhaps wasn’t some Hail Mary product or treatment out there that could swoop in and save the day.

According to information shared by the City, the tiny black beetle (2 mm in size) lives in symbiosis with a fungus called Fusarium euwallaceae. This fungus provides a food source for the beetle and its larvae, but in susceptible trees, it blocks the vascular tissue, causing branch dieback and tree death.

The top five most vulnerable trees in Cape Town include Boxelder maple, London Plane, English Oak, Liquid Amber and Coral trees. These species are all reproductive hosts of PSHB. In other words, trees in which PSHB can successfully reproduce, and which eventually die due to the infestations. (In case you were wondering, non-reproductive host trees are attacked by the beetle, but PSHB reproduction is not successful).

The City’s management protocol prescribes that highly infested trees be cut down and chipped on the property where it has been found and then carefully removed under cover of heavy-duty plastic to “an appropriate site” where the biomass will be solarised or incinerated.

For some deeper insights into this threat, People’s Post turned to Prof Francois Roets of the Conservation Ecology and Entomology department at Stellenbosch University. This is how the conversation went:

Define a highly infested tree?

This depends on many definitions. In California, it is a reproductive tree that has more than 50 holes. The problem comes in when you have a small tree with 50 holes (that will be in serious trouble) vs a big tree with 50 holes (that could be okay for a while). Expert opinion that takes the tree species and size into consideration will be best, but this is not an exact science as tree stress could also play a role.

Once infested, how long does it take for a tree to die?

Observational studies throughout South Africa show that different tree species differ in their time to demise. Again, tree stress may play a role. Boxelder trees often die within a year of first detection. English oak trees usually around four years and other species like plane trees can die either within two years (if trees are small and stressed) or much longer. I have seen heavily-infested plane trees still alive after seven years, maybe even longer. The problem lies in exactly that, especially when testing chemicals; each tree species reacts differently.

Some homeowners might be tempted to put their infested trees on “life-support”, hoping that a wonder product will come along in a year or two. Is this feasible or dangerous?

Leaving a tree in which the beetles are breeding en masse, for example, a heavily infested breeding host, is probably the worst idea when trying to manage a pest like this. It creates ample opportunities for attack of additional trees and to be accidentally transported to new areas.

Once the insect is in a tree, it is nearly impossible to get rid of it. When beetle numbers are high, they tend to probe almost anything – even tree species they would normally avoid – which increases their impact on the surrounds. Think of Covid and when we tried to flatten the curve: the exact same argument applies here.

The City has traced the original Newlands infestation (“Concern as tree-killing beetle spotted in southern suburbs”, People’s Post, 26 January) to firewood moved from Somerset West. How does the beetle spread?

The beetle can move with humans in a multitude of different ways, including firewood. For the most effective dispersal of lots of individuals, you need only a small piece of infested wood. For example, a 500 g piece of wood submitted for analysis to colleagues contained over 40 females, each of which can start a new infestation.

The beetle also spreads from private property to property via natural dispersal. They often seem to fly around 400 m, at least in the lab. Wind will certainly aid their dispersal but we do not know how far. We have tracked the advancement of the beetle infestation front at about 3 km per year in Somerset West.

During the community online meeting, there was talk that some specimens within a species, for example, English oak, may be more resistant, perhaps due to some genetically linked advantage. What does the science say?

I have literally only seen three individuals that seem to have resistance from thousands of dead oaks throughout the country. The problem is that these may be hybrids with less susceptible oak species so it is really difficult to know why. A resistant tree is resistant – it never has many holes in it so is easy to identify.

There are many people who have opted to “wait and see”. Even when applying chemicals, it at first may seem as if it is working but, inevitably, the beetle gets through the defences and nearly all English oaks will die, some faster than others. My advice: don’t roll the dice and have an 80% chance – or even higher – of adding to the problem.

Mention was made of an advanced programme PSHB bio-control in the Free State during the online meeting. What are the prospects of this research?

It is a shared research programme between our department, Rhodes University and colleagues at FABI, University of Pretoria. We also have input from colleagues in California and Vietnam. If anything is found, it will take many years – certainly longer than it will take for thousands of trees to die in the city.

If you have a highly infested breeding host, the tree will most likely die no matter what you do. Biological control cannot save a severely compromised tree and getting an agent will take many years to develop – if at all possible. Rather be proactive and plant a tree next to it that would be able to replace it. We all love trees but dead and dying trees are very dangerous to people and property.

The City says the use of pesticides and fungicides is only showing limited effects. How wary do property owners need to be of products on the market that promise to eradicate PSHB?

So many people suddenly have a cure here in South Africa. This despite 10 more years and millions of rands worth of more research going into this in the USA – without a “cure”. Our own research shows that there is limited effect at this stage. Of all the products that we have tested, we only managed to postpone infestations by about a year. Once inside and breeding, the options are even fewer.

There are some ridiculous claims out there. With extraordinary claims one needs extraordinary evidence, none of which any of my colleagues in South Africa or abroad or I have seen to date.

Based on observation of hundreds of trees and many tree species in South Africa and in the USA, chemical control for the beetle is currently not a workable option. It is expensive, dangerous and simply not effective – especially in the long run.

If not effective, it only contributes to the problem. Trees that should have been removed to reduce the number of beetles in an area are left – with a resultant increase in beetle numbers that can infect new trees and more areas.

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