A recommendation to remove Boxelder (Acer negundo) from private properties in the southern suburbs before they become infested with the Polyphagous Shot Hole Borer beetle (PSHB) – particularly if located within a kilometre of a confirmed infestation site – has left homeowners with a tough decision to make.
Speaking at a “Celebrate the biodiversity of the Western Cape” event, co-hosted by The World Wide Fund for Nature (WWF) and the Friends of the Arderne Gardens (Fotag), Francois Krige, chair of Fotag, shared that, as part of pre-emptive measures, Fotag were planning to remove Boxelders within the garden.
“They are referred to as ‘amplifier’ species. To borrow from Covid terminology, they are the super-spreaders of PSHB. Removing them also helps us to have more resources available for fewer and more special trees,” Krige said.
Prof Francois Roets of the Conservation Ecology and Entomology department at Stellenbosch University, explains that a category three invasive species is regulated by activity.
“An individual plant permit is required to undertake any of the following restricted activities (import, possess, grow, breed, move, sell, buy or accept as a gift) involving a category three species. No permits will be issued for cat three plants to exist in riparian (the interface between land and a river or stream) zones.”
Roets says an amplifier species for PSHB is any tree species that the beetle regularly breeds in and that causes their numbers to amplify rapidly in the environment.
“This includes Boxelder but also a few others such as English Oaks,” he says.
According to his own observations and that of other researchers in South Africa and abroad, Roets says it is well-known that PSHB is particularly fond of Boxelder.
“And it breeds on masse in there. The beetles find these trees easily – in many cases before anything else – and then get a chance to amplify their numbers very rapidly.”
Roets says, once infested, Boxelders usually die within two years of the first infestation.
“During this time, they have a chance to multiply and start infesting surrounding trees at a faster rate than they could have if the Boxelder was not there.”
He adds, although researchers cannot yet confirm that the tree-killing beetles actively seek out this species, they do seem to find them very easily.
“Acer negundo is a tree species that the beetle knows quite well from its native environment as both are naturally found there.”
And he says the “within a kilometre of a confirmed infestation site” rule of thumb may not even be sufficient.
“The actual distance can be debated as I believe even one kilometre may not be enough – they can fly about 400 m in the laboratory, but the spread in Somerset-West was about three kilometre per year, aided likely by wind but also by the many breeding hosts available, including Boxelder.”
But be it one km or three, Roets says, going on what he has observed, the beetles are almost guaranteed to find close-lying Boxelders.
“If not in a year – and by year I mean a summer season – then in two or three. When they find it, that tree becomes a strong part of the problem in mitigating the impact of the beetle over time.”
He explains that trees are stagnant so it would be incorrect to say Boxelders “spread” PSHB from one place to the next.
“But they help PSHB populations to explode which gives PSHB many more chances of finding other hosts. In that sense they most definitely help the beetles to spread faster and have more impact.”
On whether the immediate removal of Boxelders would assist significantly in slowing down the spread of PSHB in the southern suburbs, Roets says it is difficult to determine what “significant reduction in spread” would entail in this case.
“But a reduction in amplifier hosts – such as a preferred Boxelder host – will slow the impact down. By how much, we cannot determine directly at this stage. Basically, you will be taking away its favourite meal, but there are also other meals around.”
Roets says, with all of this information at their disposal, it makes sense to remove Boxelders throughout Cape Town – whether infested or not.
“These trees help the spread and numbers of the beetle in landscapes. The beetle is already here, it is almost guaranteed to find the Boxelders quite rapidly. It will, more than likely, kill these and, in the meantime, they will be able to breed very fast, giving them more chances to spread further and faster.”
