Pictured Above: In pear orchards in the Western Cape, Matthew Addison is leading research into the shot hole borer.
Scientists warn that the deadly polyphagous shot hole borer beetle has the potential to decimate the country’s indigenous and agriculture tree population, as eight out of South Africa’s nine provinces have reported sightings of the pest.
It was first reported in trees in the Pietermaritzburg botanical gardens in 2017.
Scientists have called it the “perfect invader”, not without good reason. Thought to have arrived on South Africa’s shores in 2012, but first seen in 2017, the polyphagous shot hole borer (PSHB) has the capacity to inflict savage levels of destruction on this country’s trees.
Currently, pear and apple orchards are the most infested commercial crops in South Africa, along with plane and oak trees in Stellenbosch and Johannesburg. While there have been no reports of the beetle in macadamia orchards, the beetle’s modus operandi, and lack of regard by the public, make it an imminent threat to all trees countrywide.
Double whammy
Pear trees in the Lourensford orchards are marked where shot hole borer beetles have burrowed, to track infestation.
The beetle, Euwallacea fornicatus, enters the environment carrying an onboard fungus (Fusarium euwallaceae), which means selected host trees face a double onslaught of pest and pathogen. The PSHB beetle operates in a mutualistic relationship with the F. euwallaceae fungus, which provides larvae and adults with their only food source. The beetle carries the fungal spores in containers behind the jaw and seeds them in tunnels bored deep into the host tree.
The tree then reacts to the injury by compartmentalising the area, which, considering the ability of the beetle to create a network of tunnels, can disconnect significant areas within the tree. The fungus, successfully established, grows into the wood, taking out nutrients and disrupting the tree’s vascular system, preventing the flow of water and nutrients. The infested tree will show symptoms of suffering from drought and branches will die back.
“It is the double attack that makes this pest/pathogen complex such a lethal one,” says Matthew Addison, crop protection manager at Hortgro, and member of a research team at Stellenbosch University (SU), looking at the invasive spread of PSHB.
“The larvae hatch, more tunnels are built, and the fungus spreads. Thus, the beetle and the fungus collaborate to effectively ringbark the tree on the inside, killing it through starvation.”
PSHB is thought to have arrived in South Africa via shipping pallets that landed in the Richards Bay harbour.
“PSHB is found in living wood but it can survive for a while after you cut the wood and it dries out. This adds a significant challenge in controlling the pest, as the beetles can survive for long periods in chopped wood, and then spread when it is transported.”
The beetle was originally detected in Pietermaritzburg as part of an international sentinel tree programme that functions as an early warning system to pick up pests and pathogens. After scientists conducted a wider search, they found evidence of range extension. It now occurs in eight of the country’s nine provinces.
“The biggest impact so far has been in northern Johannesburg, where it has whacked a lot of urban trees. George, in the Tsitsikamma area, has also been quite heavily affected,” Addison says.
Hortgro has set up a programme to monitor the spread of the beetle in South Africa, which is currently experiencing the largest global outbreak of this pest. Concerns revolve around urban trees, trees in the agricultural sector, and indigenous trees in forests and bushveld.
Sap being released by trees is often the first sign that these tiny black beetles are present.
A powerful pest
The female beetles are only 1.8mm-2.5mm long, often described as the size of a sesame seed, and males are even smaller at 1.5mm-1.67mm in length. This means they are not easy to detect on a tree and even less so once they have tunnelled in.
Outward signs of its presence are the tunnel entrances, round holes 1mm across and small enough to fit the tip of a ballpoint pen. The holes may show oozing and sugary deposits, with some dark staining.
Powerful weapons in the PSHB beetle’s arsenal are the 30-day life cycle and the ability of the female to reproduce without a male. A mated female produces male and female offspring that leave to start new colonies. An unmated female can produce all male offspring and is then able to mate with her sons to produce male and female offspring. Hence the beetle’s scientific name – E. fornicatus. So, a female beetle, mated or not, can start a new population. This takes out sterilisation as a control option.
The beetle can spread at a rate of about four kilometres a year on its own. That spread becomes even more rapid when the beetle is helped by human vectors in vehicles transporting PSHB beetle-infested wood from one area to another.
Management and control
“There is no known curative treatment; pyrethroids only delay it,” says Addison. He cites the example of California, where a lot of money has been spent in the past 12 years looking at curative treatments, from stem injections to spray treatments. They have succeeded only in delaying infestation.
Biological control until now has been unsuccessful, although it is being investigated by the research team at SU. Another possible solution would be to focus on the fungus, which is controllable. “The problem is, the beetle goes in so far and you can’t kill it with systemic fungicide because the tree is trying to block off that section of vascular tissue,” Addison adds.
“The easiest way to protect trees is to remove all of the primary reproductive hosts.”
He says in municipal areas, the insurance implications are profound as there are some big trees growing on pavements in urban areas that could fall on houses or vehicles.
“Dead trees must be removed, bearing in mind that they are infested with beetle and are a danger to other trees. The recommended method is to chip the tree material and transport it – covered – to a dedicated site where it should be composted. Alternatively, chipped infested tree waste may be put into clear plastic bags and stored in the sun.”
There is consensus that educating the public about PSHB is critical. “We have the biggest incursion in the world; there are people out there who don’t even know it exists and there are dead trees in their town,” says Addison. “Informed people can help us limit infestation by reporting the presence of the beetle and by not transporting firewood.”
The development of methods to protect orchards is critical and needs to be done without applying treatment that will lead to other problems.
Addison says PSHB has an aggregation pheromone (rather than a sex pheromone) which is imported from a Canadian lab and used with some success. It is currently being synthesised by an operation in Tzaneen, Limpopo, for local production. “Once that is running it will open some other doors in terms of attract-and-kill, or a barrier, or drawing them on to trees they can’t breed in on the outside of the orchards. This gives me some currency to work with.”
Selected species
The PSHB beetle shows a strong preference for popular garden and street trees like English oak, poplar, plane, and box elder. These trees are the primary, or highly susceptible, reproductive hosts where the beetle can breed successfully because the fungus prefers them. “There is a range of trees that can act as hosts, but also those that reject the fungus,” Addison says.
The Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria puts trees into three groups: highly susceptible reproductive hosts, reproductive hosts, and non-reproductive hosts. There are currently 83 species listed as highly susceptible reproductive hosts: 41 are indigenous and 42 are aliens. Listed among the indigenous species are many iconic South African trees, like the Outeniqua yellowwood, the pod mahogany, the coral tree, and the Natal fig. Among the aliens are avocados, pecans, pears, plums, almonds, and apricots.
“In pecans and avocadoes, the beetle tends to attack branches rather than stems, while in pear trees it attacks the stem only,” says Addison.
The danger to the macadamia nut sector is currently unknown, although traps hung around orchards show that the beetle is present in these areas. Addison says there are other borers in macadamias that are possibly not killers, and macadamias may be pre-adapted to fighting off this threat.
While we pray this is so, it is clear that vigilance is key to containing PSHB beetle.
