Joburg is being eaten by aliens, and we should be panicking but we’re not. At least, not yet.
A silent, subtle threat to biodiversity is spreading across the country almost unnoticed – and this isn’t something removed from us in the way of, say, rhino poaching that’s happening in the wilderness areas we visit only occasionally. It’s hitting us in our gardens and our cities. It’s killing our trees, and it’s chomping its way through one of the world’s biggest urban forests – Johannesburg.
We’ve known about the teeny-tiny (smaller than a match-head) polyphagous shot hole borer (PSHB) beetle here in Knysna for some years. It attacked all the oak trees – many of which were planted back in the Victorian era – that shaded the older streets and homes. The municipality was forced to fell them, because they’d become so soft and spongy that they could fall over at any time and possibly damage property or worse, injure or kill someone.
We thought they were specific to our oaks, but they’re not. After scientists from the Forestry and Agricultural Biotechnology Institute (FABI) found the bug in the iconic avenue of London plane trees at the KwaZulu-Natal National Botanical Gardens in Pietermaritzburg, they systematically surveyed the trees in all South Africa’s botanical gardens – and found that ‘it has become alarmingly apparent that PSHB is well established, thus has more than likely been present in South Africa for some years. FABI believes this pest presents a very serious threat to the health of trees in urban, agricultural and natural environments.’
How does it affect the trees?
It’s important to understand that the bug alone isn’t the problem. The adult female beetles farm the fungus, Fusarium euwallaceae, which they carry with them wherever they go. While the beetles burrow into the trees to establish brood galleries for their eggs, the fungus colonises the gallery walls, and begins working away at the wood.
Then, when the bugs lay their eggs, they (the adults) have a feast on the fungus’s rich crop of denatured wood – as do their developing larvae once they’ve been hatched.
The fungus, of course, clogs the trees’ food and water channels – and the combination of burrowing and clogging does to the tree what cholesterol in our bloodstreams does to us humans. It’s not a pretty picture.
No. Not all of them, but FABI has recorded shot hole borer beetles in around 60 tree species in South Africa – including oaks and planes, but also in indigenous species like yellowwoods, coral trees, river bushwillows, thorn trees, tree fuchsias, indigenous willows, Cape chestnuts, bush tickberry, keurboom, and others. It’s not only in our front yards, either. FABI reports: ‘Some of these observations come from semi-natural areas including the Garden Route Botanical Gardens in George, and the Pledge Nature Reserve in Knysna.’
Nor is it a problem only in South Africa. It’s been recorded in places as diverse as Israel and California, where it’s been found on commercially important crops like avocados. Shock! Horror! Imagine a world without avos!
By now you’re probably out in your prize-wining garden, staring at your trees. What are you looking for? Tiny entry and exit holes in the bark (think less than half the thickness of a matchstick), which are usually surrounded by sawdust, and bleeding (nectar or blobs of goo oozing from the bark) are usually your first clues. But since the combination of boring and fungus weakens the core of the tree, branches may snap off, revealing the beetles’ galleries and the webs of the black fungus that lines them.
How does it spread?
Speaking to a group of environmentalists during a visit to Knysna earlier this year, FABI’s Prof. Wilhelm de Beer, a mycologist and fungal biologist, said that the beetle, which comes from Southeast Asia, was probably spread around the world in untreated timber such as that used for making shipping pallets. Once here, though, the beetle can spread through the movement of infested branches – taking your diseased wood to the municipal dump, for example – or through the movement of infested nursery plants.
FABI’s website notes: ‘We have recently observed PSHB attacking containerised trees in the nursery environment. The potential for spreading over long distances through the sale and movement of nursery stock is cause for serious concern.’
So what’s being done about this?
We’ve known for some time that increases in the movement of plants and plant material around the world are potentially huge threats to our biodiversity, which is why Scientists have established the International Plant Sentinel Network which, according to its website, is
being developed to facilitate collaboration among institutes around the world, with a focus on linking botanic gardens and arboreta, national plant protection organisations (NPPOs) and plant health scientists. The aim will be for these institutes to work together in order to provide an early warning system of new and emerging pest and pathogen risks.
And, since our South African institutions are joining up as members, this means that we’ll be able to access the latest science on the subject. But the crucial words here are ‘will be’. We’re only at the start of our journey.
That said, however, here at home, the Department of Agriculture, Forestry and Fisheries, the Department of Environmental Affairs and various other stakeholders have formed a steering committee to develop an action plan for combatting the problem, to conduct surveys and monitor the spread of the problem, to plan and conduct trials on chemical treatments, and to create an awareness campaign (especially to stop the movement of infested plant material – dead or alive).
What can you do?
Prof. de Beer noted in his talk in Knysna that control requires cutting down and removing infected plants, and the establishment of dedicated dumping sites where infected wood can be burned, fumigated or solarised. The latter is the simple but surprisingly effective strategy of sealing the infected plant material in plastic bags and leaving them in the sun to heat up. He said, too, that it is possible to treat economically valuable trees, or historically or culturally important specimens, with fungicides and pesticides, but none have yet been registered for this particular problem in South Africa.
Some people believe that the beetles could be lured and trapped, but this will only become possible when we fully understand their life cycle and breeding seasons, so that we can plan our campaign for the right time of year. And, anyway, it might not work. Insects communicate very effectively through pheromones, so they may be able to tell each other to stay away from our bug traps.