Polyphagous shothole borer: this ‘smaller-than-a-bullet’ beetle could destroy South Africa’s biodiversity

An uninvited visitor has entered our borders, and, like most unwanted guests, it’s going to be difficult to show him the door. While he stays, he’s bringing fungal diseases with him, which pose a very real threat to us as it destroys trees. Meet, the polyphagous shothole borer (Euwallacea fornicatus), a tiny but potentially disastrous beetle.

What is the polyphagous shothole borer?

These beetles are an invasive species in California, Israel, and South Africa. Adults reach a length of around 2.5mm. It may sound small, but it’s 2.5mm of jaws that digest wood with a hearty appetite, enough to threaten our trees!

But that’s not all. These beetles have a symbiotic relationship (meaning both parties benefit each other) with a fungus. On its own, the fungus is fairly harmless, but the beetle and the fungus together are a lethal combination well capable of destroying entire forests of trees.

How polyphagous shothole borer get here?

The tiny beetle was discovered and identified at the Pietermaritzburg Botanic gardens during a disease survey in 2017. Prior to this, it wasn’t seen in South Africa. Since then, it has been found in a few trees in Johannesburg’s roadsides and gardens. The beetle and its fungal counterpart pose a very serious risk to the trees of Johannesburg – a city which is currently recognised as one of the largest urban forests in the world, boasting no fewer than 10 million trees.

What kind of threat does it pose?

The fungus, with the help of the polyphagous shothole borer, infests trees while the beetles bore away at the wood simultaneously. The beetle itself doesn’t kill the tree, but the fungus it carries affect the tree’s vascular system. This means water and nutrients can’t reach the tree, starving it until it dies.

Polyphagous shothole borer breeds in the wood, always choosing a live host (trees). Some of the most commonly affected crops include pecan nut trees, avocado trees, peach trees, orange trees, and grape vines.

Alongside the agricultural sector, this beetle also threatens the natural and indigenous forests of South Africa. In Sandton, concern was rising as a number of trees were simply dying. Many of them were oozing sap and sported borer holes. After DNA testing, it was confirmed that the polyphagous shothole borer was responsible (despite no beetles being found). This happened in 2015, 2 years before the beetles were identified at Pietermaritzburg Botanic Gardens.

According to Greenpop.org “the number of trees the beetle has killed in Johannesburg as well as Knysna, this beetle could potentially be one of South Africa’s largest ecological tragedies. In addition, the beetle is currently infesting over 200 tree species from 28 plant families worldwide.”

Keeping in mind trees are a vital link in the intertwined circle of our biodiversity. If trees are affected, giraffe, elephants, and ecotourism could come to a devastating halt, too. South Africa is the biodiversity capital of the world (299 species of mammals and 858 species of birds) and all these species depend on trees to flourish.

What can we do to get rid of polyphagous shothole borer and stop it from spreading?

Because of the way this bug can devastate farmers as well as urban and natural forests, there is lots of research being conducted to find new ways to control this pest. Right now, pesticides have proven ineffective because the beetles simply bore deeper into the trees to dodge the toxins. The only way to prevent them from spreading is to cut down affected trees and burn them. This is sad, but right now there is no alternative.

What can you do to help?

If you see signs of borer or ill health in your own trees, cut down the trees and burn them. Alternatively, you can but the branches into small pieces and seal them in refuse bags. Leave the bags in the full sun as heat kills the beetle and its larvae.

The symptoms to look out for include:

You are also encouraged to report any suspected polyphagous shothole borer cases. Please include a GPS coordinate of the tree, photos of the symptoms, and the name of the tree (if you know it). These details can be emailed to diagnostic.clinic@fabi.up.ac.za. Please do not move any plant matter that may be affected, we need to try and contain the contagion.