A sesame-sized invader is transforming iconic green districts, while scientists struggle to contain its hidden assault before more trees disappear.
The polyphagous shot-hole borer is rapidly spreading across the planet and destroying urban and natural tree cover. This dangerous invasive beetle has already become a serious threat to South Africa, where millions of trees and entire green urban areas are at risk.
The consequences are especially visible in Stellenbosch, South Africa, known as the “City of Oaks.” Thousands of English oaks, along with other trees, have already died or must be removed because of the infestation.
The insect is about the size of a sesame seed, yet it causes devastating damage. It bores tunnels into wood, leaving tiny holes beneath the bark. The beetle also introduces a fungus into the plant that causes Fusarium dieback. It disrupts the movement of water and nutrients inside the tree, causing the infected plant to gradually die.
Why the polyphagous shot-hole borer is called the “perfect invader”
The pest is believed to originate in Southeast Asia. However, it has now been found on every continent except Antarctica. A study published in the Journal of Pest Science in June links its global spread to the movement of timber, live plants, and packaging materials.
As of July, the polyphagous shot-hole borer had attacked around 680 species of trees and other woody plants. Its danger lies not only in its broad diet but also in its ability to rapidly establish new populations.
In South Africa, the pest was first noticed in KwaZulu-Natal province in 2012, and its presence in the country was officially confirmed in 2017. Since then, the beetle has been found in eight of the country’s nine provinces. Francois Roets, a professor of conservation ecology and entomology at Stellenbosch University, does not rule out that the insect is already present in Limpopo – the only province where it has not yet been recorded.
This little beetle combines all the characteristics we asked students to build into a model invasive species.
– Francois Roets
A female polyphagous shot-hole borer can establish a colony without a male. She lays unfertilized eggs that develop into males, and later mates with them. This reproductive system, combined with its tiny size and ability to live unnoticed inside wood, makes the pest exceptionally difficult to control.
How many trees can this dangerous beetle destroy?
After infestation, trees die at different rates depending on the species. The most vulnerable plants may not survive even one year after the pest appears.
- English oaks typically die within four years of infestation;
- box elder maples often wither in less than 12 months;
- in Johannesburg, satellite images as early as 2022 showed entire suburban streets where trees had disappeared.
At present, the most effective way to slow the insect’s spread is to remove infested and already dead trees promptly. They must be removed before the beetles can fly or move on to healthy plants.
Economic losses caused by the invasive borer
In 2022, economists and ecologists from Stellenbosch University and the University of Pretoria estimated South Africa’s potential losses from the spread of the polyphagous shot-hole borer. According to their calculations, damage could reach 275 billion rand over the following decade, or around $18.5 billion at the exchange rate at the time.
- potential losses amount to 275 billion rand;
- this equals approximately $18.5 billion at the exchange rate at the time;
- the expected impact is estimated at 0.66% of South Africa’s GDP;
- of 225 million urban trees, at least 65 million may need to be disposed of.
The financial consequences extend beyond cutting down, transporting, and disposing of infested wood. The loss of trees intensifies urban overheating, reduces street shade, and destroys habitats for birds, insects, and other species.
Can the pest’s spread be stopped?
Some countries have managed to contain local outbreaks. Germany and the Netherlands eliminated isolated cases of infestation in greenhouses and botanical gardens. However, once the beetle reaches open ecosystems, eradicating it completely becomes nearly impossible.
Australia faced this problem after the polyphagous shot-hole borer was discovered in Perth in 2021. Four years of large-scale removal and chipping of infested trees failed to stop the insect’s rapid continued spread.
Scientists are searching for biological methods to control the pest population. Possible measures include native parasitic wasps, mites capable of feeding on the fungus, and nematodes. Laboratory trials have produced some results, but using such methods in nature requires caution to ensure that the new control measure does not itself become an environmental threat.
Which regions are at risk?
Since 2022, the polyphagous shot-hole borer has also become established in Spain, Uruguay, and Turkey. Researchers emphasize that the beetle can adapt to both subtropical and temperate climates. As a result, risks are increasing for new areas in the Americas, Africa, and the Mediterranean.
Climate warming could further expand the areas suitable for the pest. Francois Roets believes that the loss of old trees could also encourage the planting of more diverse and resilient species. However, containing the “perfect invader” will require swift action, funding for research, and stricter controls on the movement of timber between regions and countries.
