Can tiny beetles kill big trees? An emerging problem in South Africa

Posted on May 08, 2023

These beetles, known as polyphagous shot hole borers (Euwallacea fornicatus), are currently killing trees in South African cities at an alarming rate. It is not certain how exactly these beetles entered and spread throughout the county, but it is thought to have happened via wooden packaging that arrived at the country’s harbours from Asia. These wooden crates and pallets were then transported by trains and trucks to their final far-flung destinations, where the beetles started flourishing. In consequence, since being first spotted on London plane trees in KwaZulu-Natal in 2017, polyphagous shot hole borers have spread to eight of our nine provinces and attacked as many as 144 different native and exotic tree species. Outbreaks of this beetle are currently most severe in George, Johannesburg, and Cape Town.

The polyphagous shot hole borer belongs to a group of insects known as the bark and ambrosia beetles, which make complex tunnel systems in dead and dying trees (Figure 1). Only 0.5-4 mm in size, bark and ambrosia beetles are typically inconspicuous members of natural fauna, which spend their lives within their dark tunnels and rarely venture outside, except when searching for a new tree to colonize. Consequently, their cylindrical bodies have become adapted to living in narrow spaces and they have developed razor-sharp mouthparts that allow them to shave off wood.

Figure 1: Beetle tunnels in the bark (A) and in the wood (B) of trees caused by bark or ambrosia beetles (Photo: Wilma Nel).

Sawdust produced by the beetles while excavating their tunnels have very low levels of nutrients, and few beetles can survive by feeding on it. Therefore, instead of eating the wood, the beetles grow fungi in their tunnels, which they eat instead. Spores (reproductive fungal structures) of these bark or ambrosia beetle-associated fungi are carried by the beetles in special pouches on their bodies from tree to tree and are planted into new tunnels where they are allowed to multiply and reproduce. To ensure that the fungus only produces the highest quality spores for eating, the beetles tend to their fungus gardens like farmers, planting, weeding and harvesting. These fungi proliferate deep into the wood of the tree to gather nutrients, which they transport via thread-like mycelium back to the beetles’ tunnel to make high-quality food, also known as ambrosia. However, in this process, the fungi can clog the tree’s water-transporting vessels or rot the actively growing part of the stem, accidentally causing the tree to die (Figure 2).

Figure 2: Fungal partner of the polyphagous shot hole borer clogging water-transporting vessels in the wood castor oil (Photo: Wilma Nel)

The polyphagous shot hole borer uses a fungus as its food source, that causes a deadly disease in trees known as Fusarium wilt. As the fungus grows in the tree to feed the beetles, it blocks the water-transporting vessels, which causes the branches and trunk to die. Most typically, one will find the polyphagous shot hole borer and its deadly fungal friend attacking exotic trees in urban forests, such as London plane, Chinese maple, and oak. However, it also attacks and kills many of our native tree species, such as bushwillow and coral trees. So far, most native trees that are attacked are on the outskirts of forests or within urban areas close to roadsides. Traffic pollution, soil compaction, drought as well as the recent excessive rains and flooding has left many trees weakened and open to beetle attack. With more trees becoming available for attack, beetle populations skyrocket allowing individuals to co-operatively push further into new areas.

The beetle’s rapid spread and the wide range of tree species it can attack is cause for concern and so far, there are no known effective control measures for the beetle or its fungal partner. At its current pace, the tree landscapes we are used to in South Africa might be completely changed in the next decade. To limit further spread of the beetle, it is recommended that affected trees be cut down and the wood burned or solarized to prevent tree-to-tree movement. To limit the spread of the beetle over large distances, it is recommended that movement of raw wood, braai wood or wooden products between regions in South Africa be stopped, especially into areas where the beetle is not known.

Unchecked population growth and spread of the polyphagous shot hole borer could result in unprecedented tree death in our native ecosystems and landscapes. Similar scenarios of large-scale tree death due to accidentally introduced wood-and-bark-boring beetles have occurred in other countries. For example, in the early 20th century the large elm bark beetle found its way to Europe and North America from Asia. Large, ancient forests of Elm used to be a common occurrence on these continents, but nowadays spotting one of these giants is a rarity. This is because when the beetle and its fungus were introduced, the fungus caused a severe wilting disease, known as Dutch elm disease, which almost wiped out the entire elm population on both continents.

Climatic conditions can also influence the beetle’s impact on our ecosystem. Under natural conditions, most bark and ambrosia beetles preferentially make their homes in weak and dying trees and thus rarely pose a threat to natural forests, orchards, or plantations. However, global climate change, episodic drought events, heat waves and storms cause healthy trees to become stressed and ideal homes for the beetles and the fungi that they carry. This can be seen in the Northern hemisphere, where populations of bark boring beetles are exploding due to hot, dry weather, killing millions of conifer trees in Russia, Belarus, Scandinavia, Canada, and the United States of America (Figure 3). This is a big concern, as it not only affects local ecosystems, but also the decomposition of the wood results in high greenhouse gas emissions, like those of massive forest fires.

Spruce forest killed by bark beetles in the Austrian Alps in Europe (Photo: Almuth Hammerbacher)

To mitigate the potential effects of the polyphagous shot hole borer in South Africa, forest pathologists and entomologists at the University of Pretoria are doing research to determine the beetle’s movement into native forest ecosystems. They are also working on various potentially affected tree crops, such as avocado and macadamia, to determine the extent of the damage that the beetle will have on these commercially important trees. Since there are no established control measures, young researchers from the University of Pretoria have also come up with creative projects where they are developing unconventional control measures to stop the beetle’s current rate of spread.

In addition, the University of Pretoria and other institutions around the country formed a national response team to share knowledge and resources to find effective solutions for the problem. This national response team is also a platform for communication between provinces and government departments. Moreover, the discovery of the polyphagous shot hole borer in various other countries, such as in the United States of America, Israel, and most recently Australia has also led to the formation of a global polyphagous shot hole borer research network. This network allows international scientists from all the affected regions to meet on a regular basis to discuss research results and together develop new ways forward in the ongoing fight against this minute invader.

- Author Wilma Nel, Almuth Hammerbacher

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