The polyphagous shot-hole borer is a small but vicious pest wreaking unprecedented havoc worldwide. Since August 2021, it has spread into Western Australia. Originally found in East Fremantle, this marine pest is now considered a significant threat. It especially threatens native forest trees such as the marri and karri. The invasive pest can live in harvested timber for as long as seven months. With a flying range of just 30m, its rapid spread across the entire Perth metropolitan area is all the more concerning.
Globally, the polyphagous shot-hole borer has been problematic for over a decade in regions such as California, Israel, and South Africa. Its clever adaptation to digest wood with the help of a particular fungus makes it so that the stick insect can dominate even the most cellulose-rich ecosystems. Experts on the ground are still deeply worried about the serious threat it poses to Western Australia’s endangered ecosystem and agricultural production.
Impact on Native Trees
Prominent among its favorite hosts are the marri (Corymbia calophylla) and karri (Eucalyptus diversicolor). We can only speculate on the long-term impact of this pest’s introduction to the region on the health of these species.
Dr. Evans, a researcher studying the borer’s effects with the Indiana DNR, said that the damage caused by the pest depends on the species of tree. “So there’s obviously local variation, probably a combination of the climate, the soil type, and water availability that changes the susceptibility of the plants,” he stated.
By the way, the pest has done extensive damage to Moreton Bay fig trees in Perth. Whether long-term impacts will affect avocado trees is still unknown, keeping farmers and scientists alike in suspense as to their susceptibility.
Management Strategies and Challenges
The Western Australian government recently revealed a strategic change of plan in its efforts to contain the polyphagous shot-hole borer. Rather than continuing on the path toward eradication as had previously been attempted, officials are now focusing on controlling the pest’s advance. This decision comes after a long series of insecticide trials proved only marginally successful at best in controlling the borer. Entomologists and plant pathologists are working to identify better strategies by testing new combinations of insecticides with fungicides to provide more effective control.
Following the management example of Dr. Evans, she gave us a deeper understanding of her field. “We’ll never get rid of [the need to] chop and chip. I think there are going to be some trees that are just too far gone, and chop and chip is the only option,” he remarked. He emphasized that careful pruning may help save some infested trees, although it is not without increased costs and reduced productivity.
The pest’s life history, especially its exceptional ability to survive treatment within cut wood, makes management particularly challenging. Chipping infested wood to less than 2.5 centimeters can result in a death rate exceeding 99 percent for the borer. The problem of infestation extends beyond their fields and into diverse ecosystems.
Understanding the Pest’s Biology
Learning about the biology behind the polyphagous shot-hole borer has given us a better idea of why it is so resilient and spreading quickly. This little beetle— it’s just 1.5 millimeters long— has a largely secretive life spent inside trees. “When you’re only 1.5 millimeters long and you spend most of your life living inside a tree, you’re not going to be an acrobat,” Dr. Evans commented on its limited mobility.
Once inside a tree, the borer must make use of fungus to help it digest the wood. “No animal has evolved the capacity to do this on its own—they always do it with a microbial friend,” Dr. Evans explained. This partnership increases the borer’s fitness and capacity to damage multiple species of trees.
Specifically, Dr. Evans pointed out areas of the southeastern U.S. that have proactively controlled the pest by deploying strong tactics. So far, what Perth’s future under such conditions might look like is still speculative beyond 20 years. “Now we don’t know how badly affected the trees are going to become because normally when the beetles are found infesting the trees, they get cut down and chipped,” he stated.
