Initial assessment of the polyphagous shot hole borer (PSHB) in indigenous Afrotemperate forests

Abstract

The polyphagous shot hole borer or PSHB (Euwallacea fornicatus) is an invasive ambrosia beetle native to Southeast Asia. In 2017 it was discovered in Pietermaritzburg, South Africa, where it was found attacking several ornamental tree species, and has since spread into eight of the nine provinces in the country. It has also started moving into indigenous forests of the southern Cape attacking native trees. However, as the beetle is known to attack a wide variety of trees, with varying levels of damage to different species, its impact in native forests cannot be predicted. This study aimed to establish long–term monitoring plots to gain insight into the distribution, host range and potential ecological impacts of PSHB in indigenous Afrotemperate forests in the Southern Cape. We aimed to monitor which species of indigenous trees are attacked by PSHB, which species are potential reproductive hosts and what makes some forest patches more prone to attack than others. Our results showed that PSHB is spreading into natural Afrotemperate forest patches throughout the Southern Cape area. Although the spread appears to be slow, this may be the beginning stages of the invasion as a consistent increase in the number of attacked and infected trees was seen over the one–year monitoring period. Many indigenous tree species were identified as hosts, and preliminary phylogenetic analyses showed that there may be some family level relationships between preferred hosts. The beetle appears to select certain tree species, and it was able to reproduce in almost half of all the host trees it attacked. The high number of potential reproductive hosts could allow the beetle to rapidly establish and spread through natural forest areas. Natural forests close to urban trees are more at risk of attack as many ornamental trees planted in towns and cities are susceptible to PSHB attack and act as reservoirs for the beetle, allowing the population to increase rapidly and spill over into natural areas. Forest patches with a high species richness had a lower mean number of infected trees, likely due to the lower number of potential hosts available to the beetle. In patches where the number of potential host trees was high, there was an increased number of PSHB attacks. The data also suggest that trees with a larger diameter, that are likely more easily located, and trees that are already under stress or in poor health, are targeted by PSHB. The PSHB beetle poses a major risk to natural forest health, not only in Afrotemperate forests, but to native forests throughout Africa. Control options for the beetle are limited and expensive. However, this study was only a preliminary investigation of the impacts PSHB is having in Afrotemperate forests. Long term monitoring over a larger area is imperative if the true impacts of PSHB are to be quantified, and to better understand its behavior, host range and the eventual fate of attacked trees.