Escalating threat: increasing impact of the polyphagous shot hole borer beetle, Euwallacea fornicatus, in nearly all major South African forest types

Abstract The polyphagous shot hole borer (PSHB; Euwallacea fornicatus ), is an invasive ambrosia beetle and poses a significant threat to a wide range of tree species globally. Despite its potential impact, research on the beetle's spread and impacts in natural ecosystems remains limited. This study examines the interactions between PSHB and native forest ecosystems in two regions in South Africa. Over 5 years, PSHB invaded all but one forest type with colonization being recorded on numerous native tree species, often resulting in severe infestations and sometimes mortality. Many tree species and families had higher than expected infestation rates. An increase in PSHB-attacked trees and infestation severity was observed over the course of the study with trees having a ~ 7.5% increased chance of PSHB infestations per year and PSHB holes increasing by over 10% annually. Significant temporal and regional effects on PSHB infestations were also noted. There were higher infestation levels in the tropical KwaZulu-Natal region than in the more temperate Western Cape. Monitoring plots closer to PSHB source populations had higher infestation rates. Higher tree species richness resulted in lower PSHB attacks, whereas higher densities of competent host species led to increased infestations. This study underscores the importance of extended monitoring of invasive species and provides key insights for the potential management of PSHB in natural forest ecosystems. The ecological effects of this invasion may be severe, with many important tree species sustaining infestations. Over time this invasion could have adverse effects to ecosystem functioning and resilience.