Fungi associated with the ambrosia beetle Xyleborus perforans (Coleoptera: Curculionidae: Scolytinae) on drought-stressed Pinus in New South Wales, Australia

Ambrosia beetles have co-evolved symbiotic relationships with an array of fungal partners. Mutualistic fungal partners are often highly successful in vertical transmission between beetle generations. These persisting relationships can alter beetle behaviour, resulting in the opportunity to occupy new ecological niches and to spread geographically. In Australia, ambrosia beetles are not currently considered a significant pest in commercial Pinus plantations, where the bark beetle Ips grandicollis is known as the primary invader of stressed trees. However, in 2019, ambrosia beetles Xyleborus perforans and X. bispinatus, co-occurring with I. grandicollis, were found to have colonised a large proportion of drought-stressed trees in commercial Pinus plantations in north-east New South Wales. In this study, X. perforans (the most prevalent of two Xyleborus spp.) was collected from infested dead and dying trees in two NSW Pinus plantations. Fungal isolates of suspected Pinus pathogens were recovered from beetle mycangia and exoskeletons as well as ambrosia beetle galleries. Morphological examination and multilocus sequence analysis identified five fungi associated with X. perforans: Fusarium parceramosum, Fusarium aff. solani, Ophiostoma ips, Raffaelea deltoideospora and Sporothrix pseudoabietina. For Australia, this is the first report of F. parceramosum, as well as the first records of O. ips, R. deltoideospora and S. pseudoabietina being vectored by Xyleborus. Pathogenicity tests were performed on seedlings of three Pinus spp., with O. ips producing significantly longer lesions than the other fungi. This study demonstrates the potential for seemingly harmless ambrosia beetles to vector plant pathogens in Australian forests, providing a mode of disease transmission that should be considered in plantation management and forest biosecurity.