Combining multiple lines of evidence to elucidate the origin and introduction pathway of bitou bush ( Chrysanthemoides monilifera subsp. rotundata ) in Australia

Understanding the historical context of biological invasions can improve weed management outcomes. In this study, we aim to identify the introduction pathway of bitou bush ( Chrysanthemoides monilifera subsp. rotundata ) into Australia and its biogeographical origin in southern Africa by combining multiple lines of evidence from genomic tools and historical documentation. Geographic structure of genomic diversity based on SNPs supported the previous analysis of the invasion pathway of bitou bush between the two countries and within Australia, namely that all Australian material originated from the southern part of the South African distribution. Our synthesis of historical records points to the introduction of this plant into eastern Australia in Newcastle, New South Wales, from its native range in South Africa, via dry shipping ballast in about 1900. Variation in the chloroplast genome was also informative as to the biogeographical origin of Australian material and the context of the introduction. Ten unique haplotypes were discovered in South Africa with only one occurring throughout Australia, indicating an introduction from a single source population to eastern Australia. The matching haplotype was from East London, a port in South Africa with documented shipping connections to Newcastle in eastern Australia, where the weed was first recorded. Historical records suggest that the most plausible explanation for the origins of the isolated bitou bush population in Western Australia is via the shipping of steel billets or landscape plantings associated with shipping companies. The most likely introduction pathway linked the eastern Australian steel processing ports of Newcastle or Port Kembla to the Western Australian port of Kwinana in 1995. Discovering the origin and pathway of bitou bush invasions in Australia opens new opportunities for sourcing biological control agents with a higher chance of impact as well as identifying additional quarantine measures to improve outcomes and reduce long-term costs to management.