Paving the way for American Bramble Biocontrol: Integrating genetic, ecological, and socio-economic surveys of invasive Rubus in South Africa

Several taxa of alien Rubus (Rosaceae) brambles are invasive in South Africa. Chief among these are taxa that have been referred to in the past as Rubus cuneifolius (American bramble) but are now distinguished as at least three entities from two sections. Two previous biological control initiatives on brambles in South Africa did not result in the release of biological control agents. However, recent taxonomic and phylogenetic studies have improved our understanding of American brambles and other species in South Africa, increasing the chances of a successful biological control programme. This study aimed to conduct pre-introductory surveys for future biological control research to 1) sample American bramble in their invasive range in South Africa to genetically characterise the taxa, assess their geographic distribution and abundance and determine their morphological traits, 2) record the invertebrate herbivore and pathogen assemblages on both native and alien Rubus taxa and lastly to 3) conduct cost-benefit assessments of Rubus in South Africa through stakeholder engagements. Field surveys found that the invasive populations are dominated by Rubus sect. Cuneifolii and Rubus sect. Arguti, further both can be regarded as abundant and damaging. There was no evidence of spatial or elevational separation of the two taxa as previously thought. Rubus sect. Cuneifolii can be separated from genotypes within R. sect. Arguti by having more upright stems compared to the curving stems of R. section Arguti . The herbivore surveys found that there is a higher species richness on native Rubus, with genetic delineation using the cytochrome c oxidase (CO1) region determining 23 species on native Rubus compared to only 11 on alien Rubus . No alien insect species recorded could be considered as viable biological control agents. Finally, the cost-benefit evaluation of Rubus in South Africa showed that invasive populations have significant negative economic impacts, including harbouring pests, impeding human and animal activities, reducing crop yields, and encroaching on grazing land. However, Rubus taxa also benefit the private sector through commercial and personal berry production, as well as their potential use in brewing alcohol. The overall economic impact assessment reveals a cost-benefit ratio is 13.5:1 in favour of costs, with benefits concentrated in the Free State province and costs primarily in KwaZulu-Natal province, highlighting the complex nature of the invasive species' economic effects. This study concludes that biological control of Rubus sect. Cuneifolii and R . sect. Arguti is feasible and may provide a suitable management option to mitigate the impact of the weed in South Africa.