Asphodelus fistulous L., a newly discovered plant invader in South Africa: Assessing the risk of invasion and potential for eradication

Abstract

Background: Naturalised populations of Asphodelus fistulosus (onion weed) were recorded in South Africa for the first time during the early 1990s. Initial records lodged in 2012 indicated the presence of two populations. Five additional populations were found between 2012 and 2016, as a result of surveys and the distribution of awareness materials. All populations in South Africa occurred along roadsides, but in other parts of the world the species has demonstrated the ability to spread into adjacent native vegetation and crop fields.Objectives: The aim of this study was to assess the risk of invasion and potential for eradication.Method: A risk assessment tool was used to establish invasion risk. Of the seven known populations, five test populations were selected to gather data on the feasibility for eradication. Randomised fixed plots were used to monitor the response of A. fistulosus populations to mechanical and chemical plant control methods and to track spread over time. The germinability of seeds was also tested.Results: The Standard Australian Risk Assessment method for invasive alien plants gave a relatively high score for the threat posed by this species. In this assessment, a threshold score is used to indicate sufficient invasive risk to fail a species as part of a preborder risk assessment. Invasiveness elsewhere (Australia and USA) contributed to the relatively high score. The bioclimatic modelling map highlighted the south-western region of South Africa as most suitable climatically for A. fistulosus. Both mechanical and chemical control methods were shown to be effective in killing live plants. Results, based on plant removal and monitoring, over a four-year research period suggest that suppression of reproduction is possible, partly as a result of high detectability and ease of control.Conclusion: It is recommended that A. fistulosus be listed as a National Environmental Management: Biodiversity Act 10 of 2004 1a invasive species (eradication target) under national legislation, thus requiring compulsory management. We estimate that extirpation of all known populations of A. fistulosusmay be possible with continual effort at an annual investment of approximately ZAR 50 000 per year. Further surveillance for undiscovered populations and monitoring of known populations must be conducted to produce a definitive comment about the feasibility of countrywide eradication.