Biological control of Australia Acacia spp. in South Africa using seed-attacking agents reduces the cost of other control interventions

Australian Acacia species are invasive in South Africa and traditionally controlled through felling followed by herbicide application to the stumps. In some instances, a cleared area is burnt or felled, and biomass is removed where affordable. Seed-attacking biological control agents have been released against these Acacia species to reduce spread. Here, we studied the changes in the abundance of Acacia saligna (Labill.) H. L. Wendl. (Fabaceae) and Acacia cyclops A. Cunn. ex G. Don (Fabaceae), in the presence of biological control, to assess whether investments made in the development of the biological control agents contributed to conventional control. Biological control had no meaningful impact in the short-term (up to six years) and resulted in no material cost reductions in other control interventions over this period. However, for the medium-term (up to 12 years), substantial cost reductions in alternative control methods were recorded. Considering the 12-year assessment period, the most realistic estimated savings per hectare of land treated amounted to USD304. Savings were higher (USD322) where both burn scars and areas with evidence of biomass removal were included. For the long-term (22 years), the impact of biological control was substantial, with the cost of control being reduced by between USD831 and USD827/ha for two scenarios using the Le Maitre (1998. An analysis of invasion processes and risks and a strategy for modelling invasions by alien plant species at a national and regional scale. Appendix 7. In D. B. Versfeld, D. C. Le Maitre, & R. A. Chapman (Eds.), Alien invading plants and water resources in South Africa: A preliminary assessment. Report TT99/98. Water Research Commission) and Higgins et al. (2000. Using a dynamic landscape model for planning the management of alien plant invasions. Ecological Applications, 10(6), 1833–1848) models as benchmarks. Biological control of these two Acacia species contributed materially to reducing the cost of conventional control methods, but this saving was realised in the medium to long-term.