Implementing a system in South Africa for categorizing the outcomes of weed biological control

The biological control of weeds, i.e. invasive alien plant species, depends on the release and establishment of host-specific plant-damaging insect, mite, or pathogen species from the country of origin of the target weed. These 'natural enemies' (agents) variously damage their target hosts and may reduce their vigor and invasiveness. Over time, populations of the target weed may be influenced and these outcomes can be observed and/or measured. The impacts of biocontrol agents and the outcomes of biological control are usually summarized using brief descriptive terms which have been previously defined and widely applied but are unsatisfactory from several viewpoints. Recently, a conceptual model was put forward that illustrates and categorizes the possible results of agent impacts at a plant population level. The details of releases of agents and their impacts against 54 species of target weeds in South Africa, since 1913, are given as elaborations of that system. Outcomes are categorized for different regions and habitats, according to measures of changes in density, biomass, area occupied, and rate of spread of the targeted weed species. The categorizations have been updated and verified through extensive consultations with local experts. The traditional descriptive assessments of outcomes in weed biological control have now been replaced in South Africa by implementing this more-rigorous system which may, with modifications, have wider applicability, in other regions or countries.