Salvinia molesta D.S. Mitchell (Salviniaceae) is a damaging free-floating invasive alien macrophyte native to South America. The biological control programme against S. molesta by the weevil Cyrtobagous salviniae Calder and Sands (Erirhinidae) has been successful in controlling S. molesta infestations in the introduced range, however, there is some debate as to how biological control success is measured. This study measured the response of epilithic algae and aquatic macroinvertebrate communities in a S. molesta-dominated state and subsequently where the weed had been cleared by biological control, as a proxy for ecosystem recovery in a before–after control–impact mesocosm experiment. The restored treatment (S. molesta and C. salviniae) demonstrated epilithic algae and aquatic macroinvertebrate recovery during the “after” biological control phase, defined as similar to the control treatment. Comparatively, the impacted treatment (100% S. molesta) showed a drastic decline in biodiversity and shifts in community assemblages. We conclude that the biological control effort by C. salviniae facilitated biodiversity recovery of the impacted treatment. Furthermore, epilithic algae and aquatic macroinvertebrate communities were reliable biological indicators for measuring ecological impacts of invasion and ecosystem recovery following biological control, and thus represent potential tools for evaluating biological control success and ecological restoration.

Biocontrol is widely considered an effective management solution for mitigating the negative impacts of invasive alien plants (weeds). Globally, post-release evaluations to assess individual biocontrol programmes are generally lacking and there have been persistent calls within the biocontrol of weeds literature to increase the quantity and quality of post-release evaluation studies. South African biocontrol researchers have prioritised post-release evaluation studies, with a significant proportion of funding dedicated to this purpose. In this study we review post-release evaluations of weed biocontrol programmes in South Africa that have been published in the last ten years, discuss the different ways these evaluations have been conducted, and identify gaps for future research. Post-release evaluations have been conducted at different scales, including physiological changes within individual plants, plant growth parameters, plant population dynamics and landscape level changes. In most cases, the results of these studies indicated that biocontrol has reduced invasions according to these metrics. While the reduction in the invasion is assumed to alleviate negative ecological and socio-economic impacts, this is usually not directly measured. Evaluations of the socio-economic and ecosystem level benefits of biocontrol were limited to just a few examples on aquatic weeds. More studies that investigate the landscape, socio-economic and ecosystem level changes due to biocontrol are required, especially for terrestrial weeds, if the true scale of the benefits provided by biocontrol are to be understood.