This special issue is the fourth decadal review of biological control of invasive alien plants (biocontrol of weeds) in South Africa, following those published in 1991, 1999 and 2011. Including this introduction, there are 24 papers covering the weed biocontrol programmes, or important developments in the science and practice, from the period 2011-2020. Seventy-two target weed species are covered, including 25 species on which projects were initiated during the past decade. Developments in regulations, mass-rearing and implementation, and community engagement are also reviewed. An updated catalogue of agents released, rejected and under consideration is presented and reflects the most recent methods of quantifying success in weed biocontrol. Key events over the last decade include the hosting of the XIV International Symposium on Biological Control of Weeds to celebrate 100 years of weed biocontrol in South Africa, as well as the establishment of the Centre for Biological Control at Rhodes University. The science and practice of weed biocontrol has expanded significantly in the past decade, with growth in the number of researchers and practitioners, increased funding, and an increased number of scholarly outputs. Unlike many other countries in the world, South Africa has largely avoided constraints due to restrictive and risk averse legislation and bureaucracy, and has continued to release new biocontrol agents at a similar rate to that in previous years. Much of the success of weed biocontrol in South Africa is due to the sustained and increasing support of the Natural Resource Management Programme of the Department of Forestry, Fisheries and the Environment (Working for Water Programme). However, gaps in funding, where no funds are available for months at a time, are a major concern as the weed biocontrol community loses human capital in these periods, and research programmes suffer significant set-backs. Weed biocontrol is an essential component of South Africa's strategy to reduce the negative impacts of invasive alien plants and has contributed significantly towards the protection of the country's ecosystems, indigenous biodiversity, water security, agricultural productivity, and society in general. If the trend of increasing support for weed biocontrol in South Africa continues, we can expect that the benefits for the country at large will increase substantially in the future.

There are 14 species of Australian Acacia now known to be invasive in South Africa, ten of which are under some form of biological control (biocontrol). The biocontrol agents introduced against this group include a fungal pathogen, Uromycladium morrisii Doungsa-ard, McTaggart, Geering & R.G. Shivas (Pucciniales: Raveneliaceae) for Acacia saligna (Labill.) H.L.Wendl., and ten insect species, including two bud-galling wasps, five seed-feeding weevils and three flower-galling fly species, all of which supress reproductive output of their hosts. There are also two native fungal pathogens that have become associated with the introduced acacias, and which have been developed for potential mycoherbicide use. Screening and importation of new agents has seen limited activity in the ten-year period reviewed here (i.e., 2011–2020). Most attention has been focussed on efforts to evaluate the effectiveness of the established agents and, importantly, to gain an understanding of the role of seeds in the population dynamics of the host plants.

Cactaceae are among the most problematic invasive alien plants in South Africa, posing serious negative consequences to agriculture and natural ecosystems. Fortunately, South Africa has a long and successful history of controlling cactus weeds using biological control (biocontrol). This paper reviews all the biocontrol programmes against invasive alien Cactaceae in South Africa, focusing on the decade since the last review published in 2011, up to, and including 2020. Biocontrol programmes against 16 target weeds are summarised, all of which rely on either the galling mealybug, Hypogeococcus sp. (Pseudococcidae), or various species or intraspecific lineages of cochineal insects (Dactylopius spp., Dactylopiidae) as agents. New agents are being considered for the three target weed species, Opuntia elata Salm-Dyck, Opuntia megapotamica Arechav. and Trichocereus spachianus (Lem.) Riccob., while permission to release a new agent against Cylindropuntia pallida (Rose) F.M. Knuth has recently been granted. The biocontrol agent, Dactylopius opuntiae (Cockrell) ‘stricta’, which has been utilised for the successful control of Opuntia stricta Haw., has shown some promise as an agent against one of the worst cactus weeds in the country, the North Cape/Free State variety of Opuntia engelmannii Salm-Dyck. Post-release monitoring and recent observations of the status of control for the 11 other cactus weeds, all of which have well-established agents, are provided. Taxonomic uncertainties and misidentifications of both target weeds and agents has been a constraint to biocontrol efforts, but this has been partially overcome through the use of genetic techniques. Biocontrol is particularly successful in controlling cactus weeds compared to most other taxonomic groups, and it is likely that past successes can be repeated with new target weeds. Mass-rearing and redistribution of agents are essential to gain the maximum possible benefit from cactus biocontrol agents, and recent increases in mass-rearing outputs have been beneficial.

Cestrum (Solanaceae) species have become invasive in conservation and agricultural areas in South Africa, resulting in the initiation of a biological control (biocontrol) programme against these species in 2007. Of the four Cestrum species recorded in South Africa, Cestrum laevigatum Schltdl. and C. parqui L'Herit., which are both commonly referred to as inkberries, have become invasive while C. aurantiacum Lindl. and C. elegans (Brongn.) Schltdl. are still isolated at a few sites in the country. The biocontrol programme against Cestrum species (commonly called cestrum) was initially focussed on pathogens associated with these plants. Although the fungus Uromyces cestri Bertero ex Mont. (Pucciniales: Pucciniaceae) was found to be a promising agent for cestrum, the pathogen project was shelved due to lack of capacity. Field surveys conducted in Argentina from 2012 to 2020 revealed a total of eight phytophagous insects that could be candidate agents. Among these is a leaf-feeding flea beetle Epitrix sp. (Coleoptera: Chrysomelidae) that has been tested for host specificity. Out of 47 plant species tested to date, Epitrix sp. has only fed and developed on three Cestrum species, suggesting that it is suitable for release against cestrum in South Africa. Permission to release Epitrix sp. will be sought once it has been identified to the species level or it has been described as a new species. Two chrysomelid beetle species, a root-feeding flea beetle (Diphaulaca sp.) and an unidentified leaf-feeding flea beetle, were also collected on C. parqui, but attempts to rear the former were unsuccessful. Three other leaf-miners collected from C. parqui include: Acrocercops leucographa Clark (Lepidoptera: Gracillariidae), an unidentified moth (Lepidoptera: Argyresthiidae) and Liriomyza sp. nr. schlingerii (Diptera: Agromyzidae). A glass-winged butterfly (Lepidoptera: Nymphalidae) was the only herbivore that may be of biocontrol value on C. laevigatum. Whilst it is essential to extend the surveys to other regions of the native range, it is also important to prioritize the most promising potential biocontrol agents for further testing in South Africa. Given the suite of potential biocontrol agents in the native range, there are good prospects for the biocontrol of cestrum species in South Africa.

Several weed species within the asteraceous tribe Eupatorieae, all with a neotropical origin, are invasive in South Africa. Three of these form the subject of this review paper: Chromolaena odorata (triffid weed), Campuloclinium macrocephalum (pompom weed), and Ageratina adenophora (crofton weed). The three species invade different habitats and regions, and all have biological control (biocontrol) agents established on them. Ageratina adenophora was the first of these weeds to be subjected to a biocontrol programme in South Africa, with two agents (an insect and a pathogen) released and established in the 1980s. Two biocontrol agents were established on C. odorata in the early 2000s, and a third one, first released in 2011, has persisted for at least eight years – all three are insects. One insect biocontrol agent was established on C. macrocephalum in 2013, although a pathogen had appeared on the weed several years earlier. Chromolaena odorata and A. adenophora are under substantial control in certain habitats, but negligible in others. The biocontrol agent on C. macrocephalum released in 2013 is causing significant damage to the plant where it has established well. Several other biocontrol agents have been released on C. odorata but have failed to establish. For all three weedy Eupatorieae, it is considered desirable to establish additional biocontrol agents, so as to increase the level of control of these priority targets in South Africa. An additional biocontrol agent has already been approved for release against C. macrocephalum, while one is close to being approved for C. odorata. There are several possibilities for additional biocontrol agents for A. adenophora.

Although Hakea sericea Schrad. & J.C. Wendl. (Proteaceae) remains a significant invasive species in parts of South Africa, efforts made to curtail its spread over the past decade have shown varying levels of success. Here, we describe progress-to-date with the insect agents and factors that have contributed to their success or failure. In particular we cover: (i) research on the ability of the seed-feeding weevil, Erytenna consputa Pascoe (Coleoptera: Curculionidae) to recolonise burnt areas after fire; (ii) the impacts and dispersal ability of the seed-feeding moth, Carposina autologa Meyrick (Lepidoptera: Carposinidae); (iii) the impacts and future prospects for the stem-boring beetle, Aphanasium australe (Boisduval) (Coleoptera: Cerambycidae); and (iv) the establishment discrepancy observed for the flowerbud-feeding weevil, Dicomada rufa Blackburn (Coleoptera: Curculionidae). In general, findings show: (i) E. consputa is able to quickly re-establish populations after fires; (ii) that C. autologa has a modest impact and disperses slowly; (iii) A. australe is performing relatively well in the field, with healthy localised populations in South Africa which should be left to increase before harvesting and redistribution of the beetle commences; (iv) climate mismatch between native and introduced ranges is a problem for D. rufa, along with other possible causes for establishment discrepancies which are still under investigation. As a result of these findings, we provide suggestions for possible management plans for some of the agents, so as to maximize resources and increase the overall impact of the H. sericea biological control programme in South Africa.

Lantana camara L. ‘lantana’ continues to pose economic and ecological challenges in South Africa. The present review provides a brief overview of biological control (biocontrol) agents established in South Africa, including their combined impact and individual performance of four agents released during the recent past. In total, 16 biocontrol agents are fully established on lantana in South Africa, and three of these are either indigenous or inadvertently introduced into the country. Despite the establishment of these agents, biocontrol of lantana is not effectively reducing the density of the weed, particularly in the inland regions of the country. However, recent studies showed that biocontrol is causing a significant reduction in plant growth, biomass and seed production at various sites in the coastal region of KwaZulu-Natal (KZN) and the continental area of Mpumalanga province. The sap-sucking mirid Falconia intermedia has been observed to resurge at a number of sites following its crashing countrywide after its release in 1999. Falconia intermedia is, however, still confined to the warm low-lying regions of the country. The flower-galling mite Aceria lantanae has dispersed widely throughout South Africa and some neighbouring countries. The mite reduced flower production of susceptible varieties by up to 97% in the humid coast of KZN. Eleven years after its release, the root-feeding flea beetle Longitarsus bethae is still largely confined to the vicinity of its release sites, where it is severely damaging lantana. Although populations of the petiole weevil Coelocephalapion camarae collapsed at many sites following its release, the weevil has persisted for over 15 years at two isolated sites in KZN province. Efforts are being made to mass rear and redistribute L. bethae and C. camarae in order to improve their establishment and dispersal rate. To cope with varietal resistance and climatic extremes, attempts to find new strains of the existing and promising biocontrol agents are continuing. To justify the continued development of additional agents, the ecological and economic benefits of biocontrol should be assessed, particularly in the coastal region where biocontrol is more effective.

Despite decades of biological control (biocontrol) endeavours against Leptospermum laevigatum (Gaertn.) F.Muell. (Myrtaceae), the weed remains a significant problem in areas within the Western Cape Province and parts of the Eastern Cape Province. Being predominantly invasive within coastal dune fynbos, the species is notoriously difficult to manage. Whilst the past 10-year period has seen limited focus and funding for this specific project, this review addresses possible reasons for the lack of success of the biocontrol programme over the past three decades, which has relied on two agents, a leaf-mining moth, Aristea thalassias (Meyrick) (Lepidoptera: Gracillariidae) and a flower-galling midge, Dasineura strobila (Dorchin) (Diptera: Cecidomyiidae). Although both agents are widespread and occur across the range of L. laevigatum, the plant continues to grow aggressively despite their presence. Findings to date suggest that mortality resulting from parasitism or predation of the agents' immature stages is not the underlying problem for the limited control of the plant and that other factors must be at play. However, further investigation to better understand the dynamics of the insect populations and ecology of this weed, is needed to help to clarify the questions that, to date, remain unanswered. Other potential agents are also currently under consideration, notably a scale insect, Callococcus leptospermi (Maskell) (Homoptera: Coccoidea). In its native range (Australia) C. leptospermi, has a patchy distribution and variable levels of abundance, reasons for which are still under investigation. With no foreseeable biocontrol breakthrough in the near future, an integrated approach to managing L. laevigatum is probably the only short to medium-term solution for this species.

The Northern Temperate Weeds programme is a novel biological control (biocontrol) programme started in 2017, with the aim of reducing the impacts of northern temperate weeds that are common, widespread and problematic in the high elevation grasslands of South Africa. As these regions are the most important systems for water security, providing nearly 50% of all water run-off, it is surprising that these species were not targeted for biocontrol previously. Thus far, research has focused on biocontrol feasibility as well as ecological and socio-economic impact studies on several northern temperate weeds, including Pyracantha angustifolia (Franch.) C.K.Schneid, Rosa rubiginosa L., Cotoneaster spp., Rubus spp., (all Rosaceae) and Salix spp. (Salicaceae). In addition, research conducted in the USA and Europe on the natural enemies associated with two of these species, Robinia pseudoacacia L. and Gleditsia triacanthos L. (both Fabaceae and native to the USA), have provided South African researchers with the necessary foundation to initiate programmes against these weeds. Research in South Africa is currently focused on pre-release studies on R. pseudoacacia, using the leaf miner Odontota dorsalis Thunberg (Coleoptera: Chrysomelidae) and the black locust midge Obolodiplosis robiniae Haldeman (Diptera: Cecidomyiidae), and on G. triacanthos, using a seed bruchid, Amblycerus robiniae F. (Coleoptera: Chrysomelidae). Progress with these programmes and potential constraints that may limit success, are discussed.

The annual herb, Parthenium hysterophorus L. (Asteraceae: Heliantheae) is a severe terrestrial invader globally. Infestations reduce crop yield, limit available grazing, hinder conservation efforts, and affect human and animal health in Africa, Asia and Australia, and on associated islands. Due to the impact and threat of further invasion of P. hysterophorus, a biological control (biocontrol) programme was initiated in 2003 in South Africa. This review discusses the research and implementation activities undertaken on the insect agents from 2011 to 2020. During this period, the stem-boring weevil Listronotus setosipennis Hustache (Coleoptera: Curculionidae), leaf-feeding beetle Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) and seed-feeding weevil Smicronyx lutulentus Dietz (Coleoptera: Curculionidae), were found to be host specific and approved for release. Releases of mass-reared insect agents have been concentrated particularly in north-eastern South Africa, where P. hysterophorus infestations are most prolific. Post-release monitoring studies indicated localised establishment and impact of L. setosipennis and S. lutulentus. Listronotus setosipennis persisted through severe drought conditions, and although it disperses slowly, larval feeding is structurally damaging. Establishment of S. lutulentus is improving, reducing seed production where it is established. Zygogramma bicolorata resulted in defoliation at a few sites, but establishment has been poor and the beetle has been absent since 2019. Although a combination of fungal and insect agents were demonstrated to reduce P. hysterophorus, additional natural enemies could improve control. Consequently, the stem-galling moth Epiblema strenuana Walker (Lepidoptera: Tortricidae) and root-crown boring moth Carmenta sp. nr. ithacae (Beutenmüller) (Lepidoptera: Sesiidae) remain under evaluation. The management of P. hysterophorus in South Africa has been guided by the development of a national strategy, which incorporates multiple management methods, including biocontrol. International collaborations have intensified as a growing number of countries begin to utilize biocontrol to manage P. hysterophorus. Despite the progress towards biocontrol of P. hysterophorus during this period, increased utilisation of approved agents and the introduction of additional agents are necessary to achieve greater control.

Historically, invasive alien grasses have not been considered a major threat in South Africa, and as a result, very few resources are allocated to their management. However, there is an increasing awareness of the severe environmental and socio-economic impacts of invasive grasses and the need for appropriate management options for their control. South Africa has a long history of successfully implementing weed biological control (biocontrol) to manage invasive alien plants, however much like the rest of the world, invasive grasses do not feature prominently as targets for biocontrol. The implementation and early indicators of success of the few grass biocontrol programmes globally and the finding that grasses can be suitable targets, suggests that biocontrol could start to play an important role in managing invasive alien grasses in South Africa. In this paper, we evaluated the prospects for implementing novel grass biocontrol projects over the next ten years against 48 grasses that have been determined to represent the highest risk based on their current environmental and economic impacts. The grasses were ranked in order of priority using the Biological Control Target Selection system. Five grasses were prioritised – Arundo donax L., Cortaderia jubata (Lem.) Stapf, Cortaderia selloana (Schult & Schult) Asch. & Graebn., Nassella trichotoma (Hack. ex Arech.), and Glyceria maxima (Hartm.) Holmb., based on attributes that make them suitable biocontrol targets. Arundo donax has already been the target of a biocontrol programme in South Africa. We reviewed the progress made towards the biocontrol of this species and discuss how this programme could be developed going forward. Moreover, we outline how biocontrol could be implemented to manage the remaining four high-priority targets. While biocontrol of grasses is not without its challenges (e.g. unresolved taxonomies, conflicts of interest and a lack of supporting legislation), South Africa has an opportunity to learn from existing global research and begin to invest in biocontrol of high-priority species that are in most need of control.

South Africa was the first country to deploy biological control (biocontrol) against invasive Prosopis populations. Developments in this regard have been ongoing, and have been reviewed, at approximately 10-year intervals, since 1991. This review spans the period 2011-2020, a timespan globally characterised by increased awareness of the impacts of invasive Prosopis populations, and recognition of the need for improved management. Concerted international collaboration has resulted in enhanced clarity on phylogenetic relationships within the Leguminosae and the phylogenetic placement of Prosopis. These advances have improved the framework for interpreting the host range of potential agents and for evaluating risk. At the outset of the biocontrol programme, in the 1980s, only agents that consumed mature seeds were considered. The intention was to reduce the invasiveness of Prosopis while simultaneously retaining it as a usable resource. The programme was subsequently expanded to investigate agents that prevent pod set or maturation of seed. More recently, potential agents that damage the vegetative growth of the plants have been included in response to recognition in South Africa, that there is no other route to successful management of Prosopis. There is a wealth of largely unexplored potential in this regard.

Solanum mauritianum Scop. (Solanaceae), colloquially referred to as bugweed in South Africa, remains a widespread invasive tree of global significance. Although biological control (biocontrol) efforts were undertaken from 1984 -2003 in South Africa, the programme eventually only released two agents, the sap-sucking lace bug Gargaphia decoris Drake (Hemiptera: Tingidae) and flowerbud-feeding weevil Anthonomus santacruzi Hustache (Coleoptera: Curculionidae). To date, these agents have been relatively ineffective in controlling S. mauritianum, largely due to low establishment success due to climatic incompatibility in relation to the widespread distribution of S. mauritianum. This has prompted the revival of S. mauritianum biocontrol research in 2018, with the programme focused largely on sourcing additional agents from climatically suitable regions in the plant's native range in South America. Climate matching between cooler regions of South Africa and known S. mauritianum sites in South America identified Uruguay as a promising source of new agents. Field collections in Uruguay focused mainly on Anthonomus spp. but included stem-boring and shoot-galling weevils. Low incidence in the field and difficulties in culturing candidate species temporarily precluded research into stem-boring and shoot-galling candidates, but the rearing and assessment of the flowerbud-feeding weevil Anthonomus morticinus Clark (Coleoptera: Curculionidae) is ongoing. Host-specificity testing of A. morticinus has thus far confirmed a narrow host range, suggesting it has potential as a new agent.

Tecoma stans (L.) Juss ex Kunth var. stans (Bignoniaceae) is an invasive shrub or small tree from Central America that continues to extend its range in all nine provinces of South Africa and in neighbouring countries. The weed has been a target for biological control (biocontrol) in South Africa since 2003. The gall-forming rust fungus Prospodium transformans (Ellis & Everh.) Cummins (Pucciniales: Uropyxidaceae) was released in 2010, but failed to establish. Two leaf-feeding agents, Mada polluta (Mulsant) (Coleoptera: Coccinellidae) and Pseudonapomyza sp. Hendel (Diptera: Agromyzidae), were subsequently released in South Africa in 2013 and 2014, respectively. Mada polluta has become established at seven sites in the low elevation coastal regions of the Eastern Cape and KwaZulu-Natal provinces, but not in high elevation inland areas. The leaf-mining Pseudonapomyza sp. has established at seven sites in four provinces, namely the Eastern Cape, KwaZulu-Natal, Limpopo and Mpumalanga, but at low population densities. The deliberate destruction of release sites by landowners and inadvertent veld fires have confounded the establishment and proliferation of both agents, as well as progress on their post-release evaluation. A root-feeding flea beetle Heikertingerella sp. Csiki (Chrysomelidae: Galerucinae), initially collected in Mexico in 2007, was determined to be host specific and potentially effective as a new biocontrol agent of T. stans. An application for the release of Heikertingerella sp. in South Africa will shortly be submitted to the regulatory authorities. Additional mass-rearing and releases of the two established agents will be undertaken to improve their establishment and impact.

Biological control programmes against the asteraceous weeds Tithonia rotundifolia (Mill.) S.F. Blake and Tithonia diversifolia (Hemsl.) A. Gray were initiated in South Africa in 2007, and that against Tithonia tubaeformis (Jacq.) Cass in 2017, following its discovery in Mpumalanga Province (MP). Permission to release two leaf-feeding beetles, Zygogramma signatipennis Stål and Zygogramma piceicollis Stål (Chrysomelidae) against T. rotundifolia was granted in 2014, while permission to release a defoliating tortoise beetle, Physonota maculiventris Stål, against T. diversifolia was granted in 2018. The two Zygogramma spp. were released in Gauteng (GP), KwaZulu-Natal (KZN), Limpopo (LP), and MP provinces from 2014 to 2019. Zygogramma signatipennis established at only a few sites in MP, where it persisted for up to four years and later collapsed. Zygogramma piceicollis initially established in GP and perished a year later. Physonota maculiventris was released in KZN, LP and MP, and has overwintered at 50% of the release sites. Investigations to explain the demise of both Zygogramma spp. and determine ways to improve their release and establishment are underway. Host-specificity testing of a stem-boring weevil, Lixus fimbriolatus Boheman (Curculionidae), a candidate agent for T. rotundifolia, was successfully completed. An application for permission to release the weevil will be submitted to the regulatory authorities in due course. Additional host-specificity testing of agents for T. diversifolia has prioritised a leaf-mining moth, Melanocinclis sp. (Cosmopterigidae) for further assessment, while a leaf-feeding plume moth, Hellinsia sp. (Pterophoridae), and a stem-boring moth, Suleima skinnerana (Heinrich) (Tortricidae), were rejected. Investigations to determine the full identity, origin and host range of a leaf-sucking lace bug (Hemiptera: Tingidae), discovered on T. diversifolia in KZN, LP and MP, are underway. Surveys in climate-matched regions of the Central American native range of the target Tithonia spp. are imperative to identify natural enemies that may be more adaptable to persist across the plants' distribution in South Africa.

Vines and other climbing plants typically invest their resources into growth at the expense of accumulating self-supporting biomass. Adaptive traits that have arisen because of the life history needs of climbing species, such as rapid and extensive growth, as well as resilience to physical damage, make these plants highly competitive. Introduced climbing species therefore have the potential to be particularly damaging in novel ranges where they escape pressure from natural enemies. In South Africa, invasive climbing species negatively influence biodiversity and plant-community structure, and as conventional management is often difficult, biological control (biocontrol) is viewed as the only viable long-term control method. This paper consolidates the work done on biocontrol programmes against climbing species in South Africa, including Anredera cordifolia (Ten.) Steenis (Basellaceae), Cardiospermum grandiflorum Sw. (Sapindaceae), Dolichandra unguis-cati (L.) L.G.Lohmann (Bignoniaceae) and Pereskia aculeata Miller (Cactaceae). To date, these programmes have investigated some 27 potential biocontrol agents, of which nine have been approved for release in the country. Since 2010, three new agents have been introduced, and considerable progress made with post-release evaluations of all the introduced agents. Some positive results have been achieved, most notably the successful reduction in seed set of C. grandiflorum due to Cissanthonomus tuberculipennis Hustache (Curculionidae), but considerable variation in efficacy over time and between infestations has been recorded for many of the other agents. Further work may help explain the factors limiting success, leading to improved control, but in some cases, such as for A. cordifolia, new biocontrol agents should be considered.

Biological control (biocontrol) against invasive macrophytes is one of the longest standing programmes in South Africa, initiated in the 1970s against water hyacinth, Pontederia crassipes Mart. (Pontederiaceae). Since then, 15 agent species (13 insects, one mite and one pathogen) have been released against six weeds, most of which are floating macrophytes, with excellent levels of success. The release of the water hyacinth planthopper Megamelus scutellaris Berg (Hemiptera: Delphacidae) in particular, has improved biocontrol prospects for water hyacinth since 2018. In the last decade, however, a new suite of submerged and rooted emergent invasive macrophytes has been targeted. The first release against a submerged macrophyte in South Africa, and the first release against Brazilian waterweed, Egeria densa Planch. (Hydrocharitaceae), anywhere in the world, was achieved with the release of a leafmining fly, Hydrellia egeriae Rodrigues-Júnior, Mathis & Hauser (Diptera: Ephydridae). Yellow flag, Iris pseudacorus L. (Iridaceae) and Mexican waterlily, Nymphaea mexicana Zucc. (Nymphaeaceae), have also been targeted for biocontrol for the first time worldwide, and are in the early stages of agent development. Post-release evaluations, long term monitoring and controlled experiments have highlighted the need for a more holistic approach to managing aquatic invasive plants in South Africa, whose presence is largely driven by eutrophication, resulting in regime shifts between floating and submerged invaded states.

Three weed biological control (biocontrol) programmes are described, all of which are considered to be ‘transfer projects’ that were initiated elsewhere, and on which South Africa has piggybacked its biocontrol efforts. Using knowledge and expertise from international collaborators, South African weed researchers are following a long tradition of transfer projects, which has been a largely successful and practical approach to biocontrol. Two Brazilian weeds, the Brazilian pepper tree Schinus terebinthifolia and the spiderwort Tradescantia fluminensis are being targeted, along with the Old-World trees Tamarix ramosissima and T. chinensis. The potential biocontrol agents are described and ranked for the two trees according to what has been discovered elsewhere, while the agent already released against T. fluminensis is rated (as poor), and other potential agents are considered. The addition of molecular techniques, climate matching and remote sensing in transfer projects can increase the chance of successful biocontrol and the inclusion of these techniques in the three new programmes is discussed. Transfer projects are a cost-effective and pragmatic way to pick winning biocontrol programmes.

The use of plant pathogens for biological control (biocontrol) of invasive alien plants in South Africa was reviewed in 1991, 1999 and 2011. In this review, subsequent progress and projects undertaken in the years 2011 to 2020, on both classical agents using exotic pathogens and inundative agents using indigenous pathogens, are detailed. We report on the impact of several previously introduced exotic fungi, monitored during this period. A significant highlight is the completion of 30 years of annual monitoring, from 1991 until 2020, of the impact of Uromycladium morrisii Doungsa-ard, McTaggart, Geering & RG Shivas (Raveneliaceae) on the density of Acacia saligna (Labill.) Wendel (Fabaceae), with declines of up to 98% recorded at monitored sites. Post-release monitoring also suggested that Entyloma ageratinae R.W. Barreto & H.C. Evans (Entylomataceae), introduced when Ageratina riparia (Regel) R.M.King & H.Rob (Asteraceae) still had a very limited invaded range in South Africa, has prevented the weed from realizing its potential as an invader. Uromycladium woodii Doungsa-ard, McTaggart, Geering & R.G. Shivas (Raveneliaceae) has been established on Paraserianthes lophantha (Willd.) I.C.Nielsen (Fabaceae), as has Puccinia xanthii Schwein (Pucciniaceae) on Parthenium hysterophorus L. (Asteraceae). Prospodium transformans (Ellis & Everh.) Cummins (Raveneliaceae) failed to establish on Tecoma stans (L.) Juss ex Kunth var. stans (Bignoniaceae), while efforts to release Puccinia lantanae Farl. (Pucciniaceae) on Lantana camara L. (Verbenaceae) are underway. Of the established exotic pathogens that are classical biocontrol agents, two are considered to cause extensive damage, two considerable damage, and two moderate damage, to their target weeds. Puccinia arechavaletae Speg. (Pucciniaceae), an adventive fungus that is established on Cardiospermum grandiflorum Sw. (Sapindaceae) following an unknown path of introduction, is proving to be a damaging agent and the implications of its presence in the country are discussed. Early season augmentation of Puccinia eupatorii Dietel (Pucciniaceae) on Campuloclinium macrocephalum (Less.) DC. (Asteraceae) was tested to determine whether damage could be increased, but no difference in the rust's incidence and severity was evident between augmented and naturally infected plants at the end of the growth season. The identity of the indigenous fungus registered as Stumpout®, for the control of coppice growth after felling of Acacia mearnsii De Wild. (Fabaceae), was confirmed as Cylindrobasidium torrendii (Bres.) Hjortstam (Physalacriaceae) rather than C. laeve (Pers.: Fr.) Chamuris, as previously thought. The application of another indigenous fungus, Colletotrichum acutatum J.H. Simmonds (Glomerellaceae), by means of helicopter flights has facilitated its establishment on Hakea sericea Schrad. & J.C. Wendl (Proteaceae), in inaccessible mountainous terrain. For registration purposes, toxicity testing (oral rat LD50) of these indigenous fungi are required. Lethal doses of Cylindrobasidium torrendii, C. acutatum and Pseudolagarobasidium acaciicola Ginns (Phanerochaetaceae), the latter for use against Acacia cyclops G. Don. (Fabaceae), exceeded 2000 mg kg^–1 body weight and all three fungi are thus considered safe for application.

South Africa supports a rich floral diversity, with 21 643 native plant taxa that include a high proportion (76.3%) of endemic species, and many of these favoured as ornamentals, both locally and globally. Consequently, South Africa has contributed substantially to global plant invasions, with 1093 native taxa (5% of all species) naturalized in other countries. At least 80 taxa are invasive in natural or semi-natural ecosystems elsewhere, while an additional 132 taxa are potentially invasive. Of the global naturalized flora, 8.2% originate from South Africa and largely comprise species of Poaceae, Asteraceae, Iridaceae and Fabaceae. Australia, in particular, but also Europe and North America are major recipients of South African weeds. However, few countries have targeted South African plants for biological control (biocontrol), with most efforts undertaken by Australia. Previous and current targets have involved only 26 species with 17 agents (15 insects, one mite and one rust fungus) of South African origin released on five target species in Australia and the United States of America. South Africa's history of weed biocontrol, together with a large cohort of active scientists, is currently facilitating several internationally funded programmes targeting invasive plants of South African origin. In particular, the recently inaugurated Centre for Biological Control at Rhodes University and the University of KwaZulu-Natal have provided the impetus for novel efforts on five new target species and renewed efforts on four previously targeted species. In this contribution, we review the history of earlier biocontrol programmes against weeds of South African origin and the status of projects currently in progress in South Africa.

It has been documented that the continual release of high numbers of biological control (biocontrol) agents for weeds increases the likelihood of agent establishment and has been shown to reduce the time between the first release and subsequent control of the target weed. Here we review the mass-rearing activities for weed biocontrol agents in South Africa between 2011 and 2020. Some 4.7 million individual insects from 40 species of biocontrol agent have been released on 31 weed species at over 2000 sites throughout South Africa during the last decade. These insects were produced at mass-rearing facilities at eight research institutions, five schools and 10 Non-Governmental Organizations. These mass-rearing activities have created employment for 41 fulltime, fixed contract staff, of which 11 are people living with physical disabilities. To improve the uptake of mass-rearing through community engagement, appropriate protocols are required to ensure that agents are produced in high numbers to suppress invasive alien plant populations in South Africa.

In the last decade, biological control in South Africa has evolved from a classical applied science, allied to an extension service, to a more community engagement-based activity. Therefore, capacity building is important for the sustainability of biological control research and its implementation. In South Africa, a broad approach has been taken to build capacity in weed biological control, starting at grass-roots level with primary and secondary school learner programmes, through to developing research capacity at the tertiary level and enhancing technical capacity through adult education. Non-specialists are empowered through access to knowledge. The dissemination of accurate information through the most appropriate outlets has become increasingly important, including non-traditional science communication through the internet and, more importantly, social media, which has the potential to reach a far wider audience. Public understanding of biological control has the potential to contribute significantly to the green and knowledge economies of South Africa, but relies on government support for the sustainability of this discipline.

Regulation of biological control (biocontrol) is essential to ensure its continued safety and to enhance its acceptability as a key contributor to the management of damaging invasive alien plants in South Africa. Local researchers were concerned that regulators may become risk averse and over-cautious, thus preventing introductions of safe biocontrol agents, as bureaucratic impediments have contributed to the decline in the number of biocontrol releases in several other countries. In South Africa, the introduction of a transparent and inclusive review process has averted these concerns. Legislation in South Africa enables departments concerned with protecting environmental and agricultural resources, to work together to regulate potential risks. An interdepartmental committee, advised by independent specialists, facilitate the review of research into the safety of potential biocontrol agents. Regulators have reviewed and timeously assessed 26 potential biocontrol agents between 2013 and 2020. This has ensured that the considerable benefits from safe biocontrol agents are available for management of some of South Africa's worst invasive alien plants. We review the system in South Africa and suggest possible improvements to the regulatory framework.

A catalogue of natural enemies considered and released as biological control (biocontrol) agents against invasive alien plants in South Africa has been maintained for many years, and is updated here to include the period 2011-2020. The current catalogue reorders and separates data for exotic natural enemies that have been approved as classical biocontrol agents, from those on exotic natural enemies that have been considered but not released, exotic natural enemies that have been inadvertently introduced into South Africa with host plants that include target weed species, and locally sourced natural enemies that have been considered as bioherbicides. Ninety weed species have been targeted for biocontrol, with 310 exotic natural enemies considered, 136 classified as de jure or de facto classical biocontrol agents, and 92 established in the field on 66 target weeds. Ten of these species were found to have established independently of being released while research on them was in progress. An additional six exotic species have been found established on target weeds, and 11 locally sourced natural enemies have been considered for inundative or augmentative biocontrol, of which two have been developed commercially. Of the exotic natural enemies deliberately established, 35% inflict extensive damage to their host plants. Insects make up the majority of biocontrol agents (90%), with the balance of 8% being fungi and 2%, mites. Both establishment rates and damage levels vary between taxa. Five biocontrol agents previously considered established, have not been found in the field for several years and their persistence is in doubt. Outcomes of biocontrol at a plant population level are indicated for 54 target weeds on which agents have been established for over a decade. The increase in numbers of target weeds considered, as well as biocontrol agents released, over the past decade, has been enabled by continued generous funding and an efficient release-approval process.