Arid rangelands in South Africa are impacted and threatened by invasive alien plant species. These plants reduce the carrying capacity of grazing land, are major water users, and some species directly harm indigenous animals and livestock. Indigenous plants are also directly affected by invasive alien plants which alter the functioning of ecosystems and compete with indigenous plants for resources. The arid north-west of South Africa, as well as the riverine vegetation along the Orange River, is impacted by a variety of invasive alien plants, some of the most serious being various cactus species and mesquite (Prosopis sp.). The control of these invasive alien species should be a priority for farmers and conservationists in the region. In the Kalahari, boxing glove cactus, Cylindropuntia fulgida var. mammilata, was a serious problem prior to the release of the biological control agent Dactylopius tomentosus 'cholla'. Three years after the release of this agent, large boxing glove cactus infestations were completely destroyed with no live plants remaining. The agent is now redistributed by farmers and biological control practitioners to new sites when required and is providing complete and permanent control. Another serious cactus weed in the region is Devil's rope cactus, Cylindropuntia imbricata, which is controlled with a different lineage of the same species of cochineal, D. tomentosus 'imbricata'. Devil's rope cactus can be controlled using integrated control. The biological control agent will kill small plants and defoliate large trees in the first few years after release, after which large trees should be felled and left as reservoirs for the biological control agent. This integrated method results in complete control of C. imbricata in most cases. The worst invasive alien plant in riverine vegetation in the arid north-west of South Africa is mesquite (Prosopis sp.). Mesquite is a major water user and completely dominates rivers and watercourses to the detriment of indigenous flora and fauna. One biological control agent for mesquite has been released in South Africa but provides negligible control. There is however a new agent, the seed feeding weevil, Coelocephalapion gandolfoi, that will be released soon; as well as other agents that could be sourced in native distribution, which are predicted to have a significant impact on the tree populations. Mass-rearing and redistribution of biological control agents will be required in order to implement biological control on a meaningful scale in the region. Currently, the only cactus agent mass-rearing facility in the country is in the Eastern Cape and there is no mass-rearing of mesquite agents. Plans are underway to initiate mass-rearing of these species in Upington to serve the north-west of the country. This project is also an opportunity for employment, and especially for the employment of people with physical disabilities. With a large mass-rearing and redistribution effort, biological control will reduce the impact and future threat of these invasive alien plant species in the region, with significant benefits to agriculture, conservation and ecosystem functioning.

Nassella (Trin.) E.Desv. is a grass genus with over 115 mostly perennial tufted species. It is mostly native to South American countries while six species are also found in the United States and Canada. Eleven of the species have been recorded growing outside their natural range. Of these, N. neesiana, N. tenuissima and N. trichotoma are naturalised in Australia, Europe, New Zealand, South Africa and the USA. They invade disturbed areas, especially overgrazed pastures. Nasella trichotoma is generally regarded the most serious invader in Australia, New Zealand and South Africa. In South Africa, N. trichotoma and N. tenuissima are declared weeds. Infestations of N. tenuissima are mainly in the Eastern Cape Drakensberg; while N. trichotoma occurs in the Western, Eastern and Northern Cape provinces, with unconfirmed records in Limpopo, Gauteng, Free State and Mpumalanga. Nassella neesiana is currently not listed as invasive, although it is known to have naturalised. Nassella can easily grow in a wide range of climatic conditions, soil types and topographical situations, with most of the Grassland Biome in southern Africa at potential risk of invasion. Nassella can self-pollinate and outcross, thus benefiting from the advantages offered by both breeding systems to become successful invaders. Mature plants can produce as many as 90,000 seeds per plant per year, which are dispersed over long distances by animals, machinery, people, vehicles and wind. The plants have a short juvenile stage, reaching maturity within a year. The major impact of Nassella species is on natural rangelands, where they out-compete desirable pasture species and dramatically reduce carrying capacity. Their high fibre content and a low nutritive value mean they are not favoured by livestock and thus flourish at the expense of good pasture plants. The fibres can form indigestible balls which block the gut of animals making them lose condition rapidly, leading to mortalities. The sharp seeds can cause injury to livestock, including blindness and severe irritation to the skin of young lambs. The awns also reduce the quality of wool. In Australia and New Zealand, it was reported that the cost of control of heavy infestations can exceed the value of the land. Research work done in South Africa in the 1970s and 1980s, especially by M.J. Wells, led to vigorous awareness and control campaigns at that time. Nassella species are not easily distinguishable and so do not readily lend themselves to citizen science (e.g. SAPIA). Hence the need for a rigorous reassessment of Nassella in South Africa, to determine current distributions of all three species; invasiveness and drivers of invasiveness; and potential responses to climate change and various management methods. Nassella highlights the challenges presented by cryptic invaders not readily distinguishable from native grasses, such as Festuca. 

Many cactus species have been introduced in South Africa and the invasive subset has multiple negative impacts on natural resources especially the underground water in arid areas. For containment of invasion problems, convenient methods of eradication strategies ranging from mechanical to biological control are applied depending on the scale of the problem. Unexpectedly, new species and their populations displaying the invasion potential, are increasingly being detected by the Biological Invasions Directorate of South African National Biodiversity Institute. The aim of the study is to report the emerging cactus species, observed negative impacts as well as their management interventions in arid areas of South Africa. A combination of the stratified and systematic surveys for species populations were conducted in different arid localities across Western Cape, Free State, Northern Cape, and North West provinces. Drive-by surveys and guided interviews were conducted to increase the detection effort. Additional species occurrence data was obtained from different platforms of citizen science (e.g. iSpot database). Large and dense populations of a new unlisted cactus species Cylindropuntia prolifera, native to Southern California, have been detected in the Karoo region of Western Cape. Also, more new populations of Category 1a species such as Cylindropuntia pallida, Tephrocactus articulatus, Austrocylindropuntia cylindrica, Myrtillocactus geometrizans, Harrisia pomanensis, and Cylindropuntia spinosior have been detected. Cylindropuntia pallida is avoided by livestock and thus displays potential to reduce grazing land while also negatively impacting on wild animals through fatal skin damage in, for example, trapped snakes, birds and lizards. Effective management intervention by chemical methods substantially (96%) reduce the population. Post-eradication monitoring suggest that follow-up controls are needed to clear plants that might have been missed and the resprouting individuals.