Opuntia aurantiaca (jointed cactus)

Opuntia aurantiaca is a succulent shrub that grows primarily in the subtropical biome. Its native range is from northeast Argentina to Uruguay. O. aurantiaca has shown itself to be a serious invasive weed on natural grasslands in Australia and South Africa for over a hundred years, reducing carrying capacity, injuring livestock and reducing the value of animal products. It is recorded as a pest in the eastern Cape, and is occasional in the Orange Free State, Natal and Transvaal. The Australian Weeds Committee declared O. aurantiaca a Weed of National Significance in April 2012, and it was reported by the Committee to be the worst Opuntia species in Queensland and the most troublesome of all cactus species in New South Wales. It was introduced to Australia and South Africa as an ornamental species and spread rapidly via dispersal of vegetative parts. However, introduction of cochineal and the cactus moth as biological control have reduced populations in infested areas to scattered plants or patches which now have mostly only a nuisance value. Nonetheless, there is a risk of further introduction into new areas via the trade in ornamental succulents and/or its escape where already present.

The word 'aurantiaca' originally referred to the orange colour of the flowers of Opuntia aurantiaca Lindley, although the plant has bright yellow flowers. This error was the result of poor communication associated with the exchange of specimens between the collectors in South America and the British taxonomists who described the plants. The original specimen that was brought to England in 1824, was then linked to an unpublished description by a Dr Gillies whose plant it 'appeared to be', and which was described from Chile as having orange flowers. This confusion and the origin of the name of O. aurantiaca are well described by Moran et al. (1976).

Opuntia aurantiaca is an inconspicuous, perennial succulent shrublet which seldom exceeds 0.5 m in height in open pasture but can reach up to 2 m when supported in vegetation. Plants consist of one to 100 or more spiny, sausage-like, fleshy segments or joints (also known as cladodes). These are 50 to 200 mm long (but may be longer when growing under shade) and 10 to 30 mm wide. Young segments are bright green and flattened whereas older joints are often cylindrical with a corky surface. Armed with barbed spines 10-30 mm long (World Flora Online, 2024). Segments covered by soil may lose thorns and resemble an underground tuber. If above-ground parts die, or are removed, plants may grow from these underground segments. Green segments take on the function of true leaves that are only present on newly formed segments and fall away within a few months. During periods of drought, or when exposed to direct sunlight, segments take on a more reddish to purplish colour. Joints are easily detached from the parent plant and in wet conditions quickly take root when in contact with the soil surface. Flowers are bright yellow (not orange as is suggested by the species name). Flowers many, c. 4 cm long; hypanthium tubular, usually spineless; petaloid segments lemon yellow to golden yellow (World Flora Online, 2024). Fruits are initially green but are red to purple and club-shaped with age. Each fruit may contain several sterile seeds. Fruit can also take root, in a similar way to segments, when falling to the ground. Reproduction of this cactus is entirely vegetative. Sharp spines arise in groups from areoles, which also contain minute thorns or glochids. Long spines have minute, backward-directed barbs at their extremities. These can hook onto passing objects, mainly animals, facilitating dispersal of isolated joints.

This species is restricted to a small number of countries, both where native and introduced. Its native range is limited here to Argentina (Buenos Aires and Entre Rios), Uruguay and Paraguay (USDA-ARS, 2009). Reed (1977) refers to O. aurantiaca as native to the Caribbean but does not state locations. Missouri Botanical Garden (2009) includes a record of a dense clump growing near St Thomas, Jamaica dated 1979. Other herbarium records from this source are pre-1900 and are deemed invalid and thus have not been included in the distribution list. It is possible that there are also isolated individuals in botanical gardens or private collections in additional locations.

It was very fashionable in the 19th century for collectors of exotic plants in Europe to exchange specimens with other collectors. For a detailed historical account of its arrival in South Africa, see Moran and Annecke (1979), who include clear evidence that O. aurantiaca arrived in Cape Town, South Africa, probably in 1843, as part of a consignment of exotic plants from England in exchange for local plants. It is highly unlikely that the plant was introduced directly from South America. The same is probably the case with Australia although no evidence exists. In Australia, it was likely introduced in the late 1800s (NSW WeedWise, 2024). In both countries, jointed cactus was distributed as a garden ornamental to settlers who also used it as a barrier plant and for protecting graves. In South Africa, infestations are now found in practically all the subdivisions of the Savanna Biome as well as many subdivisions of the Grassland Biome, and more than one million hectares of mainly grazing land is infested. In Australia, a similar area is infested in Southeast Queensland and the eastern part of New South Wales, and in both these countries, jointed cactus continues to invade new habitats and regions. For example, it is predicted that O. aurantiaca will spread from five recorded locations in Victoria to other watercourses (Dance et al., 2003). It is also present in Jamaica and Madagascar, though it is not known if it is spreading in those countries. O. aurantiaca is also recorded as introduced in Spain (Valle, 2016; Royal Botanic Gardens Kew, 2024).

Potential introductions of O. aurantiaca (and all similar species) to new countries remain a serious threat, with the most likely pathway being the international nursery trade in succulents. Many of the potentially most dangerous cactus species are widely sold in Europe where they will not become invasive, but the semi-desert lands in developing countries to the south and east remain at immediate risk of introduction and new invasions. Due to the serious invasions in Australia and South Africa, O. aurantiaca is also on the US Federal Noxious Weed list (USDA-NRCS, 2009). It is also listed on the Weed Science Society of America list of weeds in North America (EPPO, 2024).

Dislodged joints can be dispersed over long distances by water notably along river systems.

The most effective means of natural dispersal is by detached joints adhering to both wild and domesticated animals which can disperse them over long distances, before they fall to the ground where they can take root and start a new plant colony. Most long-distance dispersal to new areas is, however, through the stock trade. Small joints stuck to the wool or skin of animals can remain there for a considerable time before being dislodged or discarded. Landowners living outside the infested areas of jointed cactus would be unaware of the potential dangers of this plant when releasing animals carrying joints on them onto new pastures, and sanitary vigilance is directed mainly at animal health and diseases and not at detecting weed propagules.

Isolated joints can be dispersed by becoming fixed to cars and tractors, their tyres or other vehicle parts, and agricultural implements.

Initially, the horticultural trade contributed to the spread of O. aurantiaca although strict legislation in some countries now prevents this from happening. The international nursery trade in succulents can still however, be seen as a pathway for the introduction of this species into new countries. Many of the potentially most dangerous cactus species are widely sold in Europe where they will not become invasive, but the proximity of semi-desert lands in developing countries to the south and east lie wide open for new invasions.

There are many closely related species in the Americas with similar stem characteristics, including Opuntia discolor (Argentina), O. curassavica (Netherlands Antilles), O. taylorii (Haiti, Dominican Republic) and O. militaris (Cuba). Some of these species take on pest proportions within their native habitats wherever the land is overgrazed or degraded. Fortunately, none of these species, beside O. aurantiaca, have been introduced to other continents.

Opuntia ventanensis A. Long, found in the province of Buenos Aires, Argentina, is distinguished from O. aurantiaca and another close congener O. salmiana by characters of stem, interareolar area, spine length, flowers and fruit (Long, 2012).

In South Africa, O. pubsecens has been confused with O. aurantiaca (Cindi and Jaca, 2016).

In Australia, O. aurantiaca looks similar to rope pear (Opuntia imbricata). The latter has dark pink flowers and grows much taller (up to 3 m high) (NSW WeedWise, 2024). It looks very similar to Harrisia cactus (Harrisia martini), a low-growing plant (usually less than 1 m tall) with every long, almost cylindrical, stem segments that have numerous pyramidal humps. These stem segments have groups of spines on most of the areoles on their surfaces. Its flowers are white or pinkish in colour and its mature fruits are bright red. O. aurantiaca also looks relatively similar to boxing glove cactus (Opuntia fulgida var. mamillata), snake cactus (Opuntia spinosior) and jumping cholla (Opuntia prolifera). Boxing glove cactus is a short shrub (usually 1-2 m tall) with distorted stem segments that have numerous raised humps and are broader towards the tips. These stem segments have groups of 0-18 spines on most of the areoles on their surfaces. It usually does not produce flowers or fruit. The snake cactus is a short shrub (0.5-2 m tall) with almost cylindrical stem segments that have numerous raised humps (0.5-1.5 cm long) and are rope-like in appearance. These stem segments have groups of 4-24 spines on most of the areoles on their surfaces. Its flowers are red to reddish-purple in colour and its mature fruits are yellow. Jumping cholla is a short shrub (0.5-2.5 m tall) with almost cylindrical stem segments that have numerous raised humps (1.5-2.5 cm long) and are rope-like in appearance. These stem segments have groups of 6-12 spines on most of the areoles on their surfaces. Its flowers are reddish-purple to pinkish purple in colour and its mature fruits are greenish in colour (Weeds of Australia, 2016).

Opuntia aurantiaca is an invader plant that can survive and grow in very diverse habitats. It prefers desert shrub and semi-dry savanna grassland vegetation with an annual summer rainfall of more than 300 mm. In Australia and South Africa, jointed cactus is also well adapted to semi-shaded indigenous forests or woodlands, provided the grass cover is not too dense. It flourishes in overgrazed and disturbed habitats and invasion is always exacerbated by an abundance of grazing animals which are the main vehicles of dispersal. It is often found along watercourses as much of the long distance dispersal is via floods. Areas unsuitable for jointed cactus include winter rainfall regions and grasslands with high rainfall. The weed has not yet invaded all environmentally suitable regions in southern Africa and Australia, and extensively managed conservation areas are particularly vulnerable to invasions.

Opuntia aurantiaca is a sterile clone and the propagules responsible for survival are cladodes and sterile fruit that can disperse and survive under extreme conditions until conditions for rooting become favourable. There are four biotypes or growth forms in its native range in South America and it was postulated that the plant could be a recent hybrid between two putative parents sharing similar habitats in Argentina, O. salmiana and O. discolor. Principal component analysis showed that the biotypes were intermediate between the putative parents regarding 88 vegetative characters studied (Venter et al., 1984).

The utilization of Crassulacean Acid Metabolism (CAM) by jointed cactus is of particular significance for its success and high survival rate under extreme semi-arid conditions. The CAM physiological characteristic allows dislodged succulent propagules (cladodes) to continue to photosynthesize even after no more water uptake is possible, ensuring survival during times of prolonged moisture shortage. This also allows detached cladodes to show a very high rooting success whenever conditions become favourable again. A predominantly shallow root system with strong horizontal extensions is a pre-adaptation to exploit light precipitation. Some roots also show vertical extensions to utilize water resources from deeper horizons. This combination is important for extending habitat use. Daylength is the main controlling factor for cladode production. Growth is therefore confined to the summer months whereas carbon (reserve) allocation to the tubers occurs mainly during autumn and winter. Flowering time November – January followed by reddish succulent fruit (World Flora Online, 2024).

Reproduction in jointed cactus is entirely vegetative which is dependent on three aspects related to its life cycle, namely, the production of sufficient propagules, their survival under extreme conditions, and their ability to root and grow under a variety of conditions. The number of propagules released by mature plants in a year can vary between 1000 and 2000. Despite a high mortality of propagules of 10-20% caused by desiccation in mid-summer, many survive long periods because of minimum water loss while continuing internal cycling of carbon dioxide through Crassulacean Acid Metabolism. The significance of this physiological activity under extreme hot and dry conditions is that dislodged cladodes are not only able to continue photosynthesis with very limited gas exchange, allowing them to survive for long periods under harsh conditions, but are also able to root and rehydrate rapidly under moist conditions and to begin carbon dioxide uptake and growth almost immediately. The easily detachable stem segments of O. aurantiaca are spread by attaching to wool or fur on animals, sticking to footwear or clothing, attaching to tyres or machinery, flowing water or by dumping plant material from gardens (NSW WeedWise, 2024).

Six natural insect enemies from South America were studied as suitable biological control agents in South Africa, but only two species of those released eventually established. In Australia, three species have become established and, in both countries, a high level of biological control is achieved. Except for the jointed cactus cochineal, Dactylopius austrinus, which is an external sap sucker and by far the most important biological control agent of this weed, all the others are all internal feeders. The diaspid scale insect Diplacaspis echinocacti [Diaspis echinocacti] is specific to Cactaceae and is frequently found on jointed cactus. It is of unknown origin and is not damaging to its host and thus plays no role in biological control. Although aggressive fungi are present on jointed cactus in Argentina and Uruguay, none has been used for biological control. The cactus moth, Cactoblastis cactorum, is one of the important natural enemies of jointed cactus but it also attacks most other Opuntia, including the commercially cultivated varieties of O. ficus-indica. C. cactorum is now regarded as a pest in these commercial plantations and measures are necessary for its control. For a full account of natural enemies associated with O. aurantiaca, refer to: Mann (1969), Moran and Annecke (1979), Zimmermann et al. (1979) and Julien and Griffiths (1998).

Grazing animals learn to avoid jointed cactus plants because of possible injury, with the result that grazing pressure is drastically increased in remaining non-infested patches. Overall, the grazing capacity of infested natural grasslands is significantly reduced with a significant reduction in carrying capacity and total livestock production. The effects of the spines, i.e. contaminated wool and animal hides with lesions originating from sores caused by injury from joints, also devalue these livestock products and further add to the reduction in returns from livestock-based activities. The sharp, barbed spines can also get stuck around the mouths of calves or lambs and prevent them from feeding. O. aurantiaca is considered to South Africa’s most expensive weed (Lee, 2001).

Erosion is increased because livestock are inclined to move along selected safe routes free of jointed cactus. The non-target effects of the non-selective herbicides have had an even greater environmental effect though, after being used in large quantities for the control of jointed cactus for over half a century in Australia and South Africa, particularly hormone herbicides in oil-based carriers. Jointed cactus prefers to grow under the protection of trees and shrubs and most non-target effects of spot-spraying treatments occur here.

Other than non-target effects of herbicides, there is no evidence that jointed cactus any longer affects biodiversity or soil properties since cochineal and the cactus moth have reduced populations in infested areas to scattered plants or patches which have primarily a nuisance value.

In a study of the impact of O. aurantiaca on biodiversity and ecosystem function in a savannah rangeland in Zimbabwe, a significant negative effect of O. aurantiaca density on the abundance, species richness, species diversity and biomass yield of grass species was observed (Kawanza et al., 2019).

In both South Africa and Australia, property values are drastically reduced when they are infested with jointed cactus. The formidable spines can easily penetrate the skin, causing sores, or can lodge deep into the wool or hair of animals resulting in injury to handlers.

Besides its ornamental value, there are no other uses for O. aurantiaca.

Already during the late 1800s, control measures became necessary to prevent grazing land from being totally invaded on stock farm in both Australia and South Africa. The first attempts at control included hand grubbing and the collection of all plants parts that were then collectively destroyed by burning. Despite these intensive measures, the infestations gradually became worse and reached a state where control was beyond the means of the landowner, which then led to state-subsidized control measures, many involving herbicides.

Herbicides were first used against jointed cactus in South Africa and Australia in the 1920s, in the form of aqueous solutions of arsenic pentoxide. Dense patches and clumps were sprayed in situ, scattered plants and isolated joints were collected and sprayed collectively in heaps. However, despite these drastic measures, the problem worsened. The first hormone-based herbicides became available in the late 1950s and new concerted efforts by local authorities were put in place by both countries to control infestations. In the 1980s, South Africa switched to MSMA as the recommended herbicide, which was considerably cheaper and more effective, while Australia switched to using trichlopyr + picloram based herbicides.

Since the 1990s, when subsidies for herbicides were gradually reduced, their use declined and more and more landowners switched to biological and integrated control as their only economically viable means to control the weed. Also, there was considerable concern raised due to the observed non-target effects of the non-selective herbicides that were used in large quantities for over half a century in Australia and South Africa, particularly the hormone herbicides in oil-based carriers. Jointed cactus prefers to grow under the protection of trees and shrubs and most non-target effects of spot-spraying treatments occurred there.

The first natural enemy that was introduced from Argentina for the biological control of several Opuntia species was the pyralid moth, Cactoblastis cactorum. It was first released in Australia in 1926 for the control of another devastating Opuntia invader, namely, O. stricta. It was also released in South Africa for the control of O. ficus-indica in 1933, and in both cases the insect switched to jointed cactus, causing considerable damage, mainly to large plants. Although feeding damage to jointed cactus was impressive, it was localized and not sufficient to control the weed.

Only a few years later, the cochineal insect Dactylopius austrinus, also originating from Argentina, was released in Australia in 1933 and in South Africa in 1935. In both countries, the impact of this biocontrol agent was spectacular and large dense infestations of jointed cactus collapsed and almost disappeared. The efficacy of the insect is, however, density dependent, and its impact declined as infestations became sparser as the wind-dispersed crawlers failed to locate new isolated plants. Initially, landowners believed that jointed cactus had become resistant to the insect and many reverted back to using herbicides for control. This had the reverse effect, however, as research has shown that this chemical control of those remaining jointed cactus selectively removed those plants that were favoured by the cochineal, with the result that the efficacy of the insect further declined (Zimmermann, 1979). The problem was also exacerbated by varying climatic tolerances of the insect, being more effective in hot and dry regions and less effective in high rainfall and moister parts.

Despite these apparent deficiencies of both biological control agents, many landowners continued to use biological control to manage their infestations successfully, particularly in the drier parts of Queensland, Australia and Karoo, South Africa. In South Africa, D. austrinus is now artificially reared in cages in large quantities and supplied to landowners who need to re-inoculate their jointed cactus where insect populations are low. It is important to maintain a high cochineal presence on jointed cactus at all times, which could involve re-introducing the insect from time to time. See Moran and Zimmermann (1984; 1991) for comprehensive accounts of biological control of O. aurantiaca.

In Australia, Dialectica scalariella or Ceutorhynchus larvatus and Octotoma scabripennis or Uroplata girardi have also been recorded for the control of O. aurantiaca (Auld, 1990). Mimorista pulchellalis moth has been imported from South America into South Africa for the biological control of O. aurantiaca (Nieman, 1991). Although it did initially become established, it is now regarded as not having become established (Klein, 2011). In addition, the moth Tucumaniatapiacola has been released in South Africa against O. aurantiaca (Hoffmann, 1981). Although the moth species T. tapiacola and Nanaia sp. were both released, they failed to become established. Hoffmann (1988) predicted that the new association of Nanaia sp. on O. aurantiaca would probably not succeed and that an extensive mass-rearing and release programme would be a waste of finances, time and effort. In terms of fungi, two isolates of Microdochium lunatum from O. leptocaulis were found to be pathogenic to O. aurantiaca (Mildenhall et al., 1987).

Integrated control methods now exist which allow the full use of biological control in tandem with selected chemical and mechanical control measures where necessary. The lifting of subsidies for herbicides, making chemical control far too expensive for most landowners, forced them to implement alternative control methods aided by additional research. Chemical and mechanical control are used more where biological control is ineffective or where such control is deemed inappropriate.