Opuntia stricta
(erect prickly pear)

Opuntia stricta is a cactus species native to the Americas that has been introduced worldwide as a popular ornamental. This species escaped from cultivation and has become invasive in many countries across Africa and Australia, but also more recently in the Mediterranean basin. Large and serious invasions have been reported in Australia, South Africa, Namibia, Yemen, India, Sri Lanka, Madagascar and lately also Spain and some North African countries. It has also become naturalized in many other regions (primarily in Africa and Asia) where it has not yet been recorded as a pest. In South Africa and Namibia large infestations have been reported,mainly in dry savanna bushlands, while in Australia all states are invaded with widespread populations invading southeastern Queensland and northeastern New South Wales. Successful biological control programmes have, however, severely reduced the spread of this species in many areas where introduced, though there continues to be a risk of further introduction through the nursery trade.

Within its natural distribution, O. stricta can be found from mainland Ecuador to mainland USA , along the Gulf of Mexico in Texas, Alabama, Louisiana and Florida, and the Atlantic coast of Florida and South Carolina (Benson, 1982), although USDA-ARS (2018) report presence as far north as North Carolina and Virginia.  O. stricta has been introduced to many other countries where it quickly naturalized and became invasive.  Now it can be found naturalized and spreading across Africa, Asia, Europe, South America, and Oceania (GRIIS, 2018; USDA-ARS, 2018).  

The variety O. stricta var. dillenii was previously reported as native to many Caribbean islands.  However, O. stricta var. dillenii is now recognized as a separate species from O. stricta, O. dillennii (Labra et al., 2003; Majure et al., 2012; World Flora Online, 2020).  Opuntia dillennii is the species occurring naturally on most islands across the West Indies while O. stricta occurs only in the Bahamas and probably Cuba (Acevedo-Rodríguez and Strong, 2012).

The risk of new introduction of O. stricta is very high.  For this species the most likely pathway for further introductions is through the horticultural and nursery trade. Several species of Opuntia (including O. stricta) are among the most popular ornamental and hedge cacti commercialized worldwide (ISSG, 2018).  Seeds of Opuntia species are available online from mail-order companies, along with many other ornamental cacti.

The natural range of O. stricta spans a large area with varying climates, habitats and soils. This species is a common element of dry, semi-arid and arid regions. It inhabits open woodlands, dry forests, semiarid thickets, rangelands, grasslands, pastures, waterways, roadsides, coastal thickets, disturbed sites and waste areas (Queensland Government, 2018). Although O. stricta is frost-tolerant, it thrives best in hot and humid conditions. This species is also drought tolerant (up to 8 months), and wild and naturalized populations can be found in areas with mean annual rainfall ranging from 300 mm to > 1200 mm.

 In Australia and South Africa this species has invaded millions of hectares and can be found in many soil types, habitats and climates (Mann, 1970; Henderson, 2001). Invasions are always enhanced by disturbances and in particular by overgrazing, and O. stricta has never invaded the well-grassed downs in Australia or high altitude grasslands in the cooler parts of South Africa. Invasions were particularly common in low-lying wooded areas in Australia and savanna bushlands in South Africa (Wells et al., 1986; Malan, 1989).

Reproductive Biology
Opuntia stricta has hermaphroditic flowers that are pollinated by insects (primarily bees). This species is self-compatible but highest levels of seed-set occur when plants are subjected to cross-pollination and open pollination (Bartomeus and Vilà, 2009).

Physiology and Phenology

Seeds of O. stricta are scarified when passing through animals’ digestive systems, germinating within four days. Seeds that are not scarified germinate best after about a year. Seedlings are very delicate and nurse plants are important for their survival, providing nutrients and protection. The optimum conditions for seed germination are 20/30°C in a 12 h /12 h day/night cycle which are typical summer conditions in its range. Most seeds will not germinate during periods when average temperatures are below 20°C (Reinhardt et al., 1999). Polyembryony in O. stricta is common (2-5%). Vegetative reproduction is by means of cladodes that dislocate easily from the mother plant and can take root and develop into new plants whenever conditions are favorable. The CAM (Crassulacean Acid Metabolism) cycle enables separated cladodes to survive for long periods. These two modes of reproduction lead to the typical 'clumps' of plants, contributing to the aggressive behavior of O. stricta in areas where it has been introduced.

Environmental Requirements

Opuntia stricta is well adapted to survive extreme drought and it uses the CAM gas exchange pattern as one method to conserve water. The key to water conservation of CAM plants is their nocturnal stomatal opening as opposed to most plants, which open their stomata during the heat of the day to photosynthesize. Carbon dioxide is fixed into organic acids during the night, which is released again into the chloroplast during the day for photosynthesis (Nobel, 1995; Nobel and Bobich, 2002). Other adaptations to drought include a protective epidermis covered by a thick waxy waterproof cuticle and a shallow root system with surface 'rain roots' enabling the plant to exploit light rains (Sudzuki Hills, 1995).

At least 17 insects and mites have been identified feeding on O. stricta within its native range (Mann, 1969), which is assumed to be a conservative estimate. The diversity of associated natural enemies is highest on mainland USA and is considerably less on Caribbean islands. Several were introduced to Australia for the biological control of the two O. stricta subspecies but only three species have eventually become established; the coreid bug, Chelinidea tabulata, the cerambycid beetle, Moneilema variolare and the cochineal, Dactylopius opuntiae (Julien and Griffiths, 1998). A further introduction of a cactus-feeding insect was made that originated from South America; the pyralid moth Cactoblastis cactorum, originating from other Opuntia species native to Argentina and which readily accepted O. stricta as a new host in Australia and other countries. This new association proved to be devastating to populations of O. stricta in Australia and ended in the most spectacularly successful biological control programme in the history of weed control (Dodd, 1940; Moran and Zimmermann, 1984).

Environmental

• Amenity
• Boundary, barrier or support
• Landscape improvement

Fuels

• Fuelwood

General

• Botanical garden/zoo

Human food and beverage

• Food additive

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• garden plant
• Potted plant

Opuntia stricta is very similar to Opuntia dillenii and intermediates between the two species have been reported. It is also similar to Opuntia ficus-indica, Opuntia streptacantha, Opuntia monacantha and Opuntia tomentosa. These species can be distinguished by the following differences (Queensland Government, 2018):

• Opuntia stricta is a low-growing plant (usually 50-100 cm tall) with relatively large flattened and elongated stem segments. These stem segments are hairless and generally have few spines (sometimes one or two large spines are present) on the small raised bumps (i.e. areoles) on their surfaces. The flowers are bright yellow and the fruit reddish-purple.

• Opuntia dillenii is a low-growing plant (usually 50-100 cm tall) with relatively large flattened and elongated stem segments. These stem segments are hairless and have groups of 1-7 large spines on most of the small raised bumps (i.e. areoles) on their surfaces. The flowers are bright yellow and the fruit reddish-purple.

• Opuntia ficus-indica is a relatively tall shrubby or tree-like plant (usually 1.5-3 m tall) with very large flattened and elongated stem segments. These stem segments are hairless and do not have any spines on the small raised bumps (i.e. areoles) on their surfaces. It has yellow flowers and reddish coloured fruit.

• Opuntia streptacantha is a relatively tall and sometimes tree-like plant (usually 2-4 m tall) with flattened and egg-shaped (i.e. obovate) to almost circular (i.e. orbicular) stem segments. These stem segments are hairless and have groups of 3-20 small white spines on most of the small raised bumps (i.e. areoles) on their surfaces. It has yellow flowers and dull red or yellowish coloured fruit.

• Opuntia monacantha is a relatively tall shrub or tree-like plant (usually 2-5 m tall) with flattened and elongated stem segments. These stem segments are hairless and have one or two large spines on most of the small raised bumps (i.e. areoles) on their surfaces. It has yellow flowers and reddish-purple fruit, and some of its stem segments droop towards the ground during fruiting.

• Opuntia tomentosa is a tall plant that is tree-like in appearance (up to 6 m tall) with flattened and elongated stem segments. These stem segments are velvety hairy (i.e. finely pubescent) and do not have any spines on the small raised bumps (i.e. areoles) on their surfaces. It has orange flowers and reddish-purple fruit. 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Mechanical control

Mechanical control of O. stricta infestations includes methods such as digging, burning, crushing and dragging out the plants with animals or machines. However, some of these methods have been tried in Australia with little success. As the smallest part of a cladode is able to root and grow when in contact with the soil, mechanical disturbances that cause the plants to break and collapse often result in considerable vegetative regrowth and even an increase in invasion.

Chemical control

Chemical control of O. stricta began in 1916 in Australia when one of the first herbicides, arsenic pentoxide, was released for control (Mann, 1970). However, the infestations were already too far advanced for chemical control to make any impact. It was also too expensive and was only applied in scattered and isolated infestations. Today, more advanced and less toxic hormone herbicides are registered for the control of various invasive Opuntia species and include picloram, trichlopyr and combinations of these mostly as water-based formulations (Pritchard, 1993). In South Africa, the inorganic arsenic-based herbicide, MSMA is used for control; small plants receive a cover spray whereas larger plants are stem-injected (Grobler et al., 2000) and MSMA is considerably less expensive than hormone-based herbicides. In South Africa and Australia, these herbicides were made available to landowners at subsidized prices. By law, landowners had to apply control measures and properties are still regularly inspected to evaluate the efficacy of control operations. Due to the high costs of chemical control, most landowners in both countries are reverting to biological or integrated control (Hoffmann et al., 1998) although chemical control is still used to kill isolated plants or small new infestations. In Portugal, stem injections of glyphosate in the summer were found to be the easiest and most effective way of controlling small invasions of O. stricta (Monteiro et al., 2005).

Biological control

The release of the cactus moth, Cactoblastis cactorum, in 1926 to biologically control O. stricta that had invaded about 24 million ha in Queensland and New South Wales, Australia resulted in spectacular control of the weed during the following nine years. Dodd (1940) wrote “…the most optimistic scientific opinion could not have foreseen the extent and completeness of the destruction. The spectacle of mile after mile of heavy (prickly) pear growth collapsing en masse and disappearing in the short space of a few years did not appear to fall within the bounds of possibility”. Today remnant infestations of O. stricta are limited to regions where C. cactorum is less effective (Hosking et al., 1994).

The cochineal, Dactylopious opuntiae, was first released in Australia in 1921 and provided excellent control of dense stands, but by 1928 there was a rapid decrease in cochineal populations following the major destruction of the weed by C. cactorum. Cochineal is still widely used as an effective biocontrol agent in areas where C. cactorum is less effective such as high-lying areas of New South Wales, Australia. The insects have limited dispersal abilities in low host-plant density situations and manual dispersal of infested cladodes to uninfested plants is necessary to ensure that maximum benefits are derived from this insect (Mann, 1970; Hosking et al., 1994). Fifty years later the biological control of prickly pears in Australia has continued to be satisfactory. Recent infestations have been of relatively minor significance except for a few localities along the coast (White, 1980). In these areas, O. stricta showed a high rate of seedling germination and resistance against the cactus moth, presumably caused by water and nutrient stress, where it grows near the ecological limits of prickly pear growth.

In South Africa, serious infestations of O. stricta only became apparent in the 1970s with large infestations reported from the Kruger National Park. After the release of C. cactorum to these remote infestations the insect became well established and had a striking effect on both the density and average size of the cactus plants in both dense and sparse infestations. However, C. cactorum caused fragmentation of mainly large plants resulting in small fragments taking root and producing many new plants, resulting in a higher density of small plants. Overall, C. cactorum has not reached levels required for satisfactory control and excessive ant predation of eggs along with baboon predation of larvae may contribute to the lack of adequate control (Hoffmann et al., 1998).

Attempts to establish the cochineal, D. opuntiae, which was so effective on O. ficus-indica, failed. In 1997, a new biotype was introduced from Australia, which showed a strong preference for O. stricta and closely related species (Githure et al., 1999; Volchansky et al., 1999). After its release in the large infestations in the Kruger National Park, their increase was dramatic, and large infestations have totally succumbed to insect attack. Control is now aimed at manually spreading the insect to all clumps and large plants as a substitute for chemical control (Hoffmann et al., 1999).

Of great concern are the new invasions reported from several developing countries which have limited resources to cope with such invasions and where the concept of biological weed control is new and viewed with scepticism. The introduction of C. cactorum will certainly never become an option in these countries because of the threat to other commercially cultivated Opuntia species, mainly O. ficus-indica. Guarantees about the sustainability of the host-specificity of the 'stricta' cochineal biotype of D. opuntiae, and that it would not feed on O. ficus-indica, are quite convincing provided that cross-breeding with other biotypes is excluded (Hoffmann et al., 2002). However, such guarantees may not convince those unaccustomed to biological weed control, to allow the introduction of host-specific cochineal biotypes for biological control. Few options for control then remain other than mechanical and chemical controls that have been shown to be impractical and uneconomical.

Integrated control

Integrated control of O. stricta comprises taking full advantage of biological and chemical control. Large plants and clumps are infested with the "stricta" biotype of D. opuntiae and efforts made to ensure that C. cactorum is always present. Chemical control can then be limited to the periphery of the infestations to prevent further spread of the weed to uninfested areas (Reinhardt et al., 1999).

27/11/2007 Updated by:

Nick Pasiecznik, Consultant, France

18/04/2018 Updated by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Washington DC, US