Opuntia stricta (erect prickly pear)

Pictures

Picture	Title	Caption	Copyright
Title Habit Caption Opuntia stricta (erect prickly pear); cladode, with yellow flowers and reddish-purple fruits. Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Habit	Opuntia stricta (erect prickly pear); cladode, with yellow flowers and reddish-purple fruits.	©Plant Protection Research Institute, Pretoria, South Africa
Title Habit Caption Opuntia stricta (erect prickly pear); habit, showing a dense infestation in the Kruger National Park, South Africa. Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Habit	Opuntia stricta (erect prickly pear); habit, showing a dense infestation in the Kruger National Park, South Africa.	©Plant Protection Research Institute, Pretoria, South Africa
Title Successful biocontrol Caption Opuntia stricta (erect prickly pear); the same infestation three years later, destroyed by the natural enemy Dactylopius opuntiae ('stricta' biotype) and Cactoblastis cactorum (cactus moth). Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Successful biocontrol	Opuntia stricta (erect prickly pear); the same infestation three years later, destroyed by the natural enemy Dactylopius opuntiae ('stricta' biotype) and Cactoblastis cactorum (cactus moth).	©Plant Protection Research Institute, Pretoria, South Africa
Title Biocontrol agent Caption Opuntia stricta (erect prickly pear); the biocontol agent Dactylopius opuntiae ('stricta' biotype) infesting O. stricta under controlled conditions. Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Biocontrol agent	Opuntia stricta (erect prickly pear); the biocontol agent Dactylopius opuntiae ('stricta' biotype) infesting O. stricta under controlled conditions.	©Plant Protection Research Institute, Pretoria, South Africa
Title Biocontrol agent Caption Opuntia stricta (erect prickly pear); the biocontrol agent, Dactylopius opuntiae ('stricta' biotype) infesting O. stricta, under controlled conditions. Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Biocontrol agent	Opuntia stricta (erect prickly pear); the biocontrol agent, Dactylopius opuntiae ('stricta' biotype) infesting O. stricta, under controlled conditions.	©Plant Protection Research Institute, Pretoria, South Africa
Title Natural enemy Caption Opuntia stricta (erect prickly pear); the natural enemy, Cactoblastis cactorum (cactus moth). Larvae. Copyright ©Plant Protection Research Institute, Pretoria, South Africa	Natural enemy	Opuntia stricta (erect prickly pear); the natural enemy, Cactoblastis cactorum (cactus moth). Larvae.	©Plant Protection Research Institute, Pretoria, South Africa

Identity

Preferred Scientific Name

• Opuntia stricta (Haw.) Haw.

Preferred Common Name

• erect prickly pear

Other Scientific Names

• Cactus strictus Haw.
• Opuntia bahamana Britton & Rose
• Opuntia bentonii Griffiths
• Opuntia dillenii Haw.
• Opuntia inermis DC
• Opuntia keyensis Britton ex Small
• Opuntia macrarthra Gibbes
• Opuntia magnifica Small

International Common Names

• English: Australian pest pear; coastal prickly pear; common pest pear
• Spanish: yaaxpakan

Local Common Names

• Cuba: tuna mansa
• South Africa: suurturksvy

EPPO code

• OPUST (Opuntia stricta)

Summary of Invasiveness

O. stricta, native from Ecuador to the USA, was introduced as an ornamental and has spread widely, mainly in southern Africa and Australia, but also more recently in the Mediterranean basin and in disturbed areas in its native Caribbean. 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.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Caryophyllales
•                         Family: Cactaceae
•                             Genus: Opuntia
•                                 Species: Opuntia stricta

Notes on Taxonomy and Nomenclature

Opuntia stricta Haworth is now generally accepted as a single species comprising two varieties (or sub-species), O. stricta var. stricta and O. stricta var. dillenii (Benson, 1982). Previously, it was divided into three independent species; O. stricta. O. inermis and O. dillenii (Britton and Rose, 1919). The variations between the varieties are considerable and many intermediate forms exist. In Florida, USA, crossing between the two varieties is common and few populations can be described as purely one or the other. Hybridizations within its native range in North America and the Caribbean add to the confusion. The varieties can best be distinguished by their spines, the type variety O. stricta var. stricta having smaller cladodes which are more sparsely covered with isolated spines, whereas O. stricta var. dillenii has large cladodes densely covered by formidable clusters of stout, yellow spines. However, the taxonomy with this genus remains confused, and although two new subgeneric taxa have been described from Mexico, ssp. esparzae (Scheinvar, 2002) and var. reitzii (Scheinvar) Scheinvar et A. Rodr. (Scheinvar and Rodriguez-Fuentes, 2000), those have yet to be confirmed.

Description

Plants are sprawling or erect, much-branched succulent shrubs reaching a height of 2 m. The cladodes (stems) are green to bluish-green, flattened, and about 10-25 cm long and usually 7.5-15 cm broad. From the areoles develop the stout, slightly curved yellowish spines, varying in numbers from entirely absent to groups of one or two or more, normally clusters. Clochids (spine clusters) are yellow and relatively few, up to 5 mm long in the spinier var. dillenii. The flowers are bright yellow and typically cactus-like, appearing during the summer months (Benson, 1982). The species is best identified by its typical pear-shaped to spherical fruit, purple-coloured at maturity, 4-6 cm long and 2.5-3 cm in diameter. Its outer surface is smooth and spineless except for a few glochids imbedded in the small areoles. The pulp is intense purple in colour and sour tasting and contains about 60 hard-coated seeds. Older plants develop woody stems which provide support to the larger plants.

Plant Type

Distribution

Within the natural distribution, O. stricta is found from mainland Ecuador to mainland USA, throughout most of the Caribbean and along the Gulf (Mexico, and Texas, Alabama, Louisiana, Florida, USA) and the Atlantic coast of Florida and South Carolina (Benson, 1982), although USDA-ARS (2007) add presence as far north as North Carolina and Virginia. It is probably present and native to more countries in the Caribbean and Central America than indicated in the distribution list. The more spiny variety (O. stricta var. dillenii) is native only to drier Caribbean Islands, though USDA-ARS (2007) also record it as present on mainland USA (Louisiana and Georgia).

O. stricta has been introduced to many other countries where it quickly naturalized and became invasive. Countries that have reported serious invasions include Australia, South Africa, Namibia (Henderson, 2001), Yemen (Ellenberg, 1982), India, Madagascar (Middleton, 1999) and lately also Spain and some North African countries (Le Houérou, 2002). It has probably naturalized also in many other countries where it has not yet been recorded as a pest and these are likely to include more countries in Africa (e.g. Eritrea, Ethiopia, Somalia and Zimbabwe) and Asia (e.g. Pakistan, Thailand). In its native range in some Caribbean countries O. stricta has also become invasive because of deforestation, erosion and overgrazing. In South Africa and Namibia large infestations have been reported from mainly dry savanna bushveld (Henderson, 2001), while in Australia all states are invaded (Parsons and Cuthbertson, 1992) with widespread invasion found throughout south-eastern Queensland and north-eastern New South Wales (Mann, 1970).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

The showy yellow flowers and the attractive purple-reddish fruit makes this cactus a favoured pot and hedge plant and it is by this means that O. stricta probably arrived in Sydney, New South Wales, Australia prior to 1839 and Rockhampton, Queensland about 1870 (Mann, 1970). It is not known when and how the species arrived in South Africa. No records of invasions of O. stricta are reported in mainland USA and it can be assumed that the native natural enemies are effective in preventing excessive population increases. However, in several Caribbean islands where deforestation and overgrazing is common, O. stricta var. dillenii has become invasive together with other similar species such as O. triacantha. O. stricta is also record as introduced, cultivated and invasive in the Galapagos Islands (PIER, 2007).

Related to the spread of O. stricta is the introduction, spread and invasiveness of a biological control agent released to control it. In order to deal with invasions of native species in the Caribbean, a biological control agent Cactoblastis cactorum was released on Nevis, St Kitts, Cayman Islands, Antigua and Montserrat in 1957 (Simmonds and Bennett, 1966), providing satisfactory levels of control of problematic Opuntia species which could not have been achieved with the depauperate native cactus-feeding insects. Unfortunately C. cactorum has spread naturally and by human interventions to many other islands and it eventually reached Florida where it dispersed rapidly west along the coast of the Gulf of Mexico. It attacks all six native Opuntia species in Florida, including O. stricta (Johnson and Stiling, 1998), and the potential devastating damage to commercial Opuntia plantations in Mexico was highlighted (e.g. Zimmermann et al., 2001). However, a study of the effect of C. cactorum on native Opuntia in St Kitts-Nevis 50 years after its introduction found that residual populations remained and concluded that that the potential impact of C. cactorum on native North American and Mexican Opuntia will be significant and variable, but not necessarily catastrophic (Pemberton and Liu, 2007).

Risk of Introduction

The most likely pathway for further introductions of O. stricta and other similar species is through the nursery trade in succulents. Seeds of Opuntia species are available from mail-order companies and via the internet, together with many other ornamental cacti.

Habitat

The natural range of the two recognized varieties of O. stricta spans a large area with varying climates, habitats and soils. Within the centre of its natural distribution, O. stricta var. stricta is confined mainly along the Florida coast on sandy soils (Benson, 1982) whereas in Australia and South Africa the same 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 neither variety has ever invaded the well-grassed downs in Australia and high altitude grasslands in the cooler parts of South Africa. Invasions were particularly common in low-lying wooded areas in Australia and savanna bushveld in South Africa (Wells et al., 1986; Malan, 1989). It would appear as if nursery plants are important for initial establishment of the delicate seedlings. Although O. stricta is frost-tolerant, it thrives best in hot and humid conditions. The species is also drought tolerant, and invading populations can be found within annual rainfall regimes of 300-1200 mm which may include an 8-month dry spell.

Habitat List

Biology and Ecology

Physiology and Phenology

O. stricta is well adapted to survive extreme drought and it uses the CAM (Crassulacean Acid Metabolism) 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).

Reproductive Biology

Reproduction in O. stricta is both sexual and vegetative. Fruits are eaten by a range of animals and seeds are scarified when passing through their 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. 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 favourable. The CAM cycle enables separated cladodes to survive for long periods. These two modes of reproduction contribute to the typical 'clumps' of plants, and all these adaptations contribute to the aggressive behaviour of O. stricta in countries of introduction, exacerbated by an almost total lack of predators.

Associations

Like many other cactus species, O. stricta also plays an important role in the natural ecosystems within its native range. It is part of the local food chain involving insects, birds and mammals that have all co-evolved with this species and that feed on its fruits and cladodes and that use it for shelter and nesting sites. Their shallow roots stabilize the soil and they are often temporary exploiters of disturbances and by doing so, become part of natural rehabilitation processes (Mellink and Riojas-López, 2002).

Climate

Air Temperature

Rainfall

Rainfall Regime

Soil Tolerances

Soil drainage

• free

Soil reaction

• acid
• alkaline
• neutral

Soil texture

• heavy
• light
• medium

Special soil tolerances

• shallow

Natural enemies

Notes on Natural Enemies

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).

Means of Movement and Dispersal

Natural Dispersal (Non-Biotic)

Dislodged cladodes usually remain near the mother plants where they take root and develop into new plants resulting in clumped infestations. Terminal cladodes dislocate easily from the mother plants and can root and develop into new plants wherever they land on soil. This explains why plants of varying sizes are found together in clumps of several metres in diameter. Seeds are also carried long distances by water after heavy rains.

Vector Transmission (Biotic)

Long-distance dispersal occurs via seeds that are carried by animals which feed on the sweet fruit (Lotter et al., 1999). Seeds are scarified when passing through animals which results in high germination rates (Reinhardt et al., 1999). In South Africa, baboons and elephants feed extensively on the ripe fruit and have contributed to the rapid dispersal of the plant in the Kruger National Park, infesting 16 000 ha in 44 years (Hoffmann et al., 1998).

Intentional Introduction

Initially, O. stricta plants were established as garden ornamentals from where they started to disperse into the environment. The 35,000 ha infested with O. stricta in the Kruger National Park, South Africa originated from one or two garden plants in the Skukuza staff village. The ability of O. stricta to reproduce vegetatively and through seeds explains its ability to spread and form dense stands so effectively wherever it establishes.

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Economic Impact

Browsing animals can sustain injuries to their mouth and gut by ingesting the fruit that are covered in small spines. Animals tend to avoid infested areas and so the land becomes useless for livestock farming and this increases grazing pressure on uninfested areas. A survey of farmers in southern Madagascar considered O. stricta to be an economic problem due to loss of livestock (Larsson, 2004). Access to infested areas is restricted for other productive uses. In 1940, Dodd reported that about 24 million hectares was infested with O. stricta in Australia of which half was so densely infested it was unproductive.

Environmental Impact

There are few assessments of the impact of O. stricta infestation on native biodiversity. A study in southern Madagascar where the this species was introduced in the late 1950s, showed a gradient of increasing plant diversity with decreasing O. stricta density as the survey moved further away from the introduction site (Brolin, 2004). Robertson (2011) investigated the local impact of O. stricta infestation on assemblages of beetle and spider species in the Kruger National Park, South Africa and recorded a negative impact on beetle diversity and abundance. 

Social Impact

O. stricta can form dense stands restricting human access to areas and causing injury from the plant's spines. In protected areas, infestations also have a negative effect on aesthetics and recreation to the degree that surveyed tourists at the Pilanesburg National Park, South Africa, said they were willing to pay an increased entrance fee (US $7 per day) for a (hypothetical) programme to control O. stricta (Nikodinoska et al., 2014).

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Highly mobile locally
• Long lived
• Fast growing
• Has high reproductive potential
• Has propagules that can remain viable for more than one year
• Reproduces asexually
• Has high genetic variability

• Damaged ecosystem services
• Ecosystem change/ habitat alteration
• Modification of successional patterns
• Monoculture formation
• Negatively impacts agriculture
• Negatively impacts livelihoods
• Negatively impacts tourism
• Reduced amenity values
• Reduced native biodiversity
• Threat to/ loss of native species
• Transportation disruption

• Competition - monopolizing resources
• Interaction with other invasive species
• Rapid growth
• Produces spines, thorns or burrs

• Highly likely to be transported internationally deliberately
• Difficult/costly to control

Uses

O. stricta makes a very attractive pot plant or when planted out in gardens in dry areas as it requires minimal watering to survive, and as such its main use has been as an ornamental. Unlike O. ficus-indica and other Opuntia species, the cladodes of O. stricta are not palatable and there are no records of any of the many browsers in the Kruger National Park, South Africa feeding on this species. Its cladodes are not reported to be eaten by any mammal even during the most severe droughts. Being unpalatable to livestock, having spines and a dense, low-growing form means that O. stricta has also been used as a hedge plant. The only commercial use recorded so far comes from extraction of betacyanine for the production of fruit juice (Merin et al., 1987). This red colorant is used in certain dairy products such as yogurts and also in the production of jams and juices.

Uses List

Fuels

• Fuelwood

Similarities to Other Species/Conditions

The variation within the broad taxon O. stricta has contributed to considerable confusion amongst botanists. At least 14 scientific names have been mentioned as pertaining to either O. stricta or O. dillenii (Mann, 1970; Anderson, 2001). The credibility of the authorship largely dictates the names likely to be accepted. Overall, two main taxa are mentioned either as one species with two subspecies or as two independent species. The general tendency is to follow the classification of Benson (1982) who recognized O. stricta var. stricta and O. stricta var. dillenii. Australian botanists spent much time in identifying the origin of their two 'pest pear' entities that have invaded 24 million ha of agricultural land in Queensland and New South Wales. They identified them as O. inermis (O. bentonii) and O. stricta and were adamant that they were dealing with two separate species that would not interbreed (Mann, 1970). Both these are now recognized as variations within one species, namely, O. stricta (Hosking et al., 1993). Others (e.g. Howard and Touw, 1982; Anderson, 2001) insist that O. stricta and O. dillenii are separate species. Studies on DNA sequencing of the entire complex are expected to clarify this confusing taxonomy.

Prevention and Control

Control

Mechanical control

Mechanical control of O. stricta infestations are futile and they have never figured prominently in the long history of control against invasions. Digging, burning, crushing and dragging out the plants with animals or machines were some of the methods tried in Australia but 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., 1993).

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., 1993). 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 var. 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 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 control that have been shown to be impractical and uneconomic.

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 (Rheinhardt et al., 1999).

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