Opuntia monacantha are shrubs or small trees up to 4 meters tall; joints glossy green when fresh, narrowly obovate to oblong-lanceolate; base somewhat cuneate, 10 to 35 centimeters long, 7.5 to 12.5 centimeters wide, margins undulate toward apex. Areoles with 1 to 3 gray or yellowish to reddish brown spines with darker tips, 1 to 7.5 centimeters, trunk areoles with 10 or more spines. Flowers 7.5 to 10 centimeters long, 5 to 7.5 centimeters in diameter; outer perianth parts yellow with a reddish median stripe, 18 to 25 milimeters long, 12 to 15 milimeters wide, inner perianth parts yellow to orange, rotate, 25 to 40 milimeters long, 12 to 40 milimeters wide; staminal filaments green to white; style green, 12 to 20 milimeters long; stigma lobes 8 to 10, cream yellow. Berries reddish purple, fleshy, conical to obovoid, 5 to 7.5 centimeters long, 4 to 5 centimeters in diameter (Wagner et al. 1999, in PIER 2006).

The accepted name of the common prickly pear is Opuntia monacantha (ITIS 2008), however, the scientific name Opuntia vulgaris is the name most frequently used in the literature reviewed for this profile. In all cases the scientific names used are faithful to the information given in the source.

The fruits of common prickly pear (Opuntia monacantha) may be consumed by humans. A very strong spirit which tastes of whiskey may be distilled from this plant (Grant 1883). It was introduced to Ascension for the purpose of enriching the soil and preventing the evaporation of moisture (Ascension Island Conservation Centre Undated).
Before synthetic dyes were produced O. monacantha plants were cultivated for the purpose of supporting populations of Dactylopius coccus. When crushed the bodies of this Mexican scale insect produce a carmine-coloured dye.

Opuntia monacantha is found in dry sites in its introduced range in the Pacific (SPREP 2000). In Fiji it is often found in agricultural or wasteland areas (Smith 1981, in PIER 2006).

Opuntia monacantha is cultivated on many islands in the Pacific.

The Global Compendium of Weeds (2007) describes Opuntia monacantha as an \"agricultural weed\", \"cultivation escape\", \"environmental weed\" and \"noxious weed\".

Biological: Cochineal (Dactylopius spp.) and Cactoblasts (Cactoblastis spp.) are the two most important biological control agents for prickly pear cacti. The two attack the cactus in a totally different manner. Cochineal species attach to the outside of the plant and sucks the moisture out of the plant. Cactoblasts are black and yellow striped grubs that tunnel into and devour the inside of the plant (North West Weeds 2007). Cactoblastis oviposits by gluing sticks of about 50 to 90 eggs on cactus spines; the gregarious larvae bore into the pads or cladodes, devouring them from the inside (Stiling 2002). Because of its oligophagous feeding habits Cactoblastis has been successful against a whole range of Opuntia species including 11 species of North American origin (Julien and Griffiths 1998, in Stiling 2002).

Following the release of Cactoblastis cactorum in Australia the prickly pear Opuntia monacantha population collapsed (Dodd 1940, in Stiling 2002). The success of Cactoblastis in Australia was followed by introductions to control pest Opuntia in South Africa, St. Helena, Hawaii, New Caledonia, Pakistan, Kenya and Ascension Island. In 1913 Dactylopius ceylonicus was introduced into South Africa and achieved control of Opuntia vulgaris within a few years. In 1928 D. ceylonicus and Dactylopius opuntiae were used as biological control agents in Mauritius against Opuntia vulgaris and O. Tuna. In 1950 the cactus moth C. cactorum was also introduced in Mauritius for the purpose of controlling O. vulgaris. In 1974 C. cactorum was introduced to Ascension in an attempt to control O. vulgaris (Ascension Island Conservation Centre Undated).