Introduction

Approximately 1600 species are recognized in the family Cactaceae and, with one possible exception, all are native to the New World (Gibson and Nobel, 1986; Wallace and Gibson, 2002). The exception is an epiphyte, Rhipsalis baccifera (previously cassutha) (J. S. Miller) Stern, which appears to be indigenous in Africa and Madagascar (Wallace and Gibson, 2002).

Most species of the Cactaceae have leafless photosynthetic stems that bear spines on modified axillary buds called areoles. The cacti are predominantly succulent and adapted to survive in extreme xeric habitats. Because of their often bizarre structures and appearances, many species are now cultivated widely around the world mostly as curiosities and ornamentals. Some species of cacti are used as a source of fruit and fodder, and as hedge plants (Casas and Barbera, 2002). The most common and commercially important species in the Cactaceae is Opuntia ficus-indica (Linnaeus) Miller, usually known as either “prickly pear” or “cactus pear”. This species is cultivated in many countries for its fruit and as fodder (Barbera et al., 1995) but also as the main host for production of the carmine cochineal insect, Dactylopius coccus Costa (Homoptera: Dactylopiidae) (Casas and Barbera, 2002), and supports a flourishing dye industry in Peru, Chile, Bolivia, and the Canary Islands (Flores-Flores and Tekelenburg, 1995).

Several species of Cactaceae, introduced either deliberately or accidentally into countries outside the Americas, have become invasive.