The negative impacts of invasive alien plants on the biodiversity, water supplies, and economy of South Africa are severe. Thousands of people are employed to clear the landscape of these invasive alien species, particularly trees, from conservation and riparian areas. The long‐term suppression of the most virulent invasive plants in South Africa will never be possible without the intervention of biological control. Furthermore, the use of specially selected and carefully tested plant‐feeding insects, mites, and pathogens as biological control agents is very safe, albeit never completely risk free. We present examples and data from South Africa to support these assertions and to show that biological control is often extremely successful and highly cost effective.

Abstract. The cactus moth, Cactoblastis cactorum (Berg) (Phycitidae) is native to South America. It was released as a biological control agent against alien Opuntia‐cacti in Australia in the 1920s, then in southern Africa, and latterly on several islands, including those in the Caribbean. In 1989, the cactus moth was discovered in Florida, in the United States of America, where it is now threatening the survival of indigenous Opuntia species. In this paper we identify some of the attributes that have contributed to the success of C. cactorum as a weed biological control agent. Many of these same qualities account for the problems that C. cactorum has caused in Florida and predispose it as a major threat to the speciose, native Opuntia‐floras of Central and North America. An estimated 79 platyopuntia (prickly pear) species are at risk: 51 species endemic to Mexico; nine species endemic to the United States; and 19 species common to both countries. Many cultivated and wild Opuntia species, that are used in various ways, are also vulnerable to attack by C. cactorum, including at least 25 species in Mexico and three species in the United States, particularly the widely exploited and culturally important cultivars of O. ficus‐indica. Some control strategies are suggested that may minimize the risk and consequences of invasion by the cactus moth. The wider implications of this threat to the practice of weed biological control and to conservation are discussed.

No abstract

1. Until recently, neither the phycitid moth Cactoblastis cactorum nor the cochineal insect Dactylopius opuntiae have been satisfactory biological control agents of Opuntia stricta in South Africa. 2. In marked contrast, both of these agents have kept O. stricta under biological control for many decades in Australia. 3. In an attempt to improve the situation in South Africa, a stock of D. opuntiae was obtained from O. stricta in Australia during 1996. 4. Host‐specificity tests confirmed that the newly imported D. opuntiae from Australia is a different biotype to the one already established in South Africa. 5. The Australian (‘stricta’) biotype thrives on O. stricta but is unable to develop satisfactorily on O. ficus‐indica, while the converse is true for the South African (‘ficus’) biotype, which thrives on O. ficus‐indica but fares poorly on O. stricta. 6. The integrity of the host‐plant specificity of the two biotypes of D. opuntiae has important implications for biological control of Cactaceae in South Africa, and has greatly enhanced prospects that O. stricta can be brought under biological control successfully.

No abstract