Long-term population studies and the development of an integrated management programme for control of Opuntia stricta in Kruger National Park, South Africa

1. A cactus, Opuntia stricta, has invaded almost 16 000 ha of conserved, natural habitat and has become a major weed problem in Kruger National Park (KNP), South Africa. 2. The main objectives in the control of O. stricta are to reduce the density of the weed and to curb long-range dispersal of seeds by preventing young plants from reaching the size (28 cladodes) at which they start to produce fruits. 3. Herbicides have failed to provide satisfactory control of O. stricta because the weed infestations are replenished from seeds in the soil and from small plants that are overlooked during spraying. 4. A phycitid moth, Cactoblastis cactorum, was released in KNP during 1988 in an attempt to control O. stricta biologically. 5. Population counts of the biological control agent and of the weed over a 5-year period showed that, even though C. cactorum has not provided complete control of O. stricta in KNP, the moderate levels of larval damage have stunted the growth of O. stricta and have considerably extended the time that the young plants take to reach sexual maturity. 6. Comparisons of modelled (i.e. with no C. cactorum) and actual populations of O. stricta showed that C. cactorum is making a substantial contribution to the control of O. stricta in residual infestations of the weed that have been treated with herbicides. 7. The need for long-term evaluation studies in biological weed control is demonstrated by the development of an integrated management programme for effective control of O. stricta.