LONG-TERM POPULATION STUDIES AND THE DEVELOPMENT OF AN INTEGRATED MANAGEMENT PROGRAM FOR CONTROL OF OPUNTIA-STRICTA IN KRUGER-NATIONAL-PARK, SOUTH-AFRICA

1. A cactus, Opuntia stricta, has invaded almost 16 000 ha of conserve d, natural habitat and has become a major weed problem in Kruger Natio nal 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-ran ge dispersal of seeds by preventing young plants from reaching the siz e (28 cladodes) at which they start to produce fruits.3. Herbicides ha ve failed to provide satisfactory control of O. stricta because the we ed infestations are replenished from seeds in the soil and from small plants that are overlooked during spraying. 4. A phycitid moth, Cactob lastis cactorum, was released in KNP during 1988 in an attempt to cont rol O. stricta biologically. 5. Population counts of the biological co ntrol agent and of the weed over a 5-year period showed that, even tho ugh C. cactorum has not provided complete control of O. stricta in KNP , the moderate levels of larval damage have stunted the growth of O. s tricta and have considerably extended the time that the young plants t ake to reach sexual maturity. 6. Comparisons of modelled (i.e. with no C. cactorum) and actual populations of O. stricta showed that C. cact orum is making a substantial contribution to the control of O. stricta in residual infestations of the weed that have been treated with herb icides. 7. The need for long-term evaluation studies in biological wee d control is demonstrated by the development of an integrated manageme nt programme for effective control of O. stricta.