EVALUATION OF CACTOBLASTIS-CACTORUM (LEPIDOPTERA, PHYCITIDAE) AS A BIOLOGICAL-CONTROL AGENT OF OPUNTIA-STRICTA (CACTACEAE) IN THE KRUGER-NATIONAL-PARK, SOUTH-AFRICA

Opuntia stricta is an increasing problem in South Africa's premier wil dlife reserve, the Kruger National Park, where the weed continues to d isperse and has formed many dense, impenetrable thickets, some of whic h extend over several hectares. Although herbicidal control measures a re being used to help control the weed, a biological control program w as initiated in 1988 when Cactoblastis cactorum was introduced into th e park. A primary objective in the control of O. stricta is to prevent fruit production by the plants so that long-range dispersal and recru itment of the weed is minimized. Opuntia stricta plants bean to fruit after approximately six years when they reach a size of 28 cladodes. C actoblastis cactorum has become well established in the park and has h ad a striking effect on both the density and average size of the O. st ricta plants in both dense and sparse infestations of the weed. The av erage size of the plants has declined as the damage caused by C. cacto rum larvae has fragmented the large plants; however, simultaneously, t he density of plants has escalated as the fragments have taken root an d produced new plants. The net effect has been that the few large plan ts have been replaced by many smaller plants, but this has not resulte d in a significant decline in fruit production. Unlike the situation i n Australia and elsewhere in the world, the population densities of C. cactorum have not reached the levels required for satisfactory contro l of the weed. While the reasons for this have not yet been determined , ant predation of C. cactorum eggs, along with baboon predation of la rvae, may contribute 60 the lack of adequate control. (C) 1998 Academi c Press.