Evaluation of Cactoblastis cactorum as a Biological Control Agent of Opuntia stricta in the Kruger National Park, South Africa

Opuntia stricta is an increasing problem in South Africa's premier wildlife reserve, the Kruger National Park, where the weed continues to disperse and has formed many dense, impenetrable thickets, some of which extend over several hectares. Although herbicidal control measures are being used to help control the weed, a biological control program was initiated in 1988 when Cactoblastis cactorum was introduced into the park. A primary objective in the control of O. stricta is to prevent fruit production by the plants so that long-range dispersal and recruitment of the weed is minimized. Opuntia stricta plants begin to fruit after approximately six years when they reach a size of 28 cladodes. Cactoblastis cactorum has become well established in the park and has had a striking effect on both the density and average size of the O. stricta plants in both dense and sparse infestations of the weed. The average size of the plants has declined as the damage caused by C. cactorum larvae has fragmented the large plants; however, simultaneously, the density of plants has escalated as the fragments have taken root and produced new plants. The net effect has been that the few large plants have been replaced by many smaller plants, but this has not resulted in a significant decline in fruit production. Unlike the situation in Australia and elsewhere in the world, the population densities of C. cactorum have not reached the levels required for satisfactory control of the weed. While the reasons for this have not yet been determined, ant predation of C. cactorum eggs, along with baboon predation of larvae, may contribute to the lack of adequate control.