Evaluation ofCactoblastis cactorum(Lepidoptera: Phycitidae) as a Biological Control Agent ofOpuntia stricta(Cactaceae) in the Kruger National Park, South Africa

Opuntia strictais an increasing problem in South Africaʹs 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 whenCactoblastis cactorumwas introduced into the park. A primary objective in the control ofO. strictais to prevent fruit production by the plants so that long-range dispersal and recruitment of the weed is minimized.Opuntia strictaplants begin to fruit after approximately six years when they reach a size of 28 cladodes.Cactoblastis cactorumhas become well established in the park and has had a striking effect on both the density and average size of theO. strictaplants in both dense and sparse infestations of the weed. The average size of the plants has declined as the damage caused byC. cactorumlarvae 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 The net effect has been that the few large plants have been replaced by many smaller plants, but this has not resulted in a significant decline in fruit production.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 ofC. cactorumhave not reached the levels required for satisfactory control of the weed. While the reasons for this have not yet been determined, ant predation ofC. cactorumeggs, along with baboon predation of larvae, may contribute to the lack of adequate control.