Abstract
Cactoblastis cactorum, a phycitid moth, has been used with considerable success as a biological control agent against several different cactus species in many parts of the world, including Opuntia stricta in Kruger National Park [KNP], South Africa. Although the moth has become widespread and abundant in KNP, its populations have not reached levels where it is able to provide the desired level of control on the alien species which has invaded more than 35 000 ha of the park and continues to spread. Although several factors, especially acquired predators and parasitoids, are known to curb the abundance of C. cactorum in South Africa, the extent to which this applies in KNP is unknown. Logistic regression and Canonical Community Analysis were used to determine the biotic and abiotic factors affecting the distribution and abundance of C. cactorum in KNP. To do this, maps of the ranges of the insect and host plant were constructed and integrated with other environmental features. The logistic regression showed that O. stricta density had no influence on whether or not C. cactorum was present in an area. The Canonical Community Analysis suggested that a high fire frequency, the herbicide control programme and the presence of small host plants may be important in limiting the distribution of C. cactorum, either directly or indirectly, through impacts on associated predators or parasitoids.
Highlights
Over the past 50 years Opuntia stricta (Haworth) Haworth has spread into an area of approximately 35 000 ha within Kruger National Park [KNP] in South Africa (Foxcroft et al, 2004), forming impenetrable thickets which smother indigenous vegetation and restrict movement of animals (Lotter and Hoffmann, 1998)
This paper reports on a study that was undertaken in KNP to determine how, and to what extent, the distribution and density of C. cactorum has been determined by properties of the host plant and the surrounding environment
The density class categories were used to measure the impact of O. stricta density on the presence or absence of C. cactorum
Summary
Over the past 50 years Opuntia stricta (Haworth) Haworth has spread into an area of approximately 35 000 ha within Kruger National Park [KNP] in South Africa (Foxcroft et al, 2004), forming impenetrable thickets which smother indigenous vegetation and restrict movement of animals (Lotter and Hoffmann, 1998). The disassociation of translocated plants from their specialist natural enemies, which continually damage native species in their natural habitats, is frequently invoked to explain why alien plants become invasive (Keane and Crawley, 2002). This seems to have been the case for various introduced cacti that have become problematic in many countries around the world. O. stricta, which originates in the south-eastern parts of North America is among the most troublesome of these translocated cactus species (Dodd, 1940)
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