Biocontrol of a prickly pear cactus in South Africa: Reinterpreting the analogous, renowned case in Australia

We report on a long-term evaluation of biological control of an invasive cactus,Opuntia stricta, in the Kruger National Park, South Africa. By forming large impenetrable thickets, this weed species posed a major threat to the integrity and biodiversity of the park, and to agroecosystems more widely. Over 22 years, from 1992 to 2013, counts were made along fixed transects at four different sites to measure the abundance ofO. strictaand the prevalence of two of its biological control agents: (a)Cactoblastis cactorum, whose larvae feed in the plants' cladodes and (b) a sap-sucking cochineal insect,Dactylopius opuntiae. With onlyC. cactorumpresent, the numbers ofO. strictacladodes and fruit remained unchanged at two of the sites but increased annually at the other two. Within 5 years of the introduction ofD. opuntiae, the numbers of cladodes and fruit decreased substantially at all the sites and the residual cactus populations have been held at inconsequentially low levels ever since. Both theC. cactorumandD. opuntiaepopulations onO. strictain South Africa were sourced from founder stocks in Australia. This allows direct comparisons of biological control ofO. strictain South Africa with the world-famous program againstO. stricta, in Queensland and in New South Wales, that peaked in the 1920s and 1930s. Synthesis and applications. Almost all accounts acclaimCactoblastis cactorumas the dominant contributor to the sustained decline of populations of prickly pears in Australia in the 1930s. Our results provide evidence that this now widely accepted conclusion is incorrect and that cochineal was and is the key role player. Managers and biological control practitioners concerned with the apparent underperformance ofC. cactorumin the suppression of invasiveOpuntiacacti should interpret the entrenched reports in the literature with circumspection. There may also be less cause for concern about the anticipated devastation of nativeOpuntiaprickly pear species in the southern United States whereC. cactorumhas become an invasive pest species.