Biological Control of Dense-Thorned Bitter Apple,Solanum sisymbriifoliumLam. (Solanaceae), in South Africa

Abstract

Solanum sisymbriifolium Lam. is a weed of disturbed areas in South Africa, particularly in the high-elevation summer rainfall areas of the country. Despite the introduction of the South American leaf-feeding tortoise beetle, Gratiana spadicea (Klug) (Coleoptera: Chrysomelidae: Cassidinae) in 1994, the weed has continued to spread. Although established in a number of regions, beetle populations appear to persist in relatively low numbers, inflicting minimal damage to the weed. Consequently, a number of variables have been investigated in order to explain this lack of efficacy, and in some cases, failure to establish. Gratiana spadicea populations in South Africa are under considerable stress both from abiotic and biotic sources. Population-limiting factors such as disturbance of overwintering sites, climatic incompatibility, and high rates of pupal parasitism translate into low numbers of the agent at the start of each growing season. Small early-season populations which are slow to build-up, coupled with asynchronous development of S. sisymbriifolium, limit the beetles' effectiveness and pre-empt any impact on the weeds' reproductive output. Consequently, G. spadicea alone is unlikely to control S. sisymbriifolium infestations in South Africa. The flower-feeding weevil Anthonomus sisymbrii Hustache (Coleoptera: Curculionidae: Anthonominae), may therefore warrant study as it could have the potential, if released, to curtail fruit production and further spread of S. sisymbriifolium.