Biological control of Solanum mauritianum Scopoli (Solanaceae) in South Africa:: a review of candidate agents, progress and future prospects

The South American tree Solanum mauritianum Scopoli (Solanaceae), naturalized in the higher rainfall regions of South Africa for over a century, was targeted for biological control in 1984. However, progress has been constrained by the problem of expanded host ranges of the candidate agents during host-specificity tests in quarantine. Despite identical problems in projects against the congeneric weeds Solanum elaeagnifolium Cavanilles and S. sisymbriifolium Lamarck, releases of insect agents on these two weeds in South Africa during the 1990s have set a precedent regarding the interpretation of routine host-specificity tests. This new approach of using risk assessments to overrule conservative evaluative methods has similarly advanced the prospects for releases on S. mauritianum. These developments influenced the introduction of several new candidate agents during 1994-1998, including defoliating, flower-feeding and stem-boring species. However, the recurrence of unacceptably broad 'physiological' host ranges in captivity has disqualified several of these and only one species, the leaf-sucking lace bug Gargaphia decoris Drake (Tingidae), was deemed suitable for release. Permission to release G. decoris was granted in early 1999. This paper reviews the biological control programme against S. mauritianum, including (i) the types of agents prioritized for release, (ii) those introduced and evaluated and (iii) procedures that may reduce the risk of rejection of otherwise suitable agents. Despite the inherent problems, prospects for the biological control of S. mauritianum have improved considerably.