Importance of preintroduction surveys in the biological control of Solanum weeds in South Africa

Three exotic Solanum weeds, Solanum elaeagnifolium Cav., Solanum mauritianum Scop. and Solanum sisymbriifolium Lam. (Solanaceae), are targets for biological control in South Africa. Preintroduction surveys of the insect herbivores attacking these, as well as native and cultivated species of Solanum, have proved useful in several ways. Although there is a diverse, mainly oligophagous, herbivore fauna on native Solanum spp., that of the exotic weeds is mainly polyphagous and low in diversity and numbers. Feeding niches on introduced weeds are either vacant or underutilized, ensuring a wide scope for the selection of exotic biocontrol agents. Similarities between potential agents and native Solanum herbivores suggest that the former may recruit native parasitoids, a fact that has to be considered in the selection process. The surveys also helped to resolve a conflict of interest, caused by most candidate agents accepting the introduced crop plant Solanum melongena L. (eggplant) in specificity tests. Although a number of native Solanum oligophages attacked neglected eggplant fields, damage to crops is precluded by the pesticide regimes aimed at several generalist pests. Imported agents are no more a risk than native Solanum feeders, which are not regarded as pests. The surveys support the view that exotic agents should be introduced to control Solanum weeds.