Assessing the risks associated with the release of a flowerbud weevil, Anthonomus santacruzi, against the invasive tree Solanum mauritianum in South Africa

Biological control of Solanum mauritianum Scopoli, a major environmental weed in the high-rainfall regions of South Africa, is dependent on the establishment of agents that can reduce fruiting and limit seed dispersal. The flowerbud weevil, Anthonomus santacruzi Hustache, is a promising fruit-reducing agent, despite ambiguous results obtained during host-specificity evaluations in quarantine. Adult no-choice tests showed that although feeding is confined to Solanum species, normal feeding and survival occurred on the foliage (devoid of floral material) of cultivated eggplant (aubergine), potato, and several native South African Solanum species. During paired-choice tests, involving floral bouquets in 10-liter containers, A. santacruzi oviposited in the flower buds of 12 of the 17 test species, including potato and eggplant, although significantly more larvae were recovered on S. mauritianum than on eight other species. Larvae survived to adults on all 12 species, with survival significantly lower on only four species than on S. mauritianum. However, during multi-choice tests, involving potted plants in a large walk-in cage, A. santacruzi consistently displayed significant feeding and oviposition preferences for S. mauritianum over all of the 14 Solanum species tested. Analyses of the risk of attack on nontarget Solanum plants suggested that, with the possible exception of two native species, none is likely to be extensively utilized as a host in the field. Also, host records and field surveys in South America have suggested that A. santacruzi has a very narrow host range and that the ambiguous laboratory results are further examples of artificially expanded host ranges. These and other considerations suggest that A. santacruzi should be considered for release against S. mauritianum in South Africa, and an application for permission to release the weevil was submitted in 2003.