Abstract
Salvinia molesta D.S. Mitchell (Salviniaceae) (giant salvinia), a floating aquatic fern of Brazilian origin, has been dispersed to much of the tropical and subtropical parts of the world since the mid-1900s, where it is invasive and damaging. Herbicide application and mechanical control of this weed are labor intensive and expensive, but biological control using the host-specific weevil, Cyrtobagous salviniae (Calder and Sands), provides an effective and sustainable solution. The weevil was first released as a biological control agent onto S. molesta in Australia in 1980 and has subsequently been released in further 22 countries affected by the weed. The biological control program against S. molesta has been an extraordinary success story where a single weevil species has resulted in the weed no longer being considered invasive in most countries in a relatively short time of under 3 years. Biological control in temperate regions of the world might require more time than in tropical and subtropical regions, and this difference is presumed to be due to greater winter mortality. Nevertheless, inundative releases of the weevil early in the growing season have resulted in more effective control and less dependence on herbicide applications. However, there are new records of infestations of S. molesta in particular in Africa, and these sites must be prioritized for biological control.