Problem statement: Salvinia molesta is an aggressive aquatic fern and one of the world’s worst weeds. It has been introduced from its native Brazil to many areas of the world, where environmental damage caused by its spread has been enormous. It chokes lakes, reservoirs, slow-moving rivers, irrigation systems, rice paddies, fishponds etc with continuous meter-thick mats of dense vegetation. In addition to rendering the water useless for normal purposes its presence can lead to an increase in mosquito populations.

Methods: Initially the weed was thought to be Salvinia auriculata, and a weevil, Cyrtobagous singularis, was collected from Trinidad and released on the weed in Africa, where it failed to control it. The weed was then correctly identified as S. molesta, and Cyrtobagous was collected from this species in Brazil and released in Queensland, where it rapidly controlled the weed. Initially it was believed that the weevil represented a different biotype of C. singularis but was soon found to be an undescribed species, distinguishable only on microscopic characters.

Lessons: Biological control was achieved only after the true identity of Salvinia had been recognized, its native range found, and a previously unknown herbivore discovered. This illustrates vividly the crucial importance of sound taxonomy for understanding and solving ecological problems.

Outcomes and Impacts: The use of the correct weevil has resulted in massive clearance and control of the weed over vast geographic areas.