Problem statement: Salvinia molesta D.S. Mitchell 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. S. molesta was initially mistaken for a different water weed species, S. auriculata Aublet, but attempts to control the invasive weed in Africa, India, Sri Lanka and Fiji using a weevil, Cyrtobagous singularis Hustache, collected from S. auriculata in Trinidad failed.

Methods: The weed was then correctly identified as S. molesta and the native range of S. molesta was discovered in Brazil in 1978. A hunt began for a pest that could be introduced as a biocontrol agent in countries where S. molesta has become an invasive alien species. A weevil, first thought to be C. singularis, was found on S. molesta that successfully decimated a large population of the water weed in Queensland, Australia. The dramatic success of this biocontrol operation was reported by Room, Harley, Forno & Sands in 1981. In 1983, however, it was reported that, in fact, the weevil was not C. singularis as originally thought, but a new species, later described as C. salvinae, distinguishable from C. singularis only on microscopic characters.

Outcomes and Impacts: The weevil C. salvinae has now been introduced in Africa, New Guinea, the US and elsewhere; the use of the correct species has resulted in massive clearance and control of the invasive water weed S. molesta over vast geographic areas.

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. Without taxonomy, funds would have been wasted on ineffective control projects.