Salvinia molesta D. S. Mitchell
& S. auriculata Aubl. -- Salviniaceae
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Salvinia molesta is a free floating aquatic fern native to Brazil, which was spread as an aquarium plant and botanical curiosity to tropical and subtropical parts of Africa, Asia, Australia, Fiji and New Guinea. It has propagated vegetatively and now ranks as one of the world's most important aquatic noxious plants (Thomas & Room 1986, Goeden & Andrés 1999).
Salvinia spread to Lake Kariba on the Zambia/Zimbabwe border in 1955 and quickly spread to cover 1,000 km2 or 21% of the lake's surface. Commonwealth entomologists conducted foreign exploration in South American for Salvinia auricularia Aublet, often called Kariba weed. Three insect species, the moth Samea multiplicatus Guenee (Lepidoptera: Pyralidae), the grasshopper, Paulinia acuminata (DeGeer) (Orthoptera: Acrididae), and the weevil Cyrtobagous singularis Hustache (Coleoptera: Curculionidae), were considered the best candidates for biological control. During 1960-1970 all three species of insects were released on Lake Kariba and elsewhere in Africa and Fiji. Only P. acuminata became established on Lake Kariba and provided limited control of the fern.
Goeden & Andrés (1999) relate how by 1970 Kariba weed was cited as one of the best documented contemporary examples of the importance of taxonomy to biological control of plants. However a pressed specimen of Kariba weed collected in 1941 and preserved in the Rio de Janeiro Botanic Garden was not Salvinia auricularia but rather a new species described as S. molesta in 1972. By 1978 the weed was determined to be a sterile hybrid that originated in southeastern Brazil. Exploration by Commonwealth entomologists resulted in the detection, study and recommendation of Samea multiplicatus, P. acuminata, and what then was thought to be a separate biotype of C. singularis, as the most promising candidate biological control agents. Introductions of the weevil were made to Australia in 1980 where it established readily and in less than two years destroyed a 200 ha infestation on Lake Moondarra in northern Australia. Further studies resulted in the weevil's description as Cyrtobagous salviniae Calder & Sands. Results of this project support the concept that the best natural enemies are obtained from the target plant species (Goeden & Kok 1986, Moran et al. 1986), a concept that is frequently challenged (Hokkanen & Pimentel 1984).
Crytobagous salviniae was transferred to Papua, New Guinea from Australia in 1982 and established only after the novel manipulation of the weevil's environment in field release cages (Goeden & Andrés 1999). This involved regular applications of urea to the caged weed which increased its nitrogen content and enhanced its nutritional suitability for the weevils which then increased their reproduction rate. The weevils continued to multiply in great numbers once the cages were opened and the weevils were allowed access to large volumes of unfertilized weeds that covered Sepik Lake to a depth of ca. 1 m. Following a large scale redistribution effort in August 1985, salvinia has been reduced from 25 km2 of water surface on the lower floodplain of the Sepik River to only 2 km2, representing the destruction of two million metric tons of salvinia in two years (Goeden & Andrés 1999).
Salvinia auriculata Aubl. in Africa has been targeted for biological control using a grasshopper, Paulinia acuminata DeGeer, a weevil, Cyrtobagous singularis Hulst., and a moth, Samea multiplicallis Guenée (Bennett 1966).
REFERENCES: [Additional references may be found at: MELVYL Library ]
Bennett, F. D. 1966. Investigations on the insects attacking aquatic ferns, Salvinia spp. in Trinidad and northern South America. Proc. S. Weed Conf. 19: 497-504.
Goeden, R. D. & L. A. Andrés. 1999. Biological control of weeds in terrestrial and aquatic environments. IN: Bellows, T. S. & T. W. Fisher (eds.), Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, New York. 1046 p.
Goeden, R. D. & L. T. Kok. 1986. Comments on a proposed "new" approach for selecting agents for the biological control of weeds. Canad. Ent. 118: 51-58.
Hokkanen, H. & D. Pimentel. 1984. New approach for selecting biological control agents. Canad. Ent. 116: 1109-21.
Moran, V. C., S. Neser & J. H. Hoffmann. 1986. The potential of insect herbivores for the biological control of invasive plants in South Africa, p. 261-68. In: I. A. W. MacDonald, F. J. Kruger & A. A. Ferrar (eds.), The Ecology and Management of Biological Invasions in Southern Africa. Oxford Univ. Press, Capetown. South Africa.
Thomas, P. A. & P. M. Room. 1986. Taxonomy and control of Salvinia molesta. Nature 320: 581-84.