Salvinia molesta

Giant salvinia
The fern Salvinia molesta floating on a pond surface
Scientific classification
Kingdom:	Plantae
Division:	Pteridophyta
Class:	Pteridopsida
Order:	Salviniales
Family:	Salviniaceae
Genus:	Salvinia
Species:	S. molesta
Binomial name
Salvinia molesta D.Mitch.

Giant salvinia completely cover Lake Wilson in Hawaii

Salvinia molesta, commonly known as giant salvinia or kariba weed after it infested a large portion of the reservoir of the same name, is an aquatic fern, native to south-eastern Brazil. It is a free floating plant that does not attach to the soil, but instead remains buoyant on the surface of a body of water. The fronds are 0.5–4 cm long and broad, with a bristly surface caused by the hair-like strands that join at the end to form eggbeater shapes. They are used to provide a waterproof covering. These fronds are produced in pairs also with a third modified root-like frond that hangs in the water.

Favoured environmental conditions

S. molesta prefers to grow in slow moving waters such as those found in lakes, ponds, billabongs (oxbows), streams, ditches, marshes and rivers. It prefers nutrient rich waters such as those found in eutrophic water or those polluted by waste water. It does not usually grow in brackish or salty waters but has been reported in streams with a tidal flow in southeast Texas. It copes well with dewatering and while it prefers to grow in moderate temperatures it will tolerate temperatures that are low or very high. the United States Geological Service believes that it could grow in Zones 7a, 8, 9, and 10 of the USDA Plant Hardiness Map.

Methods of introduction

The plant originated in southeast Brazil and was exported as part of the pet industry to be used in aquariums and garden ponds. From there it escaped or was deliberately released into the wild. It is possible that it may also have been brought in with fresh, iced fish. Once in a waterway it can be spread by infested boats which not only spread it to new areas but also break the plant up which allows it to propagate. It also is spread by waterflow.

Reproduction

It reproduces by asexual reproduction only, but it is capable of growing extremely quickly, starting from small fragments and doubling in dry weight every 2.2-2.5 days. It grows from fragments that have broken off or dormant buds that have been detached from the main plant. Each node has five buds so potential for great & rapid spread is high. It also produces spores but they are genetically defective and do not produce viable offspring.

Effect

The result of the rapid growth rate of Salvinia molesta has resulted in its classification as an invasive weed in some parts of the world such as Australia, New Zealand and parts of America. Surfaces of ponds, reservoirs, and lakes are covered by a floating mat 10–20 cm (in some rare cases up to 60 cm) thick. The plant's growth clogs waterways and blocks sunlight needed by other aquatic plants and especially algae to carry out photosynthesis thereby oxygenating the water. As it dies and decays, decomposers use up the oxygen in the water. It also prevents the natural exchange of gases between the air & the body of water the plant has invaded causing the waterway to stagnate. This can kill any plants, insects or fish trapped underneath its growth. Its ability to grow and cover a vast area makes it a threat to biodiversity. Large infestations covering a wide area may also pose a problem to migratory birds as they may not be able to recognise an infested waterway when flying over head and so may not stop at it. S. molesta also provides ideal conditions for the breeding of mosquitoes that carry disease. The growth habit of Salvinia also is problematic to human activities including flood mitigation, conservation of endangered species & threatened environments, boating and irrigation.

Control

Biological control

A tiny weevil, Cyrtobagous salviniae, found in the native habitat of S. molesta, is currently being studied as a biocontrol. C. salvinae was first used as a biological control in Australia at Lake Moondarra, a recreational lake in Mount Isa, Queensland in 1980. By mid 1981 the weevil had reduced the weed to a few small patches. It ate the leaves of the weed but preferred the buds. Its larvae ate the roots, rhizomes and the buds. As the plant died it turned brown and sank to the bottom of the waterway and decomposed. This weevil was used with success in other parts of the world (13 tropical countries) such as the Sepik River in Papua New Guinea; Sri Lanka; Wappa Dam in Queensland and lagoons (e.g. Kakadu National Park) in the Northern Territory, Australia. In Australia the moth, Samea multiplicalis was also released in the hopes that it would reduce the size of the weed population. While this moth did become established in Australia and spread it was not effective as a biological control. A third species, the grasshopper Pauliana acuminata, was considered but not released.

Mechanical control

Plants are removed by machine or harvesting equipment and by hand which is only suitable for small infestations. Harvesting equipment also can encounter difficulties as it cannot remove all of the infestation, it cannot access shallow areas and equipment can be inhibited by large masses of the plant. Once removed the plant must be dried, burnt or disposed of in a manner that ensures it will not reenter the waterway.

Chemical control

Chemical control on S. molesta is difficult as it does have some resistance to herbicides. The chemical fluridon has been successfully used but it requires prolonged contact and is not effective if it is suddenly diluted by rainwater or any other influx of water. Other chemicals such as hexazinone and diquat and double chelated copper are used together to kill S. molesta.

References

• Everitt, J.H. (2007). Weeds in South Texas and Northern Mexico. Lubbock: Texas Tech University Press. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) ISBN 0-89672-614-2monkey
• http://www.csiro.au/resources/salvinia-control.html

External links

• Salvinia molesta information from the Hawaiian Ecosystems at Risk project (HEAR)
• US Geological Survey: Giant Salvinia
• Wildland Invasive Species Team - Weed alert
• Identification and Management of Giant Salvinia (Salvinia molesta)
• Texas campaign to prevent spread: Hello Giant Salvinia, Goodbye Texas Lakes
• Species Profile- Giant Salvinia (Salvinia molesta), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Giant Salvinia.