'Giant Salvinia'         (Salvinia Molesta)

    Salvinia molesta, also known as 'Giant Salvinia', is a free-floating aquatic fern species that was recently discovered to have originated in southern Brazil where it survives in a sub-tropic climate. In Brazil, Salvinia molesta's population is very low due to natural enemies. Furthermore, very little information is known about Salvinia molesta in its native habitat. This invasive plant species was introduced to Sri Lanka in 1939 by the botany department at the University of Colombo, and has spread by humans to other areas throughout the world. In September 2000, Salvinia molesta was discovered in southeastern North Carolina in three drainage basins.

Habitat

    Giant Salvinia can be found in the S , SE and SW  United States , Africa, E and SE Asia, India, Sri Lanka, Ceylon, Australia, Papua New Guinea, Indonesia, Singapore. Salvinia has been found to occur in natural lagoons, artificial dams, swamps, drains and along the margins of rivers. Salvinia molesta is considered a serious pest in many tropical, sub-tropical and warm temperature regions near the Southern Hemisphere. 

Figure 1: Salvinia molesta is known to inhabit several southern states in the US, including North Carolina. In June 2006 Salvinia distribution was shown to be concentrated in specific areas in these southern states.

Characteristics

    Salvinia molesta is a perennial plant species that is very tough and is capable of withstanding severe winters and saline water. It can survive in temperatures between 10-40˚C. Salvinia molesta is characterized by floating leaves that range in size form 1.5-6cm in width, elliptical in shape, and connected by rhizomes. The color of the leaves are light green or brownish green, and are slightly darker near the edges. The top surface of the leaves is covered by hydrophobic papillae that allow floatation, while submerged leaves acts as roots. The un-wettable portion of the leaves allows the plant to be easily dispersed by wind. (see figure 2)

    Figure 2: The image on the left shows the hydrophobic papillae that allow the plant to float. The image on the right shows the roots that are associated with Salvinia Molesta and their characteristic elliptical shape and green color.

    Salvinia molesta rapidly colonize tropical and subtropical waters very easily because they are protected from strong winds and currents, allowing them to form dense, stable mats. These mats cut off the supply of sunlight to submerged plants as well as depress oxygen concentrations, and increase CO₂  and hydrogen sulfide concentrations leading to extinction of most benthic flora and fauna. Salvinia has been shown to have a dramatic effect on wildlife, including some invertebrates, birds, and fish that depend on open water.  By depleting nutrients in the aquatic systems it inhabits, Salvinia molesta can cause a decline in fish production. Salvinia has been found to clog water intakes, affecting agricultural irrigation.

Figure 3: The mats that are established by Salvinia molesta under optimal conditions, that significantly affect aquatic systems are shown here.

How to control 'Giant Salvinia'

    Biological control programs for this particular species of Salvinia molesta began in 1972. However, because Salvinia molesta was misidentified as Salvinia auriculata, such control programs were not effective immediately. Initially, three species of herbivores were found to attack what was thought to be S. auriculata and were analyzed  as a means of control. These organisms included the weevil Cyrtobagous singularis, the moth Samea multiplicalis and the grasshopper Paulihia acuminata (see Figure 1). The weevils destroyed 30,000 tons of Salvinia when released in Australia in 1980. The moth had no effect, and the grasshopper's effect was negligible. Beetles were captured from Salvinia molesta, which was thought to be S. Auriculata, and were soon recognized to be a new beetle species, C. Salviniae. C. Salviniae destroys more Salvinia molesta than the C. Singularis species and is now used as a means of control for this invasive plant. The beetles act by destroying buds as adults and kill rhizomes as larvae.

Figure 4 : The initial biological control methods used to target Salvinia molesta: top left: an adult Samea multiplicalis moth; top right: the Cyrtobagous singularis weevil; bottom: the grasshopper Paulihia acuminata.

The Fern Group

    Salvinia molesta is an aquatic fern that belong to the Filicophyta phylum of vascular plants. These plants are non-flowering, and are composed of roots, stems and leaves that are able to reproduce via spores, which can survive under unfavorable conditions and be dispersed. Generally the roots of ferns are thin and wiry. The stems can vary in size from those of tree ferns that are tall, narrow and can reach up to 80 feet in height, to threadlike rootstocks known as rhizomes. Apical cells located on the stems initiate growth, and are protected by the leaves and bases of the leaves. Stems are protected by an 'epidermis' of flattened, thick cells, and in some cases are also protected by scales or hairs. The leaves of ferns are feather-like with a central axis and veins. 

Figure 5: Two species of ferns: a common ground fern and a tree fern.

    There is estimated to be 9,000 to 15,000 species of Filicophyta in the world, and new species are being discovered continuously in tropical regions that are not frequently explored.  Ferns are very diverse in their methods of reproduction, habitat and form. Typically ferns have a life cycle that progresses as follows:

1. Sporophyte phase: The spores which are haploid gametes, are produced in the spore case, and when released are carried by the wind.

2. Once on the ground some spores germinate to form sexually reproducing plants known as gametophytes.

3. Next, the gametophyte produces sperm and eggs by mitosis. These sperm and eggs unite to form a zygote, which is held in the tissues of the prosthallium (gametophyte) until it grows into an independent plant.

     Ferns can be seen as vines or as plants that float, such as Salvinia Molesta. These plants grow in warm, moist areas, but a few are found in cold, dry places such as Athyrium and Cystopteris.  Ferns are very uncommonly described as weeds, except in the case of the  Pteridium genus, which can spread very quickly via underground ropelike rhizomes. Salvinia auriculata has been shown to be a pest in India, where it blocks irrigation ditches and rice paddies.

References:

Myers, J.H. and Basely, D.R. (2003). Ecology and Control of Introduced Plants. New      York: Cambridge University press (175-176)

Cronk, Q. C. B. and Fuller, J.L. (2001).  Plant Invaders: The threat to Natural Ecosystems. Sterling, VA: Earthscan Publications Ltd. (119-122).

"fern." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 30  Oct.  2007  <http://www.britannica.com/eb/article-49932>.

Figure 1, courtesy of  Buck Albert of the Unites States Geological Survey organization: http://www.usgs.org

Figure 2, courtesy of tncweeds.ucdavis.edu/potosp-2.html

Figure 3, courtesy of Kenneth Calcote, of the Mississippi Department of Agriculture and Commerce: www.galvbayinvasive.org

Figure 4 courtesy of www.invasive.org/.../2FloatingFern.html

Figure 5 courtesy of www.digitalapoptosis.com/...000718html

Links:

Invasive plants of the eastern US: www.invasive.org/.../2FloatingFern.html

United States Geological Survey: www.usgs.gov