Salvinia molesta
(kariba weed)

S. molesta is a free-floating aquatic plant native to south-eastern Brazil. It has been spread widely throughout the world during the past 50 years and is invasive in a variety of aquatic habitats, including lakes, rivers and rice paddies. Based on the environmental, economic and human health impacts, S. molesta ranks a close second behind water hyacinth on a list of the world's most noxious aquatic weeds. It has also been recently added onto the list of the world’s 100 most invasive species.

S. molesta is native to south-eastern Brazil (Forno, 1983). It has been spread widely throughout the world during the past 50 years and can be found in Africa, the Indian subcontinent, southeast Asia, Australia, New Zealand, southern USA and some Pacific islands (Thomas and Room, 1986a). 

S. molesta is spread within an aquatic system by the movement of plants by wind, water currents, floods and animals. Birds, capybara, hippopotamus and waterbuffalo have all been documented to spread Salvinia between waterways (Room and Julien, 1995; Forno and Smith, 1999). Spread between aquatic systems is assumed to be mainly by humans moving plants intentionally (as ornamentals), unintentionally as a hitchhiker on boats, or in shipments of aquatic plants and fish (Parsons and Cuthbertson, 1992; Gewertz, 1983). Increased transport of commodities in international commerce will increase the movement of S. molesta around the world.

S. molesta easily colonizes disturbed habitats including rice paddies, flood canals, artificial lakes and hydro-electric facilities (Barrett, 1989). In its native range, S. molesta occurs in artificial reservoirs, swamps, drainage channels and along river margins (Forno and Harley, 1979). It occurs most commonly in freshwater lakes, rivers, swamps, streams, ditches and water tanks (Reed, 1977; Westbrooks, 1984). It prefers stagnant or slow moving water, often in small bays and inlets of dissected shorelines and tributaries of small streams, where it is protected from wave action. It will also grow around emergent brush and trees on flooded shorelines where it is also sheltered from wave action. Though it grows as a free-floating hydrophyte, it has been observed to thrive on land in a zone of constant mist near the foot of Victoria Falls in southern Africa (Holm et al., 1977). It can also survive on mudbanks, and will tolerate some drying (Owens et al., 2004). It is quickly killed by sea water (Holm et al., 1977) but can tolerate lower concentrations of salt (Biber, 2009).

Reproductive Biology

S. molesta is a sterile pentaploid hybrid fern that forms spore sacs containing abortive spores due to anomalies during meiosis. Since the spores and megagametophytes that are produced are fairly short-lived, reproduction is exclusively vegetative (Mitchell, 1972). There is therefore speculation that all S. molesta infestations worldwide may be clones of a single genetic individual (Werner, 1988; Barrett, 1989).

Although S. molesta is sterile, it undergoes rapid vegetative reproduction by growth and fragmentation (Oliver, 1993). Mitchell (1979) reported that plant biomass can double in as little as 3-4 days in sterile culture and 8.1 days on Lake Kariba in Zimbabwe. Additional studies found doubling times of 6.2–5.3 days (dry weight) and 4.8–3.8 days (leaf area) (Sale et al., 1985). Mean relative growth rates, under a range of artificial and natural environmental conditions, may be as high as a 21.64% per day increase in leaf number and 17.16% per day increase in dry weight (Mitchell and Tur, 1975). Under favourable conditions, S. molesta can completely cover lakes and slow-moving streams and rivers with thick mats up to 1 m thick (Thomas and Room, 1986a). Live biomass ranges from 250-600 g per m² dry weight (Mitchell, 1979).

Similar surveys at Lake Naivasha, Kenya, found that some infestations of S. molesta in nutrient-rich water doubled in size in 4.5 days, with most growth occurring near papyrus stands. Older mats of S. molesta may be invaded by secondary vascular vegetation, resulting in the formation of sudd (a thick mat of plants) (Tarras-Wahlberg, 1986).

Growth Stages

Species in the S. auriculata complex progress through three phenotypes or growth stages (Ashton and Mitchell, 1989; Oliver, 1993). These major stages are controlled by age, degree of crowding, water turbulence and other abiotic factors. In addition to the three major stages, Ashton and Mitchell (1989) recognized an additional stage, which they described as a survival form, that occurs in harsh environments and grows slowly with flat, sometimes yellowish leaves.

1. Primary Phase. Leaves are small (about 10 mm in diameter) and lie flat on the water surface. This phase is observed during initial invasion and in plants recovering from damage. When the plant is introduced to a new habitat, it produces colonizing stage plants with thin stems that fragment easily and produce many new plants.

2. Secondary Growth Phase. Floating leaves grow to be about 25 mm long and wide and begin to fold upward, giving the structure a keeled shape. Leaves are cupped but do not overlap, and the lower surface is in contact with the water.

3. Tertiary Growth Phase. Leaves grow and thicken to approximately 38 mm wide and 25 mm long. The terminal bud now forms leaves which are compact, almost vertical, and acutely folded. This phase occurs when competition becomes severe at the height of the growing season. When growing conditions are ideal, the plant can reach this phase in 2-3 weeks (Sculthorpe, 1985; Holm et al., 1977). This phase has also been termed the mat stage.

Physiology and Phenology

S. molesta exhibits no lignification of tissues and therefore the plant must remain turgid for mechanical support of its organs. Air is trapped by the birdcage-like hairs that grow in close, parallel rows on the upper surface of the floating leaves, allowing S. molesta to float (Kaul, 1976).

Environmental Requirements

The growth of S. molesta is enhanced by high light intensities, relatively high water temperatures, and plenty of available nutrients (Mitchell and Tur, 1975). Water temperatures rising to 30°C, results in elevated growth rates, as does increasing the concentrations of nutrients, especially nitrogen and phosphorus (Cary and Weerts, 1983). Thus, eutrophic habitats such as nutrient-rich springs and phosphate mine reclamation wetlands and ponds in the United States are particularly suitable for rapid colonization and growth (Oliver, 1993). Infestations of Salvinia can be killed when terminal buds are exposed to temperatures below -3°C, but the leaves can survive freezing air temperatures if they are under the water surface (Whiteman and Room, 1991).

S. molesta has a low tolerance of salinity and only produces new growth at salinity levels less than 5 ppt; levels above 11 ppt are toxic (Biber, 2008). Optimum growth occurs in nutrient-rich situations, pH 6-7.5, with water temperatures between 20 and 30°C, particularly when the nitrogen source is ammonium ions (NH4+) rather than nitrate ions (NO3-) (Cary and Weerts, 1983). Under these ideal conditions, a doubling of plant dry weight in 2.2 days has been recorded in Queensland, Australia (Parsons and Cuthbertson, 1992).

Animal feed, fodder, forage

• Fodder/animal feed

Environmental

• Landscape improvement

Fuels

• Biofuels

General

• Botanical garden/zoo
• Pet/aquarium trade
• Sociocultural value

Materials

• Fertilizer

Within the S. auriculata complex, to which S. molesta belongs, all of the species are very similar in their vegetative morphology. Therefore, reproductive structures should be used for identifying species within the complex whenever possible, though plants bearing sporocarps are rarely reported in the United States (Riefner and Smith, 2009). A comparison of species within the complex is provided in the section Similarities to Other Species/Conditions.

S. molesta is one of four species that are members of the S. auriculata complex (Mitchelll and Thomas, 1972). The other species are S. auriculata, S. biloba and S. herzogii. Members of this complex all have four hairs on the tip of each papilla that are joined to form a cage-like (or eggbeater) structure on the upper surface of mature leaves. Other species in the genus have hairs on the tip of each papilla, and each hair divides at the end into three or four free arms not joined into a cage (Riefner and Smith, 2009).

All species within the S. auriculata complex are very similar in the vegetative state. Reproductive structures should therefore be used for identifying species within the complex whenever possible. A comparison of species within the complex is provided below.

S. molesta
a. Upper leaves: with aerolae approximately equal size in outer third of leaf; those of outer one third at most twice as long as wide; those of the inner two-thirds considerably longer than wide.
b. Aerolae: 5-10 in number from margin to midrib.
c. Lower leaves: petiolate, divided distally with short branches not recurved.
d. Sporocarps: Globose to apiculate, sessile to short-stalked, stalks arranged in a racemose manner along the fertile axis.

S. auriculata
a. Upper leaves: with aerolae of approximately equal length in outer two-thirds of leaf; those of outer third at most as long as wide.
b. Aerolae: 10-26 in number from margin to midrib.
c. Lower leaves: sessile or long petiolate, divided distally with short recurved branches.
d. Sporocarps: globose, long-stalked, stalks mostly attached at the apex of submersed leaves; usually <14 in number.

S. biloba
a. Upper leaves: with aerolae of approximately equal length in outer third of leaf; the outer third at most twice as long as wide, the inner two-thirds considerably longer than wide.
b. Aerolae: 8-12 in number from margin to midrib.
c. Lower leaves: petiolate, divided distally with short branches recurved or not.
d. Sporocarps: apiculate, stalked, stalks attached along the fertile axis; often >10 in number.

S. herzogii
a. Upper leaves: with aerolae six times as long as wide.
b. Aerolae: 5-10 in number from margin to midrib.
c. Lower leaves: short petiolate, divided distally with short branches not recurved.
d. Sporocarps: ovoid, apiculate, sessile, arranged more or less compactly along the fertile axis.

S. natans and S. cucculata, which are found mostly in Asia, have hairs on the tip of each papilla, and each hair divides at the end into three or four free arms.

S. molesta and Ipomoea aquatica occur in similar habitats, but occupy very distinct ecological niches. I. aquatica has emergent shoots with submerged stoloniferous components, and S. molesta occupies the space between I. aquatica plants, sheltered by them. I. aquatica appears to be more aggressive of the two, colonizing new areas before S. molesta. The submerged components of I. aquatica show some resistance to adverse conditions, ensuring perpetuation of the species when favourable conditions return. Nitrogen and phosphorous are important growth factors in both macrophytes. I. aquatica shows a high absorption capacity for ammonium-nitrogen. Potassium and calcium are relatively high while magnesium and sodium are low in these habitats (Chin and Fong, 1978).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

All imported shipments of aquatic plants, tropical fish and other similar products from infested countries should be closely examined for the presence of S. molesta. Since S. molesta reproduces vegetatively, even small undetected fragments are sufficient to permit an introduction into a new country. As with all noxious weeds, prevention is the most effective way to limit the spread of S. molesta. Production of S. molesta-free products (such as tropical fish and ornamental water plants) is the only sure way to prevent further global movement of this weed.

Chemical Control

The birdcage hairs on the upper surface of S. molesta leaves form a waterproof barrier to most herbicides. However, penetration can be enhanced by the use of a wetting agent or surfactant (Oliver, 1993). In Australia, repeated applications of paraquat in combination with a wetting agent were successful in controlling S. molesta (Miller and Pickering, 1980). Outdoor herbicide trials revealed that 8.97 kg/ha glyphosate mixed with a non-ionic surfactant controlled 99% of S. molesta 42 days post-treatment (Nelson et al., 2001). Laboratory studies also showed effective control over a broad range of application rates, including those as low as 0.45%, with mortality rates increased by the addition of surfactant (Fairchild et al., 2002).

In Malaysia, diquat was effective in controlling S. molesta (Kam-Wing and Furtado, 1977). Nelson et al. (2001) showed that 1.12 kg/ha diquat provided effective control of S. molesta and performed better than glyphosate, endothall and other combinations. However, Mitchell (1979) reported that diquat was only 1/8th as effective on S. molesta as paraquat. Complete control of S. molesta was observed 10-14 days after treatment with 2,4-D plus paraffin + calcium dodecyl benzene sulphonate using punt- or hovercraft-mounted sprayers (Julien, 1984).

In a study in New Zealand, fluridone formulations provided good control of S. molesta in outdoor tanks (Wells et al., 1986). Applications of hexazinone, ametryn and paraquat have also been effective in controlling S. molesta (Westbrooks, 1984). Terbutryn is recommended in South Africa (Vermeulen et al., 1996). In another study, foliar applications of hexazinone + surfactant resulted in complete control of S. molesta (Toth and Campion, 1979). In a greenhouse evaluation the most effective herbicide was linuron followed by diuron. Affected plants suffered localized chlorosis, necrosis, retardation of stem and leaf elongation and possibly leaf formation, and eventually died (Waithaka, 1980).

After S. molesta was found in January 1977 growing in the upper reaches of the Adelaide River, Northern Territory, Australia, a 10-year eradication program was initiated. Herbicides used to kill larger stands included paraquat, diquat, 2,4-D, and diuron + calcium dodecylbenzene sulphonate. Smaller areas of S. molesta were removed by hand and areas of overhanging vegetation along the edges of the river were burned to expose hidden plants. The last sighting of S. molesta on the river was in 1982; regular surveys were continued until 1986 to ensure that no re-infestation occurred (Miller and Pickering, 1988).

In the laboratory, detergent has been shown to damage S. molesta. In one experiment, spraying the plant with a 0.05% solution of a household detergent (linear alkyl benzene sulfonate) resulted in 85% decrease in total chlorophyll and 75% decrease in total protein within 48 hours after treatment (Chawla et al., 1989).

Physical/Mechanical Control

Manual removal of plant material as a maintenance control approach has been effective, but is very labour intensive. In India, manual removal has been used successfully to control 1,500 ha of S. molesta on a hydroelectric reservoir. In this case, it took 30 men to remove about half of the infestation over a 3-month period. Continued maintenance control required a similar operation on an annual basis (Murphy, 1988). In an infestation on the Adelaide River in the Northern Territory of Australia, the bulk of the sudd (thick mat of S. molesta and other plants) was manually removed and remaining plants along the river bank were successfully controlled with herbicides such as diquat and 2,4-D (Miller and Pickering, 1988).

Generally speaking, manual removal is only practical in the early stages of invasion (Oliver, 1993). After the plant becomes established, biomass of about 80 tons/ha and rapid regrowth make mechanical harvesting and removal impractical. According to Thomas and Room (1986a), mechanical removal is not cost-competitive with chemical control.

Floating booms and wire nets have some value in containing Salvinia spp. infestations. However, such equipment is subject to breaking under the weight of large windblown mats (Thomas, 1976).

Thomas (1990) reported the development of a simple 10 hp machine in India with a high capacity jet device that sucks, fluidizes and pumps out plant material to a desired height or location. The harvesting rate was reported to be 15 tons/hour for continuous operation, and at a lower cost than manual removal of plant material. Another mechanical harvester for S. molesta, reported by Sankaranarayanan et al. (1985), is mounted on a twin-pontooned floating platform that measures 3.6 x 1.5 m and weighs 415 kg. The harvesting capacity is 16 tons per hour and the machine can operate in water as shallow as 50 cm.

Biological Control

The salvinia weevil (Cyrtobagus salviniae) (Coleoptera: Curculionidae) has been used successfully to control Salvinia spp. (Calder and Sands, 1985; Cilliers, 1991; McFarland et al., 2004). A semi-aquatic weevil native to Paraguay, Brazil, and Bolivia, C. salviniae has been released in 16 countries to control S. molesta (Wibmer and O’Brien, 1986; Julien and Griffiths, 1998; Julien et al., 2002).  C. salviniae also feeds on S. minima (common salvinia) in the USA (Tipping et al., 2010). In South Africa, Botswana, and India, where the weevil has been introduced, S. molesta was reduced to 1% of its former area (Room, 1986a; Creagh, 1991/92; Cilliers, 1991). On the Sepik River in Papua New Guinea, introductions of C. salviniae reduced a 250 km² infestation of the plant to 1.5 km² in 18 months (Thomas and Room, 1986b). In Sri Lanka, about 80% of infestations of S. molesta had been destroyed following the release of C. salviniae in 1986 (Room and Fernando, 1992). In Texas, 651,000 larvae, pupae and adult C. salviniae weevils were released at five sites with heavy S. molesta infestations, and in nine months the populations were reduced to less than 10% of their original extent (Flores and Carlson, 2006).

However, in Northern Territory, Australia, high water temperatures in water bodies have been associated with the failure of the weevil to control the plant. The effectiveness of C. salviniae in New South Wales, Australia, has also been variable, because the cooler climate of the region is not favourable for the growth of the insect (Oliver, 1993). Intermittent success in biological control of S. molesta using C. salviniae in Australia has been attributed to alternative stable states (Schooler et al., 2011; Stone, 2011).

Insect damage to S. molesta generally increases as the water temperature increases from 16 to 30°C (Forno and Bourne, 1986). Additionally, feeding and damage by C. salviniae is dependent on levels of nitrogen in the plant. In Sri Lanka, the weevil was released in several lowland areas in 1987; however, increases in weevil numbers were low due to low levels of nitrogen in the tissues of the plant until the end of a drought. After water and nitrogen levels returned to the previous levels the following year, infestations of the plant were destroyed by the weevil (Room et al., 1989).

Another potential biological control agent is an aquatic grasshopper, Paulinia acuminata (Orthoptera: Acrididae). However, adults and nymphs feed on S. molesta, water lettuce (Pistia stratiotes) and Azolla spp., and it is of questionable value as a biological control agent because it is not monophagic and has not been shown conclusively to control S. molesta (Sands and Kassaulke, 1986). Contradicting Sands and Kassaulke (1986), it has been suggested that P. acuminata has controlled a severe infestation of S. molesta 1973 on Lake Kariba, in central Africa (Zambia and Zimbabwe). After damming of the Zambesi River created the lake, growth of S. molesta increased rapidly and covered over 1000 km² of water at the peak infestation in 1962. By 1973, the infestation suddenly fell to 77 km² and remained at this level. It has been suggested that the introduction and release of P. acuminata to the lake from Trinidad in 1970, its subsequent establishment and high population in 1973 likely contributed to the control of S. molesta (Mitchell and Rose, 1979).

In Queensland, Australia, the pyralid moth Samea multiplicalis and C. salviniae were released at separate sites for biological control of S. molesta. Although C. salviniae removed large areas of the plant, S. multiplicalis did not reduce plant growth permanently at any site. In another study in Queensland, Australia, larval densities of Samea multiplicalis at 0.8 and 1.6 per plant caused severe damage to S. molesta. In the 15 experiments, Samea multiplicalis larvae destroyed about half the leaf area and reduced plant weight and number of ramets. However, roots and rhizomes remained undamaged, no buds were destroyed, and the plants were able to continue to grow (Julien and Bourne, 1988). In the United States, where Samea multiplicalis in native, it does not completely control but significantly impacts the closely related species S. minima in conjunction with releases of C. salviniae (Tewari and Johnson, 2011).

Of all insects released for the control of S. molesta, only C. salviniae has proven effective (Room, 1986b; Oliver, 1993; McFarland et al. 2004), and at a lower cost than chemical or mechanical control (Chikwenhere and Keswani, 1997)

Recently, the fungus Simplicillium lanosoniveum has been isolated from S. molesta specimens symptomatic for Brown Spot in Taiwan, but this agent’s potential for use as biological control is untested (Chen, 2008).

Regulatory Control

On a body of water, S. molesta can be moved by wind, currents, and ships. It is moved overland for short distances by adhering (with mud) to fur and feathers of animals, to clothing and to the sides and bottoms of boats and wheels of vehicles. Sometimes the plants are used as packing for fish and other products of fresh water lakes and streams. It has been spread around the world as a contaminant of various aquatic goods such as shipments of tropical fish and aquatic plants (Holm et al., 1977). It has also been spread maliciously to interfere with fishing (Gewertz, 1983). Increased transport of commodities in international commerce will increase the movement of S. molesta around the world. Because of the great difficulties associated with its manual, chemical and biological control, regulatory prevention remains the most effective management strategy available (McFarland et al., 2004).

S. molesta is listed as a Federal Noxious Weed in USA (Anonymous, 1981) and as a state noxious weed in Florida (Ramey, 1990) and North Carolina. It is also listed as a noxious weed in all states of Australia (Parsons and Cuthbertson, 1992), Thailand (Chomchalow, 2011) and Europe (EPPO).

20/11/2013 Updated by:

Katherine Parys, United States Department of Agriculture, Southern Insect Management Research Unit, 141 Experiment Station Road, Stoneville, MS 38732, USA.

06/05/2010 Updated by:

Alison Mikulyuk, Wisconsin Dept of Natural Resources, Science Operations Center, 2801 Progress Rd, Madison, WI 53716, USA