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Plantwise Technical Factsheet

kariba weed (Salvinia molesta)

Host plants / species affected
Oryza sativa (rice)

Plants: perennial, heterosporous herbs, free floating, with microspores and megaspores produced on the same plant, green, up to 30 cm long, 5 cm wide, mat-forming, mat to 2.5 cm thick (or much thicker, depending on local conditions such as water current, waves, etc.); roots absent; stems irregularly branched, pubescent.

Leaves: short petiolate, in whorls of three, two upper and one lower; upper leaves floating, photosynthetic, entire, elliptic-ovate to rounded, with a distinct midvein, aerolate, 0.7-3 cm long, to 1.8 cm wide, apices rounded to emarginate, the aerolae either fairly uniform in size throughout, or inner longer than outer; papillae apex split into several hairs that form a birdcage-like structure which traps an air bubble when submerged, creating a non-wettable upper surface; leaves often folded in half under crowded growth conditions; lower leaves subsessile or petiolate, with or without sporocarps attached, 1.5-2.0 cm long, to 0.5 cm wide, the petiole to 3 cm long, submersed, non-photosynthetic, finely divided into linear segments (feathery), segments appearing as and functioning as roots.

Sporocarps (when present): pubescent, sessile to long-stalked, globose to ovoid, rounded to apiculate at apex, either clustered at apex of submersed leaf or arising alternately in two rows down the length of the submersed leaf similar in size, sessile or stalked, in clusters or rows on lower leaves, the sporocarp wall a modified indusium; microsporocarps inconspicuous, globular, with an internal short column, the columns basal, bearing many microsporangia; microspores minute; megasporocarps inconspicuous, globular, with many megasporangia.

Microsporangia: stalked, with one massula (group of microspores); massulae with 64 microspores.

Megasporangia: sessile, with one megaspore; megaspores to 2 mm long.

Prevention and control

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).

Summary of invasiveness

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.

Related treatment support
Pest Management Decision Guides
CABI; CABI, 2017, Portuguese language
CABI; CABI, 2017, English language
External factsheets
BioNET-EAFRINET Invasive Plant Factsheets, BioNET-EAFRINET, 2011, English language
Plant Protection Service extension factsheets, Department of Agriculture Sri Lanka, 2010, Tamil language
Plant Protection Service extension factsheets, Department of Agriculture Sri Lanka, 2010, Sinhalese language
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