Salvinia molesta

Salvinia molesta D.S. Mitch.
(Latin: molestus, annoying, troublesome, or burdensome, in reference to problems caused by the rapid vegetative growth)

Local names: giant salvinia, kariba weed, aquarium water-moss, African pyle

Floating leaves often ca. 13–30(–38) mm long but under some conditions up to 60 mm, usually cordate basally, usually notched apically, often folded and compressed into dense chains (flat when young); 2n = 5x = 45 (pentaploid; Jacono et al. 2001). Lakes and other aquatic habitats, reproducing vegetatively very effectively; primarily in East TX in Robertson (S.L. Hatch 7301, BAYLU, BRIT, TAMU, TEX-LL), Harris, Montgomery (SBSC), Liberty (M. Whitbeck 61, TAES), Orange (T. Davern s.n., TAES), Houston, Newton, Sabine, Shelby, and San Jacinto (H. Elder, pers. comm.; R. Helton, pers. comm.) cos.; it has also been reported from the Gulf Prairies and Marshes (Brazoria, Chambers, and Fort Bend, cos.—H. Elder, pers. comm.) and as far nw as Flower Mound (Denton Co.) in North Central TX (Jacono & Richerson 2005) and Wichita Co. (H. Elder, pers. comm.) in the Rolling Plains. It was first observed in TX in 1997 (Jacono 1999c), and by summer 2000 it was known in the state from 4 reservoirs and ca. 40 private water bodies (R. Helton, pers. comm.). By 2004 it had been discovered in at least 11 public water sites (H. Elder, pers. comm.) and by 2010 it was known from 17 TX lakes, including some of the state’s most popular recreational water bodies: Toledo Bend, Sam Rayburn, Caddo Lake, Sheldon Lake, Lake Texana, and Lake Conroe (TPWD 2010). Particularly large-scale infestations have been noted from Todedo Bend and the Swinney Marsh Complex on the Lower Trinity River, in Liberty County, the latter being one of the most serious in the U.S. (McFarland et al. 2004). It is continuing to spread in the state (H. Elder, pers. comm.), and in 2010 TX initiated a public awareness campaign to alert the public about this potentially devastating invasive (TPWD 2010). Now known to be present across the s U.S. in AL, AR (Peck 2011a), AZ, CA, FL, GA, LA, MS, NC, SC, VA, and TX (Weakley 2010); also introduced to Mexico (Baja Calif., Sonora, Tabasco—Moro-Olivo & Yatskievych 2009). Considered “one of the world’s worst weeds” (Jacono 1999c), it is thought to be native to South America (se Brazil—Forno & Harley 1979; Forno 1983; U.S. Geological Survey 2000) and is possibly of hybrid origin. The sporangia abort and the somatic chromosome number is 45 (5 times the base number of n = 9), the plant thus being pentaploid (Jacobsen 1983; Jacono et al. 2001b). Jacono (2001b) noted that “Salvinia molesta is not known to reproduce by spores. It reproduces vegetatively, that is, new plants develop as fragments break off from mature individuals.” It has been introduced by humans to fresh waters of tropical and subtropical areas around the world including Africa, Asia, Australia, s Europe, New Zealand, and the South Pacific, which has resulted in severe economic and environmental problems (Jacono 1999a, 1999b; Garbari et al. 2000). The plants can grow rapidly and under good conditions doubling can occur in ca. one week, with a range of four to ten days (Mitchell & Tur 1975); some authorites indicate that populations can double in a little more than two days (Moran 2004). Giant salvinia covers the surface of lakes and streams, and the floating mats shade and crowd out native plants. Additionally, the mats (sometimes to a meter thick!) reduce oxygen content, degrade water quality, and can cause physical problems including hindering boats, clogging irrigation and drainage canals, and blocking water intakes. The mats can become so dense that they are able to support the weight of a cinder block. Further, in tropical areas the mats provide habitat for mosquitoes, which transmit human diseases including encephalitis, dengue fever, and malaria (Creagh 1991). Plants used in aquaria or water gardens are among the likely original sources for the escaped populations (Thomas & Room 1986; Jacono 1999a, 1999b, 1999c; Wood et al. 2001; Moran 2004), but spread by boats or boat trailers also seems likely (as in the case of other invasive aquatics such as Hydrilla verticillata, water-thyme). It was first reported outside of cultivation in the U.S. in 1995 from a private pond in SC; in 1998 outbreaks were discovered in LA and TX, and since that time it has spread widely across the s U.S. (McFarland et al. 2004). According to R. Helton (pers. comm.), “all reservoirs in East TX are imminently threatened” by this species. In TX, it is considered a “harmful or potentially harmful exotic plant” and it is illegal to release, import, sell, purchase, propagate, or possess this species in the state (Harvey 1998). Giant salvinia has been listed as a federal noxious weed since 1984 (McFarland et al. 2004; USDA Natural Resources Conservation Service 2010), and as such is prohibited in the U.S. by federal law. Jacono (1999a) indicated that, if seen, the species should be eradicated immediately, and that the Texas Parks and Wildlife, Inland Fisheries Division should be contacted at or (409) 384-9965. Biological control by the salvinia weevil (Cyrtobagous spp.) is being tested in East TX; this weevil, native to South America, has been successfully used in control programs in various places in the Old World (Thomas & Room 1986; Moran 1992, 2004; Wood et al. 2001; R. Helton, pers. comm.; H. Elder, pers. comm.). Initial indications are that the weevil is effective in helping to control giant salvinia in TX (Flores & Carlson 2006; Tipping et al. 2008). Although some authorities have suggested that the ambiguous name S adnata Desv. be used for this species, Moran and Smith (1999) argue convincingly for the continued use of S. molesta. The common name kariba weed originated in the 1950s as a result of problems it caused associated with the Kariba Dam project on the Zambezi River between Zambia and Zimbabwe (Robinson 2010).

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