PEST STATUS OF WEED

Salvinia molesta D. S. Mitchell is a floating fern na-tive to South America that in the last half of the twen-tieth century spread widely throughout the tropics and subtropics, moved in part by the trade in orna-mental plants for fish tanks and ponds. It forms dense mats over lakes and slow moving rivers and causes large economic losses and a wide range of ecological problems to native species and communities. It is of interest in the United States because of its recent es-tablishment in east Texas. Nature of Damage Economic damage. Mats of S. molesta (referred to hereafter as salvinia) impede access to and use of wa-terways for commercial and recreational purposes and degrade waterside aesthetics (Fig. 1). Mats re-duce habitats for game birds, limit access to fishing areas, and probably alter fisheries, all with negative economic consequences. Salvinia can clog water in-takes and interfere with agricultural irrigation, water supply, and electrical generation. It provides habi-tats for vectors of human disease with serious socio-economic impacts. In developing countries, the impact of salvinia can be devastating because weed mats block the use of waterways for transportation, cutting off access to important services, farm lands, and hunting grounds. The harm from salvinia mats to fisheries also can be very significant to communities dependent on fish for local consumption (sometimes as the main source of protein) or in areas where fish sales are the main source of cash income (Bennett, 1966; Thomas and Room, 1986). Salvinia also is a weed of paddy rice that reduces production by competing for wa-ter, nutrients and space (Anon., 1987). Ecological damage. The ability to grow very quickly (Cary and Weerts, 1983; Mitchell and Tur, 1975; Mitchell, 1978/9; Room, 1986) and blanket wa-ter bodies makes salvinia an aggressive and competi-tive weed (Fig. 2). Initially, salvinia forms a single layer over water, but with continued growth the mats become multi-layered and can reach up to 1 m in thickness (Thomas and Room, 1986). Thick mats sup-port other colonizing plants, and the high biomas