In North America more than 170 species of aquatic plants are classified as weeds; 40 to 50 species are considered to be of major importance (Andres and Bennett, 1975). Managing aquatic weeds is complicated by the fact that unrelated aquatic plant species, including some native species that are beneficial, may coexist with aquatic weeds; and not all weed species may require control in every situation. Moreover, water bodies are typically subject to multiple uses and some of the control methods may be incompatible with those diverse uses.
Generally, weeding using mechanical controls or by hand using various types of cutting, hoeing, and harvesting tools is the primary method of managing aquatic vegetation in many parts of the world (Wade, 1990). The secondary method is the use of chemical herbicides of which the predominant ones are 2,4-D, diquat, glyphosate, and various copper compounds (Murphy and Barrett, 1990). The use of chemical herbicides in water can result in residue and tolerance problems in potable water, nontarget effects when herbicide-containing waters are used for irrigation, and lack of selectivity.
These weeding and herbicide uses afford only temporary solutions; thus the cost of control by these methods is recurrent. Many public water bodies are intensively managed with respect to weed control, and the cost of such operations is often high. As a general rule weed management in public waters is underwritten by public funds, thus competing for the limited tax revenues.
Biological control can be a cost-effective, long-term solution (Andres and Bennett, 1975). Host-specific as well as nonspecific agents have been successfully used as biological control agents for several aquatic weeds. The herbivorous fish, the white amur or the grass carp (Ctenopharyngodon idella), has been used for a number of years in many countries for nonselective management of aquatic weeds, especially submerged weeds (Sutton, 1977). Among the weeds preferred by this fish are hydrilla, Chara spp., southern naiad, duckweed, and many other problem weeds. Since grass carp is not native to the Americas, a sterile triploid has been bred that cannot reproduce and become permanently established. Only this hybrid is allowed to be used for aquatic weed management in certain parts of the United States, and it is readily available from commercial suppliers in North America. When used at proper stocking rates, according to the size of water bodies and the nature of the weed problem, the grass carp offers an excellent, sustainable solution (Sutton and Vandiver, 1986).
Host-specific fungal pathogens also have excellent potential as biological control agents for aquatic weeds (Charudattan, 1990b). Several pathogens have been found that are capable of controlling aquatic weeds under experimental conditions. It has also been documented that Cercospora rodmanii and C. piaropi, two related species that cause foliar diseases of water hyacinth, cause natural epidemics capable of controlling this weed (Charudattan, 1986; Martyn, 1985). Similarly, natural epidemics have been associated with decline of large popula-