tions of aquatic plants in fresh water and brackish water systems (Charudattan et al., 1990). However, despite this potential, commercial development and use of microbial agents as bioherbicides to manage aquatic weeds has lagged because of the high cost of registering microbial herbicides under Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) requirements and because of the technological difficulties in producing practical levels of control under field conditions (Charudattan, 1990b).
Certain other miscellaneous agents such as snails (Marisa cornuarientis and Pomacea australis), the manatee (Trichechus manatus), the crayfish (Orconectes causeyi), and competing plants (e.g., Eleocharis spp.) have been considered as biological-control agents for aquatic weeds. However, practical use of these agents has not been realized because of various constraints.
Herbivorous arthropods have also had an unquestionable record of success as biological-control agents of aquatic weeds. Unlike the fish, weed-control insects are highly selective, and only host-specific agents are used. Outstanding examples of weed control by insects include management of alligator weed (Alternanthera philoxeroides) by the chrysomelid beetle Agasicles hygrophila), water hyacinth (Eichhornia crassipes) by the curculionid weevils Neochetina eichhorniae and N. bruchi and the pyralid moth Sameodes albiguttalis), and giant salvinia (Salvinia molesta) by the curculionid weevil Cyrtobagous salvinia (Harley and Forno, 1990).
Biological control of aquatic weeds has tremendous potential as a nonpolluting, ecologically sensible, cost-effective, long-term alternative to mechanical and chemical controls. Although some agents, such as the fish and insects, have gained considerable recognition as successful agents and have been used on a wide scale for many years, similar success can be obtained with other agents, notably microbial pathogens. Classical biological control against aquatic weeds using fungal pathogens is another promising approach (Charudattan, 1990c).
Despite the demonstrated success in controlling alligator weed, water hyacinth, and salvinia, up to now the overall importance of biocontrol has been relatively small compared to other control methods. Continued research and regulatory support will be key to gaining any future success and benefits from biological control.
It has been noted that pesticide use can have unintended adverse effects on beneficial organisms and on other nontarget species of plants and animals that come into contact with the chemical or its residues. However, the risks to human health posed by exposure to pesticides in the environment, drinking water, or food have become primary arguments in the debate about pesticide use.
Concern about chemical pesticides was brought to the attention of the public with the publication in 1962 of Rachel Carson's Silent Spring, a powerful and