Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems

Biological Control Agents Other Than Insects

The use of plant pathogens for the biological control of aquatic weeds has many advantages. Suspensions of spores can be readily formulated and applied to the weed like a herbicide. In theory, the plant pathogens could then reproduce and become selfsustaining. If so, this approach encompasses the best advantages of biological and herbicidal controls. However, pathogens tend to be environmentally sensitive and can be rendered ineffective by extreme temperature or humidity (Center et al. 1991). Restrictions of the importation of plant pathogens from abroad tend to prohibit the use of biological controls and limit the scope to native pathogens.

Fungi

Cerospora rodmani. In 1971, the association of a native fungus, Cerospora rodmanni, with a widespread decline of water hyacinth in the Rodman Reservoir, a large impoundment of water near Orange Springs, Florida, was discovered (Conway 1976a,b). The fungus is host specific to hyacinths and caused root rot and small spots on the margin of the leaf. The spots later expand and eventually cause the leaves to wither and die. The field tests revealed a high virulence of the fungus on waterhyacinths and severe damage to the plant. Tests in the greenhouse and in the field indicated that the fungus adversely affected the water hyacinth. Because it is native to Florida, no adverse effects on commerce or on the environment are expected (Conway and Freeman 1977). Although it has been formulated as a mycoherbicide, the fungus has not been effective for the control of waterhyacinths, and research is continuing (Center et al. 1991).

Snails

However, interest in the use of snails as biological control agents has waned, probably because of the environmental risk of the purposeful propagation of prolific, generalized herbivores and because snails are intermediate hosts for certain fish parasites (Center et al. 1991; Gallaghen and Haller 1990).

Marisa cornuarietis is a large freshwater snail that was accidentally introduced into Florida. It feeds on a variety of plant species including coontail (Ceratophyllum demersum), southern naiad (Najas guadalupensis), Illinois pondweed (Potamogeton illinoensis), salvinia (Salvinia rotundifolia), and hydrilla (Hydrilla verticillata). Large numbers of this snail may control weeds in small, confined water bodies, but their effective use as control agents is limited (Center et al. 1991; Blackburn et al. 1971b).

Another small snail, Pomacea australis, has been used experimentally and at one time was thought to be a promising biological control agent of plants.

Freshwater fishes

Some freshwater fishes consume large quantities of aquatic vegetation (Avault et al. 1968). The common carp (Cyprinus carpio), goldfish (Carassius auratus), Chinese grass carp or white amur (Ctenopharyngodon idella), tilapia (Tilapia spp.), and silver dollar fish (Metynnia spp.) are used for control of aquatic vegetation in many areas of the world. The latter two genera are sensitive to cold weather and, in colder climates, must be overwintered in temperature-control tanks.

The triploid Asian grass carp (Ctenopharyngodon idella) has been used in Florida and elsewhere for the control of aquatic weeds. However, this carp is not host-specific (Schmitz et al. 1993). (For more information about the use of the grass carp as a biological control agent, see the section about nonindigenous fish species.)

Diseases, viruses, fungi, bacteria, and nematodes

Diseases, viruses, fungi, bacteria, and nematodes have received little attention in research into biocontrol of aquatic weeds. Two diseases are the cause of the decrease of Eurasian watermilfoil (Myriophyllum spicatum) in the United States, but limited research has been conducted on their use.