Eichhornia crassipes (Martius) Solms-Laubach -- Pontederiaceae
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Native to the Neotropics, Eichhornia crassipes is a floating aquatic plant which has become a naturalized pest throughout tropical areas, even extending into the temperate zone (Bock 1969). Reproduction is primarily vegetative, daughter plants forming on stolons which originate from central rhizomes. Dense mats can form quickly, the plant doubling in volume every 10-15 days under favorable conditions (Penfound & Earle 1948). This plant has the ability to completely cover lakes and slowly moving streams, which can cause major navigational, agricultural and health problems. Although herbicides are effective in controlling water hyacinth, the cost is generally prohibitive (Goeden & Andrés 1999). A biological control project was initiated by the U. S. Army Corps of Engineers and the British Ministry of Overseas Development (Bennett & Zwölfer 1968).
Two lines of research were pursued, including surveys for natural enemies in South America (Bennett & Zwölfer 1968) and a study and use of indigenous organisms in the United States (i.e., Bellura densa (Walker) (Lepidoptera: Noctuidae), Cercospora rodmani Conway (Hyphomycetes), and Acremonium zonatum (Sawada) Gams (Hyphomycetes). The South American surveys turned up a number of candidates of which three were imported: Neochetina bruchi Hustache (Coleoptera: Curculionidae), N. eichhorniae Warner and Sameodes albiguttalis Warren (Lepidoptera: Pyralidae). A fourth species, Acigona infusella (Walker) (Lepidoptera: Pyralidae), was released in Australia (Julien 1987).
Studies revealed that the native host of Belludura densa is pickerelweed, Pontederia cordata L. (Pontederiaceae), which is closely related to water hyacinth; but the larvae can severely damage water hyacinth also. Studies produced a diet on which B. densa could be mass reared, which was followed by the experimental release of large numbers of eggs and first instars in attempts to augment native populations. However, the moth had very little impact in the United States (Julien 1987).
A fungus, Cercospora rodmani, indigenous to Florida was also found. The fungus causes leaf spot, leaf necrosis and secondary root rot of the hyacinth plants. It also causes plant death when applied to hyacinth mats. This fungus is being considered for registration as a commercial mycoherbicide (Charudattan 1986). Another fungus, Acremonium zonatum, damages water hyacinth in south Florida and Louisiana, but appears to have its greatest impact when associated with the mite, Orthogalumna terebrantis Wallwork. The mite is native to South America and accidentally entered the United States after its host plant was intentionally introduced as an ornamental.
Females of Neochetina eichhorniae and N. bruchi chew holes in the leaf petioles into which they insert one or several eggs, respectively (Center 1982). The larvae tunnel beneath the epidermis and work their way down to the base of the petiole or the rhizome to which the leaf is attached, by which time they are in their third and final instar. The fully grown larvae chew their way out of the stems and move toward the surface of the water. They cut several lateral roots which ar incorporated into an underwater pupal cocoon attached to the hyacinth roots. The emerging adults leave the water from emergent plant parts where they feed, mate and oviposit (Goeden & Andrés 1999). Weevils overwinter as larvae, pupae or adults, and there is one generation per year (DeLoach & Cordo 1976).
Sameodes albiguttalis oviposits into the spongy leaf petioles, favoring areas with cuts in the epidermis or injuries made by other organisms. The young larvae feed under the epidermis, periodically exiting onto the petiole surface, crawling downward, and then re-entering the globose area of the petiole to continue feeding. The fifth and final instar excavates a pupal cell in the petiole (Goeden & Andrés 1999). There are though to be five generations per year in Argentina, with larval feeding causing the petioles to break and die, resulting in heavy mortality the following winter (DeLoach & Cordo 1978).
Neochetina eichhorniae was introduced to the United States in 1972 from near Buenos Aires, Argentina (Center 1982), and it has established throughout the range of water hyacinth in North America. This weevil also was subsequently transferred from the United States to several other countries where it is now established (e.g., Australia, Fiji, Indonesia, New Guinea, India, South Africa, Sudan and Thailand (Julien 1987). Neochetina bruchi was introduced to the United States from Argentina in 1974 and is now established in California, Texas, Louisiana and Florida. Establishment has also been confirmed in India and the Sudan (Julien 1987).
Sameodes albiguttalis was released in the United States in 1977 (Center & Durden 1981), and is established in Mississippi, Louisiana, Florida and California, and it is also established in India and the Sudan (Julien 1987).
Acigona infusella was introduced to Australia from Brazil, but failed to establish. Orthogalumna terebrantis was introduced to Egypt and Zambia, but establishment is not confirmed (Julien 1987).
Neochetina eichhorniae seems to be the major contributor to the control of water hyacinth in the United States, Australia and the Sudan (Goeden & Andrés 1999). Cofrancesco et al. (1985) documented the reduction of water hyacinth in Louisiana from about 445,000 ha in 1974 to 122,000 ha by 1980. Sameodes albiguttalis retards growth in the early stages of mat development, although its action may be sporadic and patchy (Center 1985, Julien 1987). When introduced from Buenos Aires, N. bruchi was observed to successfully control water hyacinth in an isolated reservoir in La Rioja Province, Argentina, which indicated that its control potential should not be minimized (DeLoach & Cordo 1983).
The original surveys in South America which uncovered the several important natural enemy species, were primarily performed by Dr. Aquiles Silveira-Guido of the University of Uruguay. The U. S. Department of Agriculture sponsored his searches by building special laboratory facilities at the university, and providing travel funds necessary for the searchers. Dr. E. F. Legner of the University of California, accompanied Dr. Silveira-Guido on one of his original discovery trips to southern Brazil. The researchers traveled through natural waterways in canoes in this region of heavy illegal movement of contraband from Brazil to points south. On several occasions the investigators had to confront smugglers, offering them cigarettes and casual conversation to ward off their suspicions. Dr. Legner observed and remarked to the investigators that the waterways were heavily clogged with hyacinth plants, and he wondered of what possible benefit natural enemies obtained from the area would be in biological control. In fact, the hyacinth seemed to be a necessary factor in the balance of the local ecosystem, providing food and shelter to a number of native animals, including a species of crocodile that formed nests from the leaves of the hyacinth plants. The successes achieved in countries to which the phytophagous insects were eventually transferred emphasizes one's inability to predict a biological control outcome by observations at the native site.
Biological control of water hyacinth project was the second attempt at biological control of an aquatic weed with introduced arthropods. Although focus remained on the search and importation of natural enemies from South America, the discovery of the indigenous Bellura densa and Cercospora rodmani added another dimension to the research on biological control of aquatic plants in the United States. Despite the limited success of attempts to augment natural B. densa population levels in Florida, the moth may eventually prove useful as an introduced natural enemy in other countries (Goeden & Andrés 1999).
Goeden & Andrés (1999) point out that the reduction of water hyacinth has tempted control workers to target remaining pockets of plants with herbicides. However, without proper integration this action can upset the balance between natural enemies and their hyacinth host, causing further hyacinth outbreaks (Center 1982). Buckingham & Passoa (1985) suggested that in an integrated control program when conservation of the weevils is desired, summer herbicide applications should be delayed until the greatest number of newly emerged weevils with well developed wing muscles are present. These agents are then better able to migrate to unsprayed areas. They also urged that herbicides not be applied in the spring until water temperatures were above 18°C, the threshold for wing development, although herbicidal control is more efficient if begun early in the season (Goeden & Andrés 1999).
In Bangladesh water hyacinth was a pest 50 years ago when the human population was small. Today it is mainly an asset, being used as a green manure and mulch, or buried as a fertilizer and valuable source of potash. Dried hyacinth plants are used as fuel or as cattle feed. (Uchida & Arado 1988)
For further detail on biological control effort and biologies of host and natural enemies, please also see the following (USDA 1965, Bennett 1966, 1970; Maddox et al. 1971, Brown & Spencer 1973, Andrés & Davis 1974).
REFERENCES: [ Additional references may be found at: MELVYL Library ]
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