Brazil is the most likely place of origin of water hyacinth, Eichhornia crassipes, (C. Martius) Solmn-Laubach, with a natural extension to other areas on the South American continent. The beauty of its flower led to the plants introduction into other tropical countries as a decorative plant (Barret and Forno, 1982), and finally its conversion into a weed in response to the high level of nutrients in the urban, industrial and municipal wastewater.
International experience (Harley, 1990; Gutiérrez et al. 1994;) shows that the plants reproductive capacity, adaptability, nutritional requirements and resistance to adverse environments make it impossible to eradicate, and difficult to control. A variety of methods have been tried to curb the growth of the weed. Herbicides are used most often, because they provide an immediate action tool, although, they are costly and may have toxic effects if not applied according to the manufacturers instructions. However, in severe infestations, high coverage techniques are needed, such as herbicides and mechanical control to reduce the infestation. Other means can then be used, and in this regard the inclusion of biological control agents will in time supplement current methods of water hyacinth control, ensuring a sustainable management of the weed. The use of an integrated management of water hyacinth (timely implemented, adequate techniques) and the establishment of a maintenance control programme will ensure the reduction of the infestation levels. This chapter describes the principal progress made in the control of this weed.
In the case of severe infestation, aquatic herbicides are a swift and effective technique for managing water hyacinth. There are three most commonly used aquatic herbicides: 2,4-D (2,4-dichlorophenoxy), Diquat (6,7-dihydrodipyridol [1,2 a: 2,1-c] pyrazinediumion) and Glyphosate (Isopropilamine salt of N-phosphonomethyl glycine). For their use, the approval of plant protection agencies is necessary, and must be applied strictly by trained technicians.
The formulation of 2,4-D include the granular ester (butoxyathylester: BEE) for use on submersed weeds and the liquid dimethylamine (DMA) for emergence such as water hyacinth. 2,4-D is a systemic herbicide; it is readily traslocated from foliage to roots. It inhibits cell division of new tissue and stimulates cell division resulting in growth inhibition, necrosis of apical growth and eventually total cell disruption and plant death. Control is carried out approximately two weeks later.
The diquat formulations, for aquatic macrophyte weeds use, are liquid bromide salts. Diquat is rapid absorbed by foliage (1-2 hours) causing a rapid inactivation of cells and cellular functions through release of oxidants.
Glyphosate is a non-selective systemic herbicide, readily absorbed by leaves and throughout the symplast. All plants can be eliminated after three weeks (Gutiérrez et al. 1996). Glyphosate has a low toxicity and rapid decomposition in water.
Herbicide applications are usually less expensive than mechanical control, but may have to be repeated on an annual basis owing to the fact that once plants are removed, light penetration increases, favouring the germination of water hyacinth seeds and therefore new water hyacinth reinfestation. In addition, human and ecosystem health have to be take in account when aquatic herbicides are applied to water supplies, particularly drinking water. In this way, the main problem is the use of a wetting agent and a penetrant necessary to increase the effectiveness of herbicides.
Mechanical control by using a chopper or a shredder is not recommended because fragmentation may accelerate the spread of plants and, in consequence, aggravate the problem. Mechanical harvesters can remove the plants and prevent regrowth. There are a number of manufacturers of aquatic plant-harvesting equipment but the design concept is similar: spinning knives, a collector for vegetation and shore conveyor. The cost (US$60 000 00-200 000 00) and efficiency in terms of very slow removal (large harvesting can, under ideal conditions, only harvest 1-2 acres per day), and the fragmentation (may accelerate the soaring of weeds) and the release of hydrocarbon pollutants (not evaluated) would be extremely expensive, and also perpetual.
BIOLOGICAL CONTROL OF WATER HYACINTH BY USING INSECTS
Biological control is based on the use of natural enemies of the weed to discourage its development (Deloach et al. 1989). The biological control of water hyacinth began in the 1960s and produced the classical control strategy that involves the importation of natural enemies from the point of origin of the weed. Biological control requires time for the assessment of their impact, but, once established, populations remain present and in this way, the long-term cost in weed management, is less than other control measures and less harmful to the environment.
Research into the use of biological agents for water hyacinth control includes arthropods and pathogens. In the case of arthropods, only a few insects have been found to reduce the growth of water hyacinth significantly. Of these, only the following species have been considered worthy of introduction to other countries:
the mite Orthogalumna terebrantis Wallwork (Bennet, 1981);
the moth Acigona infusella Walker (Deloach et al. 1980) and Sameodes albiguttalis (Warren) (Deloach and Cordo, 1978;
the miridae Eccritotarsus catarinensis Carvalho (Hill et al. 1999)
the weevils Neochetina eichhorniae Warner and Neochetina bruchi Hustache (Deloach and Cordo, 1976 a, b; Center et al. 1982).
These last two species are the agents that have provided the best results when used within an integral control programme (Cofrancesco et al. 1985). However, its impact has been variable. In a few cases, control of water hyacinth solely by the use of insects has been successfully reported (Deloach and Cordo, 1983; Cilliers, 1991; Van Thielen et al. 1994.). The efficacy of those arthropods was not achieved at the desired level of control because of the following factors:
Injudicious herbicide application.
In the case of S. albiguttalis, requirements of young and actively growing plants for its establishment.
In the case of Neochetina spp, plant quality might influence the abundance of Neochetina (Center and Dray, 1992, Center and Wright, 1991); weevil populations increase slowly and therefore weevil density is too low for control (Perkins, 1978). In addition, these insects have relatively long life cycles (66-75 days for N. bruchi and 96-120 for N. eicchorniae). Population build-up is slow, compared with rapid plant-growth rates (a plant reproduces every 7-10 days), therefore plant reproduction may occur far more rapidly than the damage inflicted by the weevils (Martínez et al. 2001).
In the case of E. Catarinensis and O. terebrantis, this insect has been released in far fewer countries; and little impact on plant growth has been observed (Julien and Griffiths, 1998).
High incidence of disease on insects (Cordo, 1996).
Despite the difficulties connected with insect establishment, water hyacinth remains a candidate for successful biological control, but the following considerations must be take into account:
surveying possible insect disease before and after release;
ascertaining its reproductive capacity;
continuing insect release;
releasing new insects-ecotypes;
understanding factors affecting insect population growth that regulate and maintain populations at realistic sizes.
BIOLOGICAL CONTROL OF WATER HYACINTH BY USING PATHOGENS
Because of the reproductive capacity and fast growth of water hyacinth, it has been necessary to use a set of biocontrol agents to increase the biotic stress in order to reduce population resurgence. Among the natural enemies of water hyacinth, plant pathogens can be useful because, as bioherbicides in an integrated weed control programme, they are often host- specific (no risk to crops, native plants or animals), easy to propagate and disseminate and self-maintaining, thus reducing the need for repeated applications. However as is the case for other biopesticides, microbial herbicides are inactivated in the environment by exposure to temperature, low humidity and ultraviolet radiation. In fact, the main problem of biopesticides is their large-scale production in a formulation that allows for a successful application in the field.
Among plant pathogens, fungi are the most important natural plant pathogens. Many fungal pathogens have been cited in the literature as potential biocontrol agents for water hyacinth. Among them are Cercospora piaropi (=C. rodmanii), Acremonium zonatum, Alternaria eichhorniae, Myrothecium roridum, Rhizoctonia solani and Uredo eichhorniae.
A. eichhorniae and C. piaropi, have been studied for their biology, biocontrol potential, host specificity formulation and have been tested in experimental conditions (Conway and Freeman, 1977; Freeman and Charudattan; 1984, Martínez and Charudattan; 1998, Martínez and Gutiérrez, 2001; Shabana, 1997; Shabana et al. 1997). Results indicate that damage produced by fungus is enhanced when used in combination with insects (Charudattan, 1996; Galbraith, 1987, Martyn, 1985; Freeman and Charudattan, 1984). However, no commercial bioherbicide for water hyacinth is available.
Biological control reduces weed vigour, combined with environmental conditions, phenology of the plant and integrated use of other management options. The sole use of a biocontrol agent does not in itself ensure the success of the control. The biocontrol of water hyacinth, should form part of an integral control programme that includes timely, programmed technical visits to evaluate progress. In a severe infestation, wide coverage techniques (chemical and/or mechanical control) may be needed. Once the infestation has been brought down to a manageable size, the biocontrol agents should be released in safe areas. To determine the impact of biocontrol agents on the weed, both before and after the organism has been established, an evaluation has to be made of the biomass and plant density (fresh weight per unit area and the number of individuals per unit area). The number of sampling points is based on the size of the water body. The parameters that are evaluated are: fresh weight (kg), number of plants, number of daughter plants, number of flowers, number of leaves per plant, size of the plant (cm), number of insects (adults, larvae, pupae), disease severity and disease incidence in the case of pathogens.
PHENOLOGY OF WATER HYACINTH AS A CONTROL STRATEGY
The identification of susceptible periods in the water hyacinth growth cycle can be used in managing this weed. Luu and Getsinger, (1988) have observed a marked decrease in ramet and biomass production resulting from flowering. Pieterse et al. (1976) have observed that non-flowering plants produced over twice the number of ramets and nearly double the biomass compared with flowering plants. This phenomenon could suggest that the sexual reproductive phase of the plant is the control point to take into account when a control method is applied.
The cost involved in the management of water hyacinth in the world is so high for both the economy and ecology that environmentally safe and economically sustainable controls are necessary to provide long-term solutions to weed infestation. An integrated control programme for water hyacinth (adequate techniques implemented in time) has to be structured according to the characteristics of each site. In addition, a maintenance control programme has to be implemented year after year in order to minimize the cost of weed management. This is the main problem in developing countries where there is no awareness of the danger of this weed. In large-scale water hyacinth infestation, it will be necessary to reduce the coverage of the weed by means of chemical and/or mechanical control, and at the same time to include untreated reserves where repeated releases of insects and fungi have to be carried out. The objective is to bring down population levels of water hyacinth with these biological agents and to create a level of permanent stress, thus bringing about an effective control in the long run. In a biological control programme with plant pathogens, it is first necessary to make a selection of native micro-organisms in the area where they are to be used; in order to avoid the potential hazard of introducing plant pathogens. Finally, it is important to cite the statement made by Gopal (1987): Developing countries should not encourage the propagation of this weed for utilization. The interests of humanity can only be safeguarded by seeking effective long-term control of water hyacinth, rather than by its utilization.
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