Figure 4 shows the annual scientific production and citations related to WH control methods. The research is considered relatively old and started 50 years ago. In fact, the first paper in this research area was published in 1974. The number of published research papers and citations have fluctuated over the last 2 decades, reaching a peak of 26 paper in 2020. This leads us to infer that this rise in the number of articles and citations over the years represents an increasing interest in this research area.

Our screening process reveals that accessible published research on the WH control methods in the world, is concentrated in India (9 articles), followed by South Africa (8 articles), and China (5 articles) (Figure 5). These three countries together published significant number of research papers in the past 2 decades.

Declining freshwater quality, through land-based run-off, is recognized as one of the most significant threats to the long-term health of freshwater areas. Nutrient run-off, along with sediment run-off and pesticides, is particularly crucial in many freshwater bodies. Nutrient run-off is associated with algae and WH blooms, and changes in microorganism species, which pose a risk to aquatic organisms (Emirhüseyinoğlu and Ryan, 2019).

Lake Victoria, the world’s second largest freshwater Lake, suffered from the high growth of WH. Soil erosion, nutrient runoff, as well as urban and industrial pollution have induced a rapid eutrophication of the Lake over the last 50 years. High levels of nutrients in the Lake caused a dramatic invasion of WH, considering that eutrophication is the main cause of the plant growth (Kaur et al., 2018). Reducing the loss of nutrients, sediments and pesticides surrounding freshwater bodies by managing the use of agricultural amendments, and by avoiding deforestation could reduce the end-of-catchment loads and enhance the resilience of freshwater bodies (Sharpley et al., 2006).

Drainage through agricultural soils can lead to the leaching of soluble nutrients and pesticides, which they could find their way to reach downstream waters as well as groundwater. Although nitrogen and phosphorus occur naturally, increased nitrogen and phosphorus from fertilizer leaching is harmful to the freshwater bodies and promote the proliferation of WH (George and Goldman, 2007).

The best way to manage the WH long-term effects is to better match nitrogen and phosphorus inputs to crop needs (Swallow et al., 2001). Furthermore, control of organic pollution can be also done by substituting synthetic chemicals fertilizers with organic fertilizers (Krisnawati et al., 2020). By optimizing nutrient application there is less chance of surplus nutrients being lost to waterways, where they can harm the environment, and increase the proliferation of WH.

Physical methods consist of two kinds of control: manual and mechanical removal. Manual removal is done by human, to remove the WH plants from the water bodies (Ajithram et al., 2020), while the mechanical removal method is carried out by machines. Mechanical removal of WH consists of using weed cutters, harvesters, chaining, shredder boats, and dredging process. Mowing, netting, and barriers are also used as ways of mechanical removal. Barriers in the form of booms or cables have been effectively used in some countries such as Ivory Coast, Zambia, and South Africa to protect hydropower water intake pumps and water extraction pumps (Hill and Coetzee, 2008).

Physical control remains the only method through which WH can be transformed into value-added products. It allows the collection of the raw material and its potential valorization to compost or green energy and other uses. Physical control immediately opens physical space (habitat) for fish, boat traffic, fishing and recreation (Yigermal and Assefa, 2019). A study on mechanical harvesting of WH from an Australian lagoon has allowed an increase in dissolved oxygen, which was related to a potential increase of the water inflow (Perna and Burrows, 2005). Thus, physical control is the most suitable method to remove this plant, for economic and commercial benefits. Hand removal is most effective for small infestations while mechanical harvesting can be an effective tool for removing larger infestations. However, mechanical shredding and in-situ cutting processes, where plants are left to die and decompose in the water, can decrease dissolved oxygen in the water, and then catalyze the releases of phosphorus from the sediment. This process accelerates eutrophication phenomena, and can lead to a subsequent increase in WH or algae blooms (Mitan and Merry, 2019). A previous study carried out in laboratory using a water column, designed to mimic nutrient conditions of Lake Chivero (Uganda) reported that total nitrogen and phosphorus levels increased following mechanical (shredding) control of WH. Increased nutrient concentrations may be attributed to a release of these nutrients from decomposing plants (Villamagna and Murphy, 2010). Therefore, to overcome this eutrophication problem, the mechanical control must be associated with an integrated approach of valorization to produce biofertilizers and/or bioenergy from the plant (Tewabe and Asmare, 2020).

Chemical control is an immediate and short time solution to remove WH using chemical herbicides. Chemical control has been used in several countries including Ghana, Nigeria, South Africa, Zambia, and Zimbabwe. The most effective and cheap herbicides used to reduce WH spread are glyphosate (Isopropilamine salt of N-phosphomethyl glycine), diquat (6,7-dihydrodipyridol pyrazinediumon) and 2,4-D (2,4-dichlorophenoxy) (Güereña et al., 2015).

Herbicides are very efficient to control WH (Villamagna and Murphy, 2010). However, they can affect and damage the aquatic system. It is considered harmful for the environment compared to the mechanical and the BC approaches (Ajithram et al., 2020). Furthermore, this method may lead to significant socioeconomic impacts if the quality of the freshwater is affected (Villamagna and Murphy 2010). For this reason, in many countries there are restrictions that prohibit the use of chemicals in water, intended for drinking purposes.

Using herbicides can be considered only under certain conditions such as in case of small-scale infestations and in emergency situations. Tewabe and Asmare (2020) reported that herbicides such as 2,4-D, diquat, paraquat, and glyphosate have resulted in successful control in small, single-purpose water systems such as irrigation canals and dams of around 1 ha in size. However, Souza et al. (2020) reported that the use of glyphosate herbicide at its highest recommended dose of 7.0 L ha⁻¹ or 3,360 g of acid equivalent per ha, showed a low potential of environmental impact in the control of WH in reservoirs, but it should be used properly in specific infested sites and never in overall application to the whole infested area. In addition, an integrated approach with other control methods is recommended and seems to be unavoidable in certain circumstances. For example, interactions between chemical and BCs have been demonstrated. Previous studies reported that herbicides can complement BC (Cilliers et al., 2002; Perna and Burrows, 2005; Villamagna and Murphy, 2010). Paraquat, glyphosate and 2,4-D have been used to enhance the effects of Neochetina spp. species. Herbicides reduce WH populations to low levels prior to the introduction of biocontrol agents, thereby giving the agents time to better establish itself and suppress the regrowth of the weed. Moreover, creating barriers where WH can accumulate as whole block makes herbicidal control more effective. Combining mechanical and chemical control methods is a good option that saves time and energy following small or individual plants targeted for spraying (Cilliers et al., 2002; Perna and Burrows, 2005; Villamagna and Murphy, 2010).

A recent experimental study carried out on the effect of different concentrations of acetic acid on WH, aquatic life, and the physicochemical properties of water under pond conditions on the shores of Lake Tana in Ethiopia showed that increasing acetic acid concentrations gradually by 5, 10, 15 and 20% removed WH. However, it did not affect the survival of Nile tilapia but reduced water quality. Spraying a 20% concentration of acetic acid caused complete damage to the WH, while 15 and 10% concentrations allowed the development of new daughter plants and requires follow-up spraying, whereas 5% was less effective (Birhanie et al., 2020). Table 1 summarizes the world’s most used herbicides against WH, their principles, effectiveness, duration, doses, pretreatment methods, cost, advantages, and drawbacks.

Biological control is also one of the processes employed to remove or stop the growth of WH plants. In this method, some organisms including insects, pathogenic bacteria, fungi and parasites, among others were used (Yigermal and Assefa, 2019). The aim of any BC is not to eradicate the weed, but to reduce its abundance to a level where it is no longer problematic.

Integrated control is the use of two or three control methods together (Table 4). Its includes aspects of BC, herbicide applications, physical removal as well as the management of nutrients feeding the aquatic ecosystem (Van Wyk and Van Wilgen, 2002). It was reported that the key element of this approach is first the appointment of an organization to drive the control program, the involvement of all interested and affected parties, the division of the study area into management units, and finally, the application of the appropriate control methods. The control methods combination promotes weed stands reduction and growth of natural enemies for effective BC. Moreover, the use of combined control methods, in many cases, shows that it can be cost-effective than the application of only one control method in the entire study area (Hill and Coetzee, 2008). Table 4 summarizes the advantages and disadvantages of WH control combined methods.

After WH removal, this plant constitutes a valuable biomass for biofertilizers and bioenergy production (biogas, biohydrogen, bioethanol, liquid, and solid biochar), water treatment, charcoal briquettes, paper making, fish feed, animal feed, furniture making, fiberboard making, yarn and rope manufacture. All these potential ways have been deeply reviewed (Lu et al., 2008; Ratnani et al., 2020; Harun et al., 2021).

The water hyacinth is a potential candidate for bioenergy production because it is a lignocellulosic biomass. which could be used for sugar production, and fermentation to bioethanol and biohydrogen (Rezania et al., 2017). Another research has evaluated the bioconversion of fermentable sugars derived from WH into microbial lipids and single cell proteins by an oleaginous yeast. This study indicated the feasibility of sustainable valorization of WH-derived liquid hydrolysate towards a biorefinery framework Alankar et al. (2021). By the way, Arutselvy et al. (2020), have conducted a sequential valorization strategy for dairy wastewater (DW) and WH to produce fuel and fertilizer. Anaerobic co-digestion of WH and DW improved the process efficiency and increased biomethane production. Biomethane yield from the codigestion was higher compared to the monodigestion. The residual digestate was used for pyrolysis to obtain bio-oil and biochar. Another study indicated that the biochar obtained from WH pyrolysis could serve for heavy metal adsorption, and could be used for bioremediation application (Hasan et al., 2021). In addition, an integration strategy of hydrothermal carbonization (HTC) and anaerobic digestion (AD) using WH, showed that hydrochars and process waters from HTC process, could be valorized for a solid combustion fuel and biomethane production (Brown et al., 2020).

Besides bioenergy, WH valorization showed its importance in the agricultural sector as well. A novel approach of formulating an enriched N-P-K (nitrogen-phosphorous-potassium) fertilizer from WH and other residues (wet blue leather and poultry bone) has been performed. The developed organic N-P-K fertilizer resulted in significantly higher plants compared to commercial fertilizers (Majee et al., 2019). Moreover, WH could be transformed to a high-quality forage for animal feed. A previous study showed that the cultivation of edible mushrooms on WH biomass degrades almost all the lignin-cellulose of the substrate through enzymes, and enriches it with high protein content, making the spent substrate an excellent forage (Araiza et al., 2016).

Water hyacinth has been a subject for the development of cement composites and bioplastics. A study conducted by Salas-Ruiz et al. (2019), showed that WH root ash could be used as an alternative to pozzolans in cement matrices to fix these pollutants. This developed composite is cheap, green, and promotes waste and pollutants recycling and elimination. WH with other agricultural residues (bagasse and rice straw) could be transformed to produce bioplastic with a high biodegradability rate. This ecofriendly product could be used as an alternative to synthetic plastics (Ungprasoot et al., 2021).

Results obtained from this bibliographic research showed how it is beneficent to adopt these kind of strategies for efficient WH management. An economic feasibility study performed by Wang et al. (2019), showed that integrated valorization approaches could be beneficial, especially in the case of costly valorization technics such as bioethanol production. The study confirmed that coupled use of WH as a phytoremediation plant and bioethanol feedstock is a potential response to green development strategies.

The water hyacinth in the absence of its natural enemies, causes serious problems worldwide and especially in Africa, where it has rapidly invaded rivers and freshwater bodies, especially in tropical and temperate-warm regions. It has grown enormously in the last decades, becoming an agricultural, environmental, and public health problem.

Control of this weed is difficult, and all appropriate options available should be considered. If necessary, a variety of control measures can be used, depending on the nature of the problem and the local conditions of the affected area. Therefore, available environmentally friendly, and sustainable methods should be adopted (Labrada and Fornasari, 2002).

Different control programs of this weed have been applied worldwide on large-scale. Some countries have adopted the mono-control approach, by using one control method for the whole invaded area, while others have applied integrated control approach, which consists of combining the available control methods, at the same time but separated in space, according to the site’s specificity and conditions.

According to our extensive bibliographic research carried out on the different control methods and their use worldwide, BC appeared to be effective and sustainable for controlling WH. Ndunguru et al. (2001) reported that the Neochetina weevils had an effective and long-term impact, leading to a gradual reduction in the WH vigor and thus increasing its susceptibility to competition from other aquatic vegetation. Moreover, a study conducted by Firehun et al. (2015) has shown that Neochetina weevils are a promising candidate for BC of WH under tropical conditions.

The applications of herbicides over wide areas results in large quantities of WH biomass sinking into the water (Ajithram et al., 2020). The anaerobic decomposition of this biomass decreases dissolved oxygen in the water and results in massive biodiversity damage and greenhouse gas emissions. In addition, herbicides kill non targeted plants that are native, beneficial, and necessary for healthy functioning of the Lakes’s ecosystem (Rosic et al., 2020). In Mexico a combined chemical-mechanical program, using the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and a mechanical harvester, was implemented to control WH on the Trigomil Dam. Reasonably successful results were obtained (Gutiérrez et al., 1996). In China, integrating Roundup at different concentrations (41% salt of glyphosate) and Caoganlin (10% salt of glyphosate)) with weevils (N. eichhorniae and N. bruchi) greatly suppressed the plants. The tests indicated that herbicides had to be used at a lower concentration than normal, to not kill the plants too rapidly and not deprive the insects of food and habitat. The combined biological-chemical approach was effective (Mallya et al., 2001). Nevertheless, herbicide application on WH is forbidden in many other areas of China. For this reason, an asexual reproduction inhibitor named KWH02 was invented for controlling WH. The results indicated that KWH02 is a suitable control agent in areas, where water is used for drinking and in Lakes, pools, and creeks, where the water flow rate is slow (Chu et al., 2006).

On the other hand, it was reported that only physical removal, either manual or mechanical, of WH can eliminate the plant’s biomass from the water. It is considered as a selective approach and short-term solution (George and Goldman, 2007). However, disposal of the WH biomass after the physical removal remains problematic and creates terrestrial disposal issues. Plant disposal leads to a new bloom, causing environmental risks (Gunnarsson and Petersen, 2007). If WH is properly disposed of and collected, it could be used as a primary source to generate high value-added products and energy (Ayanda et al., 2020). It can offer the economic incentives to facilitate a sustained and effective WH management program. Although, recent studies have shown that it has diverse uses (Mathew et al., 2015; Hernández-Shek et al., 2016; Sindhu et al., 2017; Priya et al., 2018). These studies further recommended that the potential of this macrophyte should be fully harnessed, which could generate significant incomes. In wide infested areas the exploitation of this plant should no longer pose problems of profitability since productivity will be ensured by harvesting a surplus of the plants. Frequent and controlled harvesting of excess WH is essential to better control the rate of water plant coverage of the water surface. Additionally, using BC combined with the physical one improved the management process. Ndunguru et al. (2001) reported that WH infestation in Lake Victoria was reduced by 78% using integrated management strategies such as BC using the Neochetina weevils and manual removal. Since the physical control leads to an immediate elimination of WH, the BC ensures the establishment of bioagents in the study area and maintain the program sustainability in case of future proliferation of the plant. To visualize the approaches feasibility, and to evaluate their effectiveness in terms of application and performance, a descriptive analysis on WH control approaches and programs used worldwide was conducted.

The objective of this analysis was to identify successful control programs around the world and to evaluate the most used and effective ones. Results obtained from previous large-scale WH control experiences were considered. The analysis was carried out in 26 countries, in America, Asia, Australia, Europe and Africa. Table 5 summarizes the bibliographic research results.

As shown in Table 5, in the 26 large-scale trials, 70 control methods have been applied. Each country adopted one or more control methods throughout the invasion period of the plant. In the experiments performed, the control methods have been used either separately or in combination.

Control methods classification in terms of application for the studied countries is presented in Figure 8. Out of the 70 control methods, the biological control method has been used 25 times with a percentage of 36%, followed by the physical treatment which has been applied 23 times with 33%, and finally the chemical and the integrated control have been applied 11 times (16%) for each one in all experiments. The most adopted control methods consisted of using the biological and the physical controls separately, followed by the integrated approach, and then the separated chemical control.

Most countries started with physical control alone followed by the biological one, and few countries have adopted the chemical and integrated control. The mono-physical control has failed in most of the case studies, while the mono-biological has been successful in the major cases. The chemical control has been successful as well; but has not been used many times. The integrated control has been used in few countries but it was successful in all of them.

The classification of the control methods in terms of effectiveness for the studied countries is presented in Figure 9. From 25 biological control programs, 21 were effective. The biological control has shown its effectiveness in some cases where it has been used alone, and has been 100% effective in all integrated program cases.

For the separate physical control, despite being used widely in several countries, only few cases (9 countries out of 23) were successful while, the chemical control has not been too much adopted, because it’s costly, harmful for the environment, and forbidden in some countries (Cabrera Walsh et al. (2017)). As shown in Figure 9, despite its negative impact, and being adopted only in few countries, the chemical control was effective.

The integrated control approach is the combination of one or more control method. It involves the test of at least two control methods simultaneously but separately in space, according to the site’s specificity and conditions. The combinations consist of integrated biological and physical control methods (IBP), integrated biological and chemical control methods (IBC), integrated physical and chemical methods (IPC) and integrated biological, physical and chemical methods (IBPC). In all case studies, 11 integrated control programs have been used and all of them were successful. The integrated methods IPB, ICB, IPCB, and IPC have been used 4, 3, 3 and 1 times respectively. As shown in Figure 10, the IPB was the most used program (37%) compared to the other integrated approaches. Although all integrated programs were successful, the combined physical and biological approach was the most effective. Therefore, it is recommended to adopt this integrated approach or to switch to an integrated “biological-physical and chemical’’ approach in case of emergency. In this latter case, the chemical control should be used properly in very specific and limited infested sites. Control methods must consider such scale dependent characteristics of WH invasions and ecosystem structure (Lu et al., 2007). Moreover, an integrated “control-valorization’’ approach is highly recommended. Awareness about WH benefits by developing new product locally based on this plant should be raised. This approach has two objectives: it will allow both WH removal and income generation. In addition, nutrient run-off reduction should be considered in management programs of WH. Because nutrient run-off is the major factor leading to the proliferation of WH in freshwater bodies, its control is necessary to decrease the vigorous growth of this plant (Chen et al., 2021). It was reported that an integrated control strategy is considered as the most effective method in South Africa, including biological control, manual removal, herbicide applications, and nutrient reduction (Paterson et al., 2019). Mitigation measures of nutrients’ input from agriculture activities and the monitoring of the streams feeding the freshwater systems should be implemented. All stakeholders and policy makers should be aware of the need to reduce the impact of pollution on freshwater bodies to succeed in managing the WH invasion.

Table 6 shows the cost/benefit of the 3 separate control methods and that of the integrated approach. It is worth mentioning here that the monetary investment per hectare (US$/ha) cleared for each separate control method was calculated based on research data (surface area infested and control cost in different countries) except for South Africa and Sudan, for which the cost of the manual and mechanical removal has been taken from previous studies (Van Wyk and Van Wilgen 2002; Jones, 2009; Tayeb and Nagat Mubarak, 2012; Ilo et al., 2020).

As shown in Table 6, the use of the physical or the chemical control separately requires high investment. However, in Sudan, Benin, South Africa and United States, the cost of the biological control was about (16.6), (20.9), (18) and (12.8 US$/ha) respectively (Van Wyk and Van Wilgen et al., 2020; De Groote et al., 2003; Tayeb and Nagata Mubarak, 2012; Wainger et al., 2018). The cost of the separate biological control seems to be the lowest compared to the other control methods. Furthermore, all successful programs consisted mainly of biological control that is considered the most cost-effective and sustainable control method but long-lasting and a slow-acting one. For example, in Benin, biological control has been conducted from 1991 until 2003 (De Groote et al., 2003). As a result of this large-scale experiment, Neochetina eichhorniae showed a significant reduction of the WH covered area.

However, biological control cannot be suitable for all regions, especially in areas that experience periodic floods and frost that cause high mortality of the bioagents since the plant is able to regrow after winter. In these regions, it is necessary to switch from a purely biological to a more integrated management approach, which includes also herbicide applications, mechanical and manual removal in addition to the biological treatment. For an effective biological control, the combined control methods promote weed stands reduction and growth of natural enemies (Hill and Coetzee, 2008).

A review on the benefits of WH (Eichhornia crassipes) for Southern Africa conducted by Ilo et al. (2020) clearly showed the economic feasibility of WH using control methods. The paper revealed that the integrated control (biological with chemical) of WH was the most cost effective method (16 US$/ha) followed by the biological (18 US$/ha) and the chemical (86 US$/ha) methods. Similar results were found in China where more than US$ 10 million/year was spent for the physical control even though it was not effective. From 2002 to 2006, the Shanghai government invested US$ 1.2 million for integrated management of WH that was effective (Chu, et al., 2006). From the obtained results, we can deduce that the separate physical and chemical controls require a big investment, while the integrated control approach gives the best return on investment and decreases the period of the program achievement.