What is biological control?

According to our extensive bibliographic research carried out on the different control methods and their use worldwide, biological control appeared to be the most effective and sustainable method for controlling water hyacinth. Ndunguru J et al.,2001, reported that the Neochetina weevils have an effective and long-term impact, leading to gradual reduction in the water hyacinth vigor and thus increasing its susceptibility to competition from other aquatic vegetation. Moreover, a study conducted by Firehun et al., 2015, has shown that, the Rift Valley weather conditions are suitable for the two Neochetina weevil species. Neochetina weevils are a promising candidate for biological control of water hyacinth under Ethiopian conditions (Firehun et al., 2015). Thus, applying biological control in the study area is strongly recommended.

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 (Gunnarsson and Petersen, 2007). Besides, it is considered as a selective approach and short-term solution. The physical control promotes weed stands reduction, and growth of natural enemies for effective biological control.

According to the high infestation in the study area (30 000 to 50 000 ha of WH), an integrated approach with combining the ‘’biological and physical’’ control is strongly recommended. Ndunguru J, et al., 2001, reported that WH infestation in Lake Victoria has been reduced by 78% using integrated management strategies such as biological control using the Neochetina weevils and, manual removal. However, disposal of the water hyacinth biomass after the physical removal remains problematic, and it creates terrestrial disposal issues. Plant disposal leads to a new water hyacinth blooms causing environmental risks on water and biodiversity.

Although, if the water hyacinth has been properly disposed-of and collected, it can be used as a primary source to generate a high value products and energy through a variety of processes (Gunnarsson and Petersen, 2007). It can offer the economic incentives to facilitate a sustained and effective water hyacinth management program. Since the harvesting cost of water hyacinth is high, an effective approach requires identifying a usage for the harvested biomass, instead of disposing it. Although water hyacinth has been described as the most troublesome weed in the world, recent studies have shown that it has diverse uses. They further recommended that the potential of this macrophyte should be fully harnessed, which could change its status from a weed to an income-generating plant. An exploitation of this

plant will no longer pose problems of profitability since productivity will be ensured by harvesting a surplus of hyacinths. Frequent and controlled harvesting of excess water hyacinths is essential to better control the rate of water plant coverage of the water surface. For this reason, an integrated ‘’control-valorization’’ approach is highly recommended for the study area.

In addition, combining herbicides in an integrated approach with others control methods is also recommended. However, it should be used properly in specific infested sites. Applications of herbicides over wide areas result in large quantities WH biomass sinking into the water. The anaerobic decomposition of this biomass decreases dissolved oxygen in the water and results in massive biodiversity damage and greenhouse gas emissions. The herbicides also kill non targeted plants that are native, beneficial, and necessary for a healthy functioning of the lake’s ecosystem.

In Mexico, where a combined chemical-mechanical programme, 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 (Gutitrrez, 1996).

Moreover, integrating Roundup at different concentrations (Roundup (41% salt of glyphosate) and Caoganlin (10% salt of glyphosate)) with weevils (N. eichhorniae and N. bruchi) greatly suppressed the plants around some release areas of China. 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 (Jianqing et al., 2001). Nevertheless, the herbicide application on WH is forbidden in many others areas of China. For this reason, an asexual reproduction inhibitor, KWH02, was invented for controlling WH. The KWH02, shows that is a suitable control agent in areas, where water is used for drinking and also in lakes, pools, and creeks, where water flow rate is slow (Jian-jun Chu, et al., 2006).

Currently, no inorganic herbicides are used to control WH on Lake Tana. A recent experimental study carried out by Melkamu Birhanie et al., 2020, 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, shows that increasing acetic acid concentrations gradually by 5%, 10%, 15% and 20% removed the WH, while it did not affect the survival of Nile tilapia but reduced water quality. Spraying a 20% concentration of acetic acid causes complete damage to the WH, while a 15% and 10% concentration of acetic acid allows the development of new daughter plants and requires following spraying, whereas 5% is less effective (Birhanie et al., 2020).

As a conclusion, it is recommended to adopt an integrated approach in the study area with combining the ‘’biological and physical’’ control, or to switch to an integrated ‘’biological-physical and chemical’’ approach in emergency situations. In the second scenario the chemical control should be used properly in specific infested sites, but never in overall application to the whole infested area.