Water Hyacinth Eichhornia Crassipes Biology Essay

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Water hyacinth Eichhornia crassipes is an invasive aquatic plant that is alien to many of the ecosystems in which it has established itself. It proliferates in waterways and forms free floating mats on top of the waterway. If not controlled, water hyacinth will cover the entirety of the waterway. This significantly reduces the flow of water, blocks sunlight from penetrating the water's surface and starves the water of oxygen causing implications for the other organisms living in the waterways, such as the fish as they don't receive enough oxygen because the layer on top of the water doesn't allow the light to penetrate through to allow the underneath plant vegetation to photosynthesis and give off oxygen in which the fish use for survival. It is also a threat to human health in that it houses insects carrying diseases such as malaria and contaminates the water source through decomposition making it unfit for human consumption. Water hyacinth is regarded as South Africa's most damaging aquatic weed.

One effective method for the eradication of water hyacinth has been the introduction of water hyacinth weevils (neochetina eichhorniae and neochetina bruchi) which feed and proliferate on the water hyacinth, reducing its reproductive rate and, provided conditions are optimal, ultimately causing the death of the weed.

Research question

The effectiveness of the water hyacinth weevil as a biological control on the proliferation of water hyacinth will be influenced by a number of factors. The question that I hope that my research will answer is "To what extent does sunlight increase the activity and reproduction of the water hyacinth weevils and result in more damage to the water hyacinth plant through increased activity and reproduction?"


My hypothesis throughout this investigation is that the weevils will do greater activity on water hyacinth plants that are grown in full sunlight.


My aim in this investigation will be to determine whether the weevils are more active and more reproductive, thereby inflicting more damage, when introduced onto water hyacinth plants grown in full sunlight and resultant higher temperatures than when introduced onto water hyacinth plants grown in lower light intensities and resultant lower temperatures.

Importance of the research

The research is important because it will provide insight into the interrelationship between climatic conditions and the biological control of water hyacinth trough the introduction of weevils. This will help environmentalists to assess whether biological control of water hyacinth through the introduction of water hyacinth weevils is the most effective method of eradication of this plant in certain prevailing weather conditions or whether alternative methods, such as mechanical or chemical methods, would be more effective in protecting the ecosystem.

Literature review

The proliferation of water hyacinth has an extremely damaging impact in many of the aquatic waterways in South Africa. For example it contributes to:

the spread of deadly diseases such as malaria,

the break down of ecosystems which adversely impacts certain industries, such as fishing, and

the poisoning of water sources as it rots when diseased or dead.

Whilst there are alternative methods of controlling the proliferation of the weed, biological control is the safest option available from an environmental perspective. Every effort should be made to understand the impact of climatic conditions on the efficacy of the available biological options. The research set out below will confirm that weather conditions, particularly the impact of sunlight, has a significant impact on the selection of a biological solution as opposed to a mechanical or chemical solution.

Water hyacinth weevils weaken the plant in their feeding and reproduction activities. This makes the plant vulnerable to other weather elements such as water movement (waves) and wind which mechanically break down the plant. Weather conditions that encourage the activity (feeding) and reproduction of the weevils improve the chances of their effectiveness in controlling water hyacinth.

Successful biological control of water hyacinth through the use of the weevils has been limited to tropical and/or sub-tropical climates.

In 1991, weevils were released in the New Years Dam in the Eastern Cape, South Africa. Three years later, the water hyacinth population had reduced by approximately 80%. However, in the Western Cape, the introduction of these insects has not been as successful and researchers have concluded that the insects are not well adapted to a Mediterranean climate.

Many of the worst water hyacinth areas in South Africa occur at high altitudes that experience cold winters and frequent frosts. Research has concluded that one type of water hyacinth weevil (Neochetina eichhorniae) is much less effective in those conditions. This is due to the fact that the active growing season of the plants, and therefore the weevils, is limited to six months. The other type of weevil (Neochetina bruchi) is much more cold tolerant and play an important role in hyacinth control in colder regions.

The seasons play an important role in the activity of the weevils. Research has shown that the damage that the weevils inflict in heightened during the summer months and reduces during the autumn and winter months.

Adult water hyacinth weevils are nocturnal creatures i.e. they are less active during the day and more active during the night. This could lead to the conclusion that the weevils would be less active in environments where sunlight is present and more active where there are sustained periods of darkness.

However, the shape of the water hyacinth plant plays a role in reducing the impact of this. The plant is a mat that blocks out the sunlight thereby reducing the difference between nocturnal and diurnal activity.

Weevils are much more productive on healthy plants. The water hyacinth thrives in sunlit and warm conditions and growth is most rapid in the temperature range of 22- 35 degrees celcius . A colder or darker environment results in less healthy plants and a less effective environment for the weevils to proliferate.

Water hyacinth weevils are an environmentally friendly method of eradicating the most environmentally unfriendly aquatic weed, the water hyacinth. However, climatic conditions, specifically sunlight and temperature, play an important role in the effectiveness of the weevils as a control agent. In assessing whether to use the weevils as a measure to eradicate the weevils, the prevailing weather conditions need to be considered. Sustained periods of cold weather and/or darkness will cause the weevils to be dormant and not reproduce. This will have a significantly adverse effect on their effectiveness as control agents. Warm weather and/or sunlight increase the rate of reproduction and feeding activity and increase their effectiveness.


The experiment will be conducted in nine containers - three containers at each of the three different stations that will be exposed to different light intensities. The containers will be the containers for each individual water hyacinth plant for the duration of the investigation. The containers will all be exactly the same. Every component that the containers will enclose will be fixed variables and these variables are:

80 litres of water

24g of 7:1:3 fertiliser

8g of iron chelates

10 weevils

A water hyacinth plant, all of the same initial plant size

Areas with different light intensities will be set up to replicate areas with different light intensities found in nature. To replicate the different light intensities, shade cloth such as that used for covering car parking areas will be used. Two large structures (one meter in width by one meter in length and by one meter in height) will be constructed. These two constructions will be the structures in which the shade cloth will be positioned over to imitate different light intensities. The shade cloth will cover every opening so that no light can enter the construction enclosing the water hyacinth plants without penetrating through the shade cloth and therefore creating different light intensities. The components required in assembling one of the constructions to position shade cloth over will be:

8 meters of wood


Tape or glue

There will be three different stations, each station getting exposed to a different light intensity. In each station there will be three containers containing the above components. Three containers are used to ensure that the investigation is reliable and accurate results and to eliminate any chance of the research materials becoming tainted.

The first station will have three containers exposed to full sunlight and will have no shade cloth to adjust the light intensity.

The second station will have three containers exposed to full sunlight but will be enclosed in one of the previously discussed constructions and will have a single layer of shade cloth covering the entire enclosure, so no light can enter the construction enclosing the three containers of water hyacinth plants without penetrating through a single layer of shade cloth. Therefore the light intensity will be less than the light intensity of the first station that is exposed to full sunlight.

And the third station will have three containers and will be setup in the same way as the second station (exposed to full sunlight and enclosed in a construction and shade cloth covering the entire enclosure) with the only difference being that instead of a single layer of shade cloth like at station two the third station with have a double layer of shade cloth therefore lessening the light intensity even more than station two.

In gathering the information, once all the apparatus has been setup, I will mark off a specific leaf on each specimen of water hyacinth plant in each individual container. The marked leaf on each plant will be approximately equal in surface area and will be found in the same position on each plant to make sure all the marked leaves are of the same estimated age. The marked leaf on each plant will be from the newest batch of leaves produced by the plant because this will be the greatest area of activity of the weevils as the new leaves contain higher volumes of nutrients for the weevils.

The data that will be collected will be the number of feeding scars caused by the weevils eating the water hyacinth plant. The data will be collected by counting the feeding scars on the leaf blade, from the petiole to the apex (excluding the stem). The data will be collected from a marked off leaf of the most recently bloomed set of leaves of each water hyacinth plant. The data will be collect in the morning before school twice a week over a duration of eight weeks. The data will therefore be collected sixteen times throughout the own project.

The data will be analysed during the investigation by inserting the readings of "the number of feeding scars on a specifically marked leaf" into a table format every time data is collected, which will be twice a week for eight weeks. Each table will have a row for each individual water hyacinth plant's reading over the period of investigation


Once the investigation period is over I will use the table in which all my results from every time I collected my data is shown and by looking at my results I have gathered during the investigation period, I will display my results on a line graph. The graph will have an x-axis heading of the data collection number (1 to 16, because if I collect my data twice a week for eight weeks, 16 sets of data will be collected), and will have a y-axis of the number of feeding scars on a specifically marked leaf. All three stations' results will be displayed on one line graph and the three stations' results line will be in a different colour to the other two stations' results. This will give a clear view on how the number of feeding scars differed from plants that were exposed to different light intensities and will therefore prove my hypothesis to be true or false. During the entire experiment I will be taking photographs of my water hyacinth plants to show the progressive increase in feeding scars over the eight weeks. The evidence will be photographed on a regular basis and included in the analysis. I will take a photograph of each individual leaf from which I am counting the number of feeding scars on and will do this for all nine plants every time I collect my data (twice a week for eight weeks).

The temperature will be recorded as well using a thermometer to get a reading of the water temperature, as the different light intensities will have an impact on the water temperature.

There are a number of ethical issues that will be considered in conducting the investigation and presenting the results derived there from, inter alia:

The conclusions reached will be unbiased and will be presented in an unambiguous manner

All inputs and references will be recorded and referenced

Other organisms in the immediate vicinity of the investigation will not be harmed or become infested

The weevils will be disposed of at the end of the investigation in a responsible and humane manner