The Weevils On The Water Hyacinths Biology Essay

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In South Africa, water hyacinth has a steep exponential growth as it is able to grow in a wide range of environmental conditions. It can be thus considered as one of the most invasive alien water weeds that threaten South Africa's waterways. It blocks the waterways, thus preventing drainage and navigation. It also impedes the growth of indigenous aquatic life in the waterways. The aquatic weeds grow in mats that cover the surfaces of the bodies of water. This prevents sunlight from reaching the aquatic plants beneath the surface, which results in their inability to photosynthesise. Water hyacinth has a widespread distribution across South Africa, thus resulting in many environmental and economic impacts, as well as difficulties with the control of the aquatic weed.

The purpose of this research task is to determine if the levels of nutrients in the water affect the amount of feeding of the weevils on the water hyacinths. Water hyacinths obtain the nutrients that are necessary for their growth from the water in which they are situated. Many of these nutrients accumulate in the water as a result from the leaching of unused nutrients in fertilizers from commercial farms, as well as the drainage from sewerage stations. This research task will determine if the levels of phosphates and nitrates in the water are increasing the amount of feeding by the weevils on the water hyacinths. It will thus indicate if methods need to be introduced to further control the concentrations of nutrients that are leaching into the water, and if the control of these nutrients will aid in reducing the difficulties in the biological control of water hyacinths. This research task will aid in making South African's aware of their contributions, which are mostly indirect, to the growth of the water hyacinths.

The findings of the research should be useful to:

The management of the commercial farms and sewerage stations, who should be made aware of the impacts that the drainage of nutrients from these establishments has on the biological control of the water hyacinths. This would aid in encouraging these establishments to take a greater interest in the control of these nutrients.

The experts and researchers who are involved with the biological control of the water hyacinths

Civilian South Africans who are passionate about the preservation of South Africa's waterways and indigenous aquatic life, so that they are made aware of the impact that they have on the environment

Literature Review:

Water hyacinth ( Eichhornia crassipes) is a free-floating aquatic plant that is widely considered as one of the world's worst water weeds. It is a magnificent Amazonian plant with beautiful and fragrant purple flowers (Joubert, 2009). The leaves of the water hyacinth are shiny and of a dark green colour, in rosettes with easily identifiable swollen petioles, which aid in keeping the plant afloat in the water. The plant was introduced into South Africa because of these aesthetic qualities. Water hyacinth first appeared in South Africa on the Cape Flats in 1908, and was almost immediately introduced into Kwa-Zulu Natal at about the same time (Joubert, 2009). Since its first appearance, the removal and control of the hyacinths has been unsuccessful in South Africa. This is problematic as the hyacinths have many negative impacts on South Africa's environment, society and economy. This literature review will indicate one of the main factors that limit the success of the control of the hyacinth, as well as how this factor encourages the growth and success of the water hyacinths.

The hyacinth plants form free-floating mats which cover the surfaces of water bodies and obstruct water ways, thus clogging water supply systems, drainage canals and hydro-power generators (Marshall, 2003). These aquatic weeds also disrupt navigation by boats (Joubert, 2003). The dense cover of weeds reduces and prevents light from penetrating through the water. This prevents submerged aquatic plants from photosynthesisng to produce food, which disrupts ecosystems within the water bodies. The prolific growth of the water weeds also causes a decrease in the oxygen and an increase in the carbon dioxide within the water bodies (Jones, 2009), which aids in the disruption of the ecological functioning of the indigenous aquatic life forms. The hyacinth also causes water loss through transpiration which is distinctly greater than the loss of water from open water through evaporation (Marshall, 2009). Due to the fact that the dense mats of this aquatic plant disrupts the flow of water, causing stagnant water to occur, water hyacinth provides suitable breeding sites for vectors of disease.

Due to all of these negative impacts of the water hyacinth, different methods of management have been implemented to assist in the control of water hyacinths. Mechanical, chemical and biological methods have all been implemented in the hope of controlling the plants (Kluge, RL. 1978). Since 1974, six species of biological control agents have been introduced into South Africa (Coetzee et al.). Two species of weevils (Neochetina eichhorniae and Neochetina bruchi), a moth ( Niphograpta albiguttalis), a mite (Orthogalumna terebrantis), a leaf sucking mirid (Eccritotarsus catarinensis), and a pathogenic fungus (Cercospora piaropi). These biological control agents damage the tissues of the plant, and they kill the roots, flowers and seeds, thereby reducing the plants resources (Coetzee et al.). Despite the high numbers of agents that are released, more than anywhere else in the world, successful biological control has not yet been achieved in South Africa. This is due to certain factors such as the removal of the weed through flooding, climatic incompatibility and the presence of many nutrient-enriched water bodies in South Africa (Coetzee et al.).

Water hyacinths spread the best in waters that are polluted with fertiliser run-off (Joubert, 2009), as they can absorb large concentrations of nitrogen and phosphorous from the water bodies. Other contributors to these high levels of nutrients and phosphates are the waste products of humans and animals (Marshall, 2003).

An investigation was conducted on the effect of pH and high phosphorous concentrations on the growth of water hyacinth by the University of Florida (Haller, WT & Sutton, DL. n.d.). Research that was previously conducted on other aquatic plants, was also analysed. The findings of the investigation, when compared to the previous research, indicated that water hyacinths absorb as much as four times the phosphorous than the other aquatic plants that had been studied (Haller, WT & Sutton, DL. n.d.). The high concentrations of phosphorous that were absorbed by the hyacinths depended on the phosphorous content in the water. This indicates that the water hyacinths do not absorb a fixed concentration of nutrients, which thus allows them to thrive in nutrient-enriched waters, as they can absorb most of these high levels of nutrients. Findings in the investigation also indicated that increased levels of phosphorous in the water, resulted in the increased growth and productivity of the water hyacinths (Haller, WT & Sutton, DL. n.d.). The investigation concluded that the introduction of water hyacinths in to water bodies that suffer from eutrophication would aid in reducing the levels of nutrients. Although this is true, it has been found that the water hyacinths thrive too well in nutrient-enriched waters and, thus, cannot be controlled. Therefore this conclusion has been invalidated as the water hyacinths would be an ineffective implementation to reduce eutrophication in water bodies.

At phosphorous levels below 0.06mg P/l the hyacinths are expected to die. Between 0.06mg P/l and 0.1mg P/l the plants can survive but are not very successful. Levels between 0.1mg P/l and 1.06mg P/l will result in the active growth and success of the hyacinths depending on nitrogen levels. Maximum growth occurs at a nitrogen concentration of 21mg N/l (Coetzee et al. 2011). This data was recorded in 2005, when the South African Water Research Commision (WRC) commenced an investigation to test certain ideas for improving the biological control of water hyacinths (Coetzee et al. 2011). The findings of the investigation were published by Byrne et al. 2010. The investigation aimed to determine the factors that were limiting the success of the biological control, and then aimed to solve these issues by implementing new ideas. Although the study was less clear, the investigation aided Coetzee et al. (2007) and Byrne (2009) in concluding that higher nutrient levels aided the water hyacinths in surviving the damage created by the biological control agents (Coetzee et al. 2011). The findings of this investigation indicated that the biological control of any aquatic plant becomes increasingly difficult as the nutrient levels, in the water in which the plants are situated, increase.

The website of the University (2013) of the Witwatersrand quotes Hill & Cilliers (1999) in stating that one of the major factors that limit the biological control of the water hyacinths is the excessive eutrophication of South African waterways. According to the National Eutrophication Monitoring Programme (n.d.) "eutrophication is the process of the excessive nutrient enrichment of waters that typically results in problems associated with macrophyte, algal or cyanobacterial growth", and in this situation, the growth of water hyacinths. The causes of eutrophication in South Africa are nutrient loads in agricultural, urban and industrial runoff, as well as in discharges from sewerage treatment plans. Eutrophication has a variety of detrimental impacts on South Africa. Examples of ecological impacts are the loss of biodiversity and the result of toxins that are released into the water from the bacteria that are thriving in the nutrient-enriched waters. Water treatment costs contribute to economic impacts. Other impacts are the disruption of the recreational uses of the water bodies, as well as human health impacts due to the toxins that are released (National Monitoring Programme, n.d.). Eutrophication is one of the major causes for the wide distribution of water hyacinths across the South Africa (Joubert, 2009). Eutrophication decreases the efficacy of biological control.

A research study was performed by the Faculty of Animal, Plant and Environmental Sciences at the University of the Witwatersrand in 2007. The aim of the investigation was to determine the impact that the concentrations of nutrients in the water had on the biological control of the water hyacinths. It was determined that at high nutrient concentrations productions of leaves and daughter plants were double than at lower nutrient levels (Coetzee et al. 2007). The lengths of stems were also double at high nutrient levels when compared with low nutrient levels. The chlorophyll content was also double at high nutrient concentrations than at low concentrations (Coetzee et al. 2007). These findings indicate that higher nutrient levels encourage the success of the water hyacinths. They also indicate that biological control would be insufficient to reduce the numbers of water hyacinths at such high concentrations of nutrients.

According to all of these research investigations, high levels of nutrients in the water promote the growth and development of the water hyacinths, as well as encourage their distribution. It is evident that this is one of the main factors that have limited the success of biological control of the hyacinths in South Africa, as South Africa's waterways and water bodies experience severe cases of eutrophication. This promotes the spread and growth of the hyacinths as the plants are better suited for survival in nutrient-enriched water. What has not been clearly indicated in the literature findings is how the nutrient levels in the water negatively affect the biological control of the hyacinths. My future research task will hopefully indicate if the concentrations of nutrients in the water directly affect the amount of feeding by the weevils on the hyacinths. This will determine one of the ways in which the concentrations of nutrients in the water impact the success of biological control. The research will also indicate the requirement for methods of the control of the levels of nutrients in the water, in order to aid in the success of the biological management of water hyacinths.


Four tubs of the same size and volume, each filled with 80 litres of tap water, will be used in this research investigation. Three healthy, free-floating water hyacinth plants of the same size and at the same stage of development will be placed into each tub, with all dead leaves and stems removed. The 80 litres will be measured using a bucket that is filled with 10 litres of water by a measuring jug. The level of the 10 litres of water will be marked off, and thus the buckets will be used to measure a volume of 10 litres of water, which will be used to accurately measure out 80 litres of water for each tub.

The following are descriptions of the contents of each tub:

Tub 1 will contain 80 litres of water, 8g of iron chelates, and three water hyacinth plants that are infested with 5 weevils each.

Tub 2 will contain 80 litres of water, 8g of iron chelates, three water hyacinth plants that are infested with 5 weevils each and 24g of 7:1:3 fertilizer.

Tub 3 will contain 80 litres of water, 8g of iron chelates, three water hyacinth plants that are infested with 5 weevils each and 48g of 7:1:3 fertilizer.

Tub 4 will contain 80 litres of water, 8g of iron chelates, three water hyacinth plants that are infested with 5 weevils each and 72g of 7:1:3 fertilizer.

The concentrations of nitrates and phosphates will be calculated using ratios, and the given information of the bag of fertilizer. The bag states that the fertilizer has a nitrate concentration of 95g N/kg and a phosphate concentration of 14g P/kg. Using ratio calculations, these concentrations will be converted to concentrations measured in mg/l, as this is the standard unit of measurement for the nutrients concentrations in water.

To measure the amount of feeding performed by the weevils, the feeding scars on the leaves will be counted at the end of each 7 day week for the duration of the experiment. To ensure that feeding scars will not be twice counted, ribbons will be tied around each leaf and stem that has already been counted so as to ensure accuracy. Feeding scars will be counted as they clearly show the amount of feeding that has occurred on each plant after each week, therefore conclusions based on the amount of feeding by the weevils will be able to be made by looking at the number of feeding scars. The feeding scars also indicate the damage that has been done to the plant, and will therefore allow for conclusions to be made on the success of the biological control of the water hyacinths. Photographs will also be taken each time readings are taken, so as to provide a digital documentation of the development of the investigation. The photographs will also provide support of the honesty of the data collection, so as to address any ethical issues surrounding the honesty of the data collection.

60 weevils will be used during the course of the investigation. Their treatment will be humane and cruelty-free, ensuring that none will be intentionally harmed by a human. This will eliminate any ethical issues surrounding the treatment and use of the weevils in the investigation.

As the investigation involves the use of invasive alien plants that are already problematic society, there are ethical issues surrounding the use of these plants. In order to eliminate the presence of these issues, careful steps will be taken in the care of the plants. Each time a reading is taken, it will be ensured that all that comes in contact with the plants and that may carry its seeds, such as hands, is cleaned and washed and ensured that all plant material is removed. The water hyacinths will remain in the tubs which ensures that they are removed from any running water, such as pipes in the gardens. Careful measures will be taken in the disposal of the plants, to ensure that they are properly disposed of in the correct manners, which includes properly cleaning all the tubs that contained the water hyacinths.

Apparatus list:

Permanent marker

4 tubs of the same size and volume


Measuring jug

Hosepipe and water source

Electronic scale

4 plastic bags

144g of 7:1:3 fertilizer

32g of iron chelates

Blue ribbon

Red, green and yellow ribbon

60 weevils

12 healthy, free-floating water hyacinth plants


Method of investigation:

Using the electronic scale measure 8g of iron chelates and place in a plastic bag.

Repeat step 1and fill the other three plastic bags with 8g of iron chelates each.

Using the electronic scale, measure 24g of 7:1:3 fertilizer and place in one of the plastic bags.

Close the bag.

Repeats steps 3-4 for measurements of 48g and 72g of 7:1:3 fertilizer to be placed respectively in the 2 other plastic bags.

Close the last plastic bag, containing only iron chelates

Set out the four tubs.

Using the permanent marker label each tub as 'tub 1', 'tub 2', 'tub 3' and 'tub 4'.

Using a measuring jug mark off 10 litres of water in a bucket using a permanent marker. Ensure that the bucket is set on level ground.

Use the bucket to fill each of the four tubs with 80 litres of water from a hosepipe.

Remove the dead stems, leaves and other dead material off of 12 individual, healthy water hyacinth plants that are of the same size and at relatively the same stage of development.

Place 3 individual water hyacinth plants in each tub.

Tag each plant to differentiate between them in order for data collection. Tie the pieces of ribbon around the longest stalk of each plant. In each tub: use a piece of red ribbon for plant 1, a piece of green ribbon for plant 2 and a piece of yellow ribbon for plant 3.

Pour the contents of the plastic bag containing only 8g of iron chelates into tub 1.

Pour the contents of the plastic bag containing 8g of iron chelates and 24g of fertilizer into tub 2.

Pour the contents of the plastic bag containing 8g of iron chelates and 48g of fertilizer into tub 3.

Pour the contents of the plastic bag containing 8g of iron chelates and 72g of fertilizer into tub 4.

Count the number of feeding scars on the leaves of a hyacinth plant by marking each of the leaves that have been counted with pieces of blue ribbon.

Record the data in a table.

Remove the blue ribbon from the plant once all of the feeding scars have been counted.

Repeat steps 18 - 20 for each plant in the tubs.

Repeat steps 18-21 on the same day every week for the next 8 weeks.

Analyse the data by plotting line graphs and determining the relationships between the variables. If the feeding scars increase as the nutrient concentrations increase, it will indicate that the weevils will have an adverse impact on the plants.

Data Collection:

The information gathered will be recorded in a table format:

Independent variable: the concentrations of nitrates and phosphates measured in mg/l.

Dependent variable: the number of feeding scars per plant