Biological Pest Control Case Study – the cassava mealybug

The Cassava plant was brought to Africa from South America in the 16th century. The next four years the starchy thickened roots became the main source of food for millions of Africans, providing up to 70% of the recommended daily intake. The cassava plant is the habitat of the cassava mealybug and is damaged by the insect.

The problem – The cassava mealybug- eating cassava plant- biological control of mealybug by using wasps.

When the cassava plant was introduced to Africa most of its predators for example mites and plant diseases, were not also introduced therefore leaving the plant free of most of its predators but in the 1970’s a pest called the cassava mealybug was accidently introduced from Latin America(2) The pest is a rare insect but it quickly spread across the entire cassava plant growing area and due to the lack of natural predators within 10 years it became the most important pest insect on cassava causing a loss of up 80% of crop. The mealybug(5) shown on left hand side of text has damaged the cassava plants by sucking sap from roots, tender leaves; petioles and fruit form the plant.

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The severely infested leaves of the cassava plant will turn yellow (3) and gradually dry out a severe attack on the cassava plant can result in shredding of leaves. On the right is a picture (4) of a cassava plant which has been infested by the mealybug. Another problem with the mealybugs is that some will inject a toxic substance while feeding causing deformation of the cassava plant and therefore there was less food to be harvest. Although timing is not regular, depending on biological events and conditions such as area under cultivation and climatic factors, it seems that major new diseases or strains of cassava disease tend to appear every 7–10 years. Table below shows the disease problem in Arica.

Biological methods/ processes- biological control of invasive species.

Cassava mosaic and mealybug control programs were introduced in the 1970s (2) to combat these two problems, the decline in crop because of the mealybug eating the plant and further decline because of the spread of disease. The Institute of Biological Control (CIBC) based in Trinidad and the International Institute of Tropical Agriculture (IITA) researched into how the mealybug was kept under control in South America. They eventually found that a tiny wasp (smaller than the head of a pin) called the Leptomastix, which laid its eggs on the mealybug was keeping the mealy bugs under control therefore protecting the cassava plant. The picture on the right shows the small wasp on the mealybug larvae.

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The picture on the left shows an infected cassava tuber dug from the field and sliced open. The wasps were able to control the cassava eating the cassava crops by the wasp laying eggs on the mealybug they grew on the mealybug, the wasp larvae then killing the mealybug this would have a huge effect on the population as seen in South America, of the mealybug and therefore restoring the natural balance. This could be the breakthrough that they were looking for and could control the mealybug.

Meeting the challenge

Tests were run on the wasp by the CABI Bioscience in the UK. There the mealybug was tested to see how highly specific they are to the cassava mealybug host because if they were not then they could also affect other insects in the continent and therefore would be too dangerous to introduce to the area. The results showed that the wasp was highly specific to the insect (mealybug) so were given the okay to be shipped to Africa so they then can be reared and then the distributed to other areas around Africa. This was a huge breakthrough as without this research it had the ability to cause huge damage on the environment and the economy because of further damage to other crops.

Appropriateness.-Has been successful with the mealybug.

The mealybug feeds on the cassava stem, petiole, and leaf near the growing point of the cassava plant. During feeding, the mealybug injects a toxin that causes leaf curling, slowing of shoot growth, and eventual leaf withering. This is the effect the cassava mealybug has on the plant resulting in a huge loss of crop due to infested plant crop. The yield loss in infested plants is estimated to be up to 60 percent of root and 100 percent of the leaves the root is the main source of food so this causes a huge problem for the crop yield. The results were a reduction in mealybug damage was seen in the first season following release in both the leaf and the root, and full control was typically achieved within 2–4 years (6) after and now after a decade (10 years) the cassava mealybug and the wasps lived in natural balance with the mealybug population down to a maximum of 10% of what they were 10 years ago at a peak of the infestation.(1) Now the cassava can grow with no or little damage from the mealybug as the wasp has stopped the 60% root damage and the 100% of leaf destruction. Yield losses as reported by farmers are averaged at about 80% during the 1983 cassava mealybug outbreak but were reduced to 43% in 1979 pre infestation levels. (7)

Implications

Environmental- An environmental implication is that when the wasps are introduced to south America there was a risk that it could have a huge impact on the food chain as by taking away one organism or gaining one can have a knock on effect on others for example of the wasp was then hunted by a predator this could leave its normal pray to multiply therefore having an over population which in turn can then have knock on effects to their pray and so on and so on until extinction of a food source. Causing problems for South America in the economy and for the environment also. This environmental implication causes an imbalance of the natural balance of wildlife; this may have a knock on effect of the overall environmental surrounding and the effect on the human population.

Economic

For the farmers of Africa the economic impact has been useful and successful by increasing dramatically there yield percentage therefore increasing the money income. Every pound of investment in the mealybug project control work has returned between 200 and 500 pounds. These benefits to the environment from this environmental solution with no costs but a huge increase in crop harvest, obviously is a clear advantage economically to the farmers. By introducing the wasps there will be a series of tests that need to be run before can be realised for example testing the wasps to see if they were highly specific to the mealybug and whether they would cause destruction on other pests in the environment. The cost of setting up the project is costly especially as it is a longitudinal study so will have a large amount of money spent which will be spent over the years as has to be monitored over the years.

Benefits to the humans

There was a desperate need for something that could stop the crop destruction on the cassava plant and by finding the wasp that ate the mealybug which was destroying the crop made the crop loss decrease dramatically There was 80% of crop lost so with the help of the cassava mealybug project has gone down to 43 % (7) this a huge benefit for the locals economically it brings in more income for them and also more food, As is a large food source for the local people. The cassava plant is the third-largest source of food carbohydrates in the tropic area. The cassava is a major food in the developing world, providing a basic diet for around 500 million people. Cassava is one of the most drought-tolerant crops, capable of growing on marginal soils. Nigeria is the world's largest producer of cassava. The cassava is a great source of carbohydrate for the people although it lacks in protein.

Risks to the environment

Although there is many reasons why there was a need for this biological control of the cassava mealybug, but there was also a huge risk of the environment being destroyed for example, if the wasp was not highly specific to the mealybug and caused a different crop to grow out of control this could also affect the natural balance of the environment resulting in and upset of the natural predators food source leading to disruption of habitat and extinction of some insects which will have a large effect on predators higher up the food chain. The wasp being introduced to the environment could have caused further damage on the environment destroying the local agriculture if it had effected another plant in the environment with could have caused further damage on the local economy and their food source. The food chain on the right shows how a small animal’s decrease of extinction would affect along the food chain resulting in effecting the highest food predator and can easily lead to extinction of this animal.

Alternative solutions

Pesticides can be used to control out breaks of mealybugs. It is not very easy to control, as the cassava mealybug is covered in hydrophobic wax which repels water-based insecticides therefore making it very difficult ton successfully irradiate the mealybug using this method of biological control. Also unless mealybugs are sprayed with the insecticide at certain times the treatment will not be effective. Firstly because mealybugs are protected from aqueous sprays by their wax coat and seek sheltered sites at certain points in which to feed, insecticides must be applied when mealybugs have least wax and are most exposed. This time is in early spring as the mealybugs will feed on the foliage and at this point the mealybugs are young and have not had enough time to produce the full thick layer of wax coating.

And therefore this leaves the mealybug unprotected from the insecticide. There are also fewest leaves at this point so is harder for the, mealybug to hide from the insecticide resulting in a higher success rate. Insecticides applied after Christmas has limited effect, Once mealybugs have established there position on the plant (usually from mid-January onwards), they are virtually impossible to remove or control with insecticides at this stage therefore this means that there has to be a lot of research gone into discovering the perfect time to use the pesticide therefore costing a lot as is a longitudinal study and may vary from year to year.

Genetic modification.

Another possibility that is being looked into it genetic modification of plants, this was done by Dr Stanford. He was looking into crop improvement and saw the power of genetic selection and how there was limited changed possible using just selective breeding. He then became involved in plant genetic engineering research and showed that there were numerous genes which were potentially useful in crop plants but they were struggling to find a method for delivering these genes into the plant genome, there was no transformation technology(8) . Further research showed that shooting DNA into cells thereby penetrating cell walls and membranes. This was called the gene gun and it was able to transform early transgenic crops. This gene gun would be able to in theory change the genetic makeup of the cassava plant so that it could offer some sort of protection against predators. This would also have risks as you do not know how the modification will affect all of its predators and could therefore have an effect on them so may cause harm in some way. Although there is less risk in destroying the natural environment as it is not causing other insects a problem.

Conclusion

In conclusion the biological control of the mealybug has had a huge effect and has caused the problem of the economy and environment to be resolved therefore keeping harmony between the organisms and the human population by keeping a natural balance between the small wasp and the mealybug.

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