Biological Pest Control Case Study - the cassava mealybug

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Kelsie Male

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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. 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. 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.

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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 ...

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