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To most of us, insects are vermin, pests and a nuisance. This is attributed to poor dissemination of information about insects. Many of us are aware of pests because we learn about them in school and are bombarded with advertisements on the need to destroy vermin. Little attention is placed on telling the public about fascinating insects that are our friends. It has been estimated that there are more friendly insects than pestilent ones. Indeed, it is because of these friendly insects that we have been able to grow food as most pest species are kept in check by them. You may already know about insects like bees that produce honey, and silkworms that produce silk. There are also the insects that are allies of farmers.

In the Cameron Highlands, an important vegetable crop cultivated by farmers is the head cabbage. Since the Highlands was opened for agriculture in the late 1930's, this temperate climate crop has been widely grown. However, unknown to farmers then, an insect from the temperate lands had accompanied the vegetable to Malaysia. This insect was the diamondback moth. The moth is a small dull-looking insect, the female of which lays large numbers of eggs on leaves of cabbages. These hatch in about five days into small green caterpillars. Feeding by large numbers, these caterpillars could destroy cabbage plants.

Initially, farmers in Cameron Highlands relied on chemical insecticides to control the pest. However, the insect soon developed a resistance to the insecticides. An ecological approach was adopted in 1970 and this resulted in the importation of several species of small parasitic wasps. These wasps will search for the diamondback moth caterpillars. When these are found, the female wasps would inject an egg into the body of the caterpillar. Soon a parasite would develop within the body of the caterpillar and eventually kill it. A new wasp would develop from each parasitized caterpillar. This form of using parasitic insects to control a pest is known as biological control.

Biological control is using 'nature to control nature'. When effectively established, biological control often would not require further intervention by farmers. The parasitic wasps are truly friends of the farmers. Farmers would save on expensive insecticides and apply them only when necessary, such as for other pests. When carrying out biological control, only friendly insects that are very `host specific' are used. This. means that there would be little or no risk of importing a pest. Often such an approach requires an investment of time and money to allow biological control scientists carry out investigations in the place of origin of the pest.

Besides using insects to control insects, scientists in Malaysia have also experimented with using insects to control weeds. Weeds are unwanted plants. Often, these weeds come about because they were introduced from another country. For example, there was interest in moving plants from the tropics of South America to Malaysia in the last century. Arising from this is the introduction of rubber trees. However, not all introductions were beneficial. One plant imported for hedge was found to grow very well in coconut plantings, and almost everywhere. This was the string-bush. In some instances, the weed competed with young coconut palms. Scientists at the International Institute of Biological Control found that the weed was regulated in its native home of Trinidad by a seed-feeding wasp and a leaf-feeding beetle. Both were carefully studied and after exhaustive testing, were imported and released into fields where the weed was present.

     
  1.

From paragraph 1, give two reasons why most of us regard insects as pests.

       
  2. From paragraph 2 and 3,
    (a) what harm does the diamondback moth do ?
    (b) why is the use of insecticide ineffective ?
       
  3. From paragraph 3,
    (a) what are "parasitic wasps" ?
    (b) how do they kill the caterpillars ?
       
  4. From paragraph 4,
    (a) explain the phrase "nature to control nature".
    (b) explain what are insects that are "host specific".
       
  5. From paragraph 5,
    (a) what damage has been caused by string-bush ?
    (b) give one example how the string-bush is controlled.
       
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Answers

 
  1. (i) It is due to poor dissemination of information on insects.
    (ii) We learn about pests in school and are influenced by advertisements on the need to destroy vermin.
       
  2. (a) It lays eggs on cabbages. When the eggs hatch into caterpillars, they eat up the cabbages.
    (b) Pests have become resistant to insecticides.
       
  3. (a) They are wasps that lay their eggs on caterpillars' body.
    (b) When the eggs hatch into wasps, the caterpillars are killed.
       
  4. (a) It means using an insect to control another insect which is a pest.
    (b) They are insects that kill a particular pest with little or no risk of posing as pests themselves.
       
  5. (a) It competes with young coconut palms.
    (b) It is controlled by introducing seed-feeding wasps and a leaf-feeding beetle.
       
 
 
 
 
 

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Comprehension 1

 

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