A plague on the pest – rabbit calicivirus disease and biological control

Activity 2

A hypothetical situation involving biological control

Assume you are an environmental scientist advising the government of Ruritasia, a developing tropical country, where the majority of the population work in non-industrialised agriculture and fishing. The country receives a large annual rainfall, grows rice and is criss-crossed with a network of rivers and canals. Because mosquito larvae live in water, mosquitoes are to be found virtually everywhere. Unfortunately, they transmit malaria, a serious disease which can be fatal, and which is caused by a single-celled parasite. The government wants to improve the health of the population and to encourage visits by tourists. Accordingly, it has tried to reduce the mosquito population. One area has been sprayed regularly with an insecticide that persists in the food chain, and canals and swamps have been drained in another area.

In the area sprayed with insecticide, the local people were upset about their cats becoming sick and dying. There was an outbreak of mice and rats in the villages. Furthermore, there was a dramatic decline in the freshwater fish that the villagers rely on for the protein in their diet. Mosquito numbers fell for a few years and then started to rise again.

In the drained area the rice harvest decreased after a few years, mosquito numbers fell (although not as much as in the insecticide-treated region) and remained at their lower level. Tourists were disappointed as several exotic bird species became rarer.

  • Write a report for the government of Ruritasia that explains the causes of these events and clearly sets out the interconnections involved.

  • The Ruritasian government wants to try a form of biological control against mosquitoes, by introducing either a predatory fish species with an enormous appetite for mosquito larvae, or an insect virus that kills mosquitoes.

  • What would you advise about the advantages and disadvantages of biological control and the type of procedures that would give the best results?

Teachers notes

  1. The following points are relevant to an explanation of the causes of the events in Ruritasia:

    • the need to control mosquito numbers because they are the vector for the malarial parasite;
    • the insecticide used was not specific to mosquitoes and affected other organisms as well;
    • the build-up of resistance to the insecticide in the mosquito population;
    • the possibility that mosquito larvae are part of the food chain for fish;
    • the possibility that the exotic birds nested on or beside the waterways that were drained;
    • the fact that the insecticide persists in the food chain.
  2. Advantages of using a form of biological control include:

    • pesticides, which are harmful to all parts of the food chain, are not needed;
    • biological control is self-perpetuating;
    • suitable biological control organisms do not attack other species;
    • usually a large proportion of the pest population is destroyed.
    Disadvantages of using a form of biological control include:

    • disruption of the food chains that include mosquito larvae;
    • the need for environmentally unfriendly follow-up operations to ensure that the mosquito population does not build up resistance to the biological control age
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Posted February 1997.