IGCSE Biology / Human influences on the environment

Pesticides and biological control revision notes

Edexcel International GCSE Biology (4BI1), chapter 15: Human influences on the environment, specification point 5.4. Free lesson.

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Pests reduce crop yield by eating crops or competing with them. Pesticides act quickly, while biological control uses living organisms to reduce pests more slowly, so the best choice depends on the situation.

Why pests are controlled

A pest is an organism that eats a crop or competes with it. If pests are not controlled, they can damage much of a harvest, reducing the farmer’s yield. Controlling pests helps protect the crop and its yield.

How pests become resistant to pesticides

Some pests are already resistant by chance because individuals in a population vary. Spraying kills most pests, but resistant ones survive and reproduce. This is natural selection: the pesticide acts as the selection pressure, so resistance becomes common over generations.

How pesticides and biological control compare

Biological control means introducing a natural predator, parasite or disease of the pest instead of using a chemical. For example, ladybirds can be released to eat aphids. Each method has benefits and drawbacks, so choose according to the problem. Speed: pesticides work within hours. Biological control may take weeks or months while the predator population builds up. How much the pest is reduced: pesticides kill a very high proportion quickly, but not all. Biological control reduces the pest population but does not remove it. Repeating the treatment: pesticides must be…

Why biological control leaves some pests

Biological control is meant to keep a pest population low, not remove every pest. Predators need prey to survive, so a small pest population helps keep the control organism alive and ready to attack new pests. If every aphid were eaten, the ladybirds would starve and die out. New aphids could then arrive without any ladybirds to control them. A low, permanent aphid population can therefore mean the method is working.

Choosing a method

There is no single method that is always better. A sudden, severe infestation on a food crop may need a pesticide because it acts fast. A long-running problem in a glasshouse may suit biological control, which can become self-sustaining. When you compare methods, link your choice to the situation and explain the trade-offs.