Pest control and Biological control

A pest is an organism that damages crop plants. Pests include insects (such as aphids that suck sap), fungi (that cause plant diseases) and weeds (which compete with crops for light, water and minerals).

Why do farmers bother to control pests?

• Pests eat or damage the crop, so less of it can be harvested.
• This reduces the yield — the amount of food a field of crops produces.
• A lower yield means less food and less profit for the farmer.

So the main reason for pest control is simple: it protects the crop so the farmer can grow more food from the same area of land. With a growing human population needing to be fed, getting a good yield matters.

There are two methods you need to compare for the Double Award: using pesticides (chemicals) and using biological control (a living predator or parasite). Each has clear advantages and disadvantages.

Exam tip. A reliable one-mark point is that pests reduce crop yield, so controlling them lets the farmer produce more food / crop.

A pesticide is a chemical sprayed onto crops to kill pests. (Pesticides that kill insects are sometimes called insecticides, but you can just call them pesticides.)

Advantages of pesticides

• They kill the pests quickly, so the crop is protected fast.
• They are easy to apply — usually just sprayed onto the crop.
• They can kill a wide range of different pests.

Disadvantages of pesticides

• They can harm other (non-target) species, not just the pest — for example helpful insects or birds.
• They can kill pollinators such as bees, which crops actually need for pollination.
• They can bioaccumulate: the chemical builds up along a food chain, becoming more concentrated in animals higher up, which can poison them.
• Pests can become resistant. By natural selection, any pests that survive the pesticide reproduce and pass on the resistance, so the pesticide stops working and a stronger one is needed.

So pesticides are fast and easy, but they are not selective — they cause wider harm to the environment and the problem can return as resistance builds up.

Exam tip. For full marks, do not just say pesticides 'are bad'. Give specific disadvantages: harm non-target species / pollinators, bioaccumulate, and pests develop resistance.

Biological control means controlling a pest by introducing a living organism that is a natural predator or parasite of that pest. For example, ladybirds eat aphids, so releasing ladybirds onto a crop can control an aphid pest.

Advantages of biological control

• It is specific: the predator/parasite usually attacks only the pest, so other species are not harmed.
• It uses no chemicals, so there is no chemical pollution and no bioaccumulation.
• It can be long-lasting: once the predator population is established, it keeps controlling the pest without repeating the treatment.
• Pests do not become resistant to it in the way they do to pesticides.

Disadvantages of biological control

• It is slow — it takes time for the predator population to build up and reduce the pest numbers.
• It does not remove all the pests; some pests must remain as food for the predator, so the pest is reduced, not wiped out.
• The introduced predator may itself become a pest — it could spread, eat other useful species, or upset the local ecosystem.

So biological control is safer and more lasting than pesticides, but it is slower and less complete, and there is a risk that the new species causes problems of its own.

Exam tip. The word examiners reward is specific (it harms only the pest). The main disadvantage to remember is that it is slow and the predator may become a pest itself.

The two methods are really opposites of each other. A clear way to remember the comparison is the table below.

The simplest way to summarise the comparison: pesticides are fast but harmful, while biological control is slow but safer and longer-lasting. In real farming, the two are sometimes used together so that the strengths of one cover the weaknesses of the other.

Exam tip. 'Compare' and 'evaluate' questions want both sides for both methods. Pair an advantage of one with the matching disadvantage of the other (e.g. pesticides are fast but biological control is slow; biological control is specific but pesticides harm other species).