How are crops protected from weeds, pests and disease in a sustainable way?
The threats that weeds, pests and diseases pose to crop productivity, the characteristics of annual and perennial weeds, the use of chemical control by selective and systemic pesticides, the problems of pesticides, and the use of cultural, biological and integrated pest management.
An SQA Higher Biology answer on crop protection, covering the threats from weeds, pests and diseases, the features of annual and perennial weeds, selective and systemic pesticides and their problems, and cultural, biological and integrated pest management.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this key area is asking
The SQA wants you to describe the threats that weeds, pests and diseases pose to crops, describe the features of annual and perennial weeds, explain the use of selective and systemic pesticides and their problems, and describe cultural, biological and integrated pest management.
Threats to crops
Annual and perennial weeds
The two types of weed compete in different ways, which affects how they are controlled:
- Annual weeds have a short life cycle, grow rapidly and produce large numbers of seeds, so they spread quickly and can build up a large seed bank in the soil.
- Perennial weeds survive for several years, often with storage organs (such as underground stems or roots) that allow them to regrow even after the top is removed, making them harder to eliminate.
Chemical control
Systemic action is useful because it reaches pests feeding on parts of the plant that a surface spray would miss, and it protects new growth too. However, pesticides can cause significant problems:
- Toxicity to other, non-target organisms, such as bees and other beneficial insects.
- Bioaccumulation and biomagnification, where a persistent chemical that is not broken down builds up in tissues and becomes more concentrated at each step up the food chain, so top predators can receive harmful doses.
- Resistance, as the few pests carrying a resistance allele survive the pesticide and pass it on, so over generations the pesticide stops working and stronger or different chemicals are needed.
Alternatives to chemical control
Other approaches include:
- Cultural control, such as ploughing, weeding and crop rotation, which disrupts the life cycles of pests and weeds.
- Integrated pest management (IPM), which combines chemical, biological and cultural methods to control pests effectively while limiting harm to the environment.
IPM is often the most sustainable approach because it does not rely on a single method, which reduces the chance of resistance developing and limits the damage to non-target species.
Examples in context
Example 1. The cane toad as a biological control gone wrong. Cane toads were introduced to control beetles damaging sugar cane crops, but they became a serious pest themselves. They spread widely, poisoned native predators that tried to eat them, and did little to control the original beetle. This is a classic warning that a non-native biological control agent can cause more harm than the pest it was meant to manage, which is why control organisms must be chosen and tested carefully.
Example 2. Integrated pest management in greenhouse tomatoes. Greenhouse tomato growers control whitefly using a parasitic wasp (a biological control) together with sticky traps and careful monitoring, turning to chemical sprays only when pest numbers rise above a threshold. By combining methods, they keep the pest in check, reduce the development of resistance, and avoid harming the bumblebees used to pollinate the crop. This shows integrated pest management working in practice.
Try this
Q1. State one way weeds reduce the yield of a crop. [1 mark]
- Cue. They compete with the crop for light, water, nutrients or space.
Q2. Explain one problem caused by using pesticides. [2 marks]
- Cue. Any one of: toxicity to non-target species, bioaccumulation up the food chain, or the evolution of resistance in the pest.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe two problems that can result from the use of chemical pesticides on crops.Show worked answer →
A 4-mark answer needs two problems, each explained.
Toxicity to non-target organisms: a pesticide may kill or harm species other than the target pest, such as pollinating insects or other wildlife.
Bioaccumulation and biomagnification: a persistent pesticide that is not broken down builds up in the tissues of organisms and becomes more concentrated at each step up the food chain, so top predators may receive harmful doses.
A further valid problem is the evolution of resistance: pests with a resistance allele survive the pesticide and reproduce, so over time the pesticide stops working.
Markers reward two clearly explained problems such as toxicity to non-target species, bioaccumulation up the food chain, or the evolution of resistance.
SQA Higher 20213 marksExplain the principle of biological control and describe one risk of using it.Show worked answer →
A 3-mark answer needs the principle plus a genuine risk.
Biological control uses a natural predator, parasite or pathogen of the pest to reduce the pest's numbers, instead of (or alongside) chemicals.
A risk is that the control organism, especially if it is not native, may itself become a pest. It might attack species other than the target, or its population may grow out of control, harming the wider ecosystem.
Integrated pest management combines biological, chemical and cultural methods to control pests while limiting such harm.
Markers reward the use of a natural enemy to reduce pest numbers and a clear risk such as the control organism becoming a pest or attacking non-target species.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)