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    Pesticides — AQA A-Level Environmental Science

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    Pesticides are chemicals used to kill pests that damage crops, but they can bioaccumulate and biomagnify in food chains, causing harm to non-target species and ecosystems. AQA A-Level Environmental Science requires students to evaluate their effectiveness, environmental persistence, and alternatives like biological control and integrated pest management (IPM).

    Pesticides explained

    Mobility describes how readily a pesticide moves away from the place where it is applied, through soil, water or air.

    Read the full explanation

    A pesticide held tightly by soil particles or organic matter is largely immobile and stays near the treated area; one that dissolves easily in water or binds weakly can be carried downwards by drainage into groundwater or sideways in run-off into streams. Mobility therefore shapes both how well a pesticide reaches its target and how far non-target organisms and water bodies are exposed. Learners should link mobility to properties such as water solubility and soil adsorption, and to the risk of leaching and contamination rather than treating it as a single fixed value.

    Your focus

    1. Define pesticide mobility in terms of movement away from the application site.
    2. Explain how water solubility and soil binding influence mobility.
    3. Relate high mobility to leaching, run-off and non-target exposure.

    Which command word is this?

    Give the precise meaning of a term, e.g., 'Bioaccumulation is the build-up of a substance within an organism over time.'

    Pesticides exam tips

    Topic Overview

    Pesticides are chemicals used to control pests in agriculture and public health. They include insecticides, herbicides, fungicides, and rodenticides. While they boost crop yields and reduce disease, many are persistent, toxic to non-target species, and can bioaccumulate and biomagnify in food chains, leading to ecological damage.

    This topic is crucial for understanding the balance between food security and environmental protection. It links to concepts like toxicology, food chains, and sustainable agriculture. AQA A-Level exams often ask students to evaluate the effectiveness of pesticides versus alternatives like biological control and Integrated Pest Management (IPM).

    Key Concepts
    • →Bioaccumulation: the build-up of a substance within an organism over time.
    • →Biomagnification: the increasing concentration of a substance at each trophic level.
    • →Persistence: how long a pesticide remains active in the environment.
    • →Selective vs non-selective pesticides: selective kill only target pests; non-selective kill a wide range.
    • →Integrated Pest Management (IPM): a sustainable approach combining biological, cultural, and chemical controls.
    Marking Points
    • Identifies mobility as the tendency of a pesticide to move through soil, water or air away from its application site.
    • Links high mobility to weak binding to soil particles and to ready dissolution and transport in water.
    • Explains that a mobile pesticide can leach downwards into groundwater or be carried in run-off to surface water.
    • Contrasts a strongly adsorbed, low-mobility pesticide that remains close to the treated area with a highly mobile one.
    • Connects mobility to the exposure of non-target organisms and to contamination of water bodies.
    Examiner Tips
    • 💡Define mobility in one sentence before applying it to a scenario.
    • 💡Use the terms leaching and run-off precisely when describing where a mobile pesticide travels.
    • 💡Link mobility to a named property such as water solubility or adsorption to soil particles to earn applied credit.
    • 💡Use specific named examples (e.g., DDT, neonicotinoids) to support your answers.
    • 💡For evaluate questions, always consider both pros and cons, and give a justified conclusion.
    • 💡Define key terms precisely; vague descriptions lose marks.
    Common Mistakes
    • Confuses mobility with persistence; keep them separate, because mobility concerns movement while persistence concerns how long a substance remains.
    • Assumes a mobile pesticide is always more effective; recognise that mobility can carry it away from the target as well as towards it.
    • Treats mobility as a fixed property of every pesticide; distinguish substances by their solubility and soil-binding behaviour.
    • Students think all pesticides are equally harmful; in fact, persistence and toxicity vary widely. For example, glyphosate is less persistent than DDT.
    • Students confuse bioaccumulation (within one organism) with biomagnification (across trophic levels).
    • Students believe pesticides only affect pests; they can also kill beneficial insects like bees and natural predators.
    Revision Plan
    1. 1Day 1: Learn definitions of bioaccumulation, biomagnification, persistence, and toxicity.
    2. 2Day 2: Study case studies of DDT and neonicotinoids, noting their environmental impacts.
    3. 3Day 3: Compare pesticides with biological control and IPM, listing advantages and disadvantages.
    4. 4Day 4: Practice exam questions, especially 6-mark evaluate questions, and self-mark using mark schemes.
    5. 5Day 5: Create a mind map linking pesticides to wider topics like biodiversity and food security.
    Exam Question Types
    • 📋Define and explain bioaccumulation and biomagnification (4-6 marks).
    • 📋Evaluate the use of pesticides in agriculture (6-9 marks).
    • 📋Calculate concentration factors from given data (2-3 marks).
    • 📋Describe an example of a pesticide and its environmental impact (4-5 marks).
    Command Word Expectations (AQA)
    Define

    Give the precise meaning of a term, e.g., 'Bioaccumulation is the build-up of a substance within an organism over time.'

    Explain

    Give reasons or mechanisms, e.g., 'DDT biomagnifies because it is lipophilic and not excreted, so it concentrates at each trophic level.'

    Evaluate

    Weigh up advantages and disadvantages, then reach a justified conclusion, e.g., 'Pesticides increase yields but harm biodiversity; therefore, IPM is preferable.'

    How Students Lose Marks (Examiner Pitfalls)
    Pitfall: Students often confuse bioaccumulation with biomagnification, leading to lost marks in explanation questions.
    ❌ Weak Answer (Loses Marks):Pesticides build up in animals and get more concentrated as they go up the food chain.
    Example improved answer:Bioaccumulation is the build-up of a persistent pesticide within an individual organism over time, because it is absorbed faster than it is excreted. Biomagnification is the increasing concentration of the pesticide at each successive trophic level of a food chain, as predators consume many contaminated prey. For example, DDT biomagnifies in peregrine falcons, causing eggshell thinning.
    Examiner Tip: Always define both terms separately and give a named example. Use the phrase 'at each trophic level' for biomagnification.
    Pitfall: Students describe pesticides as universally bad without acknowledging their role in food security, losing evaluation marks.
    ❌ Weak Answer (Loses Marks):Pesticides are harmful and should be banned because they kill wildlife.
    Example improved answer:Pesticides increase crop yields by controlling pests, which is vital for global food security. However, they can harm non-target species, persist in ecosystems, and lead to pest resistance. Therefore, a balanced approach such as Integrated Pest Management (IPM) is needed, combining limited pesticide use with biological control and crop rotation to minimise environmental damage while maintaining yields.
    Examiner Tip: For evaluate questions, always give both advantages and disadvantages, then reach a justified conclusion. Use specific examples like DDT or neonicotinoids.
    Step-by-Step Worked Solutions

    Question: Explain how DDT biomagnification can lead to the decline of peregrine falcon populations. (6 marks)

    1. 1.Step 1: Define biomagnification - the increase in concentration of a persistent pesticide at each trophic level.
    2. 2.Step 2: Describe how DDT enters the food chain - sprayed on crops, absorbed by plants, consumed by herbivores, then by predators.
    3. 3.Step 3: Explain the effect on peregrine falcons - as top predators, they accumulate high concentrations of DDT from eating contaminated prey.
    4. 4.Step 4: State the physiological effect - DDT causes eggshell thinning, leading to breakage during incubation and reduced reproductive success.
    5. 5.Step 5: Link to population decline - fewer chicks survive, so population numbers fall.
    6. 6.Step 6: Conclude that DDT is persistent and lipophilic, so it remains in fatty tissues and magnifies along the food chain.
    Final Answer: DDT biomagnifies through the food chain, reaching toxic concentrations in peregrine falcons, causing eggshell thinning and reduced breeding success, leading to population decline.

    Question: Calculate the concentration factor of a pesticide if the concentration in water is 0.0001 mg/L and in a fish it is 2 mg/L. Show your working. (2 marks)

    1. 1.Step 1: Identify the concentration in water = 0.0001 mg/L.
    2. 2.Step 2: Identify the concentration in fish = 2 mg/L.
    3. 3.Step 3: Divide fish concentration by water concentration: 2 / 0.0001 = 20,000.
    4. 4.Step 4: State the concentration factor as 20,000 times.
    Final Answer: The concentration factor is 20,000.
    Active Recall Memory Test
    What is the difference between bioaccumulation and biomagnification?
    Key Fact: Bioaccumulation is the build-up of a substance within an individual organism over time; biomagnification is the increase in concentration at each trophic level.
    Name one persistent pesticide and its effect on wildlife.
    Key Fact: DDT causes eggshell thinning in birds like peregrine falcons.
    What is Integrated Pest Management (IPM)?
    Key Fact: A sustainable approach combining biological, cultural, and chemical controls to minimise pesticide use.
    Why can pesticides lead to pest resistance?
    Key Fact: Surviving pests reproduce, passing on resistance genes, so the population becomes resistant over time.
    Frequently Asked Questions
    What is biomagnification in simple terms?
    Biomagnification is the process where a substance, like a pesticide, becomes more concentrated at each step up the food chain. For example, if a small fish eats contaminated plankton, the pesticide builds up in the fish. When a larger fish eats many small fish, the concentration gets even higher, and so on. This can lead to toxic levels in top predators like birds of prey.
    Why is DDT banned in many countries?
    DDT is banned because it is persistent, bioaccumulates, and biomagnifies, causing severe harm to wildlife. It famously caused eggshell thinning in birds like peregrine falcons and bald eagles, leading to population declines. Although it was effective against malaria-carrying mosquitoes, its environmental impact led to global restrictions under the Stockholm Convention.
    What are the alternatives to pesticides?
    Alternatives include biological control (introducing natural predators), cultural methods (crop rotation, companion planting), physical methods (hand-picking, barriers), and Integrated Pest Management (IPM). IPM combines these with minimal, targeted pesticide use. These approaches reduce environmental harm and delay pest resistance.
    How do pesticides affect non-target species?
    Pesticides can kill or harm non-target species directly, such as bees, butterflies, and birds. Herbicides reduce plant diversity, affecting herbivores and pollinators. Insecticides can kill beneficial insects and natural predators, disrupting ecosystems. Indirect effects include loss of food sources and habitat, leading to biodiversity decline.
    What is the difference between selective and non-selective pesticides?
    Selective pesticides target specific pests, leaving other organisms relatively unharmed. For example, some insecticides only affect certain insects. Non-selective pesticides kill a wide range of organisms, including beneficial insects and plants. Non-selective herbicides like glyphosate kill most plants, which can reduce biodiversity if used carelessly.
    How can I remember the difference between bioaccumulation and biomagnification?
    Think of bioaccumulation as 'building up in one organism' over time. Biomagnification is 'magnifying up the food chain' at each trophic level. A helpful mnemonic: Bioaccumulation = Body; Biomagnification = Bigger (as you go up). Both involve increasing concentrations, but bioaccumulation is within an individual, while biomagnification is across the food chain.

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