Why does spraying more pesticide often make the pest problem worse over time?
Topic 5.6 Pest Control Methods: compare chemical and biological pest control and explain the pesticide treadmill and the evolution of pesticide resistance.
A focused answer to APES Topic 5.6, covering chemical pesticides, their benefits and costs, biological control, the pesticide treadmill, pesticide resistance through natural selection, and broad-spectrum versus narrow-spectrum pesticides, with a worked resistance calculation.
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What this topic is asking
The College Board (Topic 5.6) wants you to compare chemical and biological pest control, explain the pesticide treadmill, and explain how pesticide resistance evolves by natural selection.
Chemical pesticides
How resistance evolves
This is the same evolutionary process behind antibiotic resistance, applied to crop pests.
The pesticide treadmill
Biological control
Why this matters
Pest control is a core agricultural impact (linked to the Green Revolution, Topic 5.3) and a clear case of natural selection (Topic 2.6) in action. The drawbacks of chemical control motivate integrated pest management (Topic 5.14) and sustainable agriculture (Topic 5.15), which combine biological and limited chemical methods.
Try this
Q1. Identify the evolutionary process that produces pesticide-resistant pests. [1 point]
- Cue. Natural selection.
Q2. Explain why killing a pest's natural predators with a broad-spectrum pesticide can worsen the pest problem. [2 points]
- Cue. Removing the predators that normally keep the pest in check lets surviving (often resistant) pests reproduce without control, so the pest population can rebound to even higher numbers than before.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Explain how repeated use of a chemical pesticide leads to pesticide resistance in a pest population. (b) Describe the pesticide treadmill. (c) Identify one benefit of chemical pesticides. (d) Describe one example of biological pest control.Show worked answer →
A 4-point FRQ on pest control.
(a) Explain (1 point): a pesticide kills susceptible pests but a few resistant individuals survive and reproduce; over generations the resistant trait spreads (natural selection), so the pesticide becomes less effective.
(b) Describe (1 point): the pesticide treadmill is the cycle in which growing resistance forces farmers to apply more pesticide, or stronger or new pesticides, at ever greater cost and environmental harm, while pests keep adapting.
(c) Identify (1 point): chemical pesticides act quickly, increase crop yields, are easy to apply, and protect against pest damage and disease.
(d) Describe (1 point): introducing or encouraging a natural predator or parasite of the pest (for example ladybirds to eat aphids), or using Bacillus thuringiensis (Bt) bacteria.
Markers reward natural selection for resistance, the more-pesticide cost cycle for the treadmill, a real pesticide benefit, and a valid biological-control example.
AP 2019 (style)1 marksSection I (multiple choice). A farmer notices that a pesticide that once killed nearly all the target insects now kills only a small fraction. The best explanation is that: (A) the pesticide has chemically broken down (B) the insects have evolved resistance through natural selection (C) the insects have migrated away (D) the soil has absorbed the pesticide. Justify your choice.Show worked answer →
A 1-point MCQ on pesticide resistance. The answer is (B).
Repeated pesticide use kills susceptible insects but spares a few resistant ones, which reproduce and pass on the resistant trait, so the population becomes mostly resistant (natural selection). (A), (C) and (D) do not explain why the surviving insects are now harder to kill. The trap is attributing reduced effectiveness to the chemical rather than to evolution in the pest population.
Related dot points
- Topic 5.14 Integrated Pest Management: describe integrated pest management (IPM) and explain how it combines biological, cultural, mechanical and limited chemical control.
A focused answer to APES Topic 5.14, covering integrated pest management (IPM), its combination of biological, cultural, mechanical and limited chemical controls, monitoring and economic thresholds, and its advantages over relying on pesticides, with a worked threshold example.
- Topic 5.3 The Green Revolution: describe the methods and benefits of the Green Revolution and explain its environmental costs.
A focused answer to APES Topic 5.3, covering the Green Revolution, high-yield crop varieties, fertilizers, pesticides, irrigation and mechanisation, its benefits for food supply, and its environmental costs, with a worked yield-increase calculation.
- Topic 5.4 The Impact of Agricultural Practices: explain how tillage, fertilizer use, overgrazing and other farming practices degrade soil and water.
A focused answer to APES Topic 5.4, covering how tillage, fertilizer use, overgrazing, and confined animal feeding degrade soil and water through erosion, nutrient runoff, salinisation, desertification and waste, with a worked nutrient-loading calculation.
- Topic 5.15 Sustainable Agriculture: describe sustainable farming practices that conserve soil and water and maintain long-term productivity.
A focused answer to APES Topic 5.15, covering sustainable agriculture practices (crop rotation, contour ploughing, terracing, no-till, cover crops, strip cropping, agroforestry, rotational grazing) and how each conserves soil and water and maintains productivity, with a worked erosion-reduction calculation.
- Topic 2.6 Adaptations: explain how natural selection produces adaptations and how environmental change shifts which traits are favored over time.
A focused answer to APES Topic 2.6, covering adaptations, natural selection, the role of genetic variation, structural, physiological and behavioral adaptations, specialists and generalists, and how environmental change drives evolution, with a worked selection question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)