AP Syllabus focus:
‘Pesticides and related chemicals can reduce crop damage and increase yields, but pests can become resistant through artificial selection.’
Chemical pest control uses synthetic or naturally derived chemicals to suppress agricultural pests. While it can rapidly protect yields, repeated use often drives evolutionary resistance, reducing effectiveness and encouraging escalating chemical use.
What chemical pest control is (and why it’s used)
Pesticides and target pests
Pesticide: A chemical (or biological-derived chemical) used to kill, repel, or inhibit organisms considered pests, such as insects, weeds, fungi, or rodents.
Farmers use pesticides because they can:
Reduce crop losses from herbivory, plant disease, and weed competition
Increase yields and improve crop quality (less cosmetic damage)
Provide predictable, fast action compared with slower ecological controls
Common pesticide categories (by target)
Insecticides: target insect pests (e.g., chewing/sucking insects)
Herbicides: target weeds competing for light, nutrients, and water
Fungicides: target fungal pathogens (mildews, rusts, blights)
Rodenticides: target rodents that consume or contaminate crops
How pesticides work (core ideas)
Mode of action and selection pressure
Most pesticides disrupt vital functions (e.g., nerve signalling, growth, photosynthesis, or cell membrane integrity).
Diagram of a cholinergic (acetylcholine) synapse showing neurotransmitter release, receptor binding on the postsynaptic membrane, and enzymatic breakdown/reuptake steps that terminate the signal. Many insecticides act by interfering with these synaptic processes (for example, by altering receptor function or preventing normal signal shutoff), which can lead to paralysis and death in target insects. Source
If application kills susceptible individuals, any pest with traits that reduce exposure or reduce the chemical’s effect is more likely to survive and reproduce.
Chemical control is therefore both:
an immediate management tool, and
a strong evolutionary force on pest populations
Resistance: why chemicals can stop working
Resistance as an evolutionary response
Pesticide resistance: A heritable decrease in a pest population’s sensitivity to a pesticide, causing the product to be less effective at previously successful doses.
Resistance is more likely when pest populations have:
High reproductive rates and short generation times (many insects, many weeds)
Large population sizes (more mutations and more genetic variation)
Frequent exposure to the same active ingredient or same mode of action
Artificial selection in pest management
Artificial selection: Human-driven selection in which certain traits become more common because human actions (such as pesticide application) cause differential survival and reproduction.
In pesticide resistance, artificial selection typically follows this pattern:
Natural genetic variation exists; a few pests are naturally less susceptible
Pesticide application kills most susceptible individuals
Survivors reproduce, increasing the frequency of resistance alleles
Over multiple generations, the population shifts toward predominantly resistant individuals
Illustrated sequence showing how repeated herbicide applications act as a strong selection pressure: resistant individuals survive an application, reproduce, and their offspring make up an increasing fraction of the population in subsequent seasons. The figure visually reinforces how resistance alleles rise in frequency over generations under consistent chemical exposure. Source
What accelerates resistance in real agricultural systems
Practices that increase selection pressure
Repeated use of one pesticide (or same mode of action) over many seasons
Prophylactic spraying (calendar-based applications rather than need-based)
Sublethal dosing (under-application, poor coverage, or degraded product), allowing partially resistant pests to survive
Monocultures that provide abundant, continuous habitat and food for a single pest species
Biological and genetic factors
Standing variation: resistance traits may already exist before spraying begins
Rapid gene spread: mobile pests can move resistance genes across fields
Cross-resistance: resistance to one chemical can confer resistance to related chemicals with similar modes of action
Consequences of resistance (environmental and economic)
The “pesticide treadmill”
As effectiveness declines, growers may respond by:
Applying higher doses
Spraying more frequently
Switching to newer or stronger chemicals
This can increase:
Costs of production and management
Non-target impacts (beneficial insects, soil biota, aquatic organisms via runoff)
Chemical residues and broader contamination risks
Pest resurgence and secondary pests
Broad-spectrum insecticides can reduce natural enemies (predators/parasitoids). This may:
allow the target pest to rebound rapidly after spraying, or
release a previously minor species to become a secondary pest
Slowing resistance while still using chemicals (within chemical control)
Resistance is often slowed (not eliminated) by strategies that reduce consistent selection:
Rotate modes of action across time so survivors of one chemistry are controlled by another
Use targeted applications (spot treatments, threshold-triggered spraying) to reduce unnecessary exposure
Prefer narrow-spectrum products when feasible to avoid removing beneficial organisms
Maintain correct label rates and proper application timing to avoid sublethal exposure that favours tolerant individuals
FAQ
Resistance can arise through:
Increased detoxification (e.g., elevated enzyme activity that breaks down the pesticide)
Target-site changes (mutations that reduce pesticide binding)
Reduced penetration or increased excretion
Behavioural avoidance (reduced contact with treated surfaces)
Monitoring often combines field observations with lab assays. Approaches include:
Comparing mortality across known doses over time
Tracking shifts in susceptibility baselines for a region
Genetic tests for known resistance mutations where available
Early detection helps prevent widespread fixation of resistance alleles.
Higher and more frequent applications intensify selection by removing moderately susceptible individuals, leaving only the most resistant to reproduce. This can speed the shift from partial resistance to near-total field failure, especially in fast-breeding pests.
Rotation alternates modes of action across applications or seasons, reducing consistent selection in one direction. Mixing applies two modes at once; it can slow resistance if each component remains effective, but may increase non-target exposure and can fail if cross-resistance exists.
Sometimes susceptibility partially returns if resistance carries a fitness cost (resistant individuals reproduce less without the pesticide). However, resistance can persist if costs are low, if compensatory mutations arise, or if resistant individuals continue to be favoured by related chemicals nearby.
Practice Questions
Explain how repeated use of the same pesticide can lead to pesticide resistance in a pest population. (2 marks)
Mentions that susceptible individuals are killed and resistant individuals survive (1)
States survivors reproduce/increase frequency of resistance alleles over generations due to (artificial) selection (1)
A farmer reports that an insecticide used for five years now provides poor control of an insect pest. Describe the role of artificial selection in this change and outline two chemical-use practices that could slow further resistance. (6 marks)
Identifies pre-existing genetic variation or rare resistant individuals in the pest population (1)
Explains insecticide kills susceptible individuals, resistant survive (1)
Links survival to higher reproduction and increased frequency of resistance alleles over generations (1)
Uses the term artificial selection correctly in context (1)
Practice 1 described: rotate different modes of action / avoid same chemistry repeatedly (1)
Practice 2 described: threshold-based/targeted spraying OR correct label dosing and coverage OR narrow-spectrum choice to reduce selection pressure (1)
