Soil and Agriculture
4. Pesticides and Herbicides
Learning outcomes
- I can distinguish between pesticides and herbicides.
- I can explain how these chemicals are used in agriculture.
- I can identify benefits and risks associated with their use.
- I can describe environmental impacts of agricultural chemicals.
- I can evaluate alternatives to chemical pest control.
Pesticides and Herbicides
Farmers must protect crops from organisms that compete with them, damage them, or spread disease. These organisms can reduce both the quantity and quality of food produced.
Pesticides are substances used to prevent, control, repel, or kill organisms considered pests.
Different pesticides target different organisms:
- herbicides control unwanted plants or weeds
- insecticides control insects
- fungicides control fungi
- rodenticides control rodents
Therefore:
All herbicides are pesticides, but not all pesticides are herbicides.
Agricultural chemicals can improve crop production, but their use can also create environmental and health risks. Sustainable agriculture tries to obtain the benefits of pest control while minimizing unwanted effects.
What Is a Pest?
A pest is an organism that causes unwanted damage or interferes with human activities.
Agricultural pests may include:
- insects that eat crops
- fungi that cause plant diseases
- weeds that compete with crops
- rodents that consume stored grain
- microorganisms that cause crop diseases
Whether an organism is considered a pest depends on context.
An insect may be an important part of a natural ecosystem but become a pest if its population causes serious crop damage.
Why Farmers Control Pests
Crop pests can reduce agricultural production by:
- eating leaves
- damaging roots
- consuming seeds
- damaging fruits
- spreading diseases
- competing for water
- competing for mineral nutrients
- reducing access to sunlight
Without pest management, farmers may lose part of their harvest.
Severe infestations can cause major economic losses.
What Are Herbicides?
Herbicides are pesticides designed to control unwanted plants.
Unwanted plants growing among crops are commonly called weeds.
Weeds compete with crops for resources including:
- light
- water
- nutrients
- space
Controlling weeds can therefore increase the resources available to the crop.
Selective and Non-Selective Herbicides
Herbicides can differ in how specifically they affect plants.
Selective herbicides are designed to control certain types of plants while having less effect on others when used appropriately.
For example, a herbicide might control particular broad-leaved weeds while allowing a cereal crop to survive.
Non-selective herbicides affect a much wider range of plants.
The appropriate type depends on:
- crop
- weed species
- timing
- application method
- farming system
Insecticides
Insecticides are pesticides used to control insects.
Agricultural insects may damage crops by:
- eating leaves
- boring into stems
- feeding on roots
- damaging fruits
- sucking plant fluids
- transmitting plant diseases
Insecticides can protect crops from severe insect damage.
However, they may also affect insects that are not agricultural pests.
Fungicides
Fungicides are used to control fungi that cause plant diseases.
Fungal diseases can affect:
- leaves
- stems
- roots
- fruits
- seeds
Some fungi can spread rapidly through crops, particularly under warm and humid conditions.
Fungicides may be used to prevent infection or control an existing fungal problem, depending on the product and disease.
Why Pesticides Are Used
Pesticides can provide several agricultural benefits.
They can:
- reduce crop losses
- increase crop yields
- improve crop quality
- protect stored food
- reduce competition from weeds
- control disease
- make large-scale crop production easier
These benefits can help make food production more reliable.
Crop Yield
Suppose two similar fields are affected by a serious insect pest.
Without effective pest management:
potential harvest = 10 tonnes
crop lost to pests = 3 tonnes
actual harvest = 7 tonnes
If pest management reduces the loss to 1 tonne:
actual harvest = 9 tonnes
This illustrates why pest control can have significant economic and food-production benefits.
Pesticides Do Not Only Have Benefits
Agricultural ecosystems contain many species besides the crop and target pest.
A chemical applied to a field may potentially interact with:
- soil
- water
- microorganisms
- insects
- birds
- mammals
- aquatic organisms
Environmental risk depends on factors including:
- chemical properties
- toxicity
- concentration
- application rate
- persistence
- movement through the environment
- exposure of non-target organisms
Therefore, pesticide effects must be evaluated using evidence rather than assuming all pesticides behave identically.
Target and Non-Target Organisms
The target organism is the pest the pesticide is intended to control.
A non-target organism is an organism affected even though it was not the intended target.
For example, an insecticide intended to kill a crop pest could potentially affect beneficial insects if they are exposed.
Beneficial organisms can include:
- pollinators
- predators of pests
- decomposers
- soil organisms
Protecting non-target species is an important consideration in pest management.
Pollinators
Many flowering crops depend partly or completely on animal pollination.
Important pollinators include:
- bees
- butterflies
- moths
- flies
- beetles
Poorly managed pesticide applications may expose pollinators to harmful chemicals.
Farmers can reduce exposure by considering:
- which chemical is used
- when it is applied
- where it is applied
- whether crops are flowering
- whether pollinators are active
Pesticide Drift
Pesticide drift occurs when pesticide droplets, particles, or vapors move away from the intended application area.
For example:
sprayed field → wind → nearby habitat
Drift can potentially expose:
- neighboring crops
- wildlife
- waterways
- people
Appropriate equipment and suitable weather conditions can reduce drift.
Runoff
Agricultural chemicals can sometimes be transported by water flowing across the soil surface.
This is runoff.
A possible pathway is:
pesticide applied to field
↓
heavy rainfall
↓
surface runoff
↓
stream or pond
Aquatic organisms may then be exposed.
Vegetated buffer zones can help reduce movement of some contaminants from fields into waterways.
Leaching
Some substances can move downward through soil with water.
This is called leaching.
Whether a pesticide is likely to leach depends on factors such as:
- how soluble it is
- how strongly it binds to soil
- how quickly it breaks down
- soil type
- rainfall
- irrigation
Chemicals that move deeply enough may potentially reach groundwater.
Persistence
Persistence describes how long a substance remains in the environment before it is broken down or transformed.
A highly persistent chemical may remain for a relatively long time.
A rapidly degrading chemical may disappear more quickly.
Persistence can be influenced by:
- sunlight
- temperature
- microorganisms
- water
- soil chemistry
Persistence is important because longer-lasting substances may create longer periods of potential exposure.
Bioaccumulation
Some substances can accumulate within an organism faster than they are removed.
This is called bioaccumulation.
For example:
small exposure → chemical retained
repeated exposure → concentration within organism increases
Whether this occurs depends strongly on the properties of the particular substance.
Not all pesticides bioaccumulate.
Biomagnification
Biomagnification occurs when the concentration of certain persistent substances increases at higher trophic levels in a food chain.
A simplified example might be:
water → plankton → small fish → large fish → fish-eating bird
If a persistent chemical accumulates in organisms and is transferred through feeding, predators may eventually contain higher concentrations.
Again, this occurs with particular substances and should not be assumed for every pesticide.
Historical Example: DDT
DDT is a well-known example used when studying pesticide persistence and biomagnification.
DDT was an effective insecticide and was used extensively for agricultural and disease-vector control.
However, its environmental persistence and movement through food webs caused serious ecological concerns.
High concentrations of DDT-related compounds were associated with reproductive problems, including eggshell thinning in some bird species.
This example demonstrated that a chemical can provide important benefits while also producing unexpected ecological effects.
Pesticide Resistance
One of the most important consequences of repeated pesticide use is the evolution of resistance.
Within a pest population, individuals vary genetically.
Suppose:
- A pesticide is applied.
- Most susceptible pests die.
- A few naturally resistant individuals survive.
- Survivors reproduce.
- Resistance genes become more common.
- The pesticide becomes less effective.
This is an example of natural selection.
Resistance Does Not Mean Individuals "Learn" to Survive
An individual pest does not usually become resistant because it tries to adapt.
Instead:
variation already exists → pesticide creates selection pressure → resistant individuals reproduce
Over generations, the population changes.
This distinction is important.
Herbicide-Resistant Weeds
The same evolutionary process can occur in weeds.
Repeated use of the same herbicide can strongly favor resistant individuals.
Eventually:
herbicide applied → susceptible weeds die → resistant weeds survive → resistant weeds reproduce
Farmers may then find that the herbicide no longer provides effective control.
Using multiple management strategies can reduce this selection pressure.
Environmental Effects of Herbicides
Herbicides are intended to affect plants.
If they move beyond the intended area, they may potentially affect:
- wild plants
- aquatic plants
- neighboring crops
- habitat vegetation
Loss of non-target vegetation can also indirectly affect animals that depend on those plants for:
- food
- shelter
- reproduction
Environmental effects can therefore spread through ecological relationships.
Effects on Biodiversity
Heavy reliance on broad-spectrum pest-control methods may reduce the abundance of organisms besides the target pest.
Changes in one population can affect:
- predators
- prey
- competitors
- pollinators
For example:
fewer insects → less food for insect-eating animals
Agricultural pest management therefore needs to consider the wider ecosystem.
Human Health and Safety
Pesticides are designed to affect living organisms, so safe handling is important.
Risk depends on:
hazard × exposure
A substance may be hazardous, but actual risk also depends on how much exposure occurs.
Agricultural safety measures can include:
- appropriate training
- correct storage
- following product instructions
- protective equipment where required
- preventing contamination of food and water
- respecting application restrictions
Pesticides are regulated in many countries to reduce unacceptable risks.
Pesticide Residues
A residue is a small amount of a substance remaining after application.
Food-safety systems may monitor pesticide residues in agricultural products.
Regulations can establish:
- approved uses
- application limits
- timing restrictions
- maximum permitted residue levels
This is one reason pesticides must be used according to their approved instructions.
Alternatives to Chemical Pest Control
Pesticides are only one method of controlling pests.
Alternative or complementary approaches include:
- biological control
- crop rotation
- resistant crop varieties
- mechanical control
- physical barriers
- habitat management
- monitoring
- traps
- carefully timed planting
- integrated pest management
The goal is often not to eliminate every pest organism.
Instead, the goal is to prevent pest populations from causing unacceptable damage.
Biological Control
Biological control uses living organisms to reduce pest populations.
Natural enemies can include:
- predators
- parasites
- parasitoids
- disease-causing microorganisms
For example, ladybirds can consume large numbers of aphids.
Advantages of Biological Control
Possible benefits include:
- reduced chemical use
- lower pesticide residues
- fewer effects from chemical runoff
- potentially long-lasting pest suppression
However, biological control must be carefully planned.
Introducing a new organism into an ecosystem can create unintended consequences if that organism affects non-target species or becomes invasive.
Crop Rotation
Crop rotation means growing different crops in a planned sequence.
A pest adapted to one crop may struggle if that crop is absent the following season.
For example:
Year 1 → Crop A
Year 2 → Crop B
Year 3 → Crop C
This can interrupt some:
- pest life cycles
- disease cycles
- weed patterns
Crop rotation can also provide benefits for soil fertility.
Resistant Crop Varieties
Plant breeders can develop crop varieties that are naturally more resistant to:
- insects
- fungi
- other diseases
If a crop is resistant to a particular pest, fewer pesticide applications may be necessary.
However, pests can also evolve, so relying on a single resistance mechanism indefinitely may not provide permanent protection.
Mechanical Weed Control
Weeds can sometimes be controlled physically.
Methods include:
- hand weeding
- hoeing
- mowing
- cultivation
- mechanical removal
Advantages include reduced herbicide use.
Disadvantages can include:
- labor requirements
- fuel use
- soil disturbance
- erosion risk
The best method depends on the farming system.
Physical Barriers
Some pests can be excluded using physical barriers such as:
- nets
- screens
- row covers
- traps
These approaches can reduce the need for pesticides in some crops.
They may be particularly useful in:
- greenhouses
- gardens
- high-value crops
Monitoring Pest Populations
One alternative to automatically spraying on a schedule is to monitor pest populations.
Farmers can:
- inspect plants
- count pests
- use traps
- monitor crop damage
- identify pest species
Control measures are then used when evidence shows that pest populations are likely to cause unacceptable economic damage.
This avoids unnecessary treatments.
Economic Thresholds
An economic threshold is the pest population level at which action should be considered to prevent unacceptable economic loss.
The basic idea is:
very few pests → treatment may cost more than the damage
but:
large pest population → crop damage may exceed treatment cost
This encourages evidence-based pest management rather than automatically attempting to eliminate every pest.
Integrated Pest Management
Integrated Pest Management (IPM) combines several methods to control pests while reducing unnecessary pesticide use.
IPM may include:
- identifying the pest correctly
- monitoring pest populations
- establishing action thresholds
- crop rotation
- biological control
- resistant varieties
- mechanical control
- habitat management
- targeted pesticide use when necessary
A simplified IPM process is:
IDENTIFY → MONITOR → PREVENT → EVALUATE → CONTROL → MONITOR AGAIN
Why IPM Can Be More Sustainable
IPM recognizes that complete elimination of pests may be:
- unnecessary
- expensive
- environmentally damaging
- impossible over the long term
Instead, pest populations are managed at acceptable levels.
Chemical pesticides can still be used, but they become one tool among several rather than the only method.
Example: Managing Aphids
Suppose aphids begin appearing in a crop.
Approach A
Immediately spray the entire field repeatedly with a broad-spectrum insecticide.
Possible benefits:
- rapid control
Possible drawbacks:
- cost
- effects on beneficial insects
- selection for resistance
- unnecessary treatment if aphid numbers are low
Approach B: IPM
- Monitor aphid numbers.
- Identify natural predators.
- Determine whether crop damage is increasing.
- Use biological or physical controls where practical.
- Apply a targeted insecticide only if necessary.
- Continue monitoring.
Approach B may reduce pesticide use while still protecting the crop.
Example: Managing Weeds
A farmer has a serious weed problem.
Instead of relying only on herbicides, the farmer could combine:
- crop rotation
- cover crops
- mechanical control
- competitive crop varieties
- targeted herbicide application
Using several approaches reduces dependence on any single control method.
It can also slow the evolution of herbicide resistance.
Pesticides and Food Security
Pesticides can contribute to food security by protecting crops from serious losses.
This can help:
- increase harvest reliability
- protect food during storage
- reduce economic losses
- maintain food supplies
However, sustainable food production also requires:
- healthy soils
- clean water
- biodiversity
- functioning pollinator populations
Pest management therefore involves balancing short-term crop protection with long-term ecosystem health.
Evaluating Pesticide Use
A strong evaluation considers both benefits and risks.
Benefits
Pesticides can:
- reduce crop losses
- increase yields
- improve crop quality
- control plant diseases
- control weeds
- protect stored food
Risks
Poorly managed pesticide use can:
- affect non-target organisms
- contaminate soil or water
- contribute to pest resistance
- reduce biodiversity
- expose people to hazardous substances
The conclusion should depend on:
- which pesticide is involved
- how it is used
- how much is used
- environmental conditions
- available alternatives
Example Decision
A farmer discovers a small population of insect pests.
Before applying an insecticide, useful questions include:
- What species is it?
- Is it actually causing significant damage?
- How large is the population?
- Are natural predators present?
- Is the population increasing?
- Are non-chemical controls available?
- If pesticide is necessary, can it be applied selectively?
This demonstrates the central idea of sustainable pest management:
Do not treat automatically — investigate first.
Common Mistakes
Thinking Herbicides and Pesticides Are Completely Different Categories
Herbicides are a type of pesticide.
Thinking All Pesticides Kill Insects
Only insecticides specifically target insects.
Other pesticides target plants, fungi, rodents, or other pests.
Assuming All Insects Are Pests
Many insects are beneficial.
They may act as:
- pollinators
- decomposers
- predators of pests
Assuming All Pesticides Behave the Same Way
Pesticides differ greatly in:
- toxicity
- persistence
- mobility
- target organisms
- environmental effects
Thinking Resistance Occurs Because Pests Decide to Adapt
Resistance evolves through natural selection acting on inherited variation.
Assuming Organic or Biological Control Has No Risks
Any intervention can have unintended consequences.
Biological control organisms must also be evaluated carefully.
Thinking IPM Means Never Using Pesticides
IPM can include pesticides.
The goal is to use them only when appropriate and as part of a broader management strategy.
Assuming Zero Pests Is the Goal
A small pest population may cause little economic damage.
The goal is usually to keep populations below damaging levels.
Confusing Bioaccumulation and Biomagnification
Bioaccumulation occurs within an individual organism over time.
Biomagnification involves increasing concentrations through trophic levels in a food chain.
Assuming "Natural" Automatically Means Safe
The environmental impact of any pest-control method should be evaluated using evidence.
Key Terms
Pesticide — Substance used to prevent, control, repel, or kill organisms considered pests.
Pest — Organism that causes unwanted damage or interferes with human activities.
Herbicide — Pesticide used to control unwanted plants.
Insecticide — Pesticide used to control insects.
Fungicide — Pesticide used to control fungi.
Rodenticide — Pesticide used to control rodents.
Selective herbicide — Herbicide designed to affect certain plants more strongly than others when used appropriately.
Non-selective herbicide — Herbicide affecting a broad range of plants.
Target organism — Organism a pesticide is intended to control.
Non-target organism — Organism unintentionally exposed to or affected by a pesticide.
Pesticide drift — Movement of pesticide away from its intended application area.
Runoff — Movement of water across land that can transport chemicals.
Leaching — Downward movement of dissolved substances through soil.
Persistence — Length of time a substance remains in the environment before breaking down or transforming.
Bioaccumulation — Buildup of a substance within an organism over time.
Biomagnification — Increase in concentration of certain substances at higher trophic levels.
Resistance — Inherited ability of some pests to survive a control method that previously killed most individuals.
Biological control — Use of living organisms to control pest populations.
Economic threshold — Pest population level at which management action may be justified to prevent unacceptable economic damage.
Integrated Pest Management (IPM) — Strategy combining monitoring, prevention, biological, physical, cultural, and chemical methods to manage pests sustainably.
Key Takeaways
- Pesticides are substances used to manage organisms considered pests.
- Herbicides are pesticides used specifically to control unwanted plants.
- Insecticides, fungicides, and rodenticides target other types of pests.
- Agricultural pests can reduce crop yield and quality.
- Pesticides can improve food production by reducing crop losses.
- Weeds compete with crops for light, water, nutrients, and space.
- Pesticides can sometimes affect organisms other than their intended targets.
- Pollinators and natural pest predators are important non-target organisms to protect.
- Agricultural chemicals can move through drift, runoff, or leaching.
- Persistence determines how long a chemical remains in the environment.
- Some persistent chemicals can bioaccumulate or biomagnify.
- Repeated pesticide use can select for resistant pest populations.
- Resistance evolves through natural selection.
- Alternatives include biological control, crop rotation, resistant varieties, mechanical control, physical barriers, and monitoring.
- Biological control uses natural enemies to reduce pest populations.
- Crop rotation can interrupt some pest and disease life cycles.
- Monitoring helps farmers determine whether control is actually necessary.
- Integrated Pest Management combines multiple strategies instead of relying entirely on chemicals.
- IPM does not necessarily eliminate pesticide use; it aims to make pest control more targeted and sustainable.
- Effective pest management balances crop production, economic costs, human safety, and ecosystem health.
The central idea is:
IDENTIFY THE PEST → MONITOR → PREVENT → USE THE LEAST DISRUPTIVE EFFECTIVE CONTROL → EVALUATE
rather than:
SEE A PEST → SPRAY AUTOMATICALLY.