AP Environmental Science

## Learning Outcomes

- Explain how **pesticide resistance** develops by natural selection
- Distinguish between short-term yield protection and long-term resistance problems
- Identify the meaning of **selection pressure**, **economic threshold**, and **economic injury level**
- Explain how the **pesticide treadmill** reduces effectiveness over time
- Compare resistance with secondary pest outbreaks
- Understand how **integrated pest management (IPM)** helps protect yields
- Apply these ideas to AP Environmental Science free-response and data questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand pesticide resistance and yields, with a focus on the following syllabus points:

- Know that pesticide resistance develops through **natural selection**, not because pests simply “get used to” chemicals
- Identify how repeated use of the same pesticide or mode of action increases **selection pressure**
- Explain how pesticides can increase or protect crop yields in the short run but create long-run problems
- Understand the **pesticide treadmill** and why it leads to higher costs and lower effectiveness
- Explain the difference between **economic threshold** and **economic injury level**
- Identify why broad-spectrum pesticides can kill beneficial organisms and trigger **secondary pest outbreaks**
- Understand how **IPM** reduces resistance risk while maintaining economically acceptable yields
- Recognise **Bt crops** and **refuge strategy** as a named example of resistance management

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. How does a pest population become resistant to a pesticide?
2. Why can repeated use of the same pesticide reduce its future effectiveness?
3. What is the difference between an economic threshold and an economic injury level?
4. How can pesticide use both increase yields and later threaten them?
5. Why is IPM usually more sustainable than relying only on chemical pesticides?

## Introduction

Pesticides are used because they can quickly reduce crop damage. In the short term, that often means higher yields, lower losses, and more reliable harvests. For AP Environmental Science, though, you need to go beyond “pesticides kill pests.” The exam often tests the trade-off: pesticides may protect yield now, but repeated use can drive resistance, damage non-target species, and reduce long-term control.

This topic sits at the meeting point of ecology, evolution, and agriculture. A farmer’s field is not just a place where chemicals are sprayed. It is also a place where natural selection happens. When a pesticide is used again and again, it changes which individuals survive and reproduce. That evolutionary shift is why resistance matters so much for crop yields.

> **Key Term: pesticide resistance**
> The inherited ability of some pests to survive a pesticide that previously controlled them effectively.

### How pesticide resistance evolves

Resistance develops by **natural selection**. This is one of the most tested ideas in AP Environmental Science, and students often explain it wrongly. Pesticides do not create resistant genes. Instead, resistant individuals are usually already present in a pest population at low frequency because populations show natural genetic variation.

Imagine a field with thousands of insects. Most are susceptible to a pesticide, but a few have traits that let them survive exposure. When the pesticide is sprayed, many susceptible insects die, but the resistant ones are more likely to survive. Those survivors then reproduce. Because their competitors have been removed, they may have more food and space, so the resistant trait spreads faster through the next generation.

That gives a clear cause-and-effect chain:

1. Variation already exists in the pest population.
2. Pesticide application kills more susceptible individuals.
3. Resistant individuals survive.
4. Survivors reproduce and pass on resistance.
5. The population becomes less susceptible over generations.
   > **Key Term: natural selection**
   > The process in which individuals with advantageous inherited traits survive and reproduce more successfully than others.
   >
   > **Key Term: selection pressure**
   > The environmental force that favors some individuals over others; in this case, pesticide use increases the survival advantage of resistant pests.

A common APES error is to write that pests “became immune because they were exposed.” That sounds like the individuals changed themselves. The better answer is evolutionary: the population changes because resistant individuals leave more offspring.

A second common error is to ignore timescale. One spray usually does not make a whole population resistant immediately. Resistance develops across generations. However, pests with short life cycles can evolve quickly, so the problem may appear within just a few years of repeated use.

> **Test Tip:** In FRQs, use the words “variation,” “survive,” “reproduce,” and “pass on traits.” That sequence shows the grader you understand resistance as natural selection.

### Why resistance threatens yields over time

Pesticides can increase or protect yield in the short run. If insects, weeds, or fungal pests are damaging crops, chemical control may reduce losses fast. That was part of why pesticide use expanded during the Green Revolution. Farmers could harvest more marketable crop from the same land area.

> **Key Term: crop yield**
> The amount of crop produced per unit area of land, such as kilograms per hectare or bushels per acre.

But that short-run gain can hide long-run weakness. Once resistance spreads, the same pesticide kills fewer pests. Crop damage rises again, even though the farmer is still paying for the chemical. The grower may then respond by spraying more often, using higher doses, or switching to a new pesticide. That can raise costs without restoring the original level of control.

This is the heart of the **pesticide treadmill**.

> **Key Term: pesticide treadmill**
> A cycle in which resistance leads to more pesticide use, which then increases resistance problems and reduces effectiveness over time.

The treadmill works in several linked steps:

- A pesticide works well at first and helps protect yield.
- Repeated use selects for resistant pests.
- The pesticide becomes less effective.
- The farmer increases application rate or frequency, or adopts a new chemical.
- Resistance continues to build, so control becomes harder and more expensive.

This matters economically and environmentally. The farmer may pay more for less benefit. Also, more spraying means greater risk of runoff into water, more exposure of non-target organisms, and more chances for toxins to enter food webs.

The APES course also stresses inefficiency. Only a small fraction of many pesticides actually reaches the target pest. The rest may land on soil, other plants, water, or non-target organisms. So even before resistance develops, pesticide-heavy strategies can have large environmental costs.

Resistance is not the only reason yields can fall after pesticide use. Broad-spectrum pesticides may also kill predators and parasitoids that normally keep pest species in check. If those natural enemies are removed, pest numbers may rebound fast.

> **Key Term: broad-spectrum pesticide**
> A pesticide that kills a wide range of organisms, including non-target species.
>
> **Key Term: secondary pest outbreak**
> A rapid increase of a non-target pest, often because pesticide use killed its natural predators or competitors.

That distinction is easy to miss:

- **Resistance** means the target pest evolved lower susceptibility.
- **Secondary outbreak** means ecological disruption removed biological control.

Both can reduce yield, but they happen for different reasons.

### Economic thresholds, yield decisions, and IPM

Farmers do not need to eliminate every pest. In many cases, that would cost more than the crop loss prevented. This is where **economic threshold** and **economic injury level** become important.

> **Key Term: economic injury level (EIL)**
> The lowest pest population level that causes economic damage equal to the cost of control.
>
> **Key Term: economic threshold**
> The pest population level at which action should be taken to stop the population reaching the economic injury level.

The difference is simple but examiners like it:

- **EIL** = the break-even point where pest damage equals control cost
- **Economic threshold** = the action point before the break-even point is reached

Thresholds matter because treatment takes time to work. If a farmer waits until the pest population is already at the economic injury level, the crop may still suffer losses before the control reduces numbers.

This idea helps explain why more pesticide does not always mean better yield. If a farmer sprays when pest numbers are still low, the chemical cost may be greater than the crop value saved. Unnecessary spraying also increases selection pressure, making resistance more likely later.

A useful conceptual formula sometimes used for EIL is:

EIL = C / (V × I × D × K)

Where:

- **C** = cost of control
- **V** = market value of crop
- **I** = injury per pest
- **D** = yield loss per unit injury
- **K** = proportion of pest reduction from control

You do not usually need to calculate this on APES, but you should understand what it means. If crop value is high, control may be worth doing sooner. If the pesticide works poorly because resistance has spread, the value of **K** falls, so the economics of spraying become worse.

This is why **integrated pest management** is so important.

> **Key Term: integrated pest management (IPM)**
> A pest-control strategy that combines biological, physical, cultural, and limited chemical methods to keep pests at acceptable levels.

IPM aims to manage pest populations economically with the least possible hazard to people and the environment. It does not aim to wipe out every pest. Instead, it uses monitoring and thresholds to decide when action is justified.

Typical IPM methods include:

- scouting and record keeping
- traps and barriers
- crop rotation
- intercropping or polyculture
- use of natural predators
- mulch for weed control
- pheromones or hormone interrupters
- limited, targeted pesticide use as a last resort

Why does IPM help maintain yields better over time?

First, it reduces unnecessary spraying, which lowers selection pressure for resistance. Second, it protects beneficial insects and predators that support natural pest control. Third, it spreads risk by relying on several methods instead of one chemical. If one tactic fails, the whole system does not collapse.

A good named example is **Bt crops**. These are genetically modified crops that produce insecticidal proteins from _Bacillus thuringiensis_. They can reduce insect damage and lower conventional insecticide use, which may support higher yields. But resistance can still evolve if the same Bt trait is overused.

> **Key Term: Bt crops**
> Genetically modified crops that produce insecticidal proteins from _Bacillus thuringiensis_ to kill certain pests.

To slow that process, growers use a **refuge strategy**.

> **Key Term: refuge strategy**
> Planting non-Bt or untreated areas so susceptible insects survive and mate with resistant insects, slowing the spread of resistance.

The logic is evolutionary. If resistant insects mate only with other resistant insects, resistance spreads faster. If many susceptible insects survive in refuges, their genes remain common in the population and resistance develops more slowly.

> **Test Tip:** If an FRQ asks for a sustainable way to maintain yield while reducing pesticide use, mention monitoring, economic thresholds, biological control, and crop rotation. That combination usually scores better than saying only “use less pesticide.”

### How AP Environmental Science may test this topic

This topic can appear in several ways.

In multiple-choice questions, you may need to identify why repeated use of one pesticide causes resistance. The correct answer will usually focus on natural selection and inherited variation, not on pests choosing to adapt.

In free-response questions, you may be asked to design an experiment comparing pesticides. In that case:

- the **independent variable** is the pesticide used
- the **dependent variable** is pest mortality, crop damage, or yield
- the **control group** receives no pesticide
- environmental variables such as water, pest density, shade, and crop condition should be kept constant

You may also be asked to compare chemical control with IPM. Strong answers include both yield and environmental reasoning. For example, chemical pesticides may protect yield rapidly, but IPM is better for long-term effectiveness because it slows resistance, protects beneficial organisms, and reduces pollution.

Students also need to be careful with language around yield. Resistance management does not guarantee the highest short-run yield every time. Sometimes it involves trade-offs, such as allowing low pest numbers to remain below threshold. The point is long-term yield stability and preserving control methods.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Pesticide resistance is an inherited trait that spreads through natural selection
- Pesticides do not create resistant genes; they select for resistant individuals already present
- Repeated use of the same pesticide increases selection pressure
- Pesticides may protect crop yield in the short term but reduce control effectiveness in the long term
- The pesticide treadmill means farmers may pay more for less benefit as resistance spreads
- Broad-spectrum pesticides can kill beneficial species and trigger secondary pest outbreaks
- Crop yield means crop produced per unit area of land
- The economic injury level is the break-even pest level where damage equals control cost
- The economic threshold is the action point before the economic injury level is reached
- IPM combines multiple methods and uses pesticides only when justified
- Bt crops can reduce insect damage, but resistance can still evolve
- Refuge strategy slows resistance by maintaining susceptible insects in the population

## Key Terms and Concepts

- pesticide resistance
- natural selection
- selection pressure
- crop yield
- pesticide treadmill
- broad-spectrum pesticide
- secondary pest outbreak
- economic injury level (EIL)
- economic threshold
- integrated pest management (IPM)
- Bt crops
- refuge strategy


Pest Control Methods - Pesticide resistance and yields