3.2.6 Biological Resistance

Key Knowledge: reasons for, impacts of, and strategies to combat biological resistance to herbicides, pesticides and antibiotics in Australian food and fibre industries. 

This is one of the most significant challenges in modern agriculture. When we rely too heavily on a single strategy to fight pests, diseases, or weeds, those organisms eventually adapt, making our current management strategies ineffective.

So, what actions by producers could cause resistance?

In the Australian food and fibre industries, resistance is rarely the result of a single event. It is usually caused by:

Over-reliance on one chemical group: Using the same "Mode of Action" (the specific way a chemical kills a pest) repeatedly.

Sub-lethal dosing: Applying chemicals or antibiotics at a lower rate than recommended, which fails to kill the stronger individuals and allows them to build immunity.

Poor timing: Applying treatments when the pest is too mature or the population is too high to be fully controlled.

Lack of diversity: Relying solely on chemical controls instead of using the Integrated Pest Management (IPM) pyramid.

Impacts on Food and Fibre Industries

The rise of resistant superbugs or superweeds has direct consequences for Victorian farmers and consumers:

Increased Production Costs: Farmers must buy more expensive, newer chemicals or apply treatments more frequently. They may also need to pay for extra processing such as grain cleaning or accept a lower price for their crop if it is contaminated with weed seeds.

Reduced Yields: If a weed like wild radish becomes resistant to herbicides, it stays in the paddock and competes for moisture, leading to smaller harvests. It can also contaminate crops.

Animal Welfare Concerns: If antibiotics no longer work to treat bacterial infections in livestock, animal health and productivity decline.

Loss of Markets: Some export markets have strict rules about chemical residues. If farmers have to spray more often to kill resistant pests, they risk exceeding these limits.

PRACTICE:

Check Your Understanding Use the marking scheme to evaluate your work

1. Explain why applying a pesticide at half the recommended dose might actually make the pest problem worse in the long run.

2. What is meant by the term "Selection Pressure" in the context of biological resistance?

3. Why is the livestock industry moving away from using antibiotics as a preventative measure?

PRACTICE:

Exam-Style Questions Use the marking scheme to evaluate your work

Question 1 (2 Marks)

A sheep farmer has noticed that the chemical drench used to treat intestinal worms is no longer effective.

Describe the biological process that has occurred within the worm population over time.

Question 2 (4 Marks)

Discuss how a grain farmer could use the double knock strategy to manage a population of wild radish that is showing early signs of herbicide resistance.

Key Skill: Analysing Solutions to Biological Resistance

When you see the word analyse in a question, the examiners are looking for a cause-and-effect chain. You need to show how a specific management action directly addresses the biological survival of the pest.

How to analyse a Resistance Problem: 

If you are faced with a scenario where a chemical is failing, follow this mental roadmap:

First identify the selection pressure: What is being used too often? (e.g., The same Group A herbicide).

Then explain the biological survival: Why are they surviving? (e.g., Natural genetic mutation allows some individuals to survive and reproduce).

Lastly propose and justify the solution: What is the best way to stop them? (e.g., A non-chemical control to remove the survivors).

What to Look for in a Scenario

When reading a case study or exam question, look for these red flags that indicate a resistance analysis is needed:

• History of use: The prompt mentions the farmer has used the same spray for three or more consecutive years.

• Escaping individuals: The prompt mentions that most pests died, but a small patch or a few individuals seem healthy.

• Sub-lethal dosage: The prompt mentions the farmer reduced the rate of chemical to save money or because the weather was poor.

Comparison of Potential Solutions

When analysing solutions, you should weigh up which one is most effective for the specific scenario. This table shows just one way you might organise your notes on this:

Exam Tip: Explaining the Biological Why

When answering questions about biological resistance, simply stating that a pest survived the treatment is rarely enough for full marks. To secure every available mark, you must explain the reproductive consequence of that survival.

Always follow this two-step chain of reasoning:

1. Identify the survivors: Acknowledge that a sub-lethal dose or repeated chemical use allows individuals with resistant traits to survive.

2. Explain the next generation: State that these survivors then reproduce and pass their resistant genes onto their offspring.

Remember: It is not just about who dies today; it is about who is left to breed tomorrow.