Biological resistance occurs when a target organism (weed, pest, pathogen, or parasite) evolves the ability to survive exposure to a chemical or treatment that previously killed it. Resistance is one of the most serious long-term threats to sustainable food and fibre production in Australia, affecting all sectors of the industry.
VCAA FOCUS: Students must address all three components: reasons (causes) for resistance developing, impacts on agriculture, and strategies to combat resistance. All three chemical types are relevant: herbicides (weeds), pesticides (insects, mites, fungi), and antibiotics (livestock disease).
Resistance is a product of Darwinian natural selection:
$$\text{Selection pressure} + \text{Genetic variability} \rightarrow \text{Resistance evolution}$$
| Reason | Explanation |
|---|---|
| Overuse of one chemical/mode of action | Single-tactic management applies the same selection pressure repeatedly; fastest route to resistance |
| Sub-lethal doses | Under-dosing (application errors, poor coverage) kills susceptible individuals but allows partially resistant ones to survive; selects for resistance |
| High reproduction rates | Organisms with fast lifecycles (annual weeds, insects, bacteria) produce many generations quickly; resistance evolves faster |
| Large populations | The larger the population, the greater the probability that a resistant individual exists |
| High genetic variability | Some organisms (annual ryegrass, bacteria) have high mutation rates or exchange genetic material — rapid resistance evolution |
| Horizontal gene transfer (bacteria) | Bacteria can share resistance genes with other bacteria (conjugation, transformation) — not evolution of resistance by reproduction alone |
Herbicide resistance:
- Annual ryegrass (Lolium rigidum) is the world’s most herbicide-resistant weed — resistant to 11 different herbicide modes of action in Australia
- Wild radish, wild oats, and ryegrass resistance cost Australian broadacre farmers hundreds of millions of dollars annually
- Loss of effective herbicide options means more costly, difficult, or environmentally harmful alternatives required
- Risk of untreatable weed infestations threatening the viability of grain farming
Pesticide resistance:
- Western flower thrips resistance to insecticides severely limits chemical control options in horticulture
- Spider mite resistance to acaricides (miticides) in orchards and vineyards
- Diamondback moth resistance to synthetic pyrethroids and Bt toxins in brassica crops
- Resistance can spread regionally as organisms migrate
Anthelmintic (drench) resistance in internal parasites:
- Haemonchus contortus (Barber’s Pole Worm) resistance to macrocyclic lactones (ivermectin) and benzimidazoles is widespread in Australian sheep flocks
- Combination drench resistance (resistant to 3+ classes) is increasingly common
- Affects animal welfare and production; stock cannot be protected economically
Antibiotic resistance:
- Use of antibiotics in livestock (therapeutic and preventive) can select for resistant bacteria
- Antimicrobial Resistance (AMR) — resistant bacteria may transfer resistance genes to human pathogens (e.g., E. coli, Salmonella, Staphylococcus)
- AMR is a global health emergency; Australian agriculture is part of a “One Health” framework linking animal, human, and environmental health
- Concerns include resistance in Campylobacter (poultry), E. coli (cattle), and Staphylococcus aureus in dairy
COMMON MISTAKE: Students sometimes think resistance affects only chemical companies. In reality, it affects farmers (higher costs), animals (welfare impacts), consumers (food safety), and the broader environment (increased chemical use). Frame impacts across multiple dimensions.
| Strategy | How It Combats Resistance |
|---|---|
| Rotate herbicide modes of action | Different biochemical targets; resistant individuals for one MOA are often susceptible to another |
| Herbicide mixtures | Combine two MOA groups; an individual resistant to one is likely still killed by the other |
| Non-chemical tactics | Crop rotation, competitive varieties, tillage, cover crops — reduce reliance on herbicides |
| Harvest weed seed control (HWSC) | Prevents seed return to soil; seed bank depletion reduces resistance allele frequency |
| Zero tolerance for seed set | No resistant plants allowed to set seed — critical to prevent spread |
| Resistance testing | Detect resistance early (pot bioassay, molecular testing); adjust strategy before failure is widespread |
| Strategy | Detail |
|---|---|
| Rotate insecticide/fungicide MOA groups | IRAC (insecticide), FRAC (fungicide) classification systems guide rotation |
| Use selective products | Preserve natural enemy populations — biological control suppresses resistant pests too |
| Biological control emphasis | Natural enemies are not affected by chemical resistance |
| Reduce application frequency | Only apply at economic threshold — fewer applications = less selection pressure |
| Refugia strategy | Maintain some untreated areas — susceptible individuals breed with resistant ones, diluting resistance allele frequency |
| Monitor and test | Resistance monitoring programmes; adjust promptly |
| Strategy | Detail |
|---|---|
| Responsible, targeted use | Use antibiotics only when necessary (sick animals); avoid prophylactic mass treatment |
| Veterinary prescription controls | Many antibiotics require veterinary prescription in Australia — reduces casual overuse |
| Record keeping | Record antibiotic use (species, product, dose, reason) — traceability and monitoring |
| Vaccination alternatives | Vaccines prevent diseases without creating antibiotic selection pressure |
| Biosecurity | Prevent disease introduction — reduces need for antibiotic treatment |
| Antibiotic class restrictions | Highest-priority critically important antibiotics (HPCIAs) should NOT be used in food animal production |
| WHP/ESI compliance | Withholding periods and export slaughter intervals ensure residues are absent in food products |
APPLICATION: Resistance management is fundamentally about managing evolutionary pressure. Any strategy that reduces selection pressure (rotating tools, reducing exposure frequency, maintaining refugia) helps slow resistance development across all organism types.
Biological resistance develops through natural selection when susceptible organisms are repeatedly exposed to the same control agent — a universal biological process. In Australia, herbicide resistance (especially annual ryegrass), insecticide resistance (western flower thrips, diamondback moth), drench resistance (Barber’s Pole Worm), and antibiotic resistance all pose serious threats to agricultural and horticultural viability. Combat strategies include rotating modes of action, using non-chemical alternatives, reducing unnecessary chemical use, preserving refugia, and implementing comprehensive monitoring and stewardship programmes. Managing resistance is a long-term, industry-wide responsibility with implications beyond the farm gate.
