Ch.11: Pesticide Resistance

2. Resistance Trivia (memorize the numbers)

• More than 600 pest species worldwide have some resistance (some sources say over 1,000).
• Reported in all pest groups — plant diseases, weeds, insects, arachnids, rodents.
• Synthetic organics (like DDT) came into use in the early 1940s; the first resistance cases were noted by 1947.
• Colorado potato beetle: resistant to more than 50 insecticides — a "super pest."
• Pests adapt to non-chemical controls too: the northern corn rootworm extended its diapause (a hibernation) to survive a soybean year in a corn-soybean rotation.

3. How Resistance Develops

• Pest populations are very large; individuals differ genetically through random mutation. Most mutations are harmful, but a few help an individual survive a pressure.
• A pesticide applies a selection pressure, like a predator. A tiny proportion (e.g., one out of many millions) survives due to its genetic makeup.
• Survivors breed and pass the resistant trait to some offspring. Because the pesticide kills most nonresistant individuals, resistant ones become a larger share — and with each use the resistant percentage rises until most of the population is resistant.
• Key nuance: the pesticide does NOT create resistance — it selects for resistant individuals that already existed. Resistance is genetic and inherited, not learned or caused by being sprayed.

4. Cross-Resistance vs. Multiple Resistance

• Cross-resistance: resistance to one pesticide plus other chemically related pesticides — because they kill the pest the same way (same or very similar mode of action).
• Multiple resistance: resistance to pesticides that have DIFFERENT modes of action. Example: multiple-resistant waterhemp resists triazine, ALS, and PPO herbicides.
• The distinction: cross = related, same MOA; multiple = different MOAs.

5. Mode of Action (MOA)

• Mode of action = the specific way a pesticide affects a pest — usually the biological process or enzyme it interrupts.
• Site of action = the specific biochemical site harmed (a more precise description); "site of action" and "mode of action" are often used interchangeably.
• Examples: an insecticide MOA may disrupt the nervous system or interfere with hormones; a herbicide MOA may mimic growth hormones or block converting light into food.
• Over-reliance on one active ingredient or MOA places heavy selection pressure and can select for resistant individuals.

6. The Applicator's Role

• When a product stops working well, an applicator may be tempted to use higher rates or spray more often.
• That does NOT work — it increases selection pressure and speeds resistance. And if you're already at the highest labeled rate or frequency, applying more is illegal.

7. Factors That Promote Resistance

• Frequency of resistance already in the population — none is best, but you can't control when a mutation appears.
• Low chemical diversity — always using the same pesticide or same MOA lets resistant individuals survive and breed.
• A very specific mode of action (a single site of action) — especially if used frequently.
• High persistence + frequent use — the longer a pesticide stays active, the longer it pressures the population.
• Proportion of the population exposed — if some pests are NOT exposed, susceptible individuals survive and breed, which slows resistance.
• Short life cycle — pests like aphids with many generations per year build resistance faster.

8. Resistance Management

• You can't just switch products anymore — new pesticides are more complex, harder to synthesize, and more expensive, and they too can fail.
• Assume pests can develop resistance to any pesticide — play it safe.
• Resistance management = trying to prevent, delay, or reverse resistance. It's complex and involves more than just pesticides. Core practices:
  • Use IPM — other control options alongside pesticides reduce reliance on any one product.
  • Use pesticides only when needed — resistance develops only from use, so needless use (or poor timing) raises it.
  • Mix and/or rotate MOAs — use products with different modes of action so pests resistant to one are killed by the other. Not all products can be tank-mixed — read the label or do a jar test; all products must be labeled for the site.
  • Use label rates — they vary by plant and pest. Don't use less than recommended: sublethal doses increase resistance risk.

9. Labels & MOA Charts

• Many labels show the MOA as a number code in a box on the first page — but the EPA does not require it.
• Same MOA number (for products aimed at the same pest group) = they kill the pest the same way; different numbers = different MOAs.
• Rotation example: treat waterhemp with a group 2 herbicide, then next time a group 7 herbicide labeled for the same site (different MOAs) to help delay resistance.
• If the label lists the MOA group number, you don't need the full resistance chart.

10. Where to Learn More

• Insecticide resistance: irac-online.org
• Herbicide resistance: hracglobal.com
• Fungicide resistance: frac.info

Key Vocabulary