Modes of Action and Resistance Management
Key Takeaways
- Mode of action is the biological process a pesticide disrupts; rotating modes of action, not brand names, manages resistance.
- Herbicides differ by selectivity and by contact versus systemic behavior; systemic products reach roots and rhizomes of perennials.
- Organophosphate and carbamate insecticides inhibit cholinesterase, which explains their shared poisoning signs and first-aid response.
- Resistance develops when one site of action is overused; cross-resistance can spread to related chemistry sharing that site.
- Label group numbers (herbicide group, IRAC, FRAC) confirm a true rotation, so judge rotations by group number rather than product name.
What mode of action means
A pesticide's mode of action is the way it kills or controls the pest, meaning the specific biological process it disrupts. Two products can target the same pest yet act on different processes, and that difference is the foundation of resistance management. The Texas core exam expects applicators to rotate modes of action, not just brands, to keep products working.
Mode of action also explains poisoning signs and first aid. If you know a product inhibits an enzyme in the nervous system, you can anticipate the symptoms and the medical response.
How herbicides act
Herbicides are grouped by site of action and by selectivity and contact behavior. Selective herbicides control some plants while leaving others unharmed, which is how a broadleaf herbicide can be used in a grass crop. Nonselective herbicides injure most plants they contact.
| Behavior | What it means | Field implication |
|---|---|---|
| Contact | Kills only tissue it touches | Needs thorough coverage; poor on established perennials |
| Systemic or translocated | Moves within the plant | Reaches roots and rhizomes of perennials |
| Preemergence | Acts on germinating seedlings | Applied before weeds emerge |
| Postemergence | Acts on emerged plants | Applied to growing weeds |
Common herbicide sites of action include growth regulators such as the phenoxy herbicides 2,4-D and MCPA, amino-acid synthesis inhibitors such as glyphosate, and photosynthesis inhibitors. Texas regulates several of these as regulated herbicides because of drift and volatility concerns.
How insecticides act
Many insecticides act on the nervous system. Organophosphates and carbamates inhibit cholinesterase, the enzyme that resets nerve signals, which causes overstimulation; this is why their poisoning signs overlap and why atropine is part of the medical response. Pyrethroids disrupt nerve cell sodium channels. Insect growth regulators interfere with molting and development rather than killing on contact, so they act slowly and target immature stages.
Stomach poisons must be eaten, contact poisons act through the cuticle, fumigants act as a gas, and systemics move within the plant or animal so a feeding pest is exposed. Matching the mode to the pest's behavior and life stage decides whether the product works.
Resistance and why rotation matters
Resistance develops when repeated use of one mode of action selects for the few individuals that survive, and they pass on that survival. The population gradually stops responding to that chemistry, and eventually to related products that share the site of action, which is called cross-resistance.
The countermeasure is to rotate among different modes of action, not merely different brand names. Manufacturers print group numbers on labels, such as herbicide group numbers, insecticide IRAC numbers, and fungicide FRAC numbers, so applicators can confirm they are switching the actual mode and not just the product name.
- Rotate or tank-mix products with different group numbers.
- Use IPM tactics, including nonchemical controls, to reduce selection pressure.
- Apply labeled rates; underdosing and overuse both speed resistance.
- Target the most susceptible life stage to reduce the survivors that drive resistance.
- Avoid using the same site of action season after season on the same site.
Reading the group number
The exam may show two products with different brand names but the same group number. Rotating between them does nothing for resistance because they share a site of action. Conversely, two products with different group numbers represent a genuine rotation even if they control the same pest. Always read the group number to judge a rotation plan.
Exam reasoning pattern
- Define mode of action as the biological process disrupted, not the brand.
- Choose systemic products for perennial weeds and contact products where coverage is achievable.
- Connect cholinesterase-inhibiting insecticides to their poisoning signs and first aid.
- Judge a rotation by group numbers, not product names, to avoid cross-resistance.
- Combine chemistry rotation with IPM and labeled rates to slow resistance.
If a question describes repeated use of one chemistry and declining control, the answer is resistance, and the fix is rotating to a different mode of action plus nonchemical tactics. If it asks how to manage a perennial weed regrowing from roots, a translocated systemic herbicide outperforms a contact burndown.
Selectivity, timing, and rate as resistance tools
Resistance management is not only rotation. Applying the labeled rate matters because an underdose lets more partially resistant individuals survive and reproduce, while overdosing wastes product, raises residue and drift risk, and does not overcome true target-site resistance. The label rate is calibrated to the mode of action and the susceptible stage.
Timing reinforces the chemistry. Treating the most susceptible life stage, such as young larvae or small annual weeds, leaves fewer survivors to carry resistance genes forward. A late application against hardened-off plants or mature insects often gives weak control and selects heavily for resistance.
Fungicide modes in brief
Fungicides also carry resistance risk and FRAC group numbers. Protectant or contact fungicides stay on the plant surface and must be present before infection, and they generally have multiple sites of action and low resistance risk. Systemic fungicides move into the plant and can act after infection, but many are single-site and higher resistance risk, so they are rotated or tank-mixed with a protectant to protect their useful life.
An applicator has used the same chemistry on the same field for several seasons and control is now poor at labeled rates. What is the most likely cause and the best response?
Two insecticides have different brand names but the same insecticide group number. For resistance management, what does rotating between them accomplish?