3.4 Pesticide Types, Chemical Families & Modes of Action
Key Takeaways
- Pesticides are named by the pest they control — the "-cide" suffix identifies the target, so an acaricide targets mites and a nematicide targets nematodes.
- Organophosphates and carbamates both inhibit acetylcholinesterase, which is why the two share a poisoning syndrome and why rotating between them is not a true mode-of-action rotation.
- Contact products kill only the tissue they touch, while systemic products are absorbed and translocated — a distinction that drives coverage requirements, rainfastness, and the timing of perennial weed control.
- Selectivity is a property of the product plus the rate, timing, and placement; a selective herbicide applied off-label can still injure a non-target plant.
- Biopesticides, plant growth regulators, and semiochemicals are legally pesticides in Colorado and require the same registration and licensing as conventional chemistry.
3.4 Pesticide Types, Chemical Families & Modes of Action
Why classification is an exam topic in its own right: 8 CCR 1203-2 Part 4, Subpart D — "Pesticides and Their Families" — is a required continuing-education subject area, and 40 CFR § 171.103(c)(5) requires competency in "types of pesticides," formulation types, "compatibility, synergism, persistence and toxicity," and "factors affecting effectiveness and resistance." Signal words tell you how a product can hurt you; families and modes of action tell you what it does, what it will not do, and what it must not be rotated with.
1. Naming by Target: The "-cide" Suffixes
| Type | Target pest |
|---|---|
| Insecticide | Insects |
| Acaricide (miticide) | Mites and ticks |
| Herbicide | Weeds and unwanted plants |
| Fungicide | Fungi and fungal-like pathogens |
| Bactericide | Bacteria |
| Nematicide | Nematodes |
| Rodenticide | Rats, mice, and other rodents |
| Avicide | Birds |
| Molluscicide | Slugs and snails |
| Piscicide | Fish |
| Predacide | Predatory vertebrates |
| Algaecide | Algae |
| Defoliant / Desiccant | Removes or dries plant foliage (not a pest-killer as such) |
| Plant growth regulator (PGR) | Alters plant growth, flowering, or maturation |
| Repellent / Attractant | Drives pests away or draws them in |
A Colorado licensing consequence follows directly: Category 302 covers vertebrates but not rats and mice, Category 108 covers "aquatic weeds, amphibians, fish and other pests in water," and Category 305 explicitly includes applying plant growth regulators to stored commodities. Product type and category authority are linked.
2. Insecticide Families and Modes of Action
| Family | Mode of action | Field and safety consequences |
|---|---|---|
| Organophosphates (chlorpyrifos, malathion, acephate) | Irreversibly inhibit acetylcholinesterase, so acetylcholine accumulates at nerve synapses | Cholinergic crisis: miosis, salivation, lacrimation, bronchorrhea, fasciculations. Antidote atropine, with 2-PAM for OPs. Cholinesterase baseline monitoring is warranted for routine handlers |
| N-methyl carbamates (carbaryl, methomyl, carbofuran) | Inhibit acetylcholinesterase reversibly — the enzyme spontaneously decarbamylates | Same syndrome but shorter-lived. Atropine is indicated; 2-PAM is not used for pure carbamate poisoning |
| Pyrethroids and pyrethrins (bifenthrin, permethrin, lambda-cyhalothrin) | Prolong sodium-channel opening in nerve axons | Low mammalian toxicity, extreme toxicity to fish and aquatic invertebrates; strongly adsorbed, so runoff moves them on sediment. Paresthesia (skin tingling) on contact |
| Neonicotinoids (imidacloprid, thiamethoxam, clothianidin) | Bind nicotinic acetylcholine receptors as agonists | Highly systemic; long residual; major pollinator concern because residues appear in nectar and pollen |
| Diamides (chlorantraniliprole) | Activate ryanodine receptors, causing calcium depletion and paralysis | Very selective for chewing pests; comparatively easy on beneficials |
| Insect growth regulators (methoprene, diflubenzuron) | Disrupt molting or chitin synthesis | Kill only immature stages; no knockdown of adults, so scouting timing is everything |
| Microbials (Bacillus thuringiensis) | Crystal proteins bind midgut receptors in specific insect orders | Narrow spectrum, short residual, degraded by UV; strain must match the pest order |
[!IMPORTANT] Organophosphates and carbamates are different families with the same target site. That fact drives two exam answers at once: it explains why their poisoning presentations look alike, and it means rotating from an organophosphate to a carbamate is not a resistance-management rotation.
3. Herbicide Families and Modes of Action
| Family / group | Mode of action | Practical notes |
|---|---|---|
| Synthetic auxins (2,4-D, dicamba, triclopyr, clopyralid) | Mimic natural auxin, causing uncontrolled growth | Broadleaf-selective in grass crops and turf. Volatile ester formulations can move as vapor drift in heat |
| EPSP synthase inhibitors (glyphosate) | Block aromatic amino acid synthesis | Non-selective, systemic, foliar-applied, minimal soil activity |
| ALS / AHAS inhibitors (sulfonylureas, imidazolinones) | Block branched-chain amino acid synthesis | Very low use rates; long soil residual creates rotational plant-back restrictions; resistance evolves quickly |
| ACCase inhibitors (clethodim, sethoxydim) | Block fatty-acid synthesis in grasses | Grass-selective; safe on broadleaf crops |
| Photosystem II inhibitors (atrazine, metribuzin) | Interrupt electron transport in photosynthesis | Soil-applied and root-absorbed; atrazine's mobility is the classic Colorado groundwater concern |
| PPO inhibitors (flumioxazin, carfentrazone) | Cause membrane-destroying peroxidation | Fast contact burn; poor translocation |
| Cell-division / seedling inhibitors (trifluralin, pendimethalin) | Disrupt root and shoot cell division | Pre-emergence only; must be incorporated or activated by water |
Pre-emergence versus post-emergence is a timing classification layered on top of family: a pre-emergence herbicide must be in place before the weed emerges, while a post-emergence product needs actively growing tissue. Applying a pre-emergence product to an established weed wastes the application entirely.
4. Fungicide and Rodenticide Basics
Fungicides divide by movement and by risk:
- Protectant (contact) fungicides — mancozeb, chlorothalonil, copper, sulfur — sit on the leaf surface and must be present before infection. They are multi-site inhibitors, so resistance develops slowly.
- Systemic fungicides — triazoles (FRAC 3, sterol biosynthesis inhibitors), strobilurins (FRAC 11, QoI respiration inhibitors), SDHIs (FRAC 7) — are absorbed and can act curatively. Because they are single-site, resistance can appear quickly, which is why labels commonly require tank-mixing or alternating with a multi-site protectant.
Rodenticides divide by anticoagulant status:
- Anticoagulants — first-generation (warfarin, chlorophacinone, diphacinone) and second-generation (brodifacoum, bromadiolone) — block vitamin K recycling and clotting factor synthesis. Vitamin K1 (phytonadione) is the antidote. Second-generation products are far more persistent, and secondary poisoning of raptors and pets is the dominant stewardship issue.
- Non-anticoagulants — bromethalin (uncoupler of oxidative phosphorylation), cholecalciferol (hypercalcemia), zinc phosphide (releases phosphine in the stomach). There is no vitamin K antidote for these, which is why identifying the product class matters urgently in a poisoning call.
5. Movement in the Target: Contact vs. Systemic
| Contact | Systemic | |
|---|---|---|
| What it kills | Only the tissue actually touched | Tissue reached through the plant's or animal's transport system |
| Coverage demand | Thorough coverage is essential — droplets must reach the underside of leaves, the crown, or the harborage | Lower coverage tolerance; the product redistributes |
| Rainfastness | Washes off until dry | Once absorbed, rain has little effect |
| Perennial weeds | Burns back top growth; roots resprout | Translocates to roots and rhizomes — the reason perennials are treated in late summer or fall when carbohydrates move downward |
| Speed | Fast visible effect | Slower, sometimes days |
In plants, systemic products move either acropetally in the xylem (upward with the transpiration stream, typical of soil-applied insecticides) or basipetally in the phloem (downward with sugars, typical of glyphosate and the auxins).
6. Selectivity Is Not a Fixed Property
A selective pesticide controls certain organisms with little injury to others; a non-selective or broad-spectrum product affects a wide range. But selectivity depends on more than chemistry:
- Rate. A selective herbicide at an excessive rate loses its margin and injures the crop.
- Timing and growth stage. The same product may be selective on a mature crop and damaging on seedlings.
- Placement. Directed sprays, shielded booms, hooded sprayers, and basal-bark applications create placement selectivity even with a non-selective product.
- Formulation. Ester versus amine, or emulsifiable concentrate versus granule, changes uptake and volatility.
7. Biopesticides, PGRs, and Semiochemicals Are Still Pesticides
Colorado law does not create a "natural product" exception:
- Biopesticides include microbials (Bt, Beauveria bassiana), biochemicals, and plant-incorporated protectants. They are registered pesticides, and a commercial application still requires the proper license and category.
- Plant growth regulators appear by name in the Colorado category definitions for stored commodities (305) and post-harvest potatoes (308).
- Semiochemicals — pheromones used for detection, monitoring, or mating disruption — are named in Part 4.16 as their own recertification subtopic.
- FIFRA Section 25(b) minimum-risk products are exempt from federal registration but still require Colorado registration, and in cannabis production they must additionally satisfy Part 17.04(d).
An applicator treats an alfalfa aphid population with an organophosphate in June. When the population rebounds in August, why is switching to an N-methyl carbamate a poor resistance-management choice?
Why are systemic herbicides preferred for controlling established perennial weeds such as Canada thistle in late summer?
A homeowner’s dog ingests a second-generation anticoagulant rodenticide bait. Which antidote does the veterinarian use, and why does the product class matter?
Which statement about pesticide selectivity is correct?