5.1 Pesticide Types and Modes of Action

Why pesticide characteristics are a Core competency

Oklahoma commercial Core is built on the federal commercial-applicator core in 40 CFR 171.103(c). Paragraph (c)(5) is titled Pesticides and requires practical knowledge of pesticide characteristics, including all of the following: types of pesticides; types of formulations; compatibility, synergism, persistence, and animal and plant toxicity of the formulations; hazards and residues associated with use; factors that influence effectiveness or lead to problems such as pesticide resistance; and dilution procedures. This section covers types, how products act, and the residue language that shows up on food, feed, hay, and indoor accounts. Formulations and mixing are Sections 5.2 and 5.3. Resistance management is developed in Chapter 9; extra fumigant certification is Chapter 10.

A Certified Applicator card in Oklahoma is not a cotton license, a lawn license, or a termite license by itself. The same Core exam is taken by people who will add agricultural, turf and ornamental, right-of-way, or structural 7A categories. Core questions therefore ask whether you can name what kind of pesticide you are holding and how it behaves, not whether you memorized every commodity in the Panhandle. A foliar insecticide on southwest Oklahoma cotton, a pasture herbicide on native range, a turf preemergence product in Edmond, a 7A gel bait in a Tulsa kitchen, and a 7C fumigant in stored wheat are all pesticides. They are not interchangeable tools.

FIFRA’s definition of pesticide is broader than “bug spray.” It includes substances intended to prevent, destroy, repel, or mitigate a pest, and substances intended for use as a plant regulator, defoliant, or desiccant. That is why cotton harvest aids and turf growth regulators belong in this chapter even when the goal is not killing an insect.

Types of pesticides by target organism

Name the product by the organism or process it is intended to affect. If the organism is wrong, no amount of calibration saves the application, and the use is usually off-label.

Miticides are a frequent Core trap. Many insecticides do not control spider mites, and some broad-spectrum sprays kill mite predators and make an outbreak worse. If the pest is a mite, look for miticide directions and the correct IRAC group—not a leftover pyrethroid because it is already in the truck. Cotton and pecan accounts in hot, dry Oklahoma summers are classic mite-flare settings.

Antimicrobials surprise people who think Core is only agriculture. A product that claims to kill bacteria or viruses on surfaces is a pesticide. The label still governs sites, contact times, and PPE. A hospital disinfectant is not a substitute for a 7A cockroach bait, and a turf fungicide is not a food-contact sanitizer.

Plant growth regulators, desiccants, and defoliants remain pesticides when the goal is a crop response rather than “killing a pest.” Cotton harvest-aid season in southwest Oklahoma is the practical picture: you are changing the crop on purpose, at a labeled rate, with products that can injure a neighbor’s still-green field if they move. A PGR that shortens cotton internodes is not an insecticide, even if both are applied with the same boom.

Piscicides appear because 40 CFR 171.103(c)(5)(i) says types of pesticides, not “types you personally apply this year.” Aquatic competency is a separate category. For Core, know that fish toxicants exist and that they are not interchangeable with a copper algae product or a pond herbicide.

Nematicides may be soil fumigants or non-fumigant products. Fumigant gases are a formulation class in Section 5.2; using them as a soil fumigant in the field still requires the extra certification path discussed in Chapter 10. Core only needs you to know nematodes are not insects and that a foliar insecticide will not replace a nematicide.

Rodenticides are usually baits. Secondary poisoning, tamper-resistant stations, and indoor versus outdoor placement are label issues. Zinc phosphide, anticoagulants, and cholecalciferol are different modes of action; swapping “rat bait” by color of the pellet is how pets and non-target wildlife get involved.

Selective versus nonselective

A selective pesticide controls some plants or pests without killing others when used at labeled rates on labeled sites. The pasture picture is a 2,4-D amine spray that takes musk thistle and leaves established bermudagrass. A nonselective pesticide injures or kills most plants it contacts: glyphosate as a burndown, paraquat as a contact burndown or harvest aid, or a total-vegetation product on a graveled equipment yard.

Selectivity is not a personality trait of the bottle. It is rate-dependent, species-dependent, and stage-dependent. Raise the rate, hit seedling cotton, spray a stressed ornamental, or apply in slow-drying heat, and a “grass-safe” pasture herbicide becomes a crop-loss claim. Cotton, grapes, tomatoes, pecans at the wrong timing, and many greenhouse plants are broadleaf species. A herbicide that is selective in grass pasture is still a cotton killer if it lands there.

Nonselective does not mean “safe for people” or “safe for water.” Glyphosate products differ in aquatic statements and PPE. Paraquat is nonselective, restricted-use in the United States, and carries closed-system and training requirements printed on the label. Core expects you to read those statements, not to assume the word herbicide tells you the human hazard.

Insecticides and fungicides also have selective versus broad-spectrum behavior. A selective insecticide may spare some beneficials at labeled rates; a broad-spectrum pyrethroid may knock down pests and predators together and set up a mite flare. That is a type plus mode problem, not a nozzle problem.

Contact versus systemic

Contact pesticides injure only the tissue or organism they touch. Coverage and droplet placement matter. A contact insecticide on cabbage loopers needs to hit the larvae hiding on the underside of the leaf. A contact herbicide such as paraquat desiccates green tissue it hits and does not translocate into roots to finish a perennial stand. Missed strips stay green.

Systemic pesticides are absorbed and moved inside the plant or, for some animal-health and bait products, inside the pest after ingestion. Glyphosate moves toward growing points of perennial weeds. A soil-applied neonicotinoid can move into turf or ornamental tissue. Systemics can reach hidden pests, but they also create residue questions in nectar, pollen, guttation water, and harvested commodities—topics Chapter 8 develops. For Core, remember the operational difference: missed coverage hurts contact products more; wrong growth stage, drought, or a plant that is not translocating hurts systemic products more.

Insects add two more action pictures that students mix up with “systemic.” A stomach poison must be eaten (most 7A gel baits). A fumigant acts as a gas in a space (stored-wheat 7C work). Do not call a bait “systemic” unless the label actually describes translocation or ingestion-and-spread in that pest. Do not call a residual surface spray a fumigant because it “fumes a little.”

Residual versus non-residual

Residual activity means the product remains biologically active on a surface or in soil for a useful period after application. Preemergence turf herbicides, some soil-applied cotton herbicides, termiticide soil barriers, and certain orchard insecticides are residual by design. Non-residual (short-lived) products break down or dissipate quickly. They are chosen when you need a short grazing interval, a short preharvest interval (PHI), less leftover activity before overseeding, or a structural treatment that should not remain on a food-contact surface.

Residual is not automatically better. Residual soil herbicides can injure a rotational crop or a garden planted in treated soil. Residual structural sprays leave unwanted residue if you ignore site statements. Residual insecticides also increase selection pressure for resistance, which Chapter 9 treats in full. 40 CFR 171.103(c)(5)(iii) and (v) pair persistence with resistance for that reason.

Protectant versus eradicant fungicides

Protectant fungicides sit on plant surfaces and prevent spores from infecting if coverage is in place before infection. Multi-site protectants used in peanut leaf-spot or turf programs are the textbook case: they do not rescue a deep, already-established infection in the crown or stem.

Eradicant (often called curative) fungicides can stop an infection that has already begun, usually early in the disease cycle, because they move into tissue or hit a specific biochemical step. Many modern peanut and turf products combine both behaviors on one label. Core still tests the distinction. Relying only on one FRAC group of curatives is how resistance shows up in leaf-spot and dollar-spot programs. Using a protectant after the epidemic is visible, and then blaming the brand, is a timing error, not a formulation error.

Similar language appears as preventive versus knockdown for insects and as preemergence versus postemergence for weeds. Exam wording for protectant and eradicant is usually fungal.

Mode of action versus site of action

Mode of action is the general way the pesticide affects the pest: nerve poison, photosynthesis inhibitor, growth regulator, energy-production disruptor, desiccant, membrane disruptor. Site of action is the specific biochemical target—an enzyme, a receptor, or a protein complex.

Two products can share a loose mode-of-action story and still differ. Resistance management is built on site of action. If two herbicides both inhibit ALS (acetolactate synthase), rotating their brand names is not a rotation. If two insecticides are both IRAC Group 3 pyrethroids, switching colors of the jug is not a rotation.

IRAC (Insecticide Resistance Action Committee), HRAC (Herbicide Resistance Action Committee), and FRAC (Fungicide Resistance Action Committee) publish group codes that appear on many current labels. You will see them as a number, sometimes with a letter (IRAC 1A versus 1B; FRAC M05 versus 3 or 11). Group 4 auxin herbicides (2,4-D, dicamba, picloram, triclopyr, and related products) share a plant-growth-regulator family. Glyphosate is a separate Group 9 EPSPS inhibitor. Paraquat is a photosystem I electron diverter, not an auxin. You do not need the full code tables on Core—Chapter 9 goes deeper—but you do need to know the codes exist so you do not “rotate” two Group 9s and call it stewardship.

Phytotoxicity

Phytotoxicity is pesticide injury to a plant you did not intend to injure, or injury to the crop itself. Symptoms include speckling, necrotic spots, chlorosis, stunting, leaf burn, cupping, and epinasty (twisting and strapping). Causes Core expects you to list:

• Rate too high, boom overlap, or a second pass on the same swath
• Sensitive crop, variety, or growth stage
• Solvents in emulsifiable concentrates (Section 5.2)
• Incompatible tank mix or an adjuvant the pesticide label warned against
• Hot, humid, or slow-drying conditions
• Spray that moved as particles or as vapor

Oklahoma’s most useful Core example is auxin herbicide injury on cotton and other broadleaves. 2,4-D and related growth-regulator herbicides can cup and strap cotton leaves at very low doses. Ester formulations are generally more volatile than many amine salts of the same acid. That is chemistry behavior: vapor can leave a legal application and injure a neighbor. Particle drift of fine droplets is a different movement path. Weather, inversions, and buffers are Chapter 8. County notification rules for restricted-area herbicides in southwest Oklahoma are a legal topic in Chapter 3. Do not substitute the notification rule for understanding why an auxin molecule can injure cotton. Knowing both still matters on the job; they are different exam chapters.

Turf phytotoxicity often looks like tire-track burn from an EC or from a wettable powder residue plus heat. Structural indoor plants and greenhouse accounts are injured by aerosol solvents and by herbicide tracking on boots—another reason 7A technicians still need Core herbicide literacy even if they “only do bugs.”

Persistence, residue, and tolerance

These three words are not synonyms, and 40 CFR 171.103(c)(5) tests the difference under persistence, toxicity, hazards, and residues.

Persistence is how long the pesticide remains in the environment (soil, water, vegetation, treated surfaces) in a form that can still have biological activity or be detected. Sunlight, microbes, soil pH, rainfall, temperature, and the molecule itself change persistence. A long-persistent pasture herbicide can remain in hay, manure, or soil and later injure tomatoes, beans, or other sensitive crops. Picloram-type products are the cautionary tale on many Oklahoma hay labels: the pasture looked clean, the hay was legal to cut on the interval, and the vegetable garden still died after the manure or hay was used as mulch. That is persistence plus a use-site mistake, not a mystery.

Residue is what is left on or in a treated commodity, animal, or surface. Residues are why labels list preharvest intervals, grazing and haying restrictions, “do not use on food-contact surfaces,” and livestock slaughter intervals. A cotton defoliant residue conversation is not the same as a kitchen-bait residue conversation, but both are residue questions.

Tolerance is the maximum legal residue of a pesticide on a food or feed commodity under the Federal Food, Drug, and Cosmetic Act. If EPA has not established a tolerance (or an exemption from a tolerance) for that pesticide–commodity pair, the product will not be labeled for that food crop. Applying a pasture herbicide to wheat because “it’s still a grass” is a label violation and a residue/tolerance problem. Structural 7A work has an analog: a cockroach bait labeled for commercial kitchens is not automatically labeled for a food-processing line. A grain fumigant residue conversation belongs with the 7C label, not with a general-use aerosol.

Effectiveness factors (preview only)

40 CFR 171.103(c)(5)(v) pairs effectiveness with problems such as resistance. Coverage, pest life stage, water volume, weather, formulation, water quality, and tank-mix antagonism all change results. Resistance is selection of pests that survive a site of action. You delay it by rotating IRAC/HRAC/FRAC groups, keeping non-chemical tactics, and using labeled rates—not by raising the rate of the same group until it “works again.” Chapter 9 is the IPM and resistance chapter. Here, remember two Core sentences: the wrong type of pesticide cannot be fixed with a better nozzle, and the right type used on the wrong site is still illegal.