7.4 Pesticide Types, Modes of Action & Selectivity
Key Takeaways
- 302 KAR 26:020 Section 4(5)(a) makes 'types of pesticides' a core competency separate from formulations, and the exam tests the -cide suffix families, selectivity, mobility, and timing vocabulary.
- Selective pesticides injure some organisms while sparing others at the same rate; non-selective products injure nearly everything they contact, which is a use decision, not a toxicity rating.
- Contact products act only where the spray lands, so coverage is everything; systemic products translocate in xylem or phloem, so uptake and translocation timing matter more than coverage.
- Herbicide timing vocabulary - preplant incorporated, preemergence, and postemergence - describes timing relative to the weed or crop, and applying a preemergence product after emergence is a control failure, not a rate problem.
- Mode-of-action group numbers from HRAC/WSSA, IRAC, and FRAC appear on the label and are the operational tool for rotation and resistance management.
7.4 Pesticide Types, Modes of Action & Selectivity
[!NOTE] Where this sits in the blueprint. 302 KAR 26:020 Section 4(5) requires competency in the characteristics of pesticides, and it lists (a) types of pesticides first, before (b) types of formulations. Those are two different questions. "EC" tells you how the product is packaged and mixed; "systemic non-selective postemergence herbicide, WSSA Group 9" tells you what it will do in the field.
The -cide Families
| Term | Target | Kentucky examples of use |
|---|---|---|
| Insecticide | Insects | Corn rootworm, soybean aphid, bagworms, cattle horn flies |
| Miticide / Acaricide | Mites and ticks | Two-spotted spider mite on ornamentals; ticks in public health work |
| Herbicide | Plants | Waterhemp and marestail in row crops; ground ivy in turf; brush on rights-of-way |
| Fungicide | Fungi | Frogeye leaf spot in soybeans; brown patch and dollar spot in turf |
| Bactericide | Bacteria | Fire blight in orchards |
| Nematicide | Nematodes | Root-knot and soybean cyst nematode |
| Rodenticide | Rats, mice, voles | Commensal rodents in structural work; voles in orchards and nurseries |
| Avicide | Birds | Limited, mostly regulatory and public health programs |
| Molluscicide | Slugs and snails | No-till residue slug pressure |
| Algaecide | Algae | Copper products in Category 5 aquatic work |
| Piscicide | Fish | Fisheries renovation, a regulatory or aquatic use |
| Defoliant / Desiccant | Plant tissue removal or drying | Harvest aids |
| Plant growth regulator (PGR) | Plant growth and development | Turf seedhead suppression; tree growth regulators on rights-of-way |
| Attractant / Repellent | Behavior, not mortality | Pheromone lures in monitoring traps; deer repellents |
| Antimicrobial / Disinfectant | Microorganisms on surfaces | Greenhouse and food-handling sanitation |
All of these are pesticides under FIFRA and KRS Chapter 217B - including the ones that do not kill anything. That is a routine exam trap: a pheromone attractant, a repellent, and a plant growth regulator are all regulated pesticides.
Selectivity: A Use Property, Not a Safety Rating
| Selective | Non-selective | |
|---|---|---|
| Definition | Injures certain plants or organisms while causing little or no injury to others at the same rate | Injures essentially all plants or organisms it contacts |
| Herbicide example | 2,4-D removes broadleaf weeds from grass turf | Glyphosate or glufosinate on a burndown or a right-of-way bare-ground strip |
| Insecticide example | Bacillus thuringiensis affects only certain larvae | Broad-spectrum pyrethroid or organophosphate |
| Why it matters | Lets you treat inside a desirable stand | Requires directed application, shielding, or a site with nothing to protect |
[!WARNING] Selective does not mean safe, and non-selective does not mean more toxic to people. Selectivity describes the range of target organisms injured. Glyphosate is non-selective and has a very high mammalian oral LD50; several highly selective insecticides are Toxicity Category I. Never infer a signal word from a selectivity claim.
Selectivity also has degrees. Rate selectivity means a product is selective at one rate and non-selective at a higher one. Placement selectivity means you achieve selectivity by where you put the spray - a hooded sprayer or a directed basal application makes a non-selective product behave selectively.
Contact vs. Systemic: The Coverage Question
+-----------------------------------------------------------------------------+
| CONTACT vs. SYSTEMIC BEHAVIOR |
+-----------------------------------------------------------------------------+
| CONTACT | SYSTEMIC |
| -------------------------------- | -------------------------------- |
| Acts only where spray lands | Absorbed and translocated |
| COVERAGE IS EVERYTHING | UPTAKE AND TIMING MATTER MOST |
| Finer droplets / higher volume | Coarser droplets acceptable |
| Rapid visible burn-down | Slower symptom development |
| Regrowth from untreated buds/roots | Reaches roots, rhizomes, crown |
| Ex: paraquat, contact fungicides | Ex: glyphosate, imidacloprid, |
| pyrethroid on contact | strobilurin/SDHI fungicides |
+-----------------------------------------------------------------------------+
The practical consequence: for a contact product, cutting spray volume or coarsening droplets to reduce drift can cost you control, so the drift solution is a drift-reduction nozzle at adequate volume rather than a volume cut. For a systemic product on a perennial weed such as johnsongrass or bermudagrass, control depends on translocation to the rhizome, so an application that scorches the leaves too fast actually reduces control.
Within systemics, translocation direction matters:
- Xylem-mobile (apoplastic): moves upward with the transpiration stream. Soil-applied systemic insecticides and many soil-applied herbicides work this way.
- Phloem-mobile (symplastic): moves with photosynthate, downward to roots and rhizomes. Glyphosate is the classic example, which is why it works best on actively growing weeds moving sugars downward and works poorly on drought-stressed or freshly mowed plants.
Herbicide Timing Vocabulary
| Term | Timing reference | Failure mode if mistimed |
|---|---|---|
| Preplant incorporated (PPI) | Applied and mechanically mixed into soil before planting | Poor incorporation leaves the product on the surface where it volatilizes or photodegrades |
| Preemergence (PRE) | After planting, before the weed emerges; forms a soil barrier that must be activated by rainfall or irrigation | Applying after weed emergence gives essentially no control - the weed has already passed the intercept point |
| Postemergence (POST) | After the weed has emerged, on actively growing tissue | Applying to oversized, stressed, or hardened-off weeds gives partial control and selects for resistance |
| Directed / shielded | Physically aimed to avoid the crop | Boom height or shield failure produces crop injury indistinguishable from drift |
| Layby | Late-season directed application in an established crop | - |
"Preemergence" and "postemergence" describe timing relative to the weed, not relative to the crop, unless the label says otherwise. Read which one the label means.
Growth Regulators, Biologicals, and Minimum-Risk Products
- Plant growth regulators alter growth rather than killing: turf seedhead suppression, tree growth regulators that reduce trimming cycles on utility rights-of-way, fruit thinning. They are pesticides and require licensure in the appropriate category.
- Insect growth regulators (IGRs) disrupt molting or metamorphosis. They are slow, so a structural customer who expects instant knockdown must be told what to expect - a professionalism as well as an efficacy issue.
- Microbial and biological pesticides - Bacillus thuringiensis, Beauveria bassiana, entomopathogenic nematodes, biofungicides - are registered pesticides with labels, rates, and re-entry statements like any other.
- FIFRA Section 25(b) minimum-risk products made entirely of listed active and inert ingredients (clove oil, garlic, cedarwood oil, and similar) are exempt from federal registration but are not exempt from state law; Kentucky still regulates their distribution, and they still cannot be used inconsistently with their labeling.
Mode-of-Action Group Numbers
Modern labels print a mode-of-action group number in a colored box near the top of the label. These are the operational tool for the resistance management covered in Section 9.2:
| System | Scheme | Where you see it |
|---|---|---|
| HRAC / WSSA | Herbicide Group numbers (e.g., Group 9 EPSP synthase inhibitors, Group 2 ALS inhibitors, Group 4 synthetic auxins, Group 14 PPO inhibitors, Group 15 VLCFA inhibitors) | Herbicide labels, UK Extension weed control guides |
| IRAC | Insecticide Group numbers with letters (e.g., 1B organophosphates, 3A pyrethroids, 4A neonicotinoids, 5 spinosyns, 28 diamides) | Insecticide labels |
| FRAC | Fungicide Group numbers (e.g., 3 DMI, 7 SDHI, 11 QoI/strobilurin) | Fungicide labels |
Two products with different trade names and different active ingredients can share a group number. Rotating between them is not a rotation at all - it is the same selection pressure with a new invoice. The group number, not the brand, is what you rotate.
Exam Alert: Three distinctions carry this topic: selective vs. non-selective (range of organisms injured, not toxicity), contact vs. systemic (coverage-driven vs. translocation-driven), and preemergence vs. postemergence (timing relative to weed emergence). And remember that attractants, repellents, and plant growth regulators are pesticides even though they do not kill.
An applicator must control established johnsongrass rhizomes in a Kentucky fencerow. Which product characteristic matters most, and what does it imply about application technique?
A grower rotates from a Group 2 ALS-inhibiting herbicide to a different brand with a different active ingredient, but the new label also shows Group 2. What has the grower achieved?
Which statement about pesticide selectivity is correct?