7.6 Pesticide Types, Selectivity & How Products Move in the Target
Key Takeaways
- Pesticide names follow the target: insecticide, herbicide, fungicide, rodenticide, nematicide, miticide or acaricide, bactericide, molluscicide, avicide, algaecide, and defoliants, desiccants, growth regulators and repellents.
- Selective products control some organisms while sparing others; non-selective products control a broad range, which is why a non-selective herbicide in a growing crop is a crop-destruction scenario rather than a weed-control scenario.
- Contact products act only where the spray lands and demand thorough coverage, while systemic products are absorbed and translocated inside the organism and tolerate less complete coverage.
- Herbicide timing terms are distinct and testable: preplant incorporated, preemergence, postemergence, and layby, plus harvest-aid uses such as defoliants and desiccants.
- Understanding a product as a target class plus a selectivity plus a movement pattern lets you predict coverage requirements, rain-fastness questions and crop-safety risks without memorizing individual trade names.
7.6 Pesticide Types, Selectivity & How Products Move in the Target
Core Concept: Three questions describe any pesticide well enough to predict how to use it: what does it target, how selective is it, and how does it move once it contacts the organism. Answer those three and coverage requirements, rain-fastness, crop safety, and resistance risk all follow.
Types Named for Their Target
| Type | Target |
|---|---|
| Insecticide | Insects |
| Miticide / acaricide | Mites and ticks (arachnids, not insects) |
| Herbicide | Weeds and unwanted plants |
| Fungicide | Fungi and fungal-like organisms |
| Bactericide | Bacteria |
| Nematicide | Nematodes |
| Rodenticide | Rats, mice and other rodents |
| Avicide | Birds |
| Molluscicide | Slugs and snails |
| Algaecide | Algae |
| Piscicide | Fish |
| Predacide | Predatory vertebrates |
Related product classes that are still regulated as pesticides:
- Defoliant - causes leaves to drop, used as a cotton harvest aid.
- Desiccant - dries plant tissue, used as a harvest aid in several crops.
- Plant growth regulator - alters growth rate or habit rather than killing.
- Repellent - makes a site or host unattractive rather than lethal.
- Attractant - draws the pest, often paired with a trap or bait.
- Antimicrobial and sanitizer - controls microorganisms on surfaces and in water.
Arkansas relevance: cotton defoliants and desiccants are pesticides subject to the same licensing, record, and drift rules as insecticides, which surprises candidates who think of them as harvest chemicals.
Selectivity
- Selective products control certain organisms and leave others essentially unharmed - a grass herbicide applied over a broadleaf crop, or an insect growth regulator that affects immature insects but spares adult beneficials. Selectivity may be biochemical (the crop metabolizes the herbicide, the weed does not), positional (the product is placed where only the weed's roots reach it), or physiological (differences in leaf surface or uptake).
- Non-selective products control a wide range of organisms. A non-selective herbicide in a growing crop is a crop-destruction event, which is why non-selective products dominate burndown, right-of-way, and industrial vegetation work rather than in-crop use.
Herbicide-tolerant traits create engineered selectivity: glyphosate is non-selective by nature, but a glyphosate-tolerant crop makes it functionally selective in that field. That engineered selectivity is exactly what created Arkansas's Class G and Class H problems - it put a broad-spectrum, drift-capable chemistry into in-crop use across a landscape of sensitive neighbours.
Contact Versus Systemic
| Contact | Systemic | |
|---|---|---|
| Action | Kills the tissue or organism it physically touches | Absorbed and translocated within the plant or animal |
| Coverage requirement | Thorough and complete - a missed leaf underside or a missed insect is a survivor | More forgiving; the product redistributes |
| Speed | Usually fast, visible injury in hours to days | Slower; symptoms may take a week or more |
| Best against | Annual weeds, exposed insects, surface pathogens | Perennial weeds with underground reserves, sucking insects, systemic pathogens |
| Volume and droplet size | Higher carrier volumes and finer droplets improve coverage - which conflicts with drift management | Lower volumes and coarser droplets are usually acceptable |
Why it matters against creeping perennials. A contact herbicide burns off johnsongrass or nutsedge foliage; the rhizomes and tubers are untouched and the plant regrows. A systemic herbicide is translocated to those underground structures and can actually kill the plant - which is also why systemics are applied when the plant is actively moving photosynthate downward, typically in the fall or at a specific growth stage.
Why it matters for insects. A piercing-sucking insect such as an aphid feeds on phloem sap. A contact insecticide sitting on the leaf surface is largely irrelevant unless the spray physically hits the insect; a systemic insecticide is drawn up in the sap the insect is feeding on.
In plants, translocation follows two streams: xylem-mobile products move upward with the transpiration stream (typical of soil-applied systemics), while phloem-mobile products move with sugars from leaves to roots and growing points, which is what reaches a rhizome.
Herbicide Timing Terms
| Term | When |
|---|---|
| Preplant incorporated (PPI) | Applied and mechanically mixed into the soil before planting; used for volatile or photodegradable products |
| Preplant / burndown | Applied before planting to kill existing vegetation; note Arkansas's Class H prohibition on preplant burndown dicamba April 16 through October 31, with the narrow April 16 through June 30 ground exception |
| Preemergence (PRE) | After planting but before crop or weed emergence; usually requires rainfall or irrigation for activation |
| Postemergence (POST) | After emergence; efficacy falls sharply as weeds grow past the labelled size |
| Layby | A late, directed application under the crop canopy after the crop is established |
| Directed / shielded | Spray aimed away from the crop, often with a hood or shield, allowing a product the crop would not tolerate overhead |
| Harvest aid | Defoliants and desiccants applied to prepare the crop for harvest, subject to their own pre-harvest intervals |
Residual describes how long a soil-applied product continues to control germinating weeds. Residual activity is the basis of overlapping residual programs against Palmer amaranth, and it is also the source of rotational crop restrictions.
Mode of Action Ties Back to Resistance
Mode of action is the biochemical site where the product acts - ALS inhibition, EPSPS inhibition, photosystem II inhibition, sodium channel modulation, acetylcholinesterase inhibition. Products sharing a mode of action are grouped by number: HRAC groups for herbicides, IRAC for insecticides, FRAC for fungicides, and the group number is printed on the label.
The practical rule that section 7.3 develops in full: rotating trade names is not rotation; rotating group numbers is. Two products from different manufacturers with different trade names and the same group number select for the same resistance mechanism. Read the group number box on the label, not the brand.
[!IMPORTANT] Exam Trap: Coverage Requirements Follow Movement, Not Toxicity A common distractor pairs a highly toxic product with a claim that thorough coverage is less important because the product is potent. Coverage requirements are set by whether the product is contact or systemic, not by its toxicity category. A Caution-labelled contact fungicide still needs complete coverage; a Danger-labelled systemic insecticide may not.
An applicator needs to control established johnsongrass, a creeping perennial that spreads by rhizomes, in a right-of-way. Which product characteristic matters most and why?
A grower is treating aphids, a piercing-sucking insect feeding on phloem sap in a dense soybean canopy. Which reasoning is correct?
Two herbicides from different manufacturers carry different trade names but both show Group 2 on the label. An applicator alternates between them each season to manage resistance. What is the problem?