7.4 Pesticide Types, Selectivity, and Modes of Action
Key Takeaways
- Pesticides are named for the pest group they target—insecticide, herbicide, fungicide, rodenticide, acaricide, molluscicide, avicide, nematicide, algaecide—and Ontario licence categories are written around those groups.
- A selective pesticide is toxic to some pests but has little or no effect on other pests or non-target organisms; a non-selective (broad-spectrum) product controls a wide range and puts beneficials at greater risk.
- A systemic pesticide is absorbed and moves inside the treated plant or animal to reach areas remote from the point of application, while a contact (non-systemic) product works only where it lands—so thorough coverage is essential.
- A chemical family groups pesticides with similar structure and properties, similar poisoning signs, and similar first aid, cleanup, and safety guidance; rotation for resistance must change the mode of action, not just the brand.
- Herbicides are described as contact vs systemic, selective vs non-selective, residual vs non-residual, and by timing (pre-plant, pre-emergence, post-emergence); fungicides as protectant vs eradicant; rodenticides commonly as multiple-feeding or single-feeding anticoagulants.
7.4 Pesticide Types, Selectivity, and Modes of Action
Quick Answer: Pesticides are named for the pest group they target (insecticide, herbicide, fungicide, rodenticide, and others). Beyond that, every product is described by selectivity (selective vs non-selective), movement (systemic vs contact/non-systemic), persistence (residual vs non-residual), and its chemical family / mode of action. Those four descriptors—not the brand name—decide coverage requirements, non-target risk, resistance rotation, and first-aid expectations.
Sections 7.1–7.3 covered the physical package: what form the product comes in, what equipment delivers it, and what can lawfully be mixed with it. This section covers what the active ingredient actually does once it reaches the pest. Core exam stems constantly use this vocabulary ("a contact, non-selective herbicide," "a single-feeding anticoagulant," "a systemic insecticide"), and you cannot answer a coverage or non-target question without it.
Naming by target pest group
The suffix -cide means "killer." Ontario licence categories and product labels are written around these groups, so learn them as a set.
| Type | Target group | Everyday example context |
|---|---|---|
| Insecticide | Insects | Cockroaches, ants, grubs, biting flies |
| Herbicide | Plants (weeds/unwanted vegetation) | Roadside vegetation, industrial rights-of-way |
| Fungicide | Fungi and fungal diseases | Turf disease, greenhouse crops |
| Rodenticide | Rodents | Rats and mice in and around structures |
| Acaricide / miticide | Mites and ticks | Spider mites on ornamentals |
| Molluscicide | Slugs and snails | Baits in beds and greenhouses |
| Avicide | Birds | Named in Ontario supervision rules as a high-risk Class C use |
| Nematicide | Nematodes | Soil-borne plant parasites |
| Bactericide | Bacteria | Some plant disease and industrial uses |
| Algaecide / slimicide | Algae and slimes | Industrial water systems, some aquatic uses |
| Repellent / attractant | Behaviour-modifying | Animal repellents, pheromone lures |
| Plant growth regulator | Plant development | Growth suppression where registered |
Ontario legal note: the Pesticides Act definition of "pesticide" is deliberately broad and includes things used to control, prevent, destroy, mitigate, attract or repel a pest and things used to alter the growth, development or characteristics of plant life that is not a pest. That is why repellents, attractants, and some growth regulators are regulated as pesticides (Section 2.4).
Selective vs non-selective
| Term | Meaning | Practical consequence |
|---|---|---|
| Selective | Toxic to some pests, with little or no effect on other pests or non-target organisms | Preferred in IPM when it still controls the target; spares many beneficials |
| Non-selective (broad-spectrum) | Controls a wide range of organisms in the treated area | Higher risk to beneficials, ornamentals, and non-target plants; secondary pest flare-ups |
A selective herbicide may control broadleaf weeds in turf while leaving the grass; a non-selective herbicide controls essentially all plants it contacts in the treated area. A selective insecticide may target a narrow group of insects while sparing predators and parasitoids—one of the reasons Section 9.4 prefers selective products inside an IPM program.
Exam discipline: "selective" is about the range of organisms affected. It is not a statement about toxicity to people, about Ontario class letters, or about how long the product lasts.
Systemic vs contact (non-systemic)
| Term | Meaning | What it demands from the applicator |
|---|---|---|
| Systemic | Absorbed by the treated plant or animal and moves within that organism to reach sites remote from the point of application | Placement matters more than blanket coverage; can reach pests inside stems, roots, or hidden feeding sites |
| Contact / non-systemic | Stays where it is deposited and works only on what it actually touches | Thorough, uniform spray coverage is essential—missed leaf undersides or cracks are untreated |
This distinction is one of the highest-yield ideas in the whole chapter. If a stem says "non-systemic insecticide" and asks why coverage matters, the answer is that the product only affects pests it contacts; nothing moves inside the plant to finish the job. Conversely, a systemic product can control a sap-feeding insect on a leaf you never wetted.
Systemic movement also interacts with pre-harvest intervals and residues (Section 4.4): product inside the plant is not removed by rinsing the surface.
Residual vs non-residual
| Term | Meaning | Trade-off |
|---|---|---|
| Residual | Does not break down quickly; provides longer-lasting control | Longer protection, but greater carryover, re-entry, non-target, and rotational-crop concerns |
| Non-residual | Breaks down quickly after application | Lower carryover risk, but repeat treatments may be needed |
Residual behaviour is the field-level expression of persistence, which Section 10.5 develops as pesticide fate. Labels may carry re-treatment intervals, rotational-crop restrictions, or planting-back statements precisely because a residual product is still active weeks later.
How insecticides reach the insect
| Route | How it works | Application implication |
|---|---|---|
| Stomach poison | Must be eaten/ingested by the pest | Bait placement and palatability matter more than full coverage |
| Contact | Enters through the insect's exoskeleton (cuticle) on contact | Coverage and direct contact with the pest or its runways matter |
| Fumigant | Acts as a gas entering through the respiratory system | Sealed enclosure, specialized licence categories, permits |
| Systemic | Taken up by the plant (or animal) and ingested by feeding pests | Useful against sap feeders and hidden borers where labelled |
Many products act by more than one route. Read the label rather than assuming.
Herbicide vocabulary you must be able to define
| Term | Definition |
|---|---|
| Contact herbicide | Kills only the plant parts the herbicide actually contacts |
| Systemic (translocated) herbicide | Enters the weed and moves within it, preventing growth—useful against perennials with deep roots |
| Selective herbicide | Kills certain plants without killing others |
| Non-selective herbicide | Controls essentially all plants within the treated area |
| Residual herbicide | Does not break down quickly; gives long-term weed control |
| Non-residual herbicide | Breaks down quickly in soil |
| Pre-plant | Applied before the crop is seeded or planted |
| Pre-emergence | Applied after seeding but before the desirable plants or the weeds have emerged |
| Post-emergence | Applied after emergence of the crop and/or weeds |
Ontario overlay: none of this vocabulary creates permission. A perfectly chosen selective, systemic, non-residual turf herbicide is still banned for ordinary cosmetic lawn use in Ontario unless every active ingredient is on the Allowable List or an exception applies (Chapter 11).
Fungicides and rodenticides
Fungicides
| Term | When applied |
|---|---|
| Protectant | Before the disease occurs, to prevent infection—coverage and timing are critical |
| Eradicant | Acts on disease that has already infected the plant |
A protectant used after an outbreak is usually a waste of money and product; that is why identification and monitoring (Section 9.2) precede product selection.
Rodenticides
| Term | Behaviour |
|---|---|
| Anticoagulant | Reduces the blood's ability to clot; multiple-feeding products need repeated feeding over several days |
| Single-feeding anticoagulant | Effective after only one feeding |
| Non-anticoagulant / acute | Other modes of action; Ontario names 4-amino-pyridine, strychnine, and zinc phosphide products for special licensed-exterminator presence rules |
Rodenticides carry serious secondary poisoning potential (Section 10.3). Use labelled stations and placements, and never improvise open bait piles.
Chemical families and mode of action
A chemical family is a group of chemicals with similar chemical structures and properties. Grouping matters because members of a family tend to share:
- Similar modes of action against the pest
- Similar poisoning signs and symptoms in people
- Similar first aid, cleanup, and safety guidance
- Similar resistance behaviour in pest populations
Mode of action (MoA) is how the pesticide affects the pest (for example, disrupting nerve function or blocking a specific enzyme pathway). The site of action is the specific biological target the chemical hits. Two brands with different names and different formulations can share the same MoA—which is exactly why "switching brands" is not resistance management. Rotation must change the mode of action, not the label art (Section 9.4).
Chemical-family knowledge also feeds toxicology: recognizing that a product belongs to a cholinesterase-inhibiting family tells you what poisoning presentation to watch for and why medical attention should not be delayed (Section 5.2).
Selecting among legal options
When more than one registered product could do the job, Core-style training weighs six factors:
- Effectiveness of the formulation against that pest
- Risk to the applicator, bystanders, and the environment
- Availability of the required safety and application equipment
- Convenience of storage, mixing, application, and disposal
- Possibility of injuring non-target plants, animals, or surfaces
- Cost
Notice that cost is last, and that legality is not on the list because it is a precondition, not a trade-off: the product must already be registered, correctly classed, labelled for the pest and site, and permitted under Ontario licence and cosmetic-ban rules.
Exam-style scenarios
Scenario A — Coverage. A label describes a contact, non-systemic insecticide for a foliar pest. Poor coverage of leaf undersides means untreated pests survive. The fix is better coverage technique and nozzle choice, not a higher-than-labelled rate.
Scenario B — Selectivity. A client asks for "the strongest thing you have" on a bed with active lady beetle larvae. A non-selective broad-spectrum insecticide would remove the predators already suppressing the aphids. Prefer a selective option or continued monitoring.
Scenario C — Herbicide type. A perennial weed with deep roots regrows after treatment with a contact herbicide. A systemic (translocated) herbicide, where legal and labelled for that site, is the type that moves into the root system.
Scenario D — Resistance. Two "different" products fail in sequence. Check whether they share a mode of action before blaming the pest, the weather, or the sprayer.
Scenario E — Fungicide timing. A protectant fungicide applied a week after visible infection cannot undo established disease; an eradicant product (or a different program) is the relevant category.
Memory card for Section 7.4
| Descriptor | Question it answers |
|---|---|
| Type (-cide) | Which pest group? |
| Selective / non-selective | How many kinds of organism are affected? |
| Systemic / contact | Does it move inside the organism, or only work where it lands? |
| Residual / non-residual | How long does it stay active? |
| Chemical family / MoA | How does it work, what poisoning pattern, what rotation partner? |
Learn to describe any product with all five descriptors. Combined with formulation (7.1), equipment (7.2), and mixing rules (7.3), you can predict how a job will behave before you open the jug.
What is the difference between a selective pesticide and a systemic pesticide?
A label describes the product as a contact, non-systemic insecticide. What does that mean for how it must be applied?
Which statement about herbicide and fungicide terminology is correct?