9.3 Control Tactics & Pesticide Mode of Action
Key Takeaways
- The IPM toolbox has six tactic families - cultural, mechanical or physical, biological, genetic or host resistance, regulatory, and chemical - and durable programs combine several rather than relying on one.
- Pesticide names ending in -cide identify the target group: insecticide, herbicide, fungicide, rodenticide, nematicide, miticide, avicide, molluscicide, bactericide, and algaecide.
- Selective products control some organisms while sparing others; non-selective products kill broadly, which makes them useful for total vegetation control and dangerous near desirable plants.
- Contact products kill only the tissue or organism they touch, while systemic products are absorbed and translocated internally, which is why thorough coverage matters far more for contact materials.
- IRAC, HRAC, and FRAC group numbers printed on the label identify the mode of action and are the only reliable basis for rotating chemistry - two different brand names can share one group number.
9.3 Control Tactics & Pesticide Mode of Action
Once a pest is identified and a threshold is crossed, the applicator chooses how to intervene. The federal core competency standards at 40 CFR § 171.103(c)(5) require a certified applicator to understand pesticide types, formulations, and their characteristics; competency area (c)(7) requires understanding of application methods. This section covers the decision that sits between those two: what kind of tool, and what kind of chemistry.
The Six Families of Control Tactics
| Tactic family | What it does | New Mexico examples |
|---|---|---|
| Cultural | Changes the growing or living environment so the pest cannot establish | Crop rotation to break root-knot nematode cycles in chile, irrigation scheduling that shortens leaf wetness, sanitation of fallen pecans, removing standing water that breeds mosquitoes, eliminating harborage and food in a restaurant |
| Mechanical / Physical | Physically removes or excludes the pest | Cultivation of annual weeds, mowing before seed set, screening and door sweeps to exclude rodents, sticky barriers, heat treatment for bed bugs, trapping pocket gophers |
| Biological | Uses natural enemies, pathogens, or competitors | Conserving lady beetles, lacewings, and parasitic wasps in alfalfa and pecans; Bacillus thuringiensis against caterpillars; predatory mites in greenhouses; classical biological control agents released against saltcedar |
| Genetic / Host resistance | Uses resistant or tolerant varieties and rootstocks | Nematode-resistant and disease-resistant chile and vegetable varieties |
| Regulatory | Government action to prevent introduction or spread | Quarantines and inspections; this is Category 9 (Regulatory/Quarantine Pests) work |
| Chemical | Applies a pesticide | Everything else in this guide |
Two practical rules follow. First, cultural and mechanical tactics are usually cheaper, longer-lasting, and carry no re-entry interval, so they are considered first when they are available in the time the situation allows. Second, combining tactics is what makes a program durable — a program resting entirely on one chemistry is the single most reliable way to produce resistance, which is the subject of Section 9.4.
Biological control carries one operational warning worth memorizing: broad-spectrum insecticides kill natural enemies along with the pest. Because predator and parasite populations rebuild more slowly than the pest they eat, a broad-spectrum spray can be followed weeks later by a secondary pest outbreak — spider mites flaring in pecans and cotton after a broad-spectrum application is the textbook New Mexico example — or by a pest resurgence in which the original pest returns to a higher density than before treatment.
Classifying Pesticides by Target
The suffix -cide means "killer," and the prefix names the target:
| Pesticide type | Target |
|---|---|
| Insecticide | Insects |
| Miticide / Acaricide | 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 |
| Algaecide | Algae |
| Predacide | Predatory vertebrates |
Several non--cide classes appear on labels and are still pesticides under FIFRA and the Pesticide Control Act: defoliants (cause leaf drop, used ahead of cotton harvest), desiccants (dry plant tissue), plant growth regulators, insect growth regulators, repellents, attractants (including pheromones), and antimicrobials / disinfectants.
Classifying Pesticides by Selectivity
- A selective pesticide controls some organisms while leaving closely related ones largely unharmed. A selective grass herbicide can be sprayed over a broadleaf crop; a selective broadleaf herbicide such as 2,4-D can be used in a grass pasture or lawn.
- A non-selective (broad-spectrum) pesticide kills most or all organisms in the treated group. Glyphosate on any green plant is the standard example.
Selectivity is not purely chemical. It can also be achieved through placement — a shielded sprayer, a hooded boom, a directed basal spray, or a wiper applicator uses a non-selective product selectively by controlling where it lands. In New Mexico row crops and orchards this is a common way to keep a non-selective herbicide off the crop.
Classifying Pesticides by Movement
| Class | Behavior | Coverage requirement |
|---|---|---|
| Contact | Kills only the tissue or organism it physically touches; does not move within the plant or animal | Thorough, complete coverage is essential — undersides of leaves, dense canopy interiors, cracks and crevices |
| Systemic | Absorbed and translocated internally through the plant's vascular system or the animal's circulation | Coverage still matters but uptake site matters more; a systemic reaches tissue the spray never touched |
| Translocated herbicide | Moves with the plant's sugars or water | The basis for perennial weed control described in Section 9.1 |
This distinction drives application decisions constantly. A contact insecticide against spider mites on the undersides of pecan leaves demands high carrier volume and canopy penetration; a systemic soil-applied insecticide against the same pest does not. A contact herbicide burns down the top of field bindweed and the plant returns from its deep root system in three weeks; a translocated herbicide applied in fall reaches those roots.
Classifying by Timing and Action
Herbicide timing:
- Pre-plant — applied before the crop is planted, sometimes incorporated.
- Pre-emergence — applied before the weed emerges; forms a treated zone the germinating seedling grows through. In New Mexico, pre-emergence herbicides must usually be moved into the soil by irrigation or rainfall soon after application, because intense high-elevation sunlight photodegrades exposed material on the surface.
- Post-emergence — applied to emerged, actively growing weeds.
Fungicide action:
- Protectant — must be on the plant surface before the spore lands and infects. It cannot cure an established infection.
- Systemic / curative / eradicant — has some ability to act after infection begins, within a limited window.
Insecticide route:
- Stomach poison — must be eaten; effective against chewing insects and the basis of baits.
- Contact poison — absorbed through the cuticle on contact.
- Fumigant — a gas that penetrates soil, structures, or commodities and enters through the respiratory system. Fumigants are the highest-hazard class an applicator handles and are their own category (7C) in New Mexico.
Mode of Action Groups: The Number That Matters
Mode of action (MOA) is the specific biochemical process the pesticide disrupts. Three international committees assign group codes that registrants print on the label's front panel:
| System | Covers | Code format |
|---|---|---|
| IRAC (Insecticide Resistance Action Committee) | Insecticides and miticides | Group number, sometimes with a letter (for example 3A, 4A, 28) |
| HRAC / WSSA (Herbicide Resistance Action Committee) | Herbicides | Group number (for example 2, 4, 9, 14, 15) |
| FRAC (Fungicide Resistance Action Committee) | Fungicides | Group number (for example 3, 7, 11) |
The single most useful label reading skill in resistance management: two products with completely different brand names, different manufacturers, different formulations, and different prices can carry the same group number — meaning they attack the same biochemical target and a pest resistant to one is resistant to the other. "Rotating products" by brand name is not rotation. Rotating group numbers is rotation. Section 9.4 builds the whole resistance program on this fact.
A worked example: a grower alternates three glyphosate products from three different suppliers across three seasons and believes the chemistry is being rotated. All three are Group 9. The weed population sees one mode of action for three consecutive years, which is precisely the selection pressure that produced glyphosate-resistant kochia and Palmer amaranth across the western United States.
An applicator applies a broad-spectrum insecticide to a pecan orchard for an aphid problem, and three weeks later a severe spider mite outbreak develops in the same block. What is the most likely explanation?
A grower uses three glyphosate products from three different manufacturers over three consecutive seasons and reports that the chemistry has been properly rotated. Why is this reasoning wrong?
Why does a contact insecticide applied for spider mites on pecan foliage demand far more thorough coverage than a systemic product applied for the same pest?
A fungicide label describes the product as a protectant. What does this mean for application timing?