3.4 Types of Pesticides & Mode-of-Action Classification

Types of Pesticides & Mode-of-Action Classification

40 CFR 171.103(c)(5) requires commercial applicators to demonstrate practical knowledge of the characteristics of pesticides, beginning with types of pesticides. Missouri CORE items test this two ways: matching a target pest to the right product type, and reading the mode-of-action group number that drives resistance decisions.

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1. Pesticide Types by Target Pest

The suffix -cide means "to kill." The prefix names the target.

[!IMPORTANT] A "pesticide" is the whole category, not just insecticides. Under FIFRA and RSMo 281.020, a pesticide is any substance intended for preventing, destroying, repelling, or mitigating any pest, plus defoliants, desiccants, and plant regulators. A herbicide is a pesticide. So is a rodenticide, an antimicrobial, and a plant growth regulator. Candidates lose points by treating "pesticide" as a synonym for "insecticide."

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2. Two Classification Axes That Change How You Apply

Selective vs. non-selective

• Selective products control certain species while leaving others largely unharmed — 2,4-D controlling broadleaf weeds in a grass lawn.
• Non-selective products injure nearly any plant they contact — glyphosate or glufosinate in a burndown or on a right-of-way.

Selectivity may also be placement-based: a directed or shielded application can make a non-selective product behave selectively.

Contact vs. systemic

• Contact products act where they are deposited and require thorough coverage. Fine droplets and higher spray volumes matter; a missed leaf underside is an escape.
• Systemic products are absorbed and translocated within the plant or animal. Coverage is less critical, but timing and the health of the target matter enormously — a drought-stressed weed moves very little systemic herbicide.

This distinction drives nozzle selection, carrier volume, and adjuvant choice, and it explains why the same active ingredient can fail on one job and succeed on the next.

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3. Mode-of-Action Group Numbers

Modern labels print a group number in a box near the top of the front panel. This is the single most useful number on the label for resistance management.

+-----------------------------------------------------------------------------+
|                    THE THREE GROUP NUMBERING SYSTEMS                        |
|                                                                             |
|   [HERBICIDES]   HRAC / WSSA group numbers                                  |
|     Group 1  - ACCase inhibitors            Group 9  - EPSPS (glyphosate)   |
|     Group 2  - ALS inhibitors               Group 14 - PPO inhibitors       |
|     Group 4  - Synthetic auxins             Group 15 - VLCFA inhibitors     |
|     Group 5  - Photosystem II inhibitors    Group 22 - Photosystem I        |
|                                                                             |
|   [INSECTICIDES] IRAC group numbers                                         |
|     Group 1A - Carbamates    Group 1B - Organophosphates                    |
|     Group 3A - Pyrethroids   Group 4A - Neonicotinoids                      |
|     Group 5  - Spinosyns     Group 28 - Diamides                            |
|                                                                             |
|   [FUNGICIDES]  FRAC group numbers                                          |
|     Group 3  - DMI / triazoles              Group 7  - SDHI                 |
|     Group 11 - QoI / strobilurins           Group M  - Multi-site contacts  |
+-----------------------------------------------------------------------------+

The rule that follows from the numbers: repeated use of the same group number selects for resistant individuals in the population. Rotating and tank-mixing different group numbers with overlapping activity on the target pest delays resistance. Two products with different trade names, different manufacturers, and even different chemical families are not a rotation if they carry the same group number.

Missouri's waterhemp is the cautionary case: populations now carry documented resistance to Group 2 (ALS), Group 9 (glyphosate), and Group 14 (PPO), with pressure building on Group 4 and Group 15. Multi-site Group M fungicides and overlapping residual herbicide programs exist precisely because single-site chemistries are so easily defeated.

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4. Chemical Class Predicts Toxicology

The chemical family determines what a poisoning looks like and what an emergency physician needs to know. This links directly to Chapter 4.

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5. Biologicals, Minimum Risk, and Antimicrobials

• Microbial pesticides use a living organism or its byproduct — Bacillus thuringiensis (Bt) for caterpillars, Beauveria bassiana for various insects. Generally narrow-spectrum and short-lived; still pesticides, still labeled, still subject to Missouri registration.
• Biochemical pesticides include pheromones and insect growth regulators that disrupt behavior or development rather than killing outright.
• Minimum risk pesticides (FIFRA Section 25(b)) are exempt from federal registration when they contain only listed active and inert ingredients and meet strict labeling conditions. They are not exempt from Missouri product registration, and Missouri technicians and NRUPs are specifically authorized to use minimum risk products.
• Antimicrobials — sanitizers, disinfectants, sterilants — are pesticides under FIFRA. Contact time on the label is a legal requirement, not a suggestion.