5.4 Pesticide Types, Selectivity, and Movement: Reading the Product Class

Classification Is a Shortcut, Not Trivia

An applicator who can place a product into the right classes before reading the fine print already knows roughly how it will behave: where it must land, whether coverage or uptake matters more, how long it will persist, what will be injured if it moves, and what a mistake will look like. ATCP 29.30(6) turns that into a certification standard covering pesticide types and formulations, compatibility, synergism, persistence, toxicity, hazards, resistance, mixing and dilution, residues in animal commodities, and the difference between acute and chronic risk.

1. Classification by Target Pest

Two Wisconsin-specific reminders. Antimicrobials, germicides, sanitizers, and disinfectants are pesticides under FIFRA, but an individual using only germicides, sanitizers, or disinfectants is exempt from the ATCP 29.25 individual commercial applicator license. And rodenticide bait stations must be marked under ATCP 29.58 with the name of the person responsible for maintenance and the EPA registration numbers of all pesticides currently in use, in English, resistant to deterioration, and legible throughout servicing.

2. Classification by Selectivity

Selective products injure some organisms while sparing others. Non-selective (broad-spectrum) products injure essentially anything in the group they target.

+-----------------------------------------------------------------------------+
|   SELECTIVE                              NON-SELECTIVE                      |
|   Kills target species, spares others    Kills nearly all in the group      |
|   Herbicide: broadleaf control in turf   Herbicide: total vegetation kill   |
|   Insecticide: narrow-spectrum, often    Insecticide: broad-spectrum, kills |
|     conserves natural enemies              beneficials with the target      |
|   IPM-friendly, resistance pressure       Fast knockdown, high non-target   |
|     concentrated on one MoA                 and pollinator risk             |
+-----------------------------------------------------------------------------+

Selectivity is not a safety rating. A selective herbicide can be highly toxic to fish; a non-selective one can be practically non-toxic to mammals. Selectivity tells you what survives the application, nothing more.

Selectivity also has a placement dimension. A non-selective herbicide applied with a shielded sprayer, a wick applicator, or a hooded boom becomes functionally selective because of where it is placed rather than what it is.

3. Classification by Movement

This is the distinction that drives application technique.

In plants, systemic movement occurs through the xylem (upward with the transpiration stream), the phloem (downward and outward with sugars to growing points and roots), or both. A phloem-mobile herbicide reaching an underground rhizome is the reason systemic products control quackgrass and Canada thistle while contact products only burn the tops.

The same distinction applies to insecticides and fungicides. A systemic insecticide taken up by roots protects new growth the spray never reached — and, importantly, can appear in nectar and pollen, which is why several systemic insecticides carry pollinator restrictions. A protectant fungicide sits on the leaf surface and must be present before infection; a systemic fungicide can offer limited curative activity after infection begins.

4. Classification by Timing (Herbicides)

Residual activity is a two-sided property, and the exam knows it. The same persistence that gives season-long weed control creates carryover risk for next year's rotational crop. Plant-back intervals on the label are mandatory language.

5. Compatibility, Synergism, and Persistence

ATCP 29.30(6)(b) groups four properties together, and they interact.

• Compatibility — whether two products can share a tank without physical separation, curdling, or gelling, and without chemical antagonism. Section 2.3 covers the jar test and the W-A-L-E-S sequence that manage this.
• Antagonism — the combined effect is less than the products would achieve separately. A classic case is a grass herbicide antagonized by a broadleaf herbicide in the same tank.
• Synergism — the combined effect is greater than the sum of the separate effects. Synergism can be deliberate and useful, as with piperonyl butoxide added to pyrethrins to block insect detoxification enzymes. It can also be unintentional and dangerous: a mixture can be more toxic to the applicator, the crop, or non-target organisms than either product alone.
• Persistence — how long the active ingredient remains biologically active, usually described by soil half-life (DT50). Long persistence means extended control, extended carryover risk, and extended opportunity for leaching or runoff.

Memory hook: antagonism = 1 + 1 < 2. Additive = 1 + 1 = 2. Synergism = 1 + 1 > 2. The exam will ask you to name the phenomenon from the arithmetic.

6. Reading a Class Into a Decision

Given "systemic, selective, postemergence herbicide, phloem-mobile, 30-day residual," you can predict without reading further: coarser droplets are acceptable, thorough coverage helps but is not everything, perennial control is plausible, rain within a few hours may cost you the application, there will be a plant-back restriction, and off-target movement will injure only susceptible plant species. That is the payoff for learning classification: the label tells you the numbers, but the class tells you the behavior.