6.7 Types of Pesticides and How They Work

6.7 Types of Pesticides and How They Work

Quick Answer: A pesticide's name tells you its target (herbicide, insecticide, fungicide, rodenticide, nematicide, and so on) and its mode of entry tells you how to apply it. Contact products kill only what they touch and demand thorough coverage; systemic products move inside the organism and tolerate imperfect coverage. Selective products spare some species; nonselective products do not. 40 CFR 171.103(c)(5) makes types of pesticides, factors influencing effectiveness, and causes of failure a Core competency — and 171.103(c)(7) adds that fumigants, aerial application, sodium cyanide, and sodium fluoroacetate require additional certification.

Classifying by Target Organism

The suffix -cide means "killer," and the prefix names the target:

Iowa's own definitions in IAC 21—45.1 add three product classes that are regulated as pesticides but do not kill: a defoliant causes leaves to drop, a desiccant artificially accelerates drying of plant tissue, and plant growth regulators alter development. The rule also defines "insect" broadly enough to include spiders, mites, ticks, centipedes, and wood lice, and defines "fungi" to include rusts, smuts, mildews, molds, yeasts, and bacteria.

Contact vs. Systemic

This distinction drives coverage, nozzle choice, and rescue-treatment expectations more than any other property.

An Iowa scenario that tests this: a field of established Canada thistle is treated with a contact herbicide, browns off completely in four days, and regrows in three weeks. The product worked exactly as designed — it never reached the root system. A systemic herbicide applied at the right growth stage is the correct tool.

The same logic applies to fungicides. Protectant fungicides sit on the leaf surface and must be present before infection; systemic fungicides move into tissue and can have limited curative activity. It is why a protectant applied after tar spot lesions appear rarely rescues the crop.

Selective vs. Nonselective

• Selective products injure some organisms while leaving others essentially unharmed — a postemergence grass herbicide in soybeans, or a bait targeting one insect group.
• Nonselective products injure nearly everything they contact — glyphosate on a burndown pass, or a broad-spectrum insecticide that kills the predatory insects holding aphids in check.

Selectivity is not always chemical. Placement selectivity achieves the same result mechanically: a directed or shielded spray under a canopy, a hooded boom between rows, or a wick applicator above the crop.

The IPM consequence matters. A nonselective insecticide that removes natural enemies can trigger a secondary pest outbreak or a resurgence of the original pest at higher numbers than before treatment. That is why Section 6.4 insists on treating at threshold rather than on a calendar.

Herbicide Timing Vocabulary

Timing language describes the crop and weed stage, not the chemistry, and the label sets it:

A residual PRE herbicide that never receives activating moisture is one of the most common non-resistance causes of "product failure" in an Iowa spring.

Why Products Fail

40 CFR 171.103(c)(5)(v) specifically requires competency in "factors that influence effectiveness or lead to problems such as pesticide resistance." When control is poor, work the list before blaming the chemistry:

1. Misidentification — the wrong target, or a species not on the label.
2. Wrong growth stage — weeds past the labeled height, insects in a tolerant stage, disease past the protectant window.
3. Rate or calibration error — a worn nozzle set delivering 15% off rate looks exactly like resistance.
4. Coverage failure — a contact product applied at low carrier volume into a dense canopy.
5. Environment — drought-stressed plants with thick cuticles absorb less; rainfall before the rainfast period washes product off; a residual product never activated.
6. Antagonism in the tank — hard water, an incompatible partner, a missing adjuvant the label required.
7. Resistance — the last hypothesis, not the first, and the one that changes what you do next (Section 6.5).

Products That Require Additional Certification

Federal Core competency 171.103(c)(7)(ii) requires applicators to know that fumigants, aerial application, sodium cyanide, and sodium fluoroacetate each need additional certification beyond the base credential. Iowa implements this through categories — 7C Non-Soil Fumigation, the new 12 Soil Fumigation, and 11 Aerial Application — and through IAC 21—45.22(7), which forbids a noncertified applicator from using a restricted-use pesticide as a fumigant, through aerial application, or where the product contains sodium cyanide or sodium fluoroacetate. Iowa also regulates hazardous rodenticides including sodium fluoroacetate (1080) and strychnine under IAC 21—45.20.