Chapter 3: About Pesticides

The types, by what they control

The most intuitive way to name a pesticide is by its target, and the chapter gives a working list: avicides (pest birds), defoliants (make leaves drop), fungicides (fungi), growth regulators (alter a plant's or animal's growth), herbicides (weeds), insecticides (insects and related arthropods), miticides / acaricides (mites), nematicides (nematodes, i.e., roundworms), and rodenticides (rodents). Note that a couple of these are named for a mechanism rather than an organism — a defoliant and a growth regulator describe what the chemical does to a plant.

Four different ways to group pesticides

Because one pesticide can be described several ways, the chapter lays out four grouping schemes. Each tells you something different about a product's likely behavior.

By EPA registration group. The EPA sorts pesticides into three broad buckets. Conventional pesticides are synthetic (manufactured) chemicals that are neither antimicrobials nor biopesticides; they include not just outright pest-killers but also attractants, repellents, plant-growth regulators, and defoliants, and they are the most commonly used products. Antimicrobial pesticides destroy or suppress harmful microorganisms — bacteria, viruses, fungi — but only on inanimate, non-living objects and surfaces. That "non-living" detail is the exam's favorite trap: a fungicide sprayed on a living crop is not an antimicrobial, because it's applied to a living surface, even though it controls a fungus. Biopesticides are derived from natural materials such as plants, bacteria, fungi, and certain minerals, and the EPA further splits them into biochemical, microbial, and plant-incorporated protectants.

By regulatory category. Carried over from Chapter 1, this is the legal grouping: restricted use (RUP), general use (unclassified), and minimum risk (25b) products.

By chemical family. Pesticides in the same family share a similar chemical structure, and because structure drives behavior, family members often share properties like toxicity and persistence. The caution here is that chemistry is subtle — even slight structural differences can give two family members different properties — so "same family" means often similar, never guaranteed identical.

By mode of action. A mode of action is the specific way a particular pesticide affects a pest. One insecticide might disrupt an insect's nervous system while another interferes with its hormones; one herbicide might mimic a plant's growth hormones while another blocks its ability to convert light into food. Mode of action matters most for managing pesticide resistance (covered in Chapter 11) — rotating among different modes of action is how applicators keep pests from adapting.

The chapter notes that grouping by what they control is probably the most common scheme of all — the same insecticide/herbicide/fungicide logic we started with.

Properties: chemistry you can't change (mostly)

The back half of the chapter is about six properties, and the framing matters: most of these are inherent in the chemistry and structure of the molecule and can't be altered — although certain adjuvants can nudge some of them up or down. Knowing a product's properties helps you pick the right tool and avoid predictable problems.

Selectivity describes how broad or narrow a range of organisms a pesticide controls. Broad-spectrum (nonselective) products affect a wide range of species — handy when you're fighting several pests at once. The chapter's examples are carbaryl, which kills most insects it touches, and glyphosate, which kills almost any plant. The trade-off is that broad-spectrum products can harm nontarget species, not just your target. Narrow-spectrum (selective) products control only a few, usually related pests and leave others alone; some even target a specific life stage (larva vs. adult) or feeding habit (chewing vs. piercing-sucking). Their advantage is sparing beneficial organisms like bees.

Route of entry splits pesticides into contact and systemic, and some work both ways. A contact pesticide controls a pest only when it physically touches it — an insect sprayed directly, or one that walks across a treated baseboard or leaf. A contact herbicide kills a weed by covering enough of its surface to kill the leaves and stop photosynthesis. A systemic pesticide, by contrast, is applied to one spot and then moves — translocates — throughout the plant or animal to reach the pest. Systemics can also protect: a topical flea-and-tick product absorbed into a pet's body, or a systemic insecticide taken up by a plant to guard against chewing and sucking insects.

Persistence — also called residual activity — is how long a pesticide stays active. Highly persistent products can control pests for weeks, months, or even years, while nonresidual ones break down quickly into nontoxic by-products. Long persistence is a double-edged sword: it means fewer reapplications, but the longer a chemical stays active, the more time it has to move off-target, get into the food supply, contaminate ground or surface water, or harm nontarget organisms. A product's persistence comes from its own rate of degradation plus environmental factors like temperature, pH, and moisture.

Volatility is the tendency to turn into a gas or vapor, and it rises with temperature. Because a volatile product can drift off-site as vapor, some carry label statements that require downwind buffer zones or forbid application above a certain temperature or wind speed.

Solubility measures how well a chemical dissolves in a liquid, usually water. High solubility has a convenience upside — you generally don't need to agitate the spray tank to keep the product mixed. But it's also an environmental liability: soluble products move with water, either running off the surface into streams and lakes or leaching downward through the soil to contaminate groundwater.

Sorption ties the last ideas together. It's the physical-and-chemical process of one substance becoming attached to or mixed in with another, and it covers two easily confused terms that differ by two letters. Absorption is movement into something — a paper towel soaking up water, or a chemical moving into plants, animals, soil, or wood. Adsorption is a pesticide binding onto soil particles, like paper clips sticking to a magnet. A strongly adsorbed pesticide clings to clay or organic matter, which makes it less likely to run off or leach into groundwater — but that same binding also makes it less available to control pests. That tension is a nice capstone for the chapter: the property that protects groundwater can also blunt a product's effectiveness.

How to study this chapter

1. Anchor on the definition: pesticides don't all kill, and "pesticide" is an umbrella term (herbicides, fungicides, etc., are all pesticides).
2. Keep the grouping schemes distinct: EPA groups (conventional/antimicrobial/biopesticide), regulatory category (RUP/general/25b), chemical family, and mode of action — and remember an antimicrobial acts on non-living surfaces.
3. Master the six properties and, for each, the benefit and the risk: selectivity, contact vs. systemic, persistence, volatility, solubility, and sorption.
4. Nail the two look-alikes: absorption (into) vs. adsorption (onto soil), and remember that strong adsorption reduces leaching but also lowers pest control, while high solubility increases runoff and leaching.