6.1 Types of Pesticides & How They Act
Key Takeaways
- Pesticides are named for their target: insecticide, herbicide, fungicide, rodenticide, miticide, nematicide, molluscicide, avicide, algicide, and more.
- Contact products act where they touch the pest, while systemic products are absorbed and moved within the plant or animal to reach protected feeding sites.
- Selective products control certain organisms at labeled rates while sparing others; non-selective products injure most organisms they contact.
- Fumigants are gases that penetrate confined spaces, are extremely hazardous, and in Ohio may be applied only under categories 2F, 10C, or 6D.
- Every product has a brand name, a common name for the active ingredient, and a chemical name, and the common name is what identifies the chemistry across brands.
6.1 Types of Pesticides & How They Act
The word pesticide covers any substance or mixture intended to prevent, destroy, repel, or mitigate a pest, or to act as a plant regulator, defoliant, or desiccant - the definition ODA publishes and the one in ORC 921.01.
Classification by Target Pest
| Type | Target | Ohio examples of use |
|---|---|---|
| Insecticide | Insects | Turf grubs, structural cockroaches, field crop pests |
| Miticide (acaricide) | Mites and ticks | Spider mites in ornamentals and greenhouses |
| Herbicide | Weeds | Turf broadleaf control, agricultural weed control, rights-of-way |
| Fungicide | Fungal pathogens | Turf disease, greenhouse crops, orchard scab |
| Bactericide | Bacteria | Limited use; greenhouse and orchard |
| Nematicide | Nematodes | Nursery and vegetable production |
| Rodenticide | Rats, mice, other rodents | Structural and agricultural rodent control |
| Avicide / bird repellent | Pest birds | Roost and ledge management |
| Molluscicide | Slugs and snails | Greenhouse and landscape |
| Algicide | Algae | Ponds, cooling towers, Category 3A aquatic work |
| Piscicide | Fish | Fisheries management |
| Predacide | Predatory vertebrates | Requires additional federal certification |
| Wood preservative | Wood-decay organisms | Ohio Category 4B |
| Defoliant / desiccant | Plant tissue removal | Harvest aids |
| Plant growth regulator (PGR) | Plant growth processes | Turf seedhead suppression, tobacco sucker control (Ohio Category 2E) |
| Antimicrobial (disinfectant, sanitizer, germicide) | Microorganisms on surfaces | Food plants, schools, health care |
| Attractant / repellent | Behavior modification | Pheromone traps, deer repellents |
Classification by Mode of Entry and Movement
Contact versus systemic
- Contact pesticides act where they land. A contact insecticide must touch the insect or leave a residue the insect crosses; a contact herbicide kills only the tissue it wets. Thorough coverage is everything.
- Systemic pesticides are absorbed and translocated. A systemic insecticide moves in the plant's vascular tissue so a piercing-sucking insect ingests it while feeding; a systemic herbicide moves toward growing points and storage tissue, reaching rhizomes and taproots. Coverage matters less; timing and plant activity matter more.
- Translaminar products move through the leaf from the treated surface to the opposite surface without moving through the whole plant - useful against mites and leafminers on the underside of leaves.
Other modes of entry for insecticides
- Stomach poison - must be eaten; the basis for baits.
- Contact poison - absorbed through the cuticle.
- Fumigant - enters as a gas through the respiratory system.
- Insect growth regulator (IGR) - disrupts development rather than killing outright; effects are slow but reduce reproduction.
- Desiccant - abrades or absorbs the cuticular wax layer, causing water loss; diatomaceous earth and silica aerogels.
Herbicide movement categories
- Pre-emergence - applied before weed emergence; must be activated by moisture.
- Post-emergence - applied to emerged weeds.
- Soil-applied residual - remains active in soil for weeks or months; watch label plant-back and rotational restrictions.
- Foliar-applied - taken up through leaves.
Selective versus Non-Selective
A selective pesticide controls certain organisms while causing little injury to others at labeled rates. A non-selective (broad-spectrum) product injures most organisms it contacts.
Selectivity comes from real mechanisms: differences in absorption, in translocation, in metabolic breakdown, in the target site itself, or simply in placement and timing. It is not absolute. A selective turf herbicide applied at twice the labeled rate, or during heat stress, can injure the turf it was designed to spare.
Broad-spectrum products also carry an IPM cost: they kill natural enemies along with the pest, which can trigger a secondary pest outbreak or a pest resurgence when the surviving pest population rebounds without predators.
Fumigants: A Category of Their Own
Fumigants are gases or volatile liquids that diffuse through a confined space - soil under a tarp, a structure under seal, a grain bin, a greenhouse. They penetrate where sprays cannot, and they are among the most dangerous products an applicator handles: highly toxic, often with poor warning properties, and lethal to anyone who enters an untreated-for space.
Federal rule sets separate competency standards for soil fumigation and non-soil fumigation with their own requirements for fumigant management plans, air monitoring, buffer zones, posting, and respiratory protection. In Ohio, OAC 901:5-11-02 limits fumigant application to applicators licensed in and applying within 2F Soil Fumigation, 10C Fumigation, or 6D Greenhouse Pest Control.
Biological and Reduced-Risk Products
- Microbials - living organisms or their products, such as Bacillus thuringiensis (Bt) strains selective for specific larval groups, and entomopathogenic nematodes.
- Botanicals - derived from plants, such as pyrethrins from chrysanthemum flowers, often with short residual.
- Biochemicals - pheromones and insect growth regulators.
- Minimum-risk products - a short list of active ingredients exempt from registration under FIFRA Section 25(b). Exempt does not mean unregulated in Ohio: the product still must be used per its labeling, and application for hire still requires the appropriate license.
Naming: Three Names for the Same Product
| Name type | Example | Where it appears |
|---|---|---|
| Brand / trade name | The marketing name on the front panel | Front panel, in large type |
| Common name | glyphosate, imidacloprid, propiconazole | Ingredient statement, sometimes with the mode-of-action group code |
| Chemical name | The full IUPAC chemical designation | Ingredient statement, in small type |
Learn products by common name. Dozens of brands share one active ingredient; the common name is what tells you the chemistry, the resistance group, and whether you are rotating modes of action or just rotating labels.
Why does a systemic insecticide usually outperform a contact insecticide against aphids feeding on the underside of leaves?
Under Ohio rule, which categories permit the application of fumigants?
What is the practical reason for learning pesticide products by the common name of the active ingredient rather than by brand name?