2.5 Pesticide Types, Selectivity & Mode of Uptake
Key Takeaways
- The suffix -cide means to kill, so a pesticide's name usually identifies its target: insecticide, herbicide, fungicide, rodenticide, nematicide, acaricide or miticide, molluscicide, avicide, algaecide, and bactericide.
- Selective pesticides control certain pests while leaving others largely unaffected; non-selective products control a broad range of organisms, which raises non-target and phytotoxicity risk.
- Contact pesticides act only where the spray lands and demand thorough coverage; systemic pesticides are absorbed and moved within the plant or animal, so coverage matters less but timing and uptake conditions matter more.
- Herbicide timing terms describe the crop and weed stage, not the calendar: preplant, preemergence, and postemergence, with residual products continuing to control germinating weeds after application.
- Not every pest control product is a conventional chemical: biopesticides include microbial products such as Bacillus thuringiensis, biochemical products such as pheromones, and plant-incorporated protectants, and plant regulators, defoliants, and desiccants are pesticides under FIFRA even though they do not kill a pest.
2.5 Pesticide Types, Selectivity & Mode of Uptake
FIFRA defines a pesticide broadly: any substance intended to prevent, destroy, repel, or mitigate a pest, plus any substance intended for use as a plant regulator, defoliant, or desiccant. That last clause surprises people — a product that makes a crop drop its leaves before harvest is a pesticide even though it kills nothing.
Understanding how a product is classified is not academic. The classification tells you how much coverage you need, when in the pest's life cycle to apply, how much non-target risk you are carrying, and which equipment will actually deliver the product to its target.
Classification by Target Pest
The suffix -cide comes from the Latin for "to kill," and the prefix names the target:
| Type | Controls | Everyday Example |
|---|---|---|
| Insecticide | Insects | Soil and foliar treatments for corn rootworm, soybean aphid |
| Herbicide | Weeds and unwanted plants | Waterhemp and giant ragweed control in row crops |
| Fungicide | Fungal plant pathogens | White mold in soybeans, dollar spot on turf |
| Bactericide | Bacteria | Copper products for bacterial plant disease |
| Rodenticide | Rats, mice, and other rodents | Anticoagulant baits in bait stations |
| Avicide | Birds | Nuisance bird control at feedlots and elevators |
| Nematicide | Nematodes | Soybean cyst nematode seed treatments |
| Acaricide / Miticide | Mites and ticks | Two-spotted spider mite control |
| Molluscicide | Slugs and snails | Baits in nurseries and greenhouses |
| Algaecide | Algae | Copper-based aquatic treatments |
| Piscicide / Predacide | Fish; predatory vertebrates | Regulatory lake reclamation programs |
| Antimicrobial | Microorganisms on surfaces | Disinfectants, sanitizers, cooling-tower treatments |
| Wood preservative | Wood-destroying organisms | Treatment of poles, posts, and structural lumber |
Three important products do not kill but are still pesticides:
- Plant growth regulators (PGRs) alter the rate or pattern of plant growth — turf growth suppressants, fruit-thinning agents, sprout inhibitors on stored potatoes.
- Defoliants cause leaves to drop, easing harvest.
- Desiccants dry plant tissue.
Two more categories change pest behavior rather than physiology: repellents drive pests away, and attractants — including pheromones — lure them to a trap or bait.
Selective vs. Non-Selective
- A selective pesticide controls certain pests while leaving others largely unharmed. A broadleaf herbicide applied to a lawn kills dandelion and clover but not turfgrass; an insect growth regulator disrupts immature insects but not adult beneficials.
- A non-selective (broad-spectrum) pesticide controls a wide range of organisms. Glyphosate injures nearly any actively growing green plant it contacts; a broad-spectrum pyrethroid kills the aphids you targeted and the lady beetles that were eating them.
Selectivity is a decision, not just a chemistry fact. Placement and timing create practical selectivity: a wick applicator wiping tall weeds above a crop canopy makes a non-selective herbicide behave selectively.
The trade-off matters for IPM. Non-selective insecticides remove natural enemies, which is the direct cause of secondary pest outbreaks — the spider mite flare-up that follows a broad-spectrum spray.
Mode of Uptake: Contact, Systemic, and Translocated
| Category | How It Works | What It Demands of the Applicator |
|---|---|---|
| Contact | Kills only the tissue or organism the product physically touches; not absorbed or moved | Thorough, uniform coverage is everything. Finer droplets and higher carrier volumes; missed leaf undersides are missed control |
| Systemic | Absorbed into the plant or animal and moved internally | Coverage is less critical, but the plant must be actively growing and able to absorb and move the product; rainfast interval and uptake conditions dominate |
| Translocated | A systemic that moves substantial distances — up in the xylem, down in the phloem to roots and rhizomes | Essential for perennial weeds: enough product must reach the root system, which is why late-season applications during downward translocation work best |
Insecticides add three more terms describing the route into the pest:
- Contact insecticide — absorbed through the cuticle when the insect touches treated surfaces.
- Stomach poison — must be ingested, so it works on chewing insects and is the basis for baits.
- Fumigant — a gas that enters the respiratory system; requires containment, and is the highest-hazard application class in the trade.
Fungicides split on timing rather than route:
- Protectant (preventive) fungicides sit on the plant surface and must be present before infection. Reapplication is needed as new growth emerges and as weathering removes residue.
- Curative / eradicant fungicides act on an infection already established, but usually only within a short window after infection.
Timing Terms You Must Not Confuse
Herbicide labels describe timing relative to the crop and weed, not the calendar:
- Preplant — applied before the crop is planted; may be preplant incorporated (PPI) and mixed into the soil.
- Preemergence — applied after planting but before the crop or weed emerges. Careful: a label may mean preemergence to the crop, to the weed, or to both. Read which.
- Postemergence — applied after emergence, with weed size limits that are enforceable label directions.
Residual activity is a separate axis. A residual product keeps controlling germinating weeds for days or weeks after application; a non-residual product controls only what is present at application. Residual is an asset for weed control and a liability for rotational crop restrictions and groundwater risk — which is why labels carry rotational crop intervals and, in Minnesota's sandy and karst regions, soil-texture and timing restrictions.
Biopesticides and Reduced-Risk Products
- Microbial pesticides use a microorganism as the active ingredient. Bacillus thuringiensis (Bt) is the classic example: highly effective on target caterpillar or beetle larvae, and nearly harmless to most non-targets.
- Biochemical pesticides are naturally occurring substances that control pests by non-toxic mechanisms — insect pheromones used for mating disruption or monitoring traps, and some plant extracts.
- Plant-incorporated protectants (PIPs) are pesticidal substances a plant produces from genetic material added to it, such as Bt corn traits. The EPA regulates the substance and its genetic material as a pesticide.
- Insect growth regulators (IGRs) interfere with molting or development rather than killing outright, so they are slower and stage-specific but far more selective.
Biopesticides are still pesticides. They carry labels, signal words, REIs, and legal use restrictions, and a Minnesota license is still required for for-hire application.
General Use vs. Restricted Use
Every registered product is classified by the EPA as general use (GUP) or restricted use (RUP). A product is classified restricted when, without added controls, it may cause unreasonable adverse effects to the applicator or the environment even when used as directed. An RUP label carries a boxed "RESTRICTED USE PESTICIDE" statement at the top of the front panel along with the reason for the classification. In Minnesota, the commissioner may also designate additional state restricted use pesticides and impose state-specific restrictions such as use cut-off dates or soil-texture limits, and RUP purchase requires presenting a license card or private applicator card.
A pesticide label directs the applicator to obtain complete coverage of both upper and lower leaf surfaces. What does this tell you about the product?
Which product type is the appropriate choice for controlling an established perennial weed with an extensive creeping root system?
Which of the following is a pesticide under FIFRA even though it does not kill a pest?
Why does applying a broad-spectrum insecticide frequently lead to a secondary pest outbreak such as a spider mite flare-up?