Pesticide Categories and Chemical Families: Exam Study Guide

EPA Toxicity Categories and Signal Words

Before diving into chemical families, you must know how EPA classifies pesticide toxicity. The four categories are based on oral LD50 (rat), dermal LD50, inhalation LC50, and eye/skin irritation data. Signal words on the label correspond to the most hazardous route or effect.

Key exam fact: A product that is minimally toxic orally can still carry DANGER if it causes irreversible eye damage. The signal word reflects the most hazardous route, not just the oral toxicity.

Insecticide Chemical Families

Organophosphates (OPs)

Organophosphates are among the oldest synthetic insecticides and remain heavily used in agriculture. They work by inhibiting acetylcholinesterase (AChE), the enzyme that breaks down the neurotransmitter acetylcholine in nerve synapses. When AChE is inhibited, acetylcholine accumulates, causing continuous nerve stimulation. Symptoms of OP poisoning (SLUD or DUMBELS mnemonics): Salivation, Lacrimation (tearing), Urination, Defecation, plus muscle tremors, miosis (pinpoint pupils), and in severe cases, respiratory failure.

Examples: chlorpyrifos, malathion, acephate, azinphos-methyl, phosmet. OPs generally have short environmental persistence (days to weeks) but high mammalian toxicity — many are Category I or II. Many require a cholinesterase monitoring program for regularly exposed workers.

Exam critical: OPs and carbamates are in IRAC Group 1 (acetylcholinesterase inhibitors). Rotating between OPs and carbamates does NOT prevent resistance — they share the same mode of action target site.

Carbamates

Carbamates also inhibit acetylcholinesterase, but their binding to the enzyme is reversible (OPs bind irreversibly, or very slowly reversibly). This makes carbamate poisoning somewhat more treatable, but the symptoms are the same. Examples: carbaryl (Sevin), carbofuran, methomyl, oxamyl. Carbofuran and methomyl are Category I products with strict use restrictions. Atropine is the primary antidote for both OP and carbamate poisoning.

Pyrethrins and Pyrethroids

Pyrethrins are naturally derived from chrysanthemum flowers. Pyrethroids are synthetic analogs designed to be more stable and longer lasting. Both work by disrupting sodium channel function in nerve cell membranes, causing repetitive nerve firing and eventual paralysis. They are generally low in mammalian toxicity but highly toxic to fish, aquatic invertebrates, and beneficial insects including bees.

Examples: permethrin, bifenthrin, lambda-cyhalothrin, cypermethrin, deltamethrin, esfenvalerate. Pyrethroids are classified as Type I (non-cyano) or Type II (cyano-group, more potent). Exam tip: Pyrethroids degrade rapidly in the environment under UV light and are often bound tightly to soil particles — low leaching potential, but high aquatic toxicity on contact.

Neonicotinoids

Neonicotinoids act as agonists at nicotinic acetylcholine receptors (nAChR) in insect nervous systems, causing continuous stimulation. They are selective for insect nAChRs over mammalian receptors, giving them relatively low mammalian toxicity. However, they are systemic — absorbed by plants and transported throughout plant tissues — making them present in pollen and nectar, which raises significant pollinator concerns.

Examples: imidacloprid, clothianidin, thiamethoxam, acetamiprid, dinotefuran. Many state labels include restrictions on application timing relative to bloom to protect bees. IRAC Group 4A. Resistance has developed in several whitefly, aphid, and other sucking insect populations globally.

Other Important Insecticide Groups

• Diamides (IRAC Group 28) — chlorantraniliprole, cyantraniliprole. Activate ryanodine receptors in muscle cells causing uncontrolled muscle contraction. Low mammalian toxicity, effective against lepidopteran larvae. Important for resistance management rotation.
• Spinosyns (IRAC Group 5) — spinosad, spinetoram. Derived from soil actinomycetes. Act on both nicotinic and GABA receptors. Approved for organic production. Low mammalian toxicity, some bee risk when wet.
• Avermectins (IRAC Group 6) — abamectin, emamectin benzoate. Activate chloride ion channels via glutamate-gated receptors, causing paralysis. Used in soil and foliar applications for mites and caterpillars.

Fungicide Chemical Families

Fungicides are classified by site of action using the FRAC (Fungicide Resistance Action Committee) numbering system. Multi-site fungicides (FRAC M) inhibit several metabolic processes simultaneously and have low resistance risk. Single-site fungicides target specific biochemical processes and carry higher resistance risk.

• Sterol inhibitors / DMI fungicides (FRAC 3) — propiconazole, tebuconazole, myclobutanil. Inhibit ergosterol biosynthesis in fungal cell membranes. Systemic and protective. Widely used in cereals, turf, and tree fruits. Resistance documented in some powdery mildew and Botrytis populations.
• Strobilurins / QoI fungicides (FRAC 11) — azoxystrobin, pyraclostrobin, trifloxystrobin. Inhibit mitochondrial respiration at complex III. Broad-spectrum, systemic. Resistance via the G143A mutation is common — do not apply more than two consecutive applications without rotating to another FRAC group.
• SDHI fungicides (FRAC 7) — fluxapyroxad, boscalid, penthiopyrad. Inhibit succinate dehydrogenase in the mitochondria. Relatively newer class; resistance developing in some pathogens.
• Multi-site protectants (FRAC M) — chlorothalonil (FRAC M05), mancozeb (FRAC M03), captan (FRAC M04). Disrupt multiple biochemical targets. Low resistance risk, contact action only, no systemic movement.

Herbicide Modes of Action (HRAC Groups)

Herbicides are grouped by mode of action (MOA) by the HRAC (Herbicide Resistance Action Committee). Understanding MOA groups is critical for resistance management — herbicide-resistant weeds (like glyphosate-resistant Palmer amaranth) are a major agronomic challenge. Rotating between MOA groups is the primary resistance management strategy.

• HRAC Group 9 (EPSPS inhibitors) — glyphosate (Roundup). Inhibits the EPSPS enzyme in the shikimate pathway, which produces aromatic amino acids. Non-selective, systemic. Resistance now documented in 50+ weed species worldwide.
• HRAC Group 2 (ALS inhibitors) — sulfonylureas (chlorsulfuron), imidazolinones (imazapyr). Inhibit acetolactate synthase, blocking branched-chain amino acid synthesis. Post-emergent, absorbed by leaves and roots. Resistance very common.
• HRAC Group 1 (ACCase inhibitors) — clethodim, sethoxydim, fluazifop. Inhibit acetyl-CoA carboxylase in fatty acid synthesis. Selective for grasses. Used in broadleaf crops to control volunteer cereals and annual grasses.
• HRAC Group 5 (Photosystem II inhibitors) — atrazine, simazine, metribuzin. Block electron transport in photosystem II, disrupting photosynthesis. Soil-applied and foliar. Atrazine is one of the most widely used herbicides in corn production but has significant groundwater concerns.
• HRAC Group 14 (PPO inhibitors) — fomesafen, lactofen, acifluorfen. Inhibit protoporphyrinogen oxidase, causing accumulation of phototoxic compounds that destroy cell membranes. Contact action, fast burndown.
• HRAC Group 4 (Synthetic auxins) — 2,4-D, dicamba, clopyralid. Mimic natural plant growth hormone auxin, causing uncontrolled growth and death in broadleaf weeds. Volatile dicamba formulations caused widespread off-target injury to non-tolerant soybeans and other broadleaf crops — a major regulatory issue since 2016.

Restricted-Use Pesticides vs. General-Use Pesticides

EPA classifies pesticides as either General Use or Restricted Use (RUP). RUPs may only be purchased and applied by certified applicators or by persons under their direct supervision. A product is classified as RUP if it may cause unreasonable adverse effects to the applicator or the environment without additional restrictions. Look for "Restricted Use Pesticide" at the top of the label. Examples of RUPs include many organophosphates, methyl bromide, and chlorpyrifos formulations.

Key Exam Concepts to Lock In

• OPs and carbamates share the same MOA (AChE inhibition) — rotating between them does NOT manage resistance
• Pyrethroids are highly toxic to aquatic organisms — observe buffer zones near water bodies
• Neonicotinoids are systemic and appear in pollen — follow bee protection restrictions on labels
• Signal words correspond to toxicity categories I through IV; memorize the LD50 ranges
• Rotating herbicide MOA groups is the primary resistance management tool
• RUPs require a certified applicator license to purchase and apply