Insecticide classes and IRAC mode-of-action groups
The main insecticide classes, how each one kills, IRAC group numbers with examples, selectivity and safety, and planning a mode-of-action rotation.
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Insecticides remain a major tool for protecting crops from insects, but the products used today differ greatly from those of fifty years ago. Broad-spectrum organochlorines and many organophosphates have been banned or restricted in most countries, and newer classes act on targets more specific to insects. What has not changed is that insects evolve resistance to any insecticide used too often. Knowing how each class works, and grouping products by mode of action, is the foundation of using them safely and keeping them effective.
The Insecticide Resistance Action Committee (IRAC) classifies insecticides and acaricides into numbered groups by mode of action; the group number appears on many labels. Insecticide classes and IRAC groups are core exam material in entomology and plant protection.
How insecticides enter and act
- Contact poisons enter through the cuticle when the insect touches a treated surface or is sprayed.
- Stomach poisons must be eaten, as with Bt toxins and many chitin inhibitors.
- Systemic insecticides are taken up by the plant and moved in the sap, reaching sucking insects that feed inside the plant; many neonicotinoids, sulfoximines and diamides move this way. Translaminar products move through a leaf to its underside.
- Fumigants act as gases, mainly in stored products and soil.
The major groups
| IRAC group | Class | Examples | Notes |
|---|---|---|---|
| 1A | carbamates | carbaryl, methomyl, carbofuran | inhibit acetylcholinesterase; many highly toxic, restricted |
| 1B | organophosphates | chlorpyrifos, malathion, acephate, dimethoate | inhibit acetylcholinesterase; many banned or restricted |
| 2B | phenylpyrazoles | fipronil | block GABA-gated chloride channels; toxic to bees |
| 3A | pyrethroids | cypermethrin, deltamethrin, lambda-cyhalothrin | keep sodium channels open; fast knockdown; toxic to fish and many natural enemies |
| 4A | neonicotinoids | imidacloprid, thiamethoxam, clothianidin, acetamiprid | bind nicotinic acetylcholine receptors; systemic; outdoor use of three restricted in the EU since 2018 because of bee risk |
| 4C | sulfoximines | sulfoxaflor | sap-feeding insects |
| 5 | spinosyns | spinosad, spinetoram | from a soil bacterium; caterpillars and thrips |
| 6 | avermectins and milbemycins | abamectin, emamectin benzoate | glutamate-gated chloride channels; mites, leaf miners, caterpillars |
| 7 | juvenile hormone mimics | pyriproxyfen, fenoxycarb | insect growth regulators |
| 11 | microbial disruptors of the midgut | Bacillus thuringiensis (Bt) and Bt crop proteins | eaten by caterpillars or beetle larvae; very selective |
| 15 | benzoylureas | diflubenzuron, lufenuron, novaluron | block chitin synthesis; immature stages |
| 18 | diacylhydrazines | methoxyfenozide, tebufenozide | ecdysone agonists; caterpillars |
| 22A | oxadiazines | indoxacarb | block sodium channels; caterpillars |
| 23 | tetronic and tetramic acids | spiromesifen, spirotetramat | lipid synthesis; mites, whiteflies, scales |
| 28 | diamides | chlorantraniliprole, flubendiamide, cyantraniliprole | activate ryanodine receptors in muscle; caterpillars; low toxicity to mammals |
| UN | unknown or uncertain | azadirachtin (neem) | botanical |
Groups 1 to 6, 22 and 28 act on nerves or muscles and usually act quickly. Groups 7, 15, 16 and 18 act on growth and development and act slowly, mainly on immature stages. Groups 12, 13, 20 and 21 act on respiration, and many are acaricides. Group 11 attacks the midgut.
Older classes and why they declined
- Organochlorines such as DDT (introduced in the 1940s), BHC, aldrin and endosulfan were cheap and persistent, but they accumulate in food chains and the environment. Most are now banned under the Stockholm Convention on persistent organic pollutants.
- Many organophosphates and carbamates are acutely toxic to people, causing poisonings, and are being phased out in many countries, while others remain in wide use.
Selectivity and side effects
- Narrow-spectrum products, such as Bt, diamides, spinosyns (to a degree), IGRs and many sulfoximines, spare more natural enemies and fit integrated pest management better than broad-spectrum organophosphates, carbamates and pyrethroids.
- Bees: neonicotinoids, fipronil, many pyrethroids and organophosphates are highly toxic to bees; avoid spraying crops in flower and follow label bee warnings.
- Aquatic life: pyrethroids are very toxic to fish and aquatic invertebrates.
- Resurgence and secondary pests: killing natural enemies can cause pest outbreaks, such as brown planthopper resurgence in rice after some insecticides, or mite outbreaks after pyrethroids.
Rotating modes of action
IRAC recommends treating pests in windows roughly the length of one pest generation (often about 30 days), using one mode of action within a window and switching groups for the next. Products with the same IRAC number share cross-resistance, so changing brand alone does not help.
Planning a rotation and a dose
A tomato grower faces a caterpillar pest for about 90 days, three generations. Effective options are a diamide (group 28), spinosad (group 5) and indoxacarb (group 22A).
| Window | Days | Group |
|---|---|---|
| 1 | 1–30 | 28 (chlorantraniliprole) |
| 2 | 31–60 | 5 (spinosad) |
| 3 | 61–90 | 22A (indoxacarb) |
Each generation meets a different mode of action, so insects surviving one group are unlikely to survive the next.
Dose. Chlorantraniliprole is recommended at 30 g of active ingredient per hectare; the product contains 185 g/L. Product needed = 30 ÷ 185 = 0.162 L, about 162 mL per hectare. For a 0.25 ha field, that is about 41 mL in the spray volume for that area.
Key points
- Insecticides act by contact, as stomach poisons, systemically or as fumigants.
- IRAC groups classify by mode of action; same group means cross-resistance.
- Nerve and muscle groups (1 organophosphates and carbamates, 2 fipronil, 3 pyrethroids, 4 neonicotinoids, 5 spinosyns, 6 avermectins, 22 indoxacarb, 28 diamides) act fast; growth regulators (7, 15, 16, 18) act slowly on young stages; group 11 is Bt.
- Organochlorines are largely banned; many organophosphates and carbamates are restricted.
- Choose selective products to protect natural enemies and bees; rotate groups by pest generation windows.
What the exams ask
- School-leaving and entrance papers: classification of insecticides by mode of entry and chemistry, with examples.
- Degree and research-fellowship entrances: modes of action and target sites, IRAC groups, new chemistries, IGRs, Bt, and resistance and resurgence.
- Professional examinations: choosing insecticides for pests and crop stages, dose calculation, bee safety, and rotation plans.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do some insecticides kill aphids but not caterpillars?
Many systemic products, such as neonicotinoids and sulfoximines, move in the sap and act strongly on sap-feeding insects, while some caterpillar products, such as Bt and diamides, act best when eaten with leaf tissue.
Is a natural insecticide always safe?
No. Natural products such as pyrethrins, nicotine and rotenone can be toxic to people, fish or bees. Safety depends on the substance and dose, not its origin.
Why rotate by window and not spray by spray?
Within one pest generation, rotating spray by spray exposes the same insects to several groups. Using one group per generation means each generation is selected by only one mode of action, and the next generation meets a different one.
Sources
- Insecticide Resistance Action Committee. IRAC Mode of Action Classification Scheme. IRAC, updated regularly (2024 edition).
- Sparks, T. C. and Nauen, R. "IRAC: Mode of action classification and insecticide resistance management." Pesticide Biochemistry and Physiology 121, 122–128 (2015).
- Casida, J. E. and Durkin, K. A. "Neuroactive insecticides: targets, selectivity, resistance, and secondary effects." Annual Review of Entomology 58, 99–117 (2013).
- European Commission. Implementing Regulations (EU) 2018/783, 2018/784 and 2018/785 restricting imidacloprid, clothianidin and thiamethoxam.
- Matsumura, F. Toxicology of Insecticides, 2nd edition. Plenum Press, 1985.
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