🐛 Insecticides — Types and Mode of Action

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Insecticides — Types and Mode of Action

Insecticides are chemical or biological agents used to kill, repel, or inhibit insects that damage crops. They are classified by chemical group, mode of action, and method of entry into the insect body.

Classification by Mode of Entry

• Contact insecticides — kill insects on direct contact with the body surface (e.g., Cypermethrin, Malathion)
• Stomach poisons — must be ingested to be effective (e.g., Carbaryl, lead arsenate)
• Systemic insecticides — absorbed by the plant and distributed through tissues; insects ingest while feeding (e.g., Imidacloprid, Thiamethoxam)
• Fumigants — enter through the respiratory system as gas or vapor (e.g., Aluminium phosphide, Methyl bromide)

Major Chemical Groups and Mode of Action

Chemical Group	Mode of Action	Examples
Organophosphates	Inhibit acetylcholinesterase (AChE)	Chlorpyrifos, Monocrotophos, Dimethoate
Carbamates	Reversible AChE inhibition	Carbofuran, Carbaryl, Methomyl
Pyrethroids	Sodium channel modulators	Cypermethrin, Deltamethrin, Fenvalerate
Neonicotinoids	Nicotinic acetylcholine receptor (nAChR) agonists	Imidacloprid, Acetamiprid, Thiamethoxam
Diamides	Ryanodine receptor modulators	Chlorantraniliprole, Flubendiamide
Spinosyns	nAChR allosteric modulators	Spinosad, Spinetoram
IGRs	Insect growth regulators (chitin synthesis inhibitors)	Diflubenzuron, Buprofezin, Novaluron

IRAC Classification

The Insecticide Resistance Action Committee (IRAC) classifies insecticides into groups based on their mode of action to manage resistance:

• Group 1 — AChE inhibitors (OPs and carbamates)
• Group 3 — Sodium channel modulators (pyrethroids)
• Group 4 — nAChR agonists (neonicotinoids)
• Group 28 — Ryanodine receptor modulators (diamides)

Rotating insecticides from different IRAC groups is essential to delay resistance development.

Systemic vs Contact Insecticides

Feature	Systemic	Contact
Absorption	Taken up by plant roots or foliage	Acts on insect body surface
Translocation	Moves through xylem/phloem	No translocation
Target pests	Sucking pests (aphids, whiteflies, hoppers)	Exposed chewing and crawling pests
Residual activity	Longer (weeks)	Shorter (days)
Rain fastness	High (inside plant tissue)	Low (washed off easily)

Important Considerations

• Always follow label recommendations for dosage, dilution, and waiting period
• Use personal protective equipment (PPE) during application
• Avoid spraying during flowering to protect pollinators
• Observe the pre-harvest interval (PHI) before marketing the produce

Proper understanding of insecticide types and their mode of action is critical for effective, economical, and environmentally responsible pest management.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Insecticides — chemical groups, mode of action, systemic vs contact insecticides, and IRAC classification.
Section context	Revise this lesson with the rest of Synthetic Pesticides for stronger conceptual continuity.