🌱Botanical Insecticides — Plant-Origin Pest Control

Pest Control Before the Chemical Age

The previous lesson covered synthetic chemical insecticides — their generations, formulations, and toxicology. This lesson examines an older and increasingly relevant alternative: botanical insecticides, which are derived from plants and play a central role in organic farming and IPM.

Centuries before DDT existed, Indian farmers protected stored grain by mixing neem leaves into gunny bags, and European gardeners dusted chrysanthemum flower powder on vegetables to kill caterpillars. These traditional practices used botanical insecticides — pesticides extracted from plants. Today, with rising concerns about chemical residues in food and environmental damage, botanical insecticides are making a comeback as key tools in organic farming and IPM programmes.

This lesson covers:

1. Four major botanicals — Rotenone, Pyrethrum, Azadirachtin (Neem), Nicotine
2. Source plant, plant part, active ingredient, mode of action for each
3. Other plant-derived substances — neem cake, karanja oil, custard apple
4. Comparison table across all four major botanicals

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Why Botanical Insecticides Matter

Before diving into individual chemicals, understand three properties that distinguish botanicals from synthetic insecticides:

1. Rapid biodegradation — they break down quickly in the environment, leaving minimal residues
2. Lower mammalian toxicity (with exceptions like nicotine) — safer for farmers and consumers
3. Compatibility with organic farming — neem, pyrethrum, and rotenone are approved for organic production

The trade-off is that botanicals generally have shorter residual activity, meaning they may need more frequent application than synthetics.

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The Four Major Botanical Insecticides

A. Rotenone

• Source plant: Derris elliptica (tuba root) or Lonchocarpus species
• Plant part used: Roots
• Active ingredient: Rotenone
• Mode of action: Inhibits mitochondrial electron transport chain (Complex I inhibitor) — blocks cellular respiration
• Properties: Contact and stomach poison; photodegradable

Agricultural use: Effective against aphids, caterpillars, and beetles on vegetables and fruits.

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B. Pyrethrum (Pyrethrin)

• Source plant: Chrysanthemum cinerarifolium (Dalmatian pyrethrum)
• Plant part used: Flowers — specifically found in the achenes of flowers (the seed-like fruits of the flower head)
• Active ingredient: Pyrethrins — a group of six compounds (Pyrethrin I & II, Cinerin I & II, Jasmolin I & II)
• Mode of action: Disrupts sodium channels in insect nerve membranes → rapid knockdown effect
• Properties: Contact poison; strong knockdown; rapidly photodegraded by sunlight

Agricultural use: Household insect sprays, mosquito coils, stored grain protection. Less used in open-field agriculture due to photodegradation.

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C. Azadirachtin (Neem-Based)

• Source plant: Azadirachta indica (Neem tree)
• Plant part used: Seeds and leaves
• Active ingredient: Azadirachtin
• Mode of action: Multiple actions — antifeedant, insect growth regulator, repellent, oviposition deterrent, and sterilant

Azadirachtin is the most versatile botanical insecticide, effective against over 200 insect species. Its multiple modes of action make resistance development very difficult.

The five actions of Azadirachtin:

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D. Nicotine

• Source plant: Nicotiana tabacum and other Nicotiana species (Tobacco)
• Plant part used: Roots and leaves
• Active ingredient: Nicotine (alkaloid)
• Mode of action: Mimics acetylcholine at nicotinic receptors → overstimulation of the nervous system → paralysis and death
• Properties: Contact and fumigant action; highly toxic to mammals (unlike other botanicals)

Historical use: Effective against aphids and soft-bodied insects. Largely replaced by synthetic neonicotinoids (imidacloprid, thiamethoxam) which have the same target site but lower mammalian toxicity.

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E. Lantana (Lantana camara)

• Source: Lantana camara (an invasive weed itself)
• Use: Botanical pesticide — leaves and extracts have insecticidal and repellent properties

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Comparison of the Four Major Botanicals

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Other Plant-Derived Pest Control Substances

Beyond the four major botanicals, several other plant products are used in Indian agriculture, especially in organic and traditional farming.

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When to Choose Botanicals Over Chemicals

Botanicals are the right choice when:

Key limitation: Botanicals photodegrade quickly (1-3 days in sunlight). Always spray in evening for maximum effectiveness. They need repeated applications compared to synthetic chemicals.

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Exam Tips

1. Source plant + plant part + active ingredient — these three facts are the minimum you need for each botanical. Exams test all three.
2. Rotenone = fish killer. Any question linking a botanical insecticide to fish toxicity or piscicide = rotenone.
3. Pyrethrum = knockdown. Fast-acting contact poison from chrysanthemum flowers.
4. Azadirachtin = multiple modes. If asked “which botanical has antifeedant, IGR, and repellent properties,” the answer is always azadirachtin.
5. Nicotine = high mammalian toxicity. The only major botanical that is dangerous to mammals.
6. Pyrethrum vs pyrethroid is a trick question — natural vs synthetic. Pyrethroids are 4th generation insecticides, not botanicals.
7. Neem cake as nitrification inhibitor crosses the boundary between entomology and soil science — it appears in both subject papers.

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Summary Table

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Summary Cheat Sheet