# Neem Botanical Pesticides and What Actually Works

> Neem, Botanical Pesticides and What Actually Works covers the plant-derived pesticides — what each one really does, which ones are effective, which are more dangerous than the synthetics they are…

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Section: Plants and preparation
Last updated: 2026-10-04
Publisher: Library of Ashurbanipal (Van Kush Family Research Institute), https://wiki.soapbox.community

**Neem, Botanical Pesticides and What Actually Works** covers the plant-derived pesticides — what each one really does, which ones are effective, which are more dangerous than the synthetics they are marketed against, and why **how neem oil was extracted decides whether it works at all**. The nerve-agent chemistry of organophosphates and carbamates is covered in Cholinergic Neurotransmission and Cholinesterase Inhibition and is not repeated here. Alpha.

## 1. Neem

***Azadirachta indica***, the neem tree, is the most important botanical pesticide in the world and the most frequently misused.

### What the active compound does

- The principal active is **azadirachtin**, a tetranortriterpenoid (limonoid).
- It is **not a knock-down poison**. It works as:
  - an **insect growth regulator** — it blocks **ecdysone**, the moulting hormone, so larvae fail to moult and die at the next stage;
  - an **antifeedant** — treated foliage stops being eaten;
  - an **oviposition deterrent** — females will not lay on it;
  - a **sterilant** at sub-lethal doses.
- **The practical consequence:** nothing appears to happen for days. Judged on the timescale of a contact insecticide, neem looks like a failure. It is working on the next generation, not this afternoon.
- **Neem oil also smothers** soft-bodied pests physically — and **that** effect is immediate, but it is the oil doing it, not the azadirachtin.

### Cold pressed versus solvent extracted — the point that decides everything

- **Azadirachtin lives in the seed kernel** and is **destroyed by heat** and degraded by UV and by alkaline pH.
- **Cold-pressed neem oil** retains azadirachtin — typically **1,000–3,000 ppm** in a decent crude oil — and therefore has the hormonal and antifeedant activity.
- **Hot-pressed or solvent-extracted neem**, and especially **"clarified hydrophobic extract of neem oil"** (the common US-registered form), has had **most or all of the azadirachtin removed**. It still works as a **smothering oil and fungicide**, but it is no longer an insect growth regulator.
- **So two bottles both labelled "neem oil" can be completely different products.** If the label does not state azadirachtin content, assume it has little. See Cold Pressing, Expelling and Heat: Why the Same Seed Gives Different Oils.
- **Azadirachtin concentrates** (sold at 3–4.5% as separate products) are the standardised option when the growth-regulator effect is what is wanted.

### Using it

- **Emulsify properly.** Oil and water do not mix; without a surfactant the oil floats and you spray water. A little mild soap, or a bought formulation with an emulsifier.
- **Spray at dusk or dawn.** Azadirachtin is **degraded by UV** with a half-life of hours to a few days in sunlight, and oil on leaves in full sun causes **phytotoxic burn**.
- **Mix fresh** — azadirachtin hydrolyses in water, faster at alkaline pH. Do not store diluted spray.
- **Test on a few leaves first.** Oil damages some species, and damages anything that is already heat- or drought-stressed.
- **Do not spray open flowers.** Neem is relatively kind to adult bees compared with most insecticides, but it is **not harmless** — and it is **toxic to bee larvae** if it reaches brood in contaminated pollen.
- **It is toxic to fish and aquatic invertebrates.** Keep it out of water.

### Safety and the one serious warning

- Neem oil has a long record of topical and agricultural use, and neem twigs are used as chewing sticks across South Asia.
- **⚠️ Neem oil taken internally is dangerous to infants and young children** — it causes a **Reye-like syndrome** with vomiting, encephalopathy, seizures and deaths documented in the medical literature. It should never be given orally to a child.
- **Neem is an abortifacient and has documented antifertility effects** in both sexes in animal studies. Avoid in pregnancy.
- **Neem is not azadirachtin-only:** crude oil also contains **nimbin, salannin** and sulfurous compounds responsible for its garlic-peanut smell.

## 2. The other botanicals, honestly rated

- **Pyrethrins** — from ***Tanacetum cinerariifolium*** flowers. Fast, broad-spectrum knock-down acting on sodium channels. **Extremely toxic to bees and fish**, and **non-selective** — it kills every beneficial insect you have been cultivating. Degrades in sunlight within hours, which limits residue but also means pests return. **Synthetic pyrethroids** (permethrin, deltamethrin) are the stabilised versions and are far more persistent and more damaging to beneficials.
- **Spinosad** — from the soil bacterium ***Saccharopolyspora spinosa***. Technically microbial rather than botanical. **Highly effective on thrips, caterpillars and leaf miners**, low mammalian toxicity, and **approved for organic use**. **Toxic to bees while wet** — spray at dusk and it is substantially safer once dry. One of the genuinely good options.
- **Horticultural and insecticidal soaps** — kill soft-bodied insects by disrupting the cuticle. **Contact only, no residual**, cheap, and about as safe as it gets. Needs thorough coverage and repeat applications.
- **Horticultural oils** (mineral, canola) — smother eggs and soft bodies. **Dormant oils** on bare wood in winter are one of the most effective and least damaging interventions available.
- **Diatomaceous earth** — abrades the cuticle and desiccates. Works only **dry**, kills indiscriminately including beneficials, and the **dust is a genuine respiratory hazard** — wear a mask. Use **food grade**, never pool-filter grade (calcined, crystalline silica).
- **Capsaicin, garlic, essential-oil products** — mostly **repellents** rather than insecticides. Evidence is weak and inconsistent; harmless to try, unwise to rely on.
- **Karanja oil** (*Pongamia*) — pongamol and karanjin; often blended with neem and broadly similar in use.
- **Nicotine** — historically used as a sulfate, **extremely toxic to mammals**, and **banned** in most jurisdictions. Homemade tobacco tea is a genuinely bad idea and also spreads **tobacco mosaic virus**.
- **Rotenone** — from *Derris* and *Lonchocarpus*. **Withdrawn in most countries.** A mitochondrial complex I inhibitor, devastating to fish, and the subject of **epidemiological association with Parkinson's disease**. "Natural" did not make it safe.
- **Sabadilla, ryania, quassia** — historic botanicals, largely obsolete.

**The lesson across this list:** "botanical" says nothing about safety. **Rotenone and nicotine are plant-derived and are among the most dangerous things on it; spinosad and soap are among the safest.**

## 3. How plants actually defend themselves

Every botanical pesticide is a plant's own chemical weapon, borrowed. Understanding the originals explains why they work, why insects beat them, and why some of them recruit help instead of killing.

### Constitutive versus induced

- **Constitutive** defences are always present: thick cuticle, silica, trichomes, latex, stored alkaloids and terpenes. Expensive to maintain, instantly available.
- **Induced** defences are built **after** an attack, within hours to days, and cost nothing until needed. This is why a plant that has already been chewed is harder to chew than its neighbour.
- **Two hormone systems split the labour.** The **jasmonate** pathway answers chewing insects and wounding; the **salicylate** pathway answers pathogens and sap-suckers. **They inhibit one another** — so an aphid that triggers the salicylate arm can suppress the jasmonate defences that would have stopped a caterpillar. Pests exploit this crosstalk deliberately.
- **Priming:** a plant that has met an attack, or smelled a neighbour's volatiles, responds **faster and harder** next time without paying the cost in advance.

### The chemical classes, and what they do to an insect

- **Alkaloids** — nicotine (nicotinic acetylcholine receptor agonist), caffeine (a natural insecticide in seeds and nectar), atropine. **Nerve poisons.**
- **Terpenoids** — pyrethrins (sodium channels), azadirachtin (ecdysone/moulting), limonene. Also the volatiles that do signalling (below).
- **Phenolics and tannins** — bind proteins and gut enzymes, making the leaf **indigestible rather than poisonous**.
- **Glucosinolates** (brassicas) — inert until the cell is crushed, when **myrosinase** converts them to **isothiocyanates**. The mustard "bomb" is a two-component system kept in separate compartments so the plant does not poison itself.
- **Cyanogenic glycosides** (cassava, almond, sorghum, clover) — same trick, releasing **hydrogen cyanide** on tissue damage.
- **Protease inhibitors and lectins** — block the insect's own digestive enzymes. Slow starvation rather than poisoning.
- **Latex and resin** — physical. Milkweed latex glues mandibles shut; conifer resin traps bark beetles and sets.
- **Silica and phytoliths** — grasses wear insect mandibles down abrasively. A purely mechanical defence, and one reason grasses survived grazing.

### Calling for help: the third trophic level

This is the part that connects pesticides directly to Beneficial Insects and Biological Pest Control.
- A chewed plant releases **herbivore-induced plant volatiles (HIPVs)** — a blend including **green leaf volatiles**, terpenes and methyl salicylate.
- **Parasitoid wasps and predatory mites use that blend to find the prey.** The plant is not just defending; it is **recruiting bodyguards**.
- The signal is **specific**: maize attacked by one caterpillar species releases a different blend than when attacked by another, and the wasps can tell. Insect **saliva** carries elicitors — **volicitin** in *Spodoptera* — that the plant detects to identify its attacker.
- **Below ground too:** maize roots attacked by rootworm release **(E)-β-caryophyllene**, which attracts entomopathogenic nematodes. Many commercial US maize lines had **lost** this gene — and restoring it restored the nematode recruitment.
- **Plants eavesdrop on each other.** Undamaged neighbours detecting these volatiles prime their own defences. Documented in sagebrush, lima bean, maize and poplar.
- **Practical consequence:** spraying a broad-spectrum insecticide destroys the bodyguards the plant just summoned. The plant did its part; the spray undid it.

### How insects fight back

- **Detoxification enzymes** — **cytochrome P450s**, glutathione S-transferases and esterases. These are the same enzyme families that produce **pesticide resistance**, because a synthetic insecticide is just another xenobiotic to a system already evolved to dismantle plant toxins.
- **Sequestration** — monarch caterpillars store milkweed **cardenolides** and become toxic themselves; their sodium pump carries target-site mutations that make them insensitive. Same for cinnabar moths and ragwort alkaloids.
- **Rapid excretion and gut pH** — many lepidopteran larvae run a highly alkaline gut that destroys tannins.
- **Behaviour** — **vein-cutting and trenching**: a caterpillar severs the latex canals upstream of where it intends to feed, draining the pressure first. Pure engineering.
- **Symbionts** — gut bacteria that degrade plant toxins and, in some cases, insecticides.
- **Gall-formers and leaf-miners** go **inside** the tissue, where surface sprays and many defences cannot reach.

### What this means for using a pesticide

- **Resistance is pre-adapted.** Insects have been dismantling plant chemistry for 300 million years; the P450s that handle nicotine handle neonicotinoids. This is why **mode-of-action rotation** matters more than product choice.
- **Specialists are unimpressed by the defences they evolved with.** A brassica pest is attracted **by** glucosinolates — the same compound that repels generalists is the specialist's host-finding cue. So a "repellent" is only repellent to the insects it was not co-evolved with.
- **Breeding has thinned the defences.** Selection for yield, sweetness and low bitterness has removed defensive chemistry from crops; the modern cultivar is often chemically naked compared with its wild relative. **Push-pull** systems and the maize caryophyllene case are attempts to put it back.
- **Over-fertilising with nitrogen** produces soft, protein-rich growth that is better food and comes with weaker defences — which is why an over-fed plant gets aphids. See Plant Nutrients and Diagnosing Deficiencies: Reading a Leaf.
- **Companion planting works when it has a mechanism** — masking volatiles, trap-cropping, or feeding parasitoid adults — and not when it is folklore. The best-evidenced example is **push-pull**: *Desmodium* between the rows repels stemborers and suppresses *Striga*, napier grass at the border pulls them out of the crop.

## 4. The order to try things

Integrated pest management, in the order that actually works:
1. **Identify the pest.** Everything else depends on it.
1. **Culture first** — right plant, right place, right water and feeding. Stressed plants attract pests; over-fed, nitrogen-lush growth attracts aphids specifically.
1. **Physical** — row cover, sticky traps, a jet of water, hand removal. Underrated and free.
1. **Biological** — predators and parasitoids. See Beneficial Insects and Biological Pest Control.
1. **Selective materials** — Bt, spinosad, soaps, oils, neem. These leave most beneficials alive.
1. **Broad-spectrum last, if at all.** A pyrethrin spray resets your biological control to zero and the pest, which breeds faster, returns first. That is the **pesticide treadmill**.
- **Rotate modes of action** to delay resistance — the **IRAC** group number on a label exists for this. Repeating one mode of action breeds resistance to it.

## 5. Reading a label

- **"OMRI listed"** — allowed in certified organic production. **Not a safety claim** (see rotenone).
- **Signal words** — CAUTION, WARNING, DANGER, in increasing order of acute toxicity.
- **REI** (restricted entry interval) and **PHI** (pre-harvest interval) are legally binding.
- **Bee advisory box** — if present, it means something.
- **Active ingredient percentage and the IRAC group** — the two most useful numbers on the bottle.
- **"Clarified hydrophobic extract of neem oil"** — translated: the azadirachtin has been removed. It is a smothering oil.

## Sources

- Schmutterer H. (ed.), *The Neem Tree: Azadirachta indica A. Juss. and Other Meliaceae Plants* (2nd ed., 2002) — the standard reference on azadirachtin chemistry, degradation and use.
- Mordue (Luntz) A. J. and Blackwell A., "Azadirachtin: an update", *Journal of Insect Physiology* 39 (1993).
- Sundaravalli N. et al. and subsequent case series on neem-oil poisoning and Reye-like syndrome in infants, *Indian Journal of Pediatrics*.
- US EPA registration documents for azadirachtin and clarified hydrophobic extract of neem oil.
- Tanner C. M. et al., "Rotenone, paraquat, and Parkinson's disease", *Environmental Health Perspectives* 119 (2011).
- Biondi A. et al., "Spinosad effects on non-target organisms", *Pest Management Science* 68 (2012).
- IRAC Mode of Action Classification Scheme (current edition).
- Howe G. A. and Jander G., "Plant immunity to insect herbivores", *Annual Review of Plant Biology* 59 (2008) — jasmonate/salicylate pathways and crosstalk.
- Turlings T. C. J. and Erb M., "Tritrophic interactions mediated by herbivore-induced plant volatiles", *Annual Review of Entomology* 63 (2018).
- Rasmann S. et al., "Recruitment of entomopathogenic nematodes by insect-damaged maize roots", *Nature* 434 (2005) — (E)-β-caryophyllene.
- Alborn H. T. et al., "An elicitor of plant volatiles from beet armyworm oral secretion", *Science* 276 (1997) — volicitin.
- Dussourd D. E. and Eisner T., "Vein-cutting behavior: insect counterploy to the latex defense of plants", *Science* 237 (1987).
- Agrawal A. A., *Monarchs and Milkweed* (2017) — sequestration and target-site insensitivity.
- Khan Z. R. et al., push-pull with *Desmodium* and napier grass, *Philosophical Transactions of the Royal Society B* 363 (2008).