What Is Diatomaceous Earth — 3 Rules Before You Buy

Fossilised algae, ground to powder. It kills insects by cutting them, not poisoning them — which explains both what it is good at and the long list of things it is useless for.

What it actually is

Diatoms are single-celled algae that build shells of silica. Where ancient seas and lakes dried out, those shells left thick deposits of soft rock, and that rock ground to a powder is diatomaceous earth.

Under a microscope the particles look like broken glass. On an insect they behave like it: the edges scratch through the waxy cuticle that stops the insect losing water, and the insect dehydrates over hours or days.

Nothing is poisoned. That is the whole appeal — there is no metabolic pathway for an insect to evolve around, so resistance never develops the way it does with every insecticide.

Even food grade, wear a mask when applying it. The dust is an irritant, and the particle size that makes it effective is exactly the size that reaches deep into lungs.

Where it genuinely works

The pattern is consistent: a dry surface that stays dry, and an insect that has to walk across it. Everything it fails at fails on one of those two conditions.

How to apply it

• Thin and even. A visible heap is worse than a dusting — insects walk around obvious piles, and the effect needs contact, not volume
• A puffer or bulb duster, not a scoop. Even coverage matters more than quantity
• Target the route, not the area: along skirting boards, behind appliances, in cracks, across a doorway threshold, around the base of a stem
• Reapply after any rain outdoors. Every time
• Give it days, not minutes. Death is by dehydration; nothing happens immediately
• Wear a mask, and let it settle before pets and children return

The slug question

It is sold and shelved for slugs, and it is close to the worst thing on the list for them.

• Slugs are covered in mucus that resists abrasion and wets the powder as they cross
• They are active on damp nights and after rain — precisely when the powder is inert
• A slug that does get scratched often simply produces more mucus and keeps going

What works better: hand-picking after dark, beer traps, copper tape, removing damp daytime hiding places, and encouraging ground beetles, frogs and hedgehogs — all of which diatomaceous earth kills if they walk through it.

Honest summary

A good tool for a narrow job: dry, sheltered, crawling, indoors or nearly so. In that niche it is cheap, non-toxic, resistance-proof and genuinely effective.

A poor general garden insecticide, because outdoor conditions disable it most of the time and it is completely indiscriminate when they do not. It is neither the harmless organic dust it is marketed as, nor useless — it is a specific mechanism with specific conditions, and it works exactly when those conditions are met.