Diatomaceous Earth (DE) is a fine powder derived from the fossilized remains of ancient, single-celled aquatic organisms called diatoms. This naturally occurring sedimentary rock powder is composed primarily of silica. DE is widely regarded as a safe and effective physical insecticide for managing various garden pests, making it a popular choice for gardeners seeking alternatives to chemical sprays.
Understanding Diatomaceous Earth
The insecticidal power of this white powder lies in its unique microscopic structure. Under a microscope, Diatomaceous Earth particles appear as sharp, jagged silica remnants, which are abrasive to insects but harmless to humans. This mechanical action is fundamentally different from the chemical toxicity of synthetic pesticides.
When an insect crawls over the powder, the sharp edges of the DE particles cut into its exoskeleton. The exoskeleton is coated in a protective, waxy layer of lipids that prevents water loss. The porous silica then absorbs these protective lipids and fats.
This dual action of physical damage and lipid absorption destroys the insect’s ability to retain moisture. This leads to desiccation, causing the pest to rapidly dehydrate and die. Because insects cannot develop resistance to this physical mechanism, DE remains an effective long-term pest management solution.
It is important to purchase only Food Grade Diatomaceous Earth for gardening purposes. This grade contains amorphous silica and has a very low concentration of crystalline silica, making it safe for use around food crops, people, and pets. Filter Grade DE, often used for pools, is heat-treated and contains higher, potentially harmful levels of crystalline silica. Filter Grade DE should never be used on plants or in the garden.
Targeted Pests and Application Methods
Diatomaceous Earth is effective against any pest with an exoskeleton that must crawl over the powder to reach the plant. The product must make direct physical contact with the insect to initiate the mechanical process of desiccation. Common garden invaders controlled by DE include:
- Aphids
- Slugs
- Snails
- Earwigs
- Squash bugs
- Cucumber beetles
- Cabbage worms
Dry Dusting
The most effective application method is dry dusting, which is best performed in the morning after the dew has dried or during dry weather. Using a bulb duster or a simple flour sifter, lightly coat the foliage. Focus particularly on the undersides of leaves where many pests hide. Dusting the soil surface around the base of the plant creates a barrier that prevents crawling insects from reaching the stem and leaves.
Wet Slurry Application
For hard-to-reach areas or dense foliage, DE can be applied as a wet slurry. Combine four tablespoons of DE with one gallon of water and place it into a sprayer. The mixture must be agitated frequently because the powder does not dissolve and will settle quickly at the bottom of the container.
Applying DE as a slurry is less effective initially because the product only works when dry. Once the water evaporates, a fine residue of the powder is left behind on the plant surfaces. This wet application ensures thorough coverage, but dry dusting remains the most potent form of control.
Essential Precautions and Safety
While Food Grade Diatomaceous Earth is safe for use on food crops, certain precautions must be taken to protect the user and beneficial organisms. The fine powder poses an inhalation risk because the microscopic silica can irritate the lungs. It is necessary to wear a dust mask or respirator during application to avoid breathing in the airborne particles.
The mechanical action of DE is non-selective, meaning it will harm beneficial insects and pollinators just as easily as it harms pests. To mitigate this risk, avoid applying the powder directly to the blooms or flowers of the plants. Application should also be timed for late evening or very early morning when pollinator activity is low.
DE only works when dry, and its efficacy is immediately negated by moisture. The powder must be reapplied after any significant rainfall or overhead watering. Once wet, the particles clump together, reducing their sharp, abrasive edges and rendering them ineffective against pests until they dry.
To ensure the product remains effective over time, it must be stored in a dry, sealed container away from moisture. Proper storage maintains the powder’s integrity.