Diatomaceous earth (DE) is a naturally occurring soft sedimentary rock composed of the fossilized remains of ancient single-celled algae called diatoms. These microscopic skeletons are made primarily of silica, which is mined and ground into a fine, abrasive powder. DE is an effective method for controlling caterpillars in the garden, particularly smaller ones, through physical action rather than chemical toxicity.
How Diatomaceous Earth Kills Insects
Diatomaceous earth functions as a mechanical insecticide, meaning it does not rely on chemical compounds to poison the pest. The fine powder consists of microscopic particles with sharp, jagged edges. When a caterpillar crawls across dry DE, these particles contact its protective outer layer (exoskeleton).
The exoskeleton is covered in a waxy layer that prevents internal moisture loss. The abrasive action of the DE particles scratches and pierces this coating, compromising the insect’s defense against dehydration. This physical damage allows bodily fluids to escape, leading to desiccation.
The porous silica particles absorb the lipids and moisture released from the damaged exoskeleton. This dual action of abrasion and absorption causes the caterpillar to lose water rapidly, leading to fatal dehydration. Since the killing mechanism is physical, pests cannot develop resistance to DE.
Proper Application for Garden Caterpillars
Effective use of diatomaceous earth requires direct contact, and the powder must remain completely dry. DE loses efficacy when wet because moisture softens the sharp edges of the silica particles, rendering them harmless. Apply DE only when no rain or heavy dew is expected for at least 48 to 72 hours.
The best time for application is early morning after the dew has evaporated or in the late afternoon when the air is dry. Caterpillars must be thoroughly coated with the powder. Use a hand duster or a fine-mesh sifter to apply a light, even layer of dust directly onto the foliage. Pay special attention to the underside of leaves where many caterpillars hide and feed.
Dusting the entire plant ensures that any caterpillar crawling on the plant will encounter the abrasive particles. Reapplication is necessary after any rainfall or overhead watering because the water washes the powder away or causes it to clump.
Safety Considerations and DE Grades
When purchasing diatomaceous earth for garden use, select “Food Grade” DE, which contains less than 1% crystalline silica. This amorphous silica form is considered safe for use around humans, pets, and livestock. Never use “Filter Grade” or “Pool Grade” DE.
These grades are heat-treated (calcined) for filtering capabilities, converting silica into crystalline silica, which is hazardous if inhaled and intended only for industrial systems. While food-grade DE is non-toxic, inhaling the fine powder can cause respiratory irritation. Wearing a dust mask while applying the product is a precaution to avoid irritation of the nasal passages and lungs.
The abrasive nature of DE means it will also kill beneficial insects, such as bees and ladybugs, if they come into direct contact with the powder. To protect pollinators, apply the dust in the late evening when bees are less active and avoid dusting open blossoms. Focusing the application only on areas where caterpillars are actively feeding helps minimize the risk to beneficial species.