Diatomaceous Earth (DE) is a popular, naturally occurring substance used globally as a physical pest control agent. It is a fine powder made from the fossilized remains of ancient, single-celled aquatic organisms called diatoms. Gardeners often use it as a non-toxic alternative to chemical insecticides for managing pests. A common concern is its effect on beneficial soil dwellers like earthworms, which are vital for soil health and fertility. This concern stems from the physical nature of DE.
How Diatomaceous Earth Works
Diatomaceous Earth functions as an insecticide through a mechanical, not chemical, process. The powder consists of microscopic silica particles, which are the rigid, skeletal remains of diatoms. These fragments resemble shards of broken glass, giving DE its abrasive quality.
When an insect crawls across DE, the sharp edges cut into the pest’s protective outer layer, the waxy cuticle or exoskeleton. This coating retains the insect’s internal moisture. Once the cuticle is compromised, the highly absorbent DE draws out the oils and fats that form this barrier. The combined effect of abrasion and absorption causes rapid moisture loss, leading to death by dehydration.
Why Earthworms Are Generally Safe
Earthworms are fundamentally different from the insects that Diatomaceous Earth targets. Unlike insects, which rely on a hard exoskeleton for structural support, earthworms are annelids that lack this outer armor. They possess soft, permeable skin that must be kept moist at all times.
This moist skin is covered in a layer of protective mucus, which acts as a shield against the abrasive effects of the silica particles. Earthworms breathe through their skin (cutaneous respiration), meaning they require a wet surface to absorb oxygen. Any dry material, including DE, is rendered ineffective by the worm’s naturally moist environment and protective secretion.
Because they lack the waxy cuticle that DE targets, the mechanical action responsible for insect death cannot be carried out effectively. Earthworms typically live and feed beneath the soil surface in a naturally damp environment, which mitigates the drying power of DE. The few DE particles they may ingest while tunneling through the soil usually pass harmlessly through their digestive system and are expelled in their castings.
Distinguishing Food Grade from Filter Grade DE
Not all Diatomaceous Earth products are the same; the distinction between grades is based on processing and chemical composition. The two main types are Food Grade and Filter Grade (sometimes called Pool Grade). Food Grade DE is the version recommended for use in the garden, around people, and with animals.
Food Grade DE is largely composed of amorphous silica and contains less than 1% crystalline silica. This grade is mined and minimally processed, making it safe for agricultural settings. Filter Grade DE, however, is chemically and physically altered through a high-heat treatment process called calcination.
This heat treatment changes the silica structure, resulting in a product with a high concentration of crystalline silica, sometimes over 60%. Crystalline silica is hazardous if inhaled and is intended only for industrial filtration, such as in swimming pool filters. Using Filter Grade DE in the garden is unsafe and must be avoided due to its health risks.
Safe Application Practices in the Garden
Although earthworms are protected from the direct abrasive action of DE, gardeners should use careful application techniques to minimize indirect harm. The primary indirect risk is that heavy, widespread application of dry DE could excessively dry out the surrounding soil environment. Earthworms depend on soil moisture for survival and movement, so maintaining hydration is paramount.
Targeted application is the best approach, focusing the dust only on plant foliage, stems, or specific areas where pests are active. Avoid broad dusting over the entire garden bed. Apply DE in the evening or early morning when earthworms are less likely to be on the surface and beneficial pollinators are inactive.
DE loses its effectiveness as an insecticide when it becomes wet, as the particles clump together and become less abrasive. Watering the garden as usual will not compromise earthworm safety and will naturally wash the DE deeper into the soil where it is less active. Reapplication is necessary after rain or heavy watering, but a light, focused dust layer is safer than a thick application.