Diatomaceous earth (DE) is a naturally occurring powder widely used in gardening and agriculture for non-chemical pest control. This fine substance is composed of the fossilized remains of diatoms, which are ancient, microscopic algae. DE acts as a dust insecticide against many common garden pests. Gardeners often question whether this method works effectively against slugs, which are mollusks and not insects. Determining its reliability requires understanding the specific mechanism by which this powder acts against these soft-bodied invaders.
The Mechanism of Diatomaceous Earth
Diatomaceous earth is a mechanical insecticide that does not rely on chemical toxins to kill pests. The powder is primarily made of amorphous silica, and its effectiveness stems from its microscopic structure. The particles have extremely sharp, abrasive edges that resemble tiny shards of glass.
When an insect crawls over the powder, these sharp particles adhere to its body. They score and penetrate the waxy, protective outer layer, known as the cuticle or exoskeleton. This action compromises the insect’s natural defense against water loss. The material is also highly absorbent, absorbing lipids and moisture from the insect’s body. This leads to rapid desiccation, making the substance effective against a wide range of crawling invertebrates as long as it remains dry.
Why Slugs Are Difficult to Control with Diatomaceous Earth
Slugs present a unique biological challenge because they are mollusks, not insects with hard exoskeletons. Their soft bodies are composed mostly of water, making them susceptible to desiccation, but they possess a powerful defense mechanism. The primary protection for a slug is the continuous production of a thick, viscous layer of mucus or slime. This slime acts as a physical barrier, preventing the sharp DE particles from contacting the slug’s permeable skin.
The moisture in the slime effectively neutralizes the abrasive and absorbent properties of the DE. As a slug moves across a barrier, the powder quickly absorbs the moisture, turning the DE into a wet, ineffective paste. This process allows the slug to shed the saturated particles and lay down a fresh layer of protective mucus. Consequently, DE is often an unreliable primary control method, as the slug can cross the barrier without fatal injury.
Maximizing Effectiveness Through Proper Application
Despite the challenges posed by slug biology, diatomaceous earth can be used as a deterrent and supplemental control measure if applied correctly. The primary factor for success is ensuring the powder remains completely dry. Even small amounts of moisture, such as heavy dew or light rain, render the DE useless. This happens because the silica particles absorb the water and lose their sharp, desiccating quality.
Application Steps
Application should involve creating a continuous, thick band around the plants or garden beds requiring protection. A break in the barrier, even a small one, provides a pathway for the slug to bypass the treatment. Timing the application for the evening is most effective, as slugs are nocturnal and most active after sunset. Gardeners must be prepared to reapply the powder frequently, especially after any irrigation, rainfall, or heavy morning dew.
Safety Considerations
For safety, only use food-grade diatomaceous earth in the garden. Pool-grade DE is chemically treated and contains crystalline silica, which is hazardous to inhale and unsafe for plants or pets. When applying, it is advisable to wear a mask to avoid inhaling the fine dust, which can irritate the lungs.