Diatomaceous Earth (DE) is a naturally occurring, fine, white powder used widely for natural pest control. It is derived from the fossilized remains of microscopic, single-celled aquatic organisms called diatoms. For insect control applications, only the food-grade variety of DE should be used, as industrial grades may contain harmful additives. Thrips are a common and highly destructive garden pest, and this article confirms that DE kills them by a physical process.
Understanding Thrips: The Target Pest
Thrips are extremely small insects, typically measuring 1 to 2 millimeters in length, ranging from pale yellow to dark brown or black. They feed by puncturing individual plant cells and sucking out the contents, creating visible damage. This feeding results in characteristic stippling, silvering, or bronzing on plant foliage as the cells are emptied of chlorophyll.
Their complex life cycle makes thrips difficult to manage using contact insecticides. Females lay eggs directly inside the soft tissue of leaves, flowers, or stems, where they are protected from topical treatments. The pest develops through two actively feeding larval stages, but the non-feeding prepupal and pupal stages drop to develop in the soil or leaf litter. Because they spend protected time within the plant tissue and the soil, a single application of a surface-level pesticide will miss a significant portion of the population.
How Diatomaceous Earth Kills Insects
Diatomaceous earth functions as a mechanical insecticide, meaning pests cannot develop a resistance to it. The fossilized diatoms are composed primarily of silica, and microscopically, the powder has sharp, abrasive edges. These minute shards adhere to the insect’s body as thrips crawl across the treated surface.
The primary mechanism involves physical abrasion and desiccation. As the DE particles make contact, they scratch and damage the insect’s protective outer layer, the cuticle. The highly porous and absorbent DE then soaks up the thin, waxy lipid layer that covers the insect’s exoskeleton.
This protective waxy layer regulates the insect’s internal moisture levels and prevents water loss. Once this barrier is compromised and the lipids are absorbed, the thrips rapidly lose bodily fluids. The insect consequently dies from fatal dehydration, often within 48 hours of exposure.
Application Strategies for Controlling Thrips
Controlling thrips requires a comprehensive approach that targets both the plant foliage and the soil. Dry dusting is the most common method, which involves applying the fine powder directly to the plant using a bulb duster for light, even coverage. It is particularly important to apply the powder to the undersides of leaves, as this is where thrips often hide and feed.
Since the non-feeding pupal stages occur in the growing medium, applying a layer of DE to the surface of the soil is equally important. This layer acts as a barrier, killing the prepupae and pupae as they crawl through it to complete their development into flying adults. A slurry method can also be used by mixing DE with water and spraying it onto the plants.
For the slurry method, the water acts only as a carrier to distribute the powder into hard-to-reach areas, like leaf axils and crevices. The insecticidal property of the DE only becomes active once the water fully evaporates, leaving behind a fine, dry coating of silica particles. Application should be light enough to be barely visible, as excessive caking can hinder the plant’s ability to photosynthesize.
Environmental Factors Limiting Effectiveness
The effectiveness of diatomaceous earth relies completely on its ability to remain dry and powdery on the treated surfaces. Because DE is highly absorbent, any moisture, whether from rain, heavy dew, or overhead irrigation, will cause the powder to clump and become inert. Once the DE is wet, its porous structure is saturated, and it can no longer draw moisture from the insect’s cuticle, requiring reapplication once the area dries.
High relative humidity in the air can also significantly reduce the product’s killing power, with efficacy dropping sharply when humidity rises above 65%. In moist conditions, the DE absorbs water vapor from the surrounding environment instead of from the insect, which diminishes its capacity for desiccation.
Because DE only kills by contact, it is ineffective against the eggs laid inside the plant tissue or against the pupae protected deep within the soil. Consistent, repeated application is necessary to kill newly hatched larvae and emerging adults as they become exposed to the dry powder.