Cutworms, the larval stage of several moth species, represent a common and frustrating problem for home gardeners, especially those trying to establish delicate seedlings. Diatomaceous earth (DE) is an effective physical control method for managing these destructive pests in the garden environment. DE is a naturally occurring, fine powder composed of the fossilized remains of tiny aquatic organisms called diatoms. This natural, abrasive material provides an environmentally conscious solution for protecting young plants without relying on chemical pesticides.
Identifying Cutworms and Their Damage
Cutworms are plump, soft-bodied caterpillars that are typically dull grayish or brownish, ranging from one to two inches in length when fully grown. They are known for their nocturnal feeding habits, spending their days hidden just beneath the soil surface or under garden debris. When the larvae are disturbed, they immediately curl into a tight “C” shape.
The most recognizable damage inflicted by these pests is the severing of young plant stems at or just below the soil line. This “cutting” activity often results in an entire seedling toppling over. Other species, known as climbing cutworms, may move up the stem to feed on leaves, buds, or fruit, while subterranean types target the roots and underground parts of the plant. The damage is most severe in early spring when plants are small and have tender tissue, making young transplants highly vulnerable.
How Diatomaceous Earth Kills Garden Pests
Diatomaceous earth is a mechanical insecticide, meaning it kills insects through physical interaction rather than chemical poisoning. It is derived from the fossilized silica skeletons of diatoms, which are microscopically sharp and abrasive. This fine powder is deadly on contact to cutworms or any other crawling insect.
When an insect crawls over the applied DE, the abrasive particles damage the insect’s protective waxy outer layer, or exoskeleton. The powder also absorbs the oils and fats from the insect’s cuticle. This dual action compromises the insect’s barrier against moisture loss, leading to rapid desiccation and eventual death. Since the mechanism is physical, pests cannot develop resistance to diatomaceous earth.
Best Application Techniques for Cutworm Control
The most effective use of diatomaceous earth against cutworms involves creating an impassable barrier on the soil surface, as the pests must crawl across it to reach the plant. Applying a thin, even layer of the dry powder around the base of vulnerable seedlings is the primary technique. This dust ring should completely encircle the plant, ensuring there are no gaps that would allow a cutworm to cross without contact.
Timing the application to coincide with the cutworm’s behavior maximizes the powder’s impact. Since cutworms emerge to feed at night, applying the diatomaceous earth in the evening is recommended for peak effectiveness. For new seedlings, a perimeter barrier that is about one inch wide is sufficient to protect the stem. Some gardeners also incorporate the powder lightly into the top inch or two of dry soil surrounding the plant to target pests hiding just below the surface.
Regular reapplication is necessary to maintain this defensive barrier. Any disturbance, such as heavy rain, morning dew, or overhead watering, will cause the DE to clump and lose its abrasive quality. The barrier should be checked every few days and immediately refreshed after any moisture event to ensure continuous protection.
Environmental Factors and Safety Precautions
The efficacy of diatomaceous earth is highly dependent on environmental conditions, primarily the presence of moisture. Once the powder becomes wet, its microscopic structure changes, and it ceases to be an abrasive and absorbent dust. Gardeners must reapply the product after rainfall or heavy watering to restore the protective barrier.
When purchasing DE for garden use, select food-grade diatomaceous earth, which is amorphous silica and considered safe for use around food crops. Filter-grade or pool-grade DE, which is heat-treated and contains crystalline silica, should never be used for pest control as it is harmful if inhaled.
The fine dust can irritate the lungs, eyes, and skin. Wearing a dust mask or respirator, along with gloves and eye protection, is a necessary precaution during application to prevent inhalation. While DE is a targeted control for crawling pests like cutworms, it can harm beneficial insects such as bees if applied directly to open flowers. Consequently, it should be applied only to the soil surface where the cutworms crawl, keeping it away from plant blossoms.