Diatomaceous Earth: How It Works for Pest Control

Diatomaceous earth (DE) is a naturally occurring sedimentary powder made from the fossilized remains of diatoms — microscopic aquatic organisms with silica-based shells. It is one of the most widely used natural pest control substances, and unlike many natural remedies, its mechanism of action is well understood and physically — not chemically — based.

How It Works

Under a microscope, diatomaceous earth particles have sharp, jagged edges. When crawling insects move through DE, these particles scratch and damage the waxy outer layer (cuticle) of their exoskeleton. This waxy layer is critical for preventing water loss. Once it is compromised, the insect dehydrates and dies.

This is a mechanical action, not a chemical one. Insects cannot develop resistance to DE the way they can to chemical pesticides. According to the National Pesticide Information Center (NPIC), DE kills insects through desiccation regardless of species-specific chemistry.

The process is not instantaneous. Depending on the insect species, humidity, and amount of exposure, death can take anywhere from several hours to several days after contact.

Food-Grade vs. Pool-Grade: A Critical Distinction

Two types of diatomaceous earth are commercially available, and they are not interchangeable.

Food-Grade DE

• Contains less than 1% crystalline silica (typically 0.5-2%)
• EPA-registered for use as an insecticide
• Also used as an anti-caking agent in food storage and as a dietary supplement for livestock
• This is the type used for pest control in homes and gardens
• Generally recognized as safe (GRAS) by the FDA for use in food processing

Pool-Grade (Calcined) DE

• Heat-treated, which converts amorphous silica to crystalline silica (60-70%)
• Used for swimming pool filtration
• Not for pest control use
• The high crystalline silica content makes it a serious respiratory hazard
• Inhaling pool-grade DE can cause silicosis, a serious and irreversible lung disease

When purchasing DE for pest control, verify the label states “food grade” and confirms it is intended for insect control.

Which Pests It Works Against

DE is effective against crawling insects that move through treated areas. NPIC and university extension services document effectiveness against:

• Ants — applied along trails and entry points
• Cockroaches — applied in thin layers in cracks, behind appliances, and in harborage areas
• Bed bugs — used as one component of an integrated approach (not a standalone solution for bed bugs)
• Fleas — applied to carpets, pet bedding areas, and cracks in floors
• Silverfish — applied in damp areas where silverfish hide
• Earwigs — applied around foundations and in garden areas
• Centipedes and millipedes — applied along baseboards and in basements
• Carpet beetles — applied around carpet edges and under furniture
• Slugs — applied as a barrier around garden plants (loses effectiveness when wet)

DE does not work against:

• Flying insects that do not crawl through treated areas
• Insects that do not come in direct contact with the powder
• Pests in inaccessible areas where DE cannot be applied

How to Apply

Proper application is important. Too much DE is actually counterproductive — insects will walk around large piles rather than through them.

Indoor Application

• Apply a thin, barely visible layer using a hand duster, squeeze bottle, or small paintbrush
• Target cracks and crevices along baseboards, behind outlets and switch plates, under and behind appliances, inside cabinets (away from food), and around pipe penetrations
• Apply in attics, crawl spaces, and wall voids where insects harbor
• Leave in place as long as it remains dry — DE does not expire or degrade if kept dry
• Reapply if the powder gets wet, disturbed, or cleaned up

Outdoor Application

• Apply around foundations, in garden beds, and along pathways
• Create barriers around plant bases for crawling pests
• Apply in dry conditions — rain or irrigation washes DE away and wet DE is not effective
• Reapply after rain
• Avoid application on windy days, both for effectiveness and to avoid inhalation

What Not to Do

• Do not apply in large piles or thick layers — a light dusting is more effective
• Do not apply to wet surfaces
• Do not use pool-grade DE
• Avoid applying where wind will disperse it onto unintended areas
• Do not apply directly to plants in bloom — while food-grade DE is low risk for bees, direct application to flowers where pollinators are active is not advisable

Safety Considerations

Food-grade DE is considered low-risk for humans and pets by the EPA and NPIC, but it is not harmless:

Respiratory

The primary concern is inhalation. Even food-grade DE contains small amounts of crystalline silica, and the fine powder can irritate the lungs. The NPIC notes:

• Wear a dust mask during application
• Avoid creating dust clouds
• Do not apply in areas with active air circulation (near HVAC returns, for example)
• Children and people with respiratory conditions (asthma, COPD) should not be present during application
• Once settled, the powder poses minimal inhalation risk

Skin and Eyes

DE can be drying to the skin with prolonged contact and irritating to the eyes. Gloves and eye protection are appropriate during application.

Pets

Food-grade DE is used in some pet products and livestock feed. However:

• Dogs and cats should not be present during application to avoid inhaling the dust
• Some pet owners apply DE to pet bedding for flea control — this is a common practice, though veterinary opinions vary on its advisability due to respiratory concerns
• Do not apply DE directly to pets’ fur in heavy amounts; if used, apply lightly and avoid the face

Plants

DE is not harmful to plants and is sometimes applied directly to leaves for pest control. It can, however, affect beneficial insects that walk across treated foliage, so targeted application is preferable to blanket coverage.

Effectiveness and Limitations

DE has documented effectiveness as a physical insecticide, but several limitations affect real-world use:

Moisture sensitivity: DE only works when dry. In humid environments, basements with moisture problems, or outdoor applications in wet climates, effectiveness is reduced. This is perhaps its biggest practical limitation.

Speed: DE works slowly compared to chemical pesticides. It may take days to weeks to significantly reduce an insect population. This can be frustrating for people expecting immediate results.

Coverage gaps: DE only kills insects that walk through it. If application misses key travel routes or harborage areas, it will not reach the target population.

Not a standalone solution: For established infestations, DE works in combination with sanitation, exclusion, and other methods. It is a component of integrated pest management, not a silver bullet.

No repellent action: DE does not repel insects. It kills them after contact. Insects may walk through treated areas before the desiccation effect occurs.

How Long Does It Last?

When kept dry and undisturbed, DE remains effective indefinitely. The physical mechanism does not degrade over time. Indoor applications in dry areas — behind appliances, inside wall voids, in attics — can remain active for years. Outdoor applications need reapplication after rain or irrigation.

For serious infestations, consult a licensed pest control professional.