Diatomaceous earth (DE) is an inert dust mined
from the remains of fossilized silica shells of certain algae known as diatoms.
It is composed of approximately 3% magnesium, 33% silicon, 19% calcium, 5%
sodium, 2% iron as well as other trace minerals such as titanium, boron,
manganese, copper and zirconium.
DE works in
two ways to kill an insect. 1) It absorbs water-protecting fats and oils from
the outer, waxy layer on the surface of the insect and causes desiccation, or dehydration. 2) Being highly abrasive, DE scrapes and cuts
the insect’s cuticle layer, contributing to more desiccation. However, it is virtually
nontoxic to mammals.
Around the home, dusting the powder on the floor, carpets and crevices will kill cockroaches, silverfish, ants, and fleas. Since the powder must stay dry in order to be effective, repeat applications may be necessary in humid areas; DE will be more effective in drier climates. It is also used to control beetles where grains are stored.
The same basic principles used to kill indoor insects can be applied to insects which attack plants outdoors. Some gardeners will dust plants with diatomaceous earth in order to kill insects such as aphids and beetles which feed on the plants. DE is moderately effective against slugs and snails as long as the material remains dry. The problem with outdoor use is high humidity and rainfall. When DE becomes wet, its effectiveness is diminished. DE, however, can be mixed with water as its carrier and applied to such targets as ant nests. When DE dries out naturally, it will begin to take effect. Nonetheless dusting plants with DE is the most effective application method. The dust can cause eye irritation so wearing goggles and a dust mask is recommended.
Insecticidal DE is not the same as the DE used in swimming pool filters. Other chemicals are added to pool grade DE, and the product is heat treated. This causes it to assume a crystalline form and is a respiratory hazard. Pool grade DE should never be used for pest control.
