Diatomaceous earth is obtained from deposits of diatomite, fossilised sedimentary layers of tiny phytoplankton called diatoms, many of them originating at least 20 million years ago in the lakes and seas. These small creatures have survived with few major changes until today. Two major types exist, marine and freshwater. Diatoms are all single-celled organisms, actually small plants, that photosynthesize, producing much of the world's oxygen.
Diatomaceous earth is a form of amorphous silica that can kill insects by desiccation, by absorbing the oily or waxy outer cuticle layer by direct contact. When the thin, waterproof layer is lost, the insect loses water and dies. In addition to its desiccant action, it works abrasively to rupture insect cuticles. Some insects are more vulnerable due to their anatomy and physiology. Those with large surface area to volume ratios (often smaller insects) are more susceptible.
Please note the following information is based on USA research.
Diatomaceous earth has potential as a grain protectant. It is non-toxic, provides good protection when grain is stored properly, can be easily separated from the grain, by washing, and can possibly be recycled in storage bins. Toxicity is so low that DE is not counted as a foreign substance when grain is rated by the United States Department of Agriculture.
Important factors to be considered when using diatomaceous earth for grain protection are: the amount used, type of grain, particular pest problems, moisture levels in the grain, relative humidity, temperature, and length of storage. DE is not as effective on corn or sorghum, as it does not adhere well to the grains.
When moisture content of the grain is low eg 9.25% and large concentrations of DE (3500 ppm) are used, stored products are protected better with DE than with a standard malathion treatment, with almost complete protection from insect damage for 12 months.
Very few controlled studies of diatomaceous earth use in gardens and farms have been conducted. Amgrow Tomato and Vegetable Dust containing DE is effective in controlling aphids, caterpillars, codling moth, flies and ants. The major problem with outside use, other than possible toxicity to beneficial insects, is the nuisance value of the dust. The dust always needs to be reapplied after rain. It is extremely fine and does not adhere well to foliage unless applied when the plants are slightly moistened or with an electrostatic applicator.
DE is less effective in hot, humid weather. Control is better in areas of low rainfall.
To minimise death of beneficial insects, diatomaceous earth should be applied late in the afternoon or at night. Some parasitoid wasps that control pests will be adversely affected by the dust, even more than the insect pests. Excessive application rates contribute to greater losses of beneficial insects.
Diatomaceous earth can be added in small quantities to stored vegetable seed to protect it from pests.
Ingestion of diatomaceous earth is not toxic to mammals, but care should be taken to avoid getting the dust into your lungs or eyes. Protect yourself with goggles and a face mask. The only possible health effect comes from long term chronic exposure to quantities of the inhaled dust.
It is important that only natural diatomaceous earth be used for insect control not DE treated for swimming pool filter use, as this contains crystalline silica, a dangerous respiratory hazard.