Diatomaceous earth, a fine powder composed of the broken shells of single-celled, fossilised aquatic plants called diatoms, acts as a natural insecticide. While research into its efficacy is sketchy at best, it has nonetheless gained popularity with organic farmers for use as a pesticide for plants and livestock. Diatomaceous earth is used both internally as a feed additive and externally.
Diatomaceous earth comes in two forms, calcined and amorphous. Calcined diatomaceous earth is heat-treated to increase the percentage of sharp, crystalline silica fragments that, when inhaled, sometimes cause a progressive and occasionally fatal lung condition called silicosis. Fortunately, natural (uncalcined) diatomaceous earth has very low levels of crystalline silica. While this product causes lung irritation, it is not listed as carcinogenic or associated with silicosis. If lung irritation occurs while working with it, seek fresh air immediately and see a medical professional.
Skin and Eye Irritation
Diatomaceous earth kills insects and other parasites by scratching and damaging the protective cuticle that covers their body. This damage causes the creature to dehydrate over a period of hours or days until it finally dies. This same effect damages human skin, causing dry hands when used without gloves or other protection. Diatomaceous earth also irritates the user's eyes, so protective goggles are recommended. Wash hands with soap and water if affected. Rinse eyes with clean water and remove contact lenses. See a doctor if irritation persists.
Harm to Beneficial Insects
Unfortunately, beneficial insects are just as susceptible to fatal cuticle damage after exposure to diatomaceous earth as undesirable insects. Specifically, it has been shown to be harmful to honey bees, an insect that is economically important as a crop pollinator. To minimise accidental bee deaths, don't use diatomaceous earth on plants in flower. While some bees still come into contact with the substance when used on non-flowering plants, they are not actively attracted to the area.
Poor Results in Cattle Testing
Some organic cattle producers feed their animals diatomaceous earth to combat internal parasites and coccidiosis. This is an off-label use of the product, and relying on it instead of proven coccidiostats and dewormers has been shown in one nutritional study to reduce weight gain in feedlot conditions by 9 per cent. In this same study, diatomaceous earth was shown to be ineffective against coccidia, a protozoal parasite. Relying on diatomaceous earth for internal parasite control in cattle may compromise animal health.