Diatomaceous earths, a group of natural insecticides

Diatomaceous earth (DE) is a geological deposit consisting of the fossilised skeletons of numerous species of siliceous marine and fresh water unicellular organisms, particularly diatoms and other algae. Many of these fossilised sedimentary layers originated at least 20 million years ago in the lakes and seas of the Eocene and Miocene epochs. After quarrying, crushing and milling, a fine light dust is obtained, containing porous particles with certain abrasive properties and the ability to absorb lipids to about three or more times the particle mass. Any diatomaceous earth with high oil absorbing capacity is a potential insecticide. Beyond the absorbing capacity, the size of particles, uniformity and shape of the particles, pH, and the purity of formulation affect the compound's insecticidal efficacy. Insecticidal diatomaceous earth should be a highly pure amorphous silica, having particles of equal diameter (< 10 mu m), pH < 8.5, containing the least possible number of clay particles and less than 1% crystalline silica. The particles of diatomaceous earth are easily picked up by rough bodied insects. The particles damage the cuticle through hydrocarbon absorption and abrasion making the cuticle permeable to water which rapidly leaves the insect's body causing death from desiccation. In this paper the advantages and disadvantages of diatomaceous earth as an insecticide and its scope of use are discussed. Briefly, the description of DE application, the modes of action and its uses are described. Tests results show great variation in physical properties and efficacy against insects among DEs from different geographical locations. Environmental factors affecting efficacy and the potential scope of DE use are discussed. (C) 1998 Published hy Elsevier Science Ltd. All rights reserved.