DIATOMACEOUS EARTHS, A GROUP OF NATURAL INSECTICIDES

Diatomaceous earth (DE) is a geological deposit consisting of the foss ilised skeletons of numerous species of siliceous marine and fresh wat er unicellular organisms, particularly diatoms and other algae. Many o f these fossilised sedimentary layers originated at least 20 million y ears ago in the lakes and seas of the Eocene and Miocene epochs. After quarrying, crushing and milling, a fine light dust is obtained, conta ining porous particles with certain abrasive properties and the abilit y to absorb lipids to about three or more times the particle mass. Any diatomaceous earth with high oil absorbing capacity is a potential in secticide. Beyond the absorbing capacity, the size of particles, unifo rmity and shape of the particles, pH, and the purity of formulation af fect the compound's insecticidal efficacy. Insecticidal diatomaceous e arth should be a highly pure amorphous silica, having particles of equ al diameter (< 10 mu m), pH < 8.5, containing the least possible numbe r of clay particles and less than 1% crystalline silica. The particles of diatomaceous earth are easily picked up by rough bodied insects. T he particles damage the cuticle through hydrocarbon absorption and abr asion making the cuticle permeable to water which rapidly leaves the i nsect's body causing death from desiccation. In this paper the advanta ges 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 g reat variation in physical properties and efficacy against insects amo ng DEs from different geographical locations. Environmental factors af fecting efficacy and the potential scope of DE use are discussed. (C) 1998 Published hy Elsevier Science Ltd. All rights reserved.