Rapid assessment of the insecticidal value of diatomaceous earths without conducting bioassays

The efficacy of diatomaceous earth (DE) against insects depends greatly on several physical properties of the diatom particles. Ideally, active DE should have an high amorphous silicon dioxide content with a uniform particle size (less than 10 mu m), a high oil sorption capacity, a large active surface, and very little clay and other impurities. The analysis of physical and chemical properties of DE is time-consuming and expensive, and ran be conducted only by experts at specially equipped laboratories. Therefore, in the past, bioassay was considered the most important criterion for the assessment of the efficacy of DE against insects. These methods are relatively expensive and time-consuming also, since they require an expert and a well-equipped entomological laboratory. After conducting numerous experiments with 36 different diatomaceous earths or formulations collected from the U.S.A., Mexico, Canada, Australia, Japan, China, and Macedonia (Europe), results indicate that the efficacy of DE against insects depends on different properties of the diatom particles. Properties include the ability of DE particles to reduce bulk grain density (test weight), DE tapped and loose density, the tendency of DE particles to adhere on the grain surface, particle size distribution, diatom shape, and pH. It is possible to evaluate and to predict the insecticidal value of diatomaceous earth mainly by very simple and low-cost analysis of these properties of DE without bioassays or extensive physical and chemical analyses. (C) 1997 Elsevier Science Ltd.