Using Dusts: Diatomaceous earth (FOOD GRADE) and silica aerogels are chemically inert dusts that have proven effective against fleas, killing them by dehydration. Once applied, dusts will take a few days to work. One application in the spring should be enough each year to control fleas. To use dust, sprinkle it sparingly onto carpeting, upholstery, and pet bedding. After it is sprinkled about, allow it to sit for a day or two, then vacuum as usual.