DE is highly absorbent and will then suck the moisture away from the pest until it dies of dehydration. After coming in contact with the diatomaceous earth, fleas may die in as little as 4 hours, though it is recommended to leave the DE for up to 48 hours(2 day) to ensure its effectiveness.
It remains effective as long as it stays dry, and often takes a week or more to start killing insects. Since the insects may have laid eggs by then, leaving on the diatomaceous earth for several weeks will help prevent a rebound.
