Diatomaceous Earth is the fossilized remains of ancient algal shells called Diatoms or Phytoplankton.
When insects (ants, fleas, flies, cock-roaches to name but a few) come into contact with DE it ultimately kills them. The tiny particles of the sharp and hard DE stick to the insect, abrasively rub through and serrate their epi-cuticle and joints, absorb their bodily fluids and they die of dehydration. This process is purely mechanical not chemical. As such there is no build up or chemical residue, the insect cannot build up immunity or become tolerant to it. There is no plant or livestock with-holding period for human consumption.