by Gary Pilarchik (The Rusted Gardener)
Diatomaceous earth or DE is not a poison. It is basically the sharp fossilized remains of microscopic sea creatures made up of silica. It is not harmful to us as it is a fine powder that feels like chalk. You should take precautions not create a lot of floating dust and breath it in as it can be an irritant in the lungs. However it is perfectly safe to handle. For crawling insects, it can be devastating. The microscopic dust or silica is extremely sharp and it will grind an insect's shell and body down over time.
The key to successful use of DE is dusting the areas the insects most often frequent and keeping it dry. I put this on an ant colony that invade my sunken container garden last summer. They were gone within 2 days. Fungus gnats and crawling insects can make their way into your seed starts. One strategy to reduce their numbers is to use the DE on the surfaces of your seed starts and on the leaves of the plants.
The whole key is to keep the dust dry. You want the insect to crawl across the dry dust and get the microscopic sharp silica into its joints. The sharp dust will grind in the joints and cause death. The video shows you how I use it on my seed starts.