Diatomaceous Earth
is made up of the silicate shells of microscopic sea creatures called diatoms.
With it's microscopic razor sharp edges, diatomaceous earth when spread in your garden or field physically kills and deters slugs without the use of harmful poisons.
It can be dusted or sprayed as a control for most leaf eating insects and is used to deter slugs and snails.
Natural DE makes a very effective natural insecticide. The insecticidal quality of DE is due to the razor sharp edges of the diatom remains. When DE comes in contact with the insects, the sharp edges lacerate the bugs' waxy exoskeleton and then the powdery DE absorbs the body fluids causing death from dehydration. Said more simply, DE kills insects by drying then up.

"FYI": Di-a-tom-a-ce-ous Earth sold for swimming pool filters is ineffective for insect control because it has been heated and chemically treated. It won't kill insects and it is very dangerous to breathe.