I have found that crushing eggshells and making a border of
'em around individual plants will deter slug'n'snails.....they don't like to
cross the jaggy surface.
Eggshells in large pieces around your brassica-family
vegetables (cabbage, cauliflowers, buk choy etc) deters cabbage
butterflies...they will not lay their eggs, as they think another cabbage
butterfly is there already.
However, if it is too late, and you have caterpillars on
your cabbages already...pick off all you can, and put 'em in a blender with
some water, and spray 'em back onto the cabbages. You will not get any more
eggs laid there. Of course..remember to wash your cabbages well before use!