There’s conflicting evidence on this. Some gardeners swear by crushed eggshells to deter and even kill slugs, snails, and any other crawling pests.  The sharp edges are said to cause cuts and dehydration, which supposedly kill these pests.  However, there have also been reports of slugs climbing over the crushed eggshells with seemingly no ill effects.  If you want to give it a try, crush your eggshells coarsely, to about the size of a baby aspirin and scatter them in a ring around vulnerable plants.