Slugs and Snails
Slugs and snails are attracted to the tender young shoots of seedlings and small unestablished plants. Feeding by these common garden pests is usually done at night so you may not notice anything until the following day where the seedlings could be eated down to soil level.
Product solution: Lightly sprinkle Slug and Snail Killer, Slug Bait or Natria Slug and Snail Control around susceptible seedlings.
Cultural methods: Try watering seedlings during the day so conditions are less damp and humid by the evening. Look for tell tale signs of silver trails left behind by these pests on the surface of the soil and seed trays. Place very fine horticultural grit on the surface of the seed compost to deter slugs and snails.