It is hard to say. If it's something big, then it could be deer or rabbits. If it is definitely something smaller then maybe snails, slugs, Japanese beetles, or weevils. It is hard to say for sure without knowing where you live or whether these are large bites or definitely an insect of some sort.
If it is a larger animal, then something as simple as a wind-chime may work, or motion activated sprinkler. Deer do not like scented flowers, so planting very fragrant flowers near plainer ones may do the trick.
If you are trying to keep smaller pests away, then you need to find out what they do not like. Slugs will stay away if you use oak leaf mulch. Supposedly Japanese beetles will stay away if you spray your plants with one tablespoon of dish soap per one gallon of water.