What is chewing holes in my plant leaves?

There are a number of different critters that could be munching on your plants:

• Slugs and snails are the most common cause of holes in leaves, but they often remain unseen because they feed at night. Slugs and snails tend to create through-and-through, irregular-shaped holes in leaves, not along the edges. Slug holes always have smooth green edges.
• Caterpillars are likely if you've seen butterflies that may have layed eggs on the plants. Some types of caterpillars, known as cutworms, chew through stems at soil level, causing plants to keel over. 
• Some sawfly larvae create irregular, transparent holes that do not extend all the way through a leaf. This makes the holes appear transparent. Other types of sawfly larvae cluster along leaf edges, so that up to a dozen worm-like creatures are feeding on the same leaf.

• Deer and rabbits can also be garden pests. Deer tear foliage and create jagged edges. Rabbits leave neatly clipped stems low to the ground. They also chisel away bark on woody plants, especially in fall and winter.