Slugs and snails are "gastropods." That scientific term means "stomach-foot." You know these hungry bugs fit the term if they attack your garden. If leaves have holes but no bugs, check your garden at night with a torch.
Caterpillars are butterfly and moth larvae. This diversified assortment of hole-makers includes inchworms and 4-inch hornworms. Some feed 24/7, others at night. These pests also eat leaves, stems, roots, buds, flowers, and fruits.
Chewing caterpillars lose their leaf holes as adults. Many mature chewing insects continue their harm. Chewing insects bite, shred, and grind leaves with their tooth-like mandibles to form holes.
Specialised mouthparts allow hole-making insects to eat liquids. Piercing-sucking pests include aphids and squash bugs. They use straw-like mouthparts to puncture leaves and drink sap.
Diseases cause certain leaf holes, not insects or gastropods. These leaf holes start as dots, unlike newly bitten edges or leaking holes. Spots spread swiftly when diseased. Anthracnose, Cercospora, and shot hole disease cause holes.