Hostas are ornamental garden plants that can fall victim to a number of pests, infections and diseases, and holes in the plant's leaves are most commonly the result of insect infestation, including slugs and snails. Because slugs are nocturnal, gardeners who find holes in their hostas' leaves may not have any other evidence that slugs are to blame since these insects tend to hide during the day. However, holes are a good indication that insects are to blame as other pests, such as rabbits and deer, tend to focus on the plant's shoots or simply strip leaves bare, and diseases tend to result in brown patches or white cottony patches.
While insect damage is typically the most reasonable explanation for hole-shaped damage in hosta leaves, there are alternate explanations. Specifically, hail damage can also explain round holes in hosta leaves. This possibility can be easily eliminated from consideration, though, if the gardener knows for sure that no hail or other precipitation has fallen in their area since the holes in the plant's leaves appeared.
Diagnosing insects as the problem may take some trial and error. Although slugs are nocturnal, their slime trails can dry and be visible during the day, so looking for these trails is a good way to identify their presence.