I douse my plants with a solution of 1 part ammonia:15 parts water. I tend to pour it around the base of the plant rather than right on the leaves. You can go as strong as 1:10, but the weaker solution seems to work OK and the ammonia goes farther.
Last year, however, I'm convinced that at least half of the damage to my hostas was done by earwigs. They can be trapped or dispatched with soap solution 1:40.
