Leaf miners cause unsightly tunneling beneath the surface of holly and other leaves. The adults emerge in May and that is the time to take action. Get in touch with your local Cooperative extension office (Penn State) for up to date chemical controls in your area. Any infected leaves that drop should be cleaned up right away as these contain eggs that will hatch in due course and re-infect the hollies. If you see leaves on the plant that have been tunneled, remove these as well. Unless the infestation is really severe, it is probably not necessary to resort to chemical controls.