See photo. Almost all leaves on my lilac bushes to a height of about 3 ft (in some places damage up to 6 ft) have scalloped edges that look like the work of the leaf cutter ants in nature films of Central America.
What's eating them and what can I do to stop the damage?
Arapahoe County Colorado
This leaf-edge notching is the handiwork of root weevils, probably the lilac root weevil. Root weevils feed nocturnally so they are not even on the leaves during daylight hours, when you might be looking for the cause of the leaf notching.
Feeding on leaves by weevils causes little harm to the plant, as less than 10% of the leaf surface area is removed. However, you may want to control numbers of root weevils with an insecticide, as they can become a nuisance "accidental tourist" in your house. See for more detail: http://www.ext.colostate.edu/pubs/insect/05551.html