If populations are high you can treat with insecticidal soap. You may need several applications. Only the outer leaves remain. I have a 4 foot fence around my garden. I have a Japanese lilac tree that is loosing leaves from the new growth.
Older leaves show drop-out along the veins with sections missing.
There is no evidence of insects or black decay. The leaves of the older branches have pieces or sections missing without any sign of infestation. This starts out as a yellowing in small areas along the vein and then progresses to a dropout of the leaf substance. There are no canker spots or sap leakage, and none of the leaves have turned black. Japanese tree lilacs are sensitive to broadleaf weedkillers.