The lilac borer can be a serious pest of lilacs. A few bores can stress the plant and cause dieback. Heavy infestations can kill a plant quickly. Leaves may wilt and turn reddish brown if you have a borer problem. You may see sap and sawdust where the larvae or worm entered the plant. Plants that have been wounded or are under stress are more susceptible since the eggs are almost always laid in or near wounds that provide easy entry into the plant.
Your first line of control should be to prune out and destroy infested branches. The larvae can also be removed by digging them out of the trunk with a sharp knife or crushing them by inserting a wire into the tunnel. Preventative insecticide treatments can be made in mid April by spraying the main branches. Use insecticide materials that are labeled for borer control. Contact your county Extension office, Master Gardener hotline or local nursery for more information.
Reference: MU Guide 7190 Insect Borers of Fruit Trees