Lilac borer attacks plants in the lilac genus and is called ash borer when it attacks ash trees.
This caterpillar borer keeps its tunnel free of debris by pushing frass (feces and wood debris) out onto the bark surface. The borer exit hole is circular and about 1/4 inch in diameter. On lilac, the exit holes tend to be low on the large basal stems but can be higher on the trunk. On ash, exit holes tend to be at branch crotches. On heavily attacked lilacs and ash, leaves wilt and turn brown as they die. Stem growth dries and dies. On ash, the young tree can be killed by this insect.
The adult is a clearwing moth that look like a wasp. Eggs are laid on the bark. The larvae eat their way through the bark where they feed on the phloem tissue. Eventually the larvae tunnel deeper and feed on the sapwood. The adult female borer lays almost 400 eggs in less than a week and dies soon after laying the eggs. The eggs are laid near injuries including pruning wounds.
Keep lilac shrubs properly pruned, removing stems over 2 inches in diameter.Avoid pruning lilacs and ash during summer months. Keep lilacs healthy - water and fertilize as needed to maintain plant vigor (however over fertilation can prevent flowering). Use an insecticide if needed when vanhoutte spirea is in full to late bloom. Using a phermone trap can assist in timing inseticide applications. Apply treatment two weeks after peak moth catch.