If the lilac can make some of its own roots, the trouble may be overcome in time. Insect Problems Lilac borer, Podosesia syringae . The larva bores into the stems of lilac and privet. It is white or yellowish with a dark brown head. May. On emergence, the adult leaves the pupal case protruding from the burrow.
Females lay eggs in patches on roughened places in the bark.
The larvae tunnel in the solid wood and sometimes in the pith for a distance of 8 to 10 inches. Foliage wilts and eventually the branches die back. The insects also cut across the grain so that the branch breaks. The larva pupates in the burrow after cutting a passageway to the bark. There is one generation each year.