If you are finding larvae boring into younger shoots showing signs of wilting or die-back, it is probably the sombre twig pruner affecting your trees.
These longhorn borer beetles usually go for trees that are stressed, e.g. trees suffering water stress or trees stressed by soil-borne pests (e.g. nematodes) or diseases. These indigenous beetles attack plants in the family Oleaceae, including wild and cultivated olives, ash (Fraxinus) and privet. At present there is no other control options except cutting off and destroying affected branches. There are no chemicals registered for control. By the time infestation is noticed, the damage has been done and even if chemical control was available, it would not reverse the shoot die-back. Therefore, the best way to manage the pest is to try and ensure that trees are not stressed and to remove and destroy infested shoots. Because commercial olive production is relatively new in SA, very little research has been done on indigenous olive pests, like this borer, which do not occur in the traditional olive producing countries.
-Dr. Elleunorah Allsopp, entomologist of the Agricultural Research Council (ARC)