I left this reply to a similar post/question on another forum a few days ago. You may be interested:

The immature or larval stages of insects, mainly beetles or moths, that feed on wood rather than leaves or plant juices are referred to as borers. They leave telltale evidence of their presence called frass, which resembles and includes wood (saw)dust. All the woody parts of the tree from the buds and twigs to the trunk and roots are susceptible to attack. Most borers are attracted to already stressed and weakened trees, suffering from drought, injury or disease, but a few borer species can successfully attack healthy, vigorous trees.
Once borers have a foothold, they can be difficult to eradicate. Sprays are successful only if they are applied to the tree when the adults are active and laying eggs. Consequently, the proper timing of insecticide sprays is critical for effective chemical control of borers. For this reason, you need to isolate one of the pests & have it identified so you can apply the appropriate remedy at the appropriate time. Borers are a pain.

Often, if you can find the entry holes, they can be killed by inserting a fine wire in the hole. Systemics are usually not effective. So, since borers usually attack weakened trees, the best defense is to insure the cultural requirements of light, water, and nutrient availability are being met so the tree maintains good vigor.

Al